State of Science
Bedrock fracture influences on geomorphic process
and form across process domains and scales
Daniel N. Scott* and Ellen E. Wohl
Department of Geosciences, Colorado State University, Fort Collins, CO USA
Received 2 March 2018; Revised 5 June 2018; Accepted 10 July 2018
*Correspondence to: Daniel N. Scott, Department of Geosciences, Colorado State University, 1482 Campus Delivery, Fort Collins, CO 80523, USA. E-mail: dan.
scott@colostate.edu
ABSTRACT: Fractures are discontinuities in rock that can be exploited by erosion. Fractures regulate cohesion, profoundly affecting
the rate, style, and location of Earth surface processes. By modulating the spatial distribution of erodibility, fractures can focus
erosion and set the shape of features from scales of fluvial bedforms to entire landscapes. Although early investigation focused on
fractures as features that influence the orientation and location of landforms, recent work has started to discern the mechanisms
by which fractures influence the erodibility of bedrock. As numerical modeling and field measurement techniques improve, it is
rapidly becoming feasible to determine how fractures influence geomorphic processes, as opposed to when or where. However,
progress is hampered by a lack of research coordination across scales and process domains. We review studies from hillslope,
glacial, fluvial, and coastal domains from the scale of reaches and outcrops to entire landscapes. We then synthesize this work to
highlight similarities across domains and scales and suggest knowledge gaps, opportunities, and methodological challenges that
need to be solved. By integrating knowledge across domains and scales, we present a more holistic conceptualization of fracture
influences on geomorphic processes. This conceptualization enables a more unified framework for future investigation into fracture
influences on Earth surface dynamics. © 2018 John Wiley & Sons, Ltd.
KEYWORDS: fracture; erosion; process; geomorphology; topography
Introduction
Earth’s surface can be characterized on a broad scale by discontinuities,
or fractures, which separate otherwise continuous
Earth materials. As a first-order approximation, fractures have
been hypothesized to be the dominant control on erosion rates,
effectively acting as the mechanism by which tectonic stress
shapes the landscape (Molnar et al., 2007). Fractures set the
primary boundary condition for plucking by glaciers and rivers,
which may be the most efficient mechanism of eroding
bedrock (Hallet, 1996; Whipple et al., 2000a), and in doing
so can set the speed limit for the evolution of landscapes
(Whipple, 2004). Investigators have long recognized the importance
of fractures in influencing hillslope stability (Gilbert,
1904); the location and orientation of channels from the scale
of gullies to entire river networks (Hobbs, 1905; Gilbert,
1909); and erosion rates (Bryan, 1914). However, we lack a
unified theory of how fractures impact the development of
Earth’s surface across spatial and temporal scales and across
diverse geomorphic process domains (Montgomery, 1999).
In recent years, the focus of geomorphology has shifted
towards understanding geomorphic processes utilizing
conceptual models to inform geomorphic laws that describe
the transport of Earth material across scales and domains
(Dietrich et al., 2003; Wohl et al., 2016). For processes
influenced by fractures, this effort has led to important conceptualizations
and models of surface processes such as fluvial
plucking (Chatanantavet and Parker, 2009; Lamb et al., 2015),
glacial quarrying (Hallet, 1996), coastal erosion (Naylor and
Stephenson, 2010), and hillslope stability (Clarke and Burbank,
2010; Loye et al., 2012). In these domains, we can rudimentarily
model fractures acting as controls on the rate, style, and spatial
occurrence of geomorphic processes. However, the lack of synthetic
understanding of the impacts of fractures on geomorphic
process and form is starting to limit our progress. For instance, research
into the quarrying of fracture-bound blocks by glaciers
has progressed to include fracture orientation as an explicit
control on quarrying (Lane et al., 2015), whereas research into
fluvial plucking is only just starting to suggest a potential role
of orientation in controlling erosion rate (Lamb et al., 2015).
Synthesis of the various impacts of fractures on geomorphology
will facilitate the application of knowledge across process
domains to both fundamental and applied research questions.
Here, we review current understanding of the mechanisms
by which fractures influence the rate, style, and location of
erosion, as well as feedbacks between erosion and fracture
propagation (the widening or lengthening of a fracture). We
organize our review into three sections: (1) effects of fractures
on erosion rates and styles, (2) fracture controls on the shape,
orientation, and location of landforms and erosion, and (3)
EARTH SURFACE PROCESSES AND LANDFORMS
Earth Surf. Process. Landforms 44, 27–45 (2019)
© 2018 John Wiley & Sons, Ltd.
Published online 7 August 2018 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/esp.4473
feedbacks between erosion and fracture propagation that act to
either accelerate or retard further erosion. We then synthesize
this understanding across process domains and scales and
identify logical next steps to address existing knowledge gaps.
Definition of scope
We use the definitions of Selby (1993) to clarify the meaning of
fracture as any parting that allows open space or discontinuity
between otherwise intact masses of Earth material. Specific
types of fractures such as joints (fractures with no shear along
the fracture surface), faults (fractures with displacement), and
fractures following foliation or bedding will generally not be
differentiated in terms of their impacts on geomorphic processes
(namely erosion and weathering), which tend to exploit
fractures as weak zones, regardless of their formation mechanism.
Faults will not be treated as distinct from joints other than
in the sense that they commonly correspond to areas of high
fracture density (number of fractures per unit area or length)
and potentially lithologic discontinuity.
We focus on the effects of fractures on geomorphic process
and form, although we provide a brief overview of fracture
generation. We refer readers to rock mechanics literature for a
more detailed examination of fracture generation
(Gudmundsson, 2011; Eppes and Keanini, 2017). Fractures
are formed by the response of rock to stress. The processes by
which fractures form can be roughly divided into those that affect
broad regions, due to either widespread temperature
change or broadly exerted pressures, and those that are more
local, creating more variable fracture geometry in a smaller
area. Regional fracture-forming processes tend to form more
predictable, spatially uniform, or gradually varying fracture geometry.
Local processes tend to form spatially constrained,
highly variable fracture geometries. Both sets of processes occur
in most rock masses. Complex fracture patterns can occur
from multiple discrete episodes of stress applied to a material
in different directions and magnitudes (Selby, 1993). Both compressive
and tensile stresses work to fracture rock, with fracture
patterns commonly reflecting the source, magnitude, and direction
of stress applied to the rock. Foliation or bedding can
create weaknesses in rock that may eventually become
fractures.
We consider fractures on scales up to that of a landscape (up
to 106
m), but not continental or global scales. Although there is
strong evidence that continental-scale lineaments do impact
topography (e.g. rift zones creating grabens), it is difficult to
distinguish whether such lineaments are caused by fractures
(openings or distinct weaknesses in rocks, O’Leary et al.,
1976) or simply folding. We consider timescales from days to
millions of years. As a broad approximation, these timescales
correspond directly to spatial scales in terms of geomorphic
process (i.e. geomorphic processes occurring on landscape
scales generally do not occur over a matter of days, with the
exception of catastrophic events such as volcanic eruptions or
tsunamis), and we categorize the influences of joints on geomorphic
processes using these approximate scales.
Review of the Influence of Fractures on
Geomorphic Processes and Forms
We distinguish three categories of how the characteristics of
fractures influence geomorphic processes and forms. First, the
spacing and orientation of fractures exert a strong control on
erosion rate and style. More densely fractured rock, for example,
generally erodes faster than sparsely fractured rock
(Dühnforth et al., 2010; Becker et al., 2014), and the spacing
of fractures is a first-order control on the dominance of plucking
versus abrasion in fluvial bedrock incision (Whipple et al.,
2000a). Second, fractures commonly bound landforms observed
in the field, and there is a direct connection between
erosion rate and style and the shape of landforms bound by
fractures (e.g. Hancock et al., 1998). Finally, variation in
erosion rates across the landscape can influence the rate and
spatial distribution of fracture propagation. In doing so, erosion
mediated by fractures can cause either a self-reinforcing,
positive feedback or a self-mitigating, negative feedback on
erosion rate.
This section reviews our understanding of the impacts of
fractures on geomorphology. Each of the aforementioned three
sections is organized by spatial and corresponding temporal
scale. The landscape scale refers to broad processes acting over
103
–106
m and 104
–107
years. The hillslope and valley scales
refer to processes acting over 102
–104
m and 10À1
–104
years.
Finally, the reach and outcrop scales refer to processes acting
over 10À3
–102
m and 10À2
–103
years. These distinctions are
purposefully approximate and overlapping, as many processes
span multiple scales. However, this scheme helps to organize
processes in a comprehensible way to enable comparison
and eventual synthesis.
Relationships between fracture geometry and the
style and rate of erosion
Across scales and domains, more densely fractured rocks erode
more easily than massive rocks. Fracturing controls the style of
erosion, and the removal of fracture-bound blocks is generally
more efficient than abrasion or corrosion in all geomorphic domains
(Selby, 1982; Whipple et al., 2000a; Dühnforth et al.,
2010; Naylor and Stephenson, 2010). Fracture spacing, orientation,
and variability (anisotropy) in those metrics should exert
a strong control on erosion rates. We use the term fracture geometry
to refer to the spacing between fractures, the orientation
of fractures that bound blocks, and the anisotropy of spacing
and orientation in three-dimensional space. Figure 1 illustrates
the processes explained later.
Landscape scale fracture influences on erosion rate and style
At the landscape scale, Molnar et al. (2007) suggest that tectonic
stress fracturing rock is the dominant control on erosion
rate across the landscape by regulating the susceptibility of
rock to erosive force (Figure 1a). Tectonics can be tied numerically
to erosional patterns on Earth’s surface via a stress–strain
framework that highlights the importance of regional weakening
of rock by fracturing (Koons et al., 2012). Fractures induced
by tectonic stress increase bedrock surface area susceptible to
weathering and the erosive effects of vegetation (e.g. Aich
and Gross, 2008). By bounding blocks that can then be detached
from hillslopes, fractures reduce and set the initial size
of sediment supplied to hillsides, glaciers, and rivers (Sklar
et al., 2017; DiBiase et al., 2018). By delineating zones of
weaker material, fractures focus erosion across the landscape,
resulting in incised gorges that follow fracture patterns (Pelletier
et al., 2009).
Rock erodibility is generally assumed to scale directly with
fracture density. Indeed, both direct measures and proxies of
erosion rates in fluvial systems indicate that erosion rates are
maximized in areas of more densely spaced fractures
(Figure 1b; Kirby and Ouimet, 2011; Tressler, 2011; Kirby and
Whipple, 2012). In the Colorado River basin, more densely
fractured rock generally exhibits lower channel steepness (a
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Figure 1. Summary of fracture effects on erosion rate and style, reviewed in the text, organized by spatial and temporal scale. Line drawings depict
the effects in a simplified manner, photographs illustrate examples, and we provide relevant informative references for each topic. Fractures are represented
by dashed lines, while solid lines represent surfaces. For illustrations of Figures 1c and 1h, please see Figure 6. References: [1] Molnar et al.,
2007; [2] Kirby and Whipple, 2012; [3] Krabbendam and Glasser, 2011; [4] Crompton et al., 2018; [5] Naylor and Stephenson, 2010; [6] Sklar et al.,
2017; [7] DiBiase et al., 2018; [8] Selby, 1982; [9] Moore et al., 2009; [10] Wohl, 2008; [11] Johnson and Finnegan, 2015; [12] Whipple et al., 2000a.
[Colour figure can be viewed at wileyonlinelibrary.com]
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© 2018 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, Vol. 44, 27–45 (2019)
proxy for erosion rate; Tressler, 2011; Kirby and Whipple,
2012). However, it is worth noting that this relationship is not
well-studied at the landscape scale, and recent work has indicated
that although fractures weaken rock and may help set
its overall resistance to erosion, other factors such as tensile
strength can mask the impacts of fracturing in some systems
(Bursztyn et al., 2015).
Glacial erosion rates are strongly linked to fracture density at
the landscape scale. Becker et al. (2014) show that areas of
densely fractured rock in Tuolumne Meadows, USA exhibit
low, flat surfaces, in contrast to the more sparsely fractured rock
that forms high relief cliff faces and domes. They attribute this
contrast to the dominance of glacial quarrying in densely fractured
regions versus abrasion in sparsely fractured regions.
Fracture geometry controls on glacial, coastal, and hillslope
erosion rates and styles
At the valley and hillslope scale, fracture spacing controls the
dominance of plucking versus abrasion in glacial erosion
(Figure 1c). As plucking is generally more efficient than
abrasion, erosion style acts as a threshold control on erosion
rate. Early investigators working in dominantly granitic,
exfoliated terrains noted that glacial erosion in fractured rocks
is more effective than erosion in massive rocks (Matthes,
1930; Jahns, 1943). These early studies used the presence or
lack of exfoliation sheets and the steepness of lee sides of large
glacial landforms to infer relative erosion rates. Outside of
granitic terrain, investigators noted enhanced glacial incision
in densely jointed sedimentary rocks (Crosby, 1945). Building
on observations of landforms, Olyphant (1981) found a nonlinear
inverse relationship between estimated glacial erosion
rate and average joint spacing, indicating that more closely
spaced joints erode much faster than more widely spaced joints.
Following statistical evidence of the mechanism by which
fractures influence glacial erosion rates, Iverson (1991) developed
a numerical model to explore subglacial bedrock erosion.
This model yielded new insights regarding the relationship between
water in cavities downstream of quarried steps and upstream
fracture growth, highlighting the importance of vertical
fractures and plucking in generating a stepped profile that enabled
further erosion. Building on Iverson’s model (1991,
Hallet (1996) developed an analytical model of glacial quarrying,
which suggested that not only fracturing, but continued
fracture growth, is essential to the quarrying process and high
glacial erosion rates. Importantly, the model suggested that
even in relatively massive rock with only minor fracturing,
glacially-mediated fracture-growth could enable quarrying.
Iverson (2012) recently developed a more holistic model to describe
quarrying that highlights the importance of variability in
fracture-mediated bedrock strength in determining the nonlinearity
of the relationship between erosion rate and glacier
sliding speed. In glacial settings, fracture generation by glacial
stresses and erosion likely plays a dominant role in weakening
bedrock (Leith et al., 2014b). However, glaciers also exploit
pre-existing fractures in bedrock, which in some cases can be
the dominant fractures bounding plucked blocks (Hooyer
et al., 2012).
Field evidence to quantitatively support the importance of
fracture geometry on glacial erosion rates will help to evaluate
the hypotheses raised by numerical modeling, but this is sparse.
In recent years, cosmogenic radionuclide dating has allowed a
more quantitative evaluation of the impacts of fracture spacing
on glacial erosion rates: Dühnforth et al. (2010) found that more
densely fractured sites in Yosemite National Park, USA exhibited
higher erosion rates, as suggested by beryllium-10 (10
Be)
exposure ages. Fracture orientation, in addition to spacing, is
interpreted to influence the rate of glacial erosion by
determining the dominance of plucking versus abrasion. By
simplifying bedding dip as being either in the direction of ice
flow or opposed to it, investigators have used field evidence
to infer that dip direction controls the prevalence of plucking
versus abrasion in glacial erosion (Kelly et al., 2014; Lane
et al., 2015). However, the effects of more complex orientation
variability beyond bedding dip on glacial erosion process dominance
or erosion rate have yet to be understood. Indirect evidence
relating fracture spacing to glacial erosion rate also
comes from Crompton et al. (2018), suggesting that glacial
surging (dramatic changes in ice flow velocity that may regulate
erosion rate; Smith, 1990; Humphrey and Raymond,
1994) may be controlled by fracture spacing influences on till
dynamics on the bed.
In the coastal domain, fracturing weakens rock and changes
the style of coastal retreat (Figure 1d). More densely fractured
rocks can enable coastal retreat rates twice that of less fractured
rock (Barbosa et al., 1999). Similarly, shore platforms in more
densely jointed rocks are lowered to a greater extent than
nearby, more sparsely jointed platforms (Kennedy and Dickson,
2006). Naylor and Stephenson (2010) performed a detailed investigation
of fractured bedrock exposed on coastlines. They
found that the spacing of bedding planes controlled the ability
of waves to erode portions of coastal cliff faces. More closely
spaced joint sets permitted enhanced erosion of certain sedimentary
beds, and the orientation of joint sets and their continuity
in space controls their resistance to erosion. This is a prime
example of how anisotropy in joint spacing and orientation
plays an important role in determining erosion rate and style.
Sediment delivery to rivers and glaciers may be set by fracture
spacing, orientation, and anisotropy (Figure 1e; Sklar et al.,
2017; DiBiase et al., 2018). This sediment acts as tools (enabling
erosion by abrasion) and cover (enabling alluviation
and preventing incision into bedrock) in fluvial erosion (Sklar
and Dietrich, 2004), thus influencing erosion rates. The link between
fracture spacing and the eventual size of sediment delivered
to rivers has yet to be fully understood due to the myriad of
breakdown processes that occur between the production of
sediment from bedrock, its transport downslope, and its eventual
deposition in the channel. However, a case study comparing
two sites with differing fracture density shows that fracture
density can set channel erodibility and landscape relief structure
by setting the size of sediment delivered to channels
(DiBiase et al., 2018). Numerical modeling also indicates that
sediment delivery may play a strong role in linking fracture geometry
to landscape evolution (Roy et al., 2016b).
More densely fractured hillslopes are inherently less stable
(Figure 1f; Clarke and Burbank, 2011; Loye et al., 2012; Selby,
1982) and experience higher erosion rates than hillsides in
massive rock. Although fracture geometry controls the erodibility
of hillslopes and the rates at which they erode (Selby, 1982,
1993), the literature generally focuses on how fractures control
the location, orientation, and size of mass movements, and are
hence treated in more detail later.
Fracture geometry controls on fluvial erosion rate and style
At the reach scale, fractures influence erosion rate dominantly
by controlling the spatial orientation of fluvial erosion (vertical
incision versus lateral widening), and determining whether
plucking or abrasion dominate the erosion of bedrock rivers
(Figures 1g and 1h). Work examining the density of fractures
in relationship to bedrock channel morphology has shown
how fracture density exerts a strong control on channel width,
with more densely fractured rock exhibiting wider valleys
(Ehlen and Wohl, 2002; Wohl, 2008). Multiple studies have
documented the process of subaerial weathering leading to
densely fractured sedimentary rocks (slaking) that enable
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significant erosion at channel margins, leading to widening and
the potential for strath terrace formation (Montgomery, 2004;
Johnson and Finnegan, 2015; Schanz and Montgomery,
2016). This is a prime example of surface fracturing creating
anisotropy in fracture density and erodibility, leading to nonuniform
erosion rates within a channel.
Rivers and glaciers exploit fractures to erode bedrock via
plucking. Over the last two decades, much of the research into
plucking erosion has used physical and numerical modeling to
determine thresholds for block entrainment from the bed. Four
mechanisms of entrainment have been examined: sliding
(Hancock et al., 1998; Dubinski and Wohl, 2013), vertical entrainment
(Coleman et al., 2003), pivoting about an upstreamfacing
step following vertical entrainment (Wende, 1999;
Fujioka et al., 2015), and toppling (Lamb and Dietrich, 2009).
Vertical entrainment is likely the initial entrainment mechanism
that enables the pivoting of tabular blocks about
upstream-facing steps (Wende, 1999). However, it is extremely
rare to observe cavities in the bed bound on all sides by rock
that would represent the space left by a purely vertically
entrained block (i.e. with no pivoting), and pure vertical entrainment
requires block protrusion to an extent not observed
in natural channels (Coleman et al., 2003; Lamb et al., 2015),
indicating that pure vertical entrainment without pivoting likely
does not occur in natural channels. Vertical entrainment and
pivoting about an upstream-facing step likely occurs in streams
eroding bedded lithologies that dip downstream, based on
observations of upstream-facing steps with tabular, blockshaped
voids that follow fractures oriented perpendicular to
flow (e.g. Figure 2). Wende (1999) suggests a critical flow velocity
entrainment threshold for blocks resting against an immobile
upstream-facing step on their downstream side. This
threshold is mainly a function of the block height and top surface
area, although it neglects wall friction. More tabular
blocks with large top surface areas relative to their height are
predicted to be more easily vertically entrained and then
flipped or pivoted as they move downstream. This theoretical
prediction was confirmed by flume experiments that showed
flipping to be a viable entrainment mechanism, although, depending
on the height of the upstream-facing step, blocks
may not be fully flipped after entrainment (Wende, 1999). In
contrast to the vertical entrainment synthesized by Lamb et al.
(2015), this type of entrainment requires a free surface on the
upstream side of the block. However, this shows that vertical
entrainment, at least when it precedes pivoting about an
upstream-facing step, is likely an important mechanism of
entraining blocks in fractured channels.
Both sliding and toppling entrainment are strongly dependent
on the ratio of block dimensions, primarily height and
length (Lamb and Dietrich, 2009; Dubinski and Wohl, 2013;
Lamb et al., 2015). This indicates that fracture spacing and
spacing anisotropy (deviation from cuboid fracture systems)
may exert strong controls on entrainment rates. Most existing
work focuses on cuboid systems: only recently has experimental
work examined non-cuboid fracture systems (George et al.,
2015) and concluded that block orientation relative to the flow,
determined by fracture geometry, exerts a strong control on the
entrainment threshold.
Field observations demonstrate that plucking can occur in
modes similar to those simulated in flume settings (Lamb
and Fonstad, 2010; Anton et al., 2015), and that plucking
of fractured rock is likely the only way to explain high erosion
rates in rivers. Natural channels display strong spatial
variability in plucking rates, associated with the migration
of knickpoints (Miller, 1991; Seidl et al., 1994; Lima and
Binda, 2013). This spatial and temporal variability in the rate
of erosion resulting from plucking makes it very difficult to
accurately model channel evolution due to plucking. Despite
this, numerical modeling has shown success in simulating
decadal-scale evolution of a bedrock channel (Chatanantavet
and Parker, 2009, 2011). This model uses a conservation of
mass approach by conceptualizing plucking as a process of
stripping off particles produced by weathering and fracture
propagation. Faster fracture propagation and the lack of sediment
cover enhance plucking in this model. Despite not explicitly
treating fracture geometry, this model accurately
simulates knickpoint formation and development. This indicates
that a detailed mechanistic understanding of plucking
may not be necessary for understanding channel evolution
on timescales of decades.
Figure 2. Example of upstream-facing steps in a limestone bedrock river, Marienbergbach, Austria. Flow is from bottom right to top left. Red lines
delineate major downstream-dipping joints (formed by bedding planes) that bound the downstream faces of steps. These bedding plane joints, along
with other fractures, create upstream-facing steps. Plucking may occur by the flipping or vertical pivoting of tabular blocks from the bed that can rotate
around the lips of such upstream-facing steps as per Wende (1999). [Colour figure can be viewed at wileyonlinelibrary.com]
31BEDROCK FRACTURE INFLUENCES ON GEOMORPHOLOGY
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However, to add complexity, it is important to note that entrainment
only partially determines erosion rates due to
plucking. Transport of plucked blocks, which act as alluvium
after entrainment, and the propagation of fractures (see later)
are necessary to prevent alluviation of the bed and thus enable
erosion. Lamb et al. (2015) highlight the lack of observational
data to examine this question, although Chatanantavet and
Parker (2011) have developed a model that can accommodate
variability in alluviation as a function of bed sediment and fracture
propagation, which could be used as a starting point for
further field testing. Using a critical dimensionless shear stress
formulation to describe entrainment thresholds under the aforementioned
mechanisms of entrainment, Lamb et al. (2015)
point out that sliding-dominated and especially topplingdominated
reaches are likely transport limited. The distribution
of sediment in the form of blocks in fractured bedrock rivers, especially
at the base of toppling-dominated knickpoints, seems
to support this observation. Additionally, a transport-limited
model performs well in predicting channel development in a
well-jointed substrate (Lamb and Fonstad, 2010). However,
the abundance of sustained bedrock reaches that exhibit
fracture-bound voids and plucking dominance, and that are devoid
of sediment, indicates that entrainment rate likely limits
erosion rates in many systems. It is important to note that analytical
models of plucking entrainment are generally based on
cuboid fracture sets with two fracture sets oriented normal to
flow and one oriented parallel to flow. This is an idealization
that is rarely an exact description of natural systems, and it is
important to note that non-cuboid (even subcuboid) fracture
orientations are significantly more complex.
Determining thresholds for erosion process dominance in
bedrock rivers. Bedrock river evolution is largely determined
by the dominance of plucking versus abrasion processes
(e.g. Figures 1g and 1h). Because bedrock rivers fundamentally
regulate landscape evolution, it is imperative to understand the
conditions that determine erosion process dominance. Although
field observations have indicated that bedrock channels
with closely spaced fractures are dominated by plucking
erosion and exhibit higher erosion rates than massive,
abrasion-dominated channels (Whipple et al., 2000a), a threshold
fracture spacing that enables plucking has yet to be identified.
The question of whether plucking or abrasion accounts for
the majority of the erosion in a reach is deceptively difficult to
answer. Many investigators have used the morphology of the
bed as an indicator of the relative efficiency of plucking versus
abrasion (Tinkler, 1993; Hancock et al., 1998; Whipple et al.,
2000b; Beer et al., 2016), while acknowledging (Tinkler,
1993; Hartshorn, 2002) and even directly observing evidence
(Beer et al., 2016) that plucking is a much more episodic style
of erosion than abrasion. Even in sculpted channels, where
abrasion seems to dominate, plucking may still remove more
material over long timescales (Beer et al., 2016).
The presence of sculpted bedforms only indicates that abrasion
has continued long enough to sculpt the bed; even a few
millimeters of erosion, potentially accomplished over the
course of a few years (based on observed abrasion rates on
the order of 1 to 5 mm aÀ1
in natural channels; Hancock
et al., 1998; Whipple et al., 2000a; Beer et al., 2016), can obscure
more sharply angled plucked forms. If the time between
plucking events is greater than the time needed to smooth the
bedrock, then the presence of sculpted forms in a channel cannot
be a reliable indicator of process dominance. The detailed
measurements of a bedrock gorge performed by Beer et al.
(2016) over the course of two years exemplify this observational
difficulty by showing that a single and likely infrequently
occurring plucking event dramatically exceeded rates of
erosion by abrasion, even in dominantly sculpted and massive
bedrock. A sculpted bed may simply be exhibiting a long
‘waiting time’ (Hancock et al., 1998) between plucking events.
An exception to this is when the bed substrate is entirely massive
and no fracture-bound clasts are evident in bed material:
abrasion must dominate in conditions with no fractures to create
blocks and without evidence that macroabrasion (breaking
of bedrock into blocks by the impact of large clasts) is sufficient
to fracture rock into blocks for plucking (e.g. Coyote Creek,
Utah, Wohl et al., 1999).
Although the shape of canyon walls generally preserves evidence
of erosive style in a bedrock channel (e.g. asymmetric
wall slopes may indicate lateral migration), valley wall morphology
may not indicate process dominance. Shear stress decreases
with height above the bed, and abrasion may dominate
high off the bed in a confined channel (although subaerial
weathering may produce smaller and more easily detached
blocks higher off the bed, counteracting this; Shobe et al.,
2017). As the channel incises, abrasion may be the last process
to fluvially erode the walls before the channel incises sufficiently
deeply to stop shaping the walls above a certain height
from the bed. This would result in smoothed walls that, although
they could have been exposed by plucking or abrasion
incision, only reflect the last erosive process, which may have
been abrasion.
That said, a similar conundrum may not apply to inferring the
dominance of plucking from channel form. Plucking likely
dominates in channels that are obviously blocky and exhibit
fracture-bound, concave forms (cavities left from plucked
blocks; e.g. Figure 2). Plucking is likely more episodic (due to
requiring high shear stresses to move blocks) and effective
(due to removing large blocks of material over short timescales)
than abrasion, which can be assumed to occur more consistently
through time in systems that are not entirely devoid of
sediment (Hancock et al., 1998; Whipple et al., 2000a; Sklar
and Dietrich, 2004). As such, for a channel bed to persistently
exhibit sharp, fracture-bound angles and plucked cavities,
plucking must outpace abrasion, even though it may not occur
as often.
Because plucking likely is much more effective than abrasion,
and because it can occur even in otherwise massive rocks
via fracturing due to macroabrasion (Whipple et al., 2000a;
Whipple, 2004), plucking in some form probably should be assumed
to be the default mode of eroding bedrock in the absence
of definitive evidence that abrasion dominates. In terms
of field observation, such definitive evidence may come from
the lack of plucked forms on the bed, the lack of fracture-bound
clasts in bed material, and well-developed sculpted forms in
the absence of strongly expressed fractures or evidence of
plucking.
Temporal and spatial scale can also determine process
dominance. In reconciling low, short-term, abrasion-related
erosion rates with higher long-term erosion rates from strath
terraces on the Indus River in Pakistan, Hancock et al.
(1998) note that extremely infrequent plucking events could
have eroded significant amounts of material. Over short timescales
on sculpted beds, abrasion almost certainly dominates.
However, over longer timescales, potentially on both
sculpted and blocky beds, plucking may dominate. Spatially,
plucking may only occur infrequently and across small portions
of the bed, similarly to abrasion, which varies strongly
in space depending on bedform orientation (Hancock et al.,
1998; Beer et al., 2016). Accurately determining the conditions
that lead to the dominance of episodic plucking processes
over more continuous abrasion processes is essential
for understanding and predicting the evolution of bedrock
rivers and landscapes.
32 D. N. SCOTT AND E. E. WOHL
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Fracture controls on the shape, orientation, and
location of landforms and erosion
Some of the earliest investigations into the impacts of fractures
on the development of landscapes focused on spatial correlations
between fractures and erosional forms (Hobbs, 1905;
Bryan, 1914). Fractures control the shape, orientation, and location
of landforms by two mechanisms. First, because fractures
increase the erodibility of the landscape, they tend to
focus erosion and create incisional features. Second, fractures
bound eroded blocks. As glacial plucking, fluvial plucking, or
hillslope failure remove blocks, they leave a cavity that defines
the micro- to meso-scale morphology of the eroded landscape,
commonly bound by one or more fractures. These two mechanisms
work together on multiple and overlapping temporal and
spatial scales to produce a landscape that is typically defined
by the underlying fracture network. Figure 3 illustrates the processes
explained later.
Fracture controls on the orientation and elevational distribution
of topography
At the landscape scale, one of the most noticeable impacts of
fracturing on the landscape is the correlation between fracture
orientation and stream planform orientation (Figure 3a). This
correlation has been noted in a wide variety of landscapes, including
relatively tectonically quiescent, climatically wet
limestone landscapes in the north-eastern United States
(Hobbs, 1905; Sheldon, 1912; Cole, 1930); arid sandstone
and metamorphic landscapes of the south-western United
States (Bryan, 1914; Pelletier et al., 2009); glaciated sedimentary
landscapes of Greenland (Pessl Jr, 1962); subhumid sandstone
landscapes in Australia (Baker and Pickup, 1987);
metamorphic rocks in the Southern Alps of New Zealand
(Hanson et al., 1990); sedimentary rocks of central India (Kale
et al., 1996); granitic and gneissic terrain of South Africa (Tooth
and McCarthy, 2004); and granitic terrains of the US Sierra
Nevada (Ericson et al., 2005). The ubiquity of this correlation
has led many researchers to hypothesize that underlying fractures
control the distribution of erosion on the landscape, with
the result that valleys tend to follow fractures.
However, as landscape evolution modeling has taken a leading
role in augmenting our understanding of erosional processes,
researchers have been able to draw mechanistic links
to bring causation to the aforementioned correlation between
fractures and valley orientation. One of the major difficulties
in this correlation is that, although streams generally follow
fractures, not all fractures are exploited by these streams.
Pelletier et al. (2009) address this difficulty using numerical
modeling to explore fracture-controlled drainages in metamorphic
core complexes of Arizona in the United States. They
found that tectonic tilting of the landscape was likely responsible
for the preferential exploitation of certain joint sets across
the landscape, producing the drainage pattern observed today.
In contrast, Ericson et al. (2005) found that glacial erosion
could force what are now fluvially dominated streams to follow
major joints that do not follow the range-wide slope. Earlier
modeling of glacial erosion shows that contrasts in rock
erodibility determined by fracture geometry may strongly influence
glacial valley form and the lateral distribution of erosion
across the valley (Harbor, 1995). This indicates that widespread
fracture sets can similarly influence both glacial and fluvial erosion.
Focusing on fluvial erosion, Roy et al. (2015) use
numerical modeling of fault-weakened zones and show that a
sufficient erodibility contrast (potentially due to variability in
fracture density) between a weakened zone and surrounding
rock is necessary for that weakened zone to control drainage
network development. The orientation of the weak zone also
controls the development of valley walls as the river incises.
Fracture controls on the spatial distribution of erosion are not
limited to fluvial systems. Becker et al. (2014) found that extremely
densely fractured zones caused preferential glacial
quarrying in Tuolumne Meadows, where topographic highs
correspond to areas lacking bands of fractured rock and lows
correspond to areas that exhibit these fractured zones
(Figure 3b). This provides direct evidence for Molnar et al.’s
(2007) suggestion that the mechanism by which tectonics most
influences the landscape is by fracturing rock and focusing
erosion. More densely fractured rock is more easily eroded,
leaving high elevation features in areas of sparse fracturing.
For example, topographic variations in granitic uplands
(e.g. tors) correspond to spatial variations in fracture spacing.
Fracture spacing sets the size and morphology of tor blocks produced
by weathering (Gerrard, 1976; Ehlen, 1992).
Also in the glacial domain, researchers have long recognized
that fjords tend to follow the orientation of regional fracture systems
(Figure 3c; Holtedahl, 1967; Nesje and Whillans, 1994;
Glasser and Ghiglione, 2009). Fractures enable glaciers to preferentially
erode certain parts of the landscape repeatedly across
glacial cycles, and have been proposed to be the dominant
control on fjord development, as opposed to internal glacial
dynamics (Glasser and Ghiglione, 2009). Although glacial erosion
that creates fjords appears to simply follow fractures at a
broad scale, fractures likely influence glacial erosion rates by
allowing for rapid removal of fracture bound blocks (see earlier).
Evidence for this comes from the morphology of fjord valley
floors, which exhibit knickpoints bound by fractures
(Holtedahl, 1967).
Fracture controls on the morphology of hillslopes and valleys
Glaciers carve landforms on the scale of hillslopes and valleys
that are commonly defined more by fracture orientation and
spacing than by glacial dynamics (Figure 3d). Examining glacial
valley floors using numerical modeling, Anderson (2014) shows
that because fracture spacing determines the size of blocks able
to be quarried on the bed, in turn controlling the dominance of
abrasion versus quarrying, steps with a wavelength determined
by variations in fracture spacing form periodically in the
evolution of a glacial valley. Glacial landforms are commonly
bound by dominant joint sets in a region (Matthes, 1930;
Gordon, 1981; Rastas and Seppala, 1981; Olvmo and
Johansson, 2002). Roche moutonées, commonly cited as
indicators of ice flow direction, have been observed to follow
joint sets rather than ice flow direction (Gordon, 1981). Rastas
and Seppala (1981) show that the spacing and size of roche
moutonées follow the spacing of dominant fractures, providing
an example of how underlying fracture geometry exerts the
dominant control on the dimensions of a landscape.
Hillslope morphology, and the spatial distribution of mass
movements that control hillslope evolution in steep terrain,
are determined by the spacing, orientation, and geometric anisotropy
of fractures (Figure 3e; Selby, 1982, 1993). In general,
slopes with more closely spaced fractures, and those with fractures
dipping out of the slope, accommodate sliding failure
more easily. Indeed, Moore et al. (2009) show that fracture orientation
dominates over other controls on long-term cliff retreat
rates in the Sierra Nevada. The location of avalanches and hillslope
failures typically correlates with joint sets (Figure 3f; Butler
and Walsh, 1990; Cruden, 2003; Braathen et al., 2004; Loye
et al., 2012). Mountain tops and bedrock slopes exhibit morphologies
that are a direct result of rock strength and angle of
bedding planes or joint sets that form planes of weakness and
eventual failure (Selby, 1982; Cruden, 2003; Braathen et al.,
2004). By setting the size of blocks produced by weathering
33BEDROCK FRACTURE INFLUENCES ON GEOMORPHOLOGY
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Figure 3. Summary of fracture controls on the shape, orientation, and location of landforms, reviewed in the text, organized by spatial and temporal
scale. Line drawings depict the effects in a simplified manner, photographs illustrate examples, and we provide relevant informative references for
each topic. Dashed lines represent fractures, while solid lines represent surfaces. Arrows indicate flow direction. References: [1] Pelletier et al.,
2009; [2] Tooth and McCarthy, 2004; [3] Becker et al., 2014; [4] Glasser and Ghiglione, 2009; [5] Rastas and Seppala, 1981; [6] Anderson, 2014;
[7] Loye et al., 2012; [8] Butler and Walsh, 1990; [9] Aich and Gross, 2008; [10] Velázquez et al., 2016; [11] Ortega-Becerril et al., 2016; [12] Bryan,
1914; [13] Lamb and Dietrich, 2009. [Colour figure can be viewed at wileyonlinelibrary.com]
34 D. N. SCOTT AND E. E. WOHL
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and erosion, fractures can set the slope of talus fields on
hillslopes (Bryan, 1914; Caine, 1967). A detailed analysis of
fracture geometry can yield insights into likely failure mechanisms
and eventual post-landslide morphology (Brideau et al.,
2009). Loye et al. (2012) present a detailed look at the mechanism
by which fractures influence the location of hillslope failure,
showing that not simply fracture orientation, but instead
the orientation of maximum joint frequency, can set the bulk
strength of the hillslope. This implies a strong role of fracture
anisotropy on hillslope failure probability.
Fractures can control the distribution of vegetation across
bedrock, especially in arid landscapes (Figure 3g). Vegetation
exploits fractures in bedrock as zones of enhanced soil development,
water retention, and weathering rate, harboring substrate,
water, and nutrients for plants, but only where soil
does not thickly mantle bedrock (Burkhardt and Tisdale,
1969; Loope, 1977; Yair and Danin, 1980). In arid landscapes,
fracture patterns can actually be identified via aerial photography
by tracing lines of vegetation exploiting those fractures
(e.g. Aich and Gross, 2008). The result of this enhanced vegetation
growth in fractures is seen in the physical effects of roots on
bedrock, with roots exerting force due to both swelling and
above-ground motion (Strahler, 1952; Roering et al., 2003,
2010), and chemical weathering feedbacks that influence fracture
propagation (see later). Tree throw erodes bedrock by root
exploitation of fractures and can transport significant amounts
of sediment downslope. As trees fall, they transport material
downslope. If trees are rooted into bedrock, they break off bedrock
blocks and transport them downslope (Gabet et al., 2003;
Gabet and Mudd, 2010).
Fracture controls on the reach scale morphology of rivers
At the reach scale, individual channels in a bedrock river can
exploit joints to produce anabranching planforms (Kale et al.,
1996; van Niekerk et al., 1999; Tooth and McCarthy, 2004).
In these cases, rivers erode preferentially along fractures. Tooth
and McCarthy (2004) note that both joints and foliation direct
the abrasion of bedrock, creating sculpted, multi-thread channels.
However, plucking also appears to be capable of
producing such a planform (Kale et al., 1996). Tooth and McCarthy
(2004) provide a detailed synthesis of anabranching
planform observations in bedrock and conclude that fracturing
is likely necessary for such a planform to develop in bedrock.
By providing strong heterogeneity in cross-sectional erodibility,
fractures overcome the usual positive feedback between
channelized flow, erosion of a thalweg, and further channelization,
forming a long-lived, multi-thread planform (Tooth and
McCarthy, 2004).
Similar to planform, fluvial longitudinal form can be determined
by fractures. Bryan (1914, p. 133) provides an excellent
example of a knickzone with near-vertical and near-horizontal
surfaces (forming the longitudinal profile of the knickzone) that
follow major joint sets (Figure 3i). Knickpoint or step height is
commonly strongly related to bedding thickness in sedimentary
rocks, and knickpoint lips typically follow oblique or
perpendicular-to-flow joint sets (e.g. Miller, 1991, Figure 4).
Knickpoint spacing and location have been observed to depend
strongly on the longitudinal distribution of vertical joints
(Phillips and Lutz, 2008). Lamb and Dietrich (2009) provide
evidence for plucking by toppling on knickpoints with
subvertical joints defining their faces and sufficiently deep
plunge pools as a mechanism for preserving vertical faces as
knickpoints retreat. Fracture orientation appears to strongly influence
knickpoint morphology and inferred migration rate in
multiple lithologies (Phillips and Lutz, 2008; Lima and Binda,
2013; Ortega et al., 2013). However, mechanisms of
knickpoint retreat in the presence of influential fracture systems
are poorly understood.
Within a single reach or knickpoint, fractures commonly define
the margins of bedforms, reflecting various mechanisms of
plucking and concentrated abrasion. As mentioned earlier,
sliding, toppling, flipping/pivoting, or vertical entrainment can
remove blocks from the streambed. The cavities left from
plucking create the typical morphology of the bed of a fractured
bedrock river (e.g. Figure 4). Toppling has been proposed
as a mechanism that can sustain larger vertical forms (Lamb
and Dietrich, 2009). Flume observations have shown that sliding
can similarly sustain vertical, joint-bound steps in the bed,
Figure 4. An example of a knickpoint oriented oblique to flow bound by sub-vertical joints on the Aso River, Spain (approximate location:
42.563125, 0.039353). Note the generally cuboid blocks and the voids left by plucking in the right foreground. Shading in the foreground highlights
the planar surfaces bound by joints that set the form of the knickpoint. Red shading indicates subvertical joint-bound surfaces perpendicular to flow,
and blue shading indicates subhorizontal joint-bound surfaces. [Colour figure can be viewed at wileyonlinelibrary.com]
35BEDROCK FRACTURE INFLUENCES ON GEOMORPHOLOGY
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and cross-sectional distributions of sliding rates can influence
the morphology of block bedforms at knickpoint lips (Dubinski
and Wohl, 2013). Vertical entrainment would likely produce
block-shaped holes in the bed, although such holes are not
commonly documented in real channels. As Lamb et al.
(2015) point out, other mechanisms of plucking are more likely
to dominate unless blocks protrude from the bed to a degree
not commonly seen in natural rivers. Pivoting vertical entrainment
about an upstream-facing step tends to produce and sustain
upstream-facing steps and imbricated boulder slab
bedforms in bedding-dominated bedrock rivers (e.g. Figure 2;
Wende, 1999). Sedimentary bedding in particular can form
fracture-bound planar surfaces, where the channel follows a
single sedimentary bedding plane for some length and then
moves to another sedimentary bed at a step (Miller, 1991;
Richardson and Carling, 2005).
Abrasion can also exploit fractures on the bed, creating
sculpted forms with a geometry that follows fracture orientation
or is bound by fractures (Figure 3h). Early investigations of potholes
indicated that they can exploit steeply dipping fractures
in the bed (Elston, 1918). Like many other effects of fractures
on geomorphology, investigation of this process has mostly
been limited to observational correlations between fractures
and pothole orientations, locations, and shapes (Bryan, 1920;
Springer et al., 2006; Ortega et al., 2014). More recently, detailed
geotechnical and statistical investigations of potholes
seem to confirm that potholes can exploit small-aperture fractures
on the bed, and that potholes correlate more strongly with
fracture orientation and substrate resistance than with hydraulics
(Ortega-Becerril et al., 2016). Similar to glacial landforms
on a much larger scale, potholes seem to be more reflective
of underlying substrates than the flow of material that scours
them. Other sculpted forms in bedrock channels also exhibit
fracture control, especially in the case of furrows or solution
pits following fractures on the bed (Richardson and Carling,
2005). Fractures that induce flow separation can act as seeds
for sculpted forms such as flutes (Velázquez et al., 2016).
Springer et al. (2002) suggest that fractures on the bed and walls
act to anchor sculpted forms in place, fundamentally altering
their long-term evolution.
Feedbacks between erosion and fracture
propagation
Feedbacks between erosion of the land surface and fracture
propagation regulate how fractures influence erosion rate and
style through time (e.g. Molnar, 2004). In a system with
surface-generated fractures, the ratio of the rate of erosion to
the rate of fracture propagation controls how bedrock erodibility
may change through time, as fractures must continually form
and propagate in order for block removal type erosion to continue
(e.g. Hancock et al., 1998; Chatanantavet and Parker,
2009). Figure 5 illustrates the processes explained here.
Fracture propagation feedbacks at the landscape and valley
scales
On landscape scales, relatively widespread tectonic stresses
modulated by topographic stresses on rock form and propagate
fractures (Figure 5a). Topographic stress refers to gravitational
stress near Earth’s surface generated by relief. As relief
increases, the stress exerted on ridges, hillslopes, and valley
bottoms increases. Models indicate that this stress is sufficient
to fracture bedrock (Miller and Dunne, 1996). Thus, as rivers
erode and create relief, stress increases and rock fractures,
enabling further erosion of bedrock. Although this may
appear to be an inherently positive feedback, it is important
to note that in accelerating the pace of relief generation via
fluvial incision, this fracturing can also accelerate hillslope
failure, potentially covering valley bottoms with sediment
and preventing rivers from incising bedrock (Molnar, 2004).
The direction and magnitude of this feedback depend on
the relative rates of fluvial incision versus hillslope erosion
and sediment supply, as well as the lateral stress regime induced
by regional tectonics, as variation in fracture orientation
may differentially favor the erosion of hillslopes versus
valleys.
Comparing numerical modeling to field observations tests
whether topographic stresses can be a dominant control on
rock fracture patterns. Field observations of fractures from borehole
(Slim et al., 2015) and geophysical data (St Clair et al.,
2015) find that numerically modeled fractures due to topographic
stresses generally follow patterns observed in the field,
supporting the idea of topographically induced stresses fracturing
rock and likely influencing landscape evolution.
Numerical modeling can examine the possible feedback between
topographic stress fracturing and landscape evolution.
Roy et al. (2016a) use a coupled numerical model of crustal deformation
in response to fluvial incision to suggest that incision
focuses stress and resulting rock damage (fracturing), resulting
in erodibility contrasts that control drainage network development.
Moon et al. (2017) model three-dimensional topographic
stresses to better understand the relationship between landform
orientation and tectonic stresses, finding that both the orientation
and location of fracture-rich zones depend on stress orientation
and topographic geometry. They suggest a framework
based on compressive stress and topography that generates
testable hypotheses regarding the spatial distribution (ridges
versus valleys) of topographically-induced fracturing and the
resulting direction of the feedback between topographic fracturing
and incision rate.
Topography also influences landscape evolution via
pressure-relief fracturing, or exfoliation. Pressure-relief stresses
modulated by exhumation and existing topography cause
widespread microcrack formation and eventual fracture propagation
(Figure 5b; Leith et al., 2014a). This process is best
displayed in granitic lithologies, where some of the first observations
of the process were made (e.g. Dale, 1923; Matthes,
1930; Jahns, 1943). As erosion removes exfoliated sheets and
relieves pressure on the underlying rock, fractures form subparallel
to Earth’s surface. Recently, advances have been made in
understanding the mechanisms of fracture propagation that occur
during granite exhumation. Through detailed monitoring of
exfoliating slabs, diurnal thermal stresses emerge as the most
likely candidate for actual fracture propagation. These stresses
have been observed to trigger slab failure and rock fall (Collins
and Stock, 2016).
Fracture propagation feedbacks at the reach and outcrop scales
The rate of surface fracture propagation is dependent on the
rate of exposure of bedrock. Surface fractures are generally
small-scale features in terms of the depth to which they have
a measurable aperture. As such, fracture propagation processes
that widen fractures and/or extend fracture tips generally operate
at small scales, despite their widespread effects on landscapes
(e.g. frost cracking reducing the erodibility of a
landscape; Marshall et al., 2015). The following processes all
act to exert pressure on the sides of fractures or pressure on
the surface that translates to pressure within a fracture that acts
to widen the fracture.
In cold, alpine landscapes, fracture propagation feedbacks
occur both below glaciers and in unglaciated regions.
Numerical modeling suggests that more broken rock should
36 D. N. SCOTT AND E. E. WOHL
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Figure 5. Summary of feedbacks between erosion and fracture formation and propagation, reviewed in the text, organized by spatial and temporal
scale. Line drawings depict the effects in a simplified manner, photographs illustrate examples, and we provide relevant informative references for
each topic. Dashed lines represent fractures, while solid lines represent surfaces. Arrows indicate flow direction. References: [1] Molnar, 2004; [2]
Matthes, 1930; [3] Collins and Stock, 2016; [4] Andersen et al., 2015; [5] Iverson, 1991; [6] Orlando et al., 2016; [7] Strahler, 1952; [8] Aich and
Gross, 2008; [9] Hancock et al., 1998; [10] Whipple, 2004. [Colour figure can be viewed at wileyonlinelibrary.com]
37BEDROCK FRACTURE INFLUENCES ON GEOMORPHOLOGY
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experience less restrictive water flow conditions, allowing for
more susceptibility to frost-cracking under certain conditions
(Figure 5c; Andersen et al., 2015). This may contribute to the
sustained erosion of peaks in alpine regions (Hales and
Roering, 2009). Beneath glaciers, cavity water pressure fluctuations
exert stress within fractures, propagating fractures to detach
blocks and enable transport (Figure 5d; Iverson, 1991).
This process may lead to a positive feedback whereby overdeepened
sections of the bed result in crevassing at the glacier
surface just upstream, leading to increased subglacial water
pressure fluctuation in the over-deepened section (Hooke,
1991). However, it is important to note that in postglacial landscapes,
plucked surfaces commonly follow preglacial joint sets,
potentially indicating that glaciogenic joints are not important
in forming pluckable blocks (Hooyer et al., 2012). Water pressure
at the bed exerting pressure on fracture tips, however,
likely plays an important role in decreasing friction along fracture
surfaces, making preglacial fractures easier to exploit via
plucking.
In vegetated landscapes, chemical weathering and biota play
an important role in fracture propagation. Fractures strongly influence
the pattern of rock weathering and the structure of regolith
by promoting deep water infiltration into rock. Positive
feedbacks can occur due to water table fluctuations, whereby
oxidative weathering can create small fractures that enable
the further infiltration of water and subsequent oxidative
weathering (Figure 5e; Orlando et al., 2016). As fractures grow,
more rock surface area is exposed to oxidation, enhancing fracture
generation by oxidation.
Fractures also act as a beneficial habitat condition for the existence
of certain plants when soil mantles are thin (Burkhardt
and Tisdale, 1969; Loope, 1977; Sternberg et al., 1996; Wiser
et al., 1996; Hubbert et al., 2001; Aich and Gross, 2008). Because
plant roots tend to follow fractures (Sternberg et al.,
1996; Hubbert et al., 2001; Brantley et al., 2017), they exert
both physical and chemical forcings that serve to propagate
fractures (Figure 5f). By shrinking and swelling due to water intake,
and eventually growing within fractures, roots exert pressure
along fracture walls (Strahler, 1952), probably leading to
fracture propagation. By physically enlarging fractures and
interacting with infiltrating water, roots create conditions favorable
for chemical weathering along fracture walls, further enhancing
fracture propagation and creating a positive feedback
similar to that described earlier for oxidative weathering (Phillips
et al., 2008; Brantley et al., 2017).
In rivers, two processes have been proposed for propagating
fractures. Both processes depend on the presence of sediment
as well as on at least partially exposed and fractured bed.
First, hydraulic clast wedging may act to enlarge fractures
through the process of pushing a clast into a fracture
(Figure 5g). The clast acts as a wedge, exerting high pressure
on the fracture side walls, which likely results in cracking at
the fracture tip (Hancock et al., 1998). This process has thus
far only been inferred from the observation of clasts wedged
tightly in fractures on the bed and walls of bedrock rivers. It
is unclear whether these clasts are bashed into fractures by
larger, saltating clasts or whether hydraulic forces serve to
slightly widen fractures during high magnitude floods,
allowing clasts to be emplaced within the fracture and trapped
as the fracture closes, acting as ratchets that prevent the fracture
from closing back to its original state after being widened
(Hancock et al., 1998).
Second, coarse, saltating particles impart high pressures on
channel beds when they impact the bed, likely causing
macroabrasion, or the formation and propagation of fractures
in the bedrock (Figure 5h; Chatanantavet and Parker, 2009;
Whipple, 2004). The stress imparted by particles impacting
the bed can serve to both form impact fractures, which can create
small blocks able to be plucked from the bed, and exert
stress on blocks bound by pre-existing fractures, potentially
detaching those blocks and allowing entrainment.
Synthesizing Current Understanding of
Fracture Influences on Landforms and
Landscapes to Identify Future Directions
Fractures have been investigated at all scales in all relevant
geomorphic process domains strongly influenced by the presence
of bedrock. Here, we bring together these investigations
to present a group of related ideas and knowledge gaps to make
it easier to use lessons learned from diverse process domains
and scales to inform future investigation. Addressing the
knowledge gaps identified here will be difficult without acknowledging
the similarities between fracture influences on
geomorphic processes at various scales and in various domains.
Table I presents a list of what we find to be the most
pressing questions and knowledge gaps related to fracture influences
on geomorphic processes.
In terms of research in specific process domains, our literature
review broadly reveals a bias towards glacial, fluvial, and
hillslope domains. While there has been some research into
fracture influences on coastal geomorphology (see earlier),
both the coastal and aeolian domains remain ripe for basic research
into this topic.
Process dominance in eroding bedrock
The dominance of plucking versus abrasion in glacial and fluvial
domains is likely strongly related to fracture geometry
(Whipple et al., 2000a; Anderson, 2014). More widely spaced
fractures produce larger blocks that generally require more
stress to entrain and transport, although the relationship
between block entrainment and block size is complex
(Dubinski and Wohl, 2013; Lamb et al., 2015). If blocks are
too big for the flow to entrain and transport, plucking may yield
in dominance to abrasion, whereby the blocks are eroded gradually
through time. In this case, however, it is still possible that
surface fracture generation (macroabrasion in rivers, bed stress
and water pressure fluctuation beneath glaciers) can break
down large blocks to the point at which they can be plucked
faster than abraded. Holding fracture density constant, orientation
also likely plays a strong role in determining whether
blocks can be plucked at a rate faster than the bed can be
abraded. A system with only one or two fracture sets will likely
produce larger blocks than one with three or more fracture sets.
Similarly, the aspect ratio of blocks strongly influences the entrainment
mechanism for those blocks (Lamb et al., 2015),
and the predicted shear stress needed to entrain the blocks. A
good field example of this comes from the Christopher Creek
drainage (Wohl, 2000), where reaches with upstream-dipping
beds tend to exhibit higher gradients, implying higher resistance
to erosion, than reaches with downstream-dipping beds.
This could imply that systems dominated by vertical entrainment
and pivoting about an upstream-facing step or sliding
(downstream-dipping reaches) are more erodible than those
dominated by sliding or toppling (upstream-dipping reaches).
Other than fracture geometry, wall friction (Dubinski and Wohl,
2013; Lamb et al., 2015), tensile strength (Sklar and Dietrich,
2001; Bursztyn et al., 2015), and sediment supply and caliber
(Sklar and Dietrich, 2004) all likely play a role in determining
whether abrasion versus plucking dominates in a given system.
38 D. N. SCOTT AND E. E. WOHL
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Glacial systems seem to share many characteristics with fluvial
systems in terms of the dominance of plucking versus abrasion.
There appears to be a threshold fracture spacing (scaled to
the erosive power of the flow) that determines whether
plucking is possible. In both systems, there are mechanisms
for generating fractures in bedrock to enable plucking
(macroabrasion in fluvial systems, subglacial water pressure
fluctuations or ice-sliding driven shear stress in glacial systems),
but the contribution of such autogenic fracturing to erosion
rate, especially in systems with pre-existing fractures, is poorly
understood. Finally, fracture orientation appears to play a role
in determining the dominance of plucking versus abrasion
and erosion rate in both fluvial (where it can affect plucking entrainment
mechanisms, Wende, 1999; Lamb et al., 2015) and
glacial (where it can affect the surface area exposed to plucking
versus abrasion, Kelly et al., 2014; Lane et al., 2015) systems.
The progress made in each domain varies, but given these
similarities, we suggest that future investigations into process
dominance consider results from both domains, as it is likely
that such a synthetic approach could result in more wellinformed
ideas to better understand the impact of fracture geometry
on process dominance.
The potential dominance of plucking versus abrasion and the
aforementioned ideas are summarized conceptually in Figure 6
by considering both the scale of erosivity (via dimensionless
shear stress, or some other metric representative of erosive
power) and the scale and nature of fracturing (e.g. many fractures
along a single channel reach versus a few sparsely distributed
fractures across a landscape). As Figure 6 implies, the
relationship between dimensionless shear stress and process
dominance is likely non-linear, as there are probably a set of
thresholds (in block size, fracture orientation, wall friction,
etc.) that define the transition from abrasion to plucking. This
conceptualization greatly simplifies the characteristics that
likely play a role in determining process dominance. We
emphasize that a model for predicting whether abrasion or
plucking will dominate in a given system has yet to be developed.
Such a model should integrate understanding from glacial
and fluvial erosion and ideally apply to both domains, as
similar ideas have arisen in both domains (e.g. that fracture orientation
and spacing relative to the direction and magnitude of
flow strongly influence how easily blocks may be plucked). A
better prediction of process dominance is essential for accurately
parameterizing landscape evolution models that seek to
produce realistic predictions while acknowledging pre-existing
or high-flow generated discontinuities in rock. We suggest the
conceptualization of Figure 6 as being useful to contextualize
and draw similarities between investigations at varying scales
and in varying domains.
Identifying relevant scales for understanding
fracture influences on geomorphic processes
The question of whether abrasion dominates over plucking is
fundamentally a question of scale. At small temporal scales,
abrasion can easily dominate, as plucking can be infrequent.
However, over long temporal scales, stress will likely exceed
the plucking threshold or that threshold stress may be sufficiently
decreased by surface fracturing producing smaller
blocks, engendering potentially rare but effective plucking episodes
(Figure 6). It is also possible that the duration between
plucking events is long enough that abrasion does more work
over the course of long time-periods. It is important for landscape
and morphodynamic modeling to identify the temporal
thresholds that separate process dominance to ensure that
models accurately parameterize the importance of abrasion
versus plucking.
With regard to spatial scale, the abundance and depth of surface
fractures may be the dominant fracture geometry
Table I. A list of prominent questions that present future opportunities for developing our understanding of fracture impacts on geomorphic
processes, organized by general topic
Topic Questions
Process dominance in eroding bedrock
• Under what conditions does plucking dominate over abrasion in glacial and fluvial erosion?
• Can we infer process dominance from channel form (i.e. sculpted versus blocky forms)?
• Which fractures (of what orientation relative to flow or gravity) matter most in determining the erodibility of a pluckable block?
• In the case of downstream-dipping beds, when does sliding entrainment dominate over vertical entrainment and pivoting about an
upstream-facing step?
• What is the mechanism by which fractures influence channel planform, and do fractures influence planform in both abrasion and
plucking dominated channels?
Identifying relevant scales for understanding fracture influences on geomorphic processes
• For a given process, at what scales is fracture geometry relevant, and at what scale should it be measured?
• Under what conditions do surficially generated fractures versus pre-existing, deep fractures dominate in influencing erosion rates
and styles?
• At what spatial scales and magnitudes of erosive stress do fractures dominate over flow dynamics in determining the shape and
orientation of landforms and bedforms?
Understanding fracture geometry influences on erosion rates
• Can erodibility be described by fracture geometry alone, or are variables that are more difficult to measure necessary (e.g. fracture
continuity, aperture, roughness)?
• What is the nature of the relationship between fracture characteristics and erodibility across process domains?
• How does fracture geometry influence the mechanism by which blocks are plucked by a flow (i.e. under what geometries
do various mechanisms dominate), and does plucking mechanism regulate erosion rate?
• How do fractures affect erosion rates due to abrasion in rivers and glaciers?
• How is knickpoint migration affected by fracture geometry?
• How can we translate work on idealized, cuboid fracture systems to natural systems with varying fracture geometry?
Understanding feedbacks on fracture propagation
• Under what conditions do topographically induced stress fractures act as a positive versus negative feedback on incision?
• Does hydraulic clast wedging play a role in fracture propagation, how widespread is this process, and how does it function?
• Does vegetation become more effective at propagating fractures when fractures grow larger (i.e. when roots within fractures grow),
which may imply a positive feedback?
39BEDROCK FRACTURE INFLUENCES ON GEOMORPHOLOGY
© 2018 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, Vol. 44, 27–45 (2019)
parameter controlling a process (e.g. Chatanantavet and Parker,
2011), whereas the location or spacing of only the deepest or
most persistent fractures may best relate to other processes
(e.g. Hooyer et al., 2012; Ortega-Becerril et al., 2016). We currently
lack an understanding of which fracture characteristics
are relevant for a given spatial scale to best predict the erosion
rate of a given process.
There remains an open question as to the importance of various
fracture sets at different spatial scales. Analytical work examining
individual blocks indicates that fracture characteristics
that set block height, protrusion above the bed, and length
likely set entrainment thresholds and, in detachment-limited
systems, erosion rates (Lamb et al., 2015). Results at the valley
to catchment scales, however, indicate that subvertical fractures
oriented subparallel to stream planform primarily determine
planform and potentially erosion rate (Pelletier et al.,
2009). In general, it is still unclear which fracture set orientations
relative to flow direction dominantly control erosion rates.
It is also unclear whether orientation controls plucking erosion
to the same degree as average fracture density (which sets the
mean size of blocks on the bed). Although work on hillslopes
has indicated that certain orientations of fractures lead to a
higher likelihood of failure (Brideau et al., 2009; Loye et al.,
2012), similar progress has yet to be made in the glacial or fluvial
domains. Fracture continuity, aperture, and wall friction
also have not been thoroughly investigated in terms of their impacts
on glacial and fluvial erosion.
Understanding fracture geometry influences on
erosion rates
Across domains, the orientation of erosive forces relative to
fracture orientations can determine how easily blocks are removed
from bedrock. Many studies document how ice or water
flow directions or simply the orientation of hillslopes relative to
fracture orientations influence the development of bedforms
and the style of erosion (e.g. Lamb and Dietrich, 2009; Naylor
and Stephenson, 2010; Loye et al., 2012; Lane et al., 2015).
However, a conceptual model of how fracture orientation impacts
the erodibility of the landscape has yet to be developed.
Lamb et al. (2015) make an important first step towards such
a model by deriving phase diagrams for the fluvial entrainment
of blocks under varying block aspect ratios. A complete phase
diagram showing the erodibility of blocks based on all possibilities
of fracture orientation and spacing anisotropy, even just for
cuboid fracture systems, would likely be extremely complex.
Therefore, we suggest moving in a direction of identifying key
fracture geometry variables (e.g. the ratio of block height to
Figure 6. Conceptual, hypothesized diagram of the factors influencing the dominance of plucking versus abrasion in a fluvial or glacial system. This
diagram assumes that abrasion can be dominant over the timescale of interest. The ordinate describes the erosivity of the process shaping the landscape
(quantifiable by, for example, dimensionless shear stress). The abscissa describes both fracture density (sparse fractures being widely spaced
and dense fractures being closely spaced) and the susceptibility of fractures to plucking due to their orientation relative to flow. Resistant might describe
tetrahedral blocks with faces oriented mainly parallel to flow that experience low drag, while susceptible may describe cuboid blocks on a
knickpoint lip, prone to sliding or toppling. Although fracture density and susceptibility (orientation) are represented on the same axis here for simplicity,
we do not mean to imply that the two are correlated. Plucking dominates whenever erosivity is high enough to erode blocks of a given size
(represented by fracture density) and orientation (represented by susceptibility). Pictures show field examples that we hypothesize to fit in various parts
of the diagram. Pictures show: (a) a glacially plucked and abraded valley bottom with low fracture density that was still dominated by plucking below
Dog Tooth Peak, Wind River Range, WY; (b) a densely jointed and dominantly glacially plucked surface with a small modern glacier on the east flank
of Mount Hinman, WA; (c) an undulating, sculpted reach with no evident fractures in No Kidding Canyon (a tributary of North Wash), UT; (d) a
densely jointed and dominantly plucked reach of Outlaw Canyon (a tributary of the Yampa River in Dinosaur National Monument), CO. [Colour
figure can be viewed at wileyonlinelibrary.com]
40 D. N. SCOTT AND E. E. WOHL
© 2018 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, Vol. 44, 27–45 (2019)
length) and testing those variables to examine the components
of fracture geometry that dominantly impact erosion rate and
style.
The influence of fractures on non-plucking processes is also
a major knowledge gap. Previous investigations are dominated
by observational evidence that fractures can generate, anchor,
or guide the development of sculpted forms and abrasion erosion.
However, the relationship between fracture geometry
and rates of abrasion remains an important unknown. Specifically,
determining the effects of variation in fracture orientation,
spacing, and intrinsic properties (continuity, aperture, wall
roughness) on abrasive erosion rates would be a major step towards
an integrated understanding of bedrock erosion
processes.
Understanding feedbacks on fracture propagation
Topographically-induced stress fractures are probably the least
well understood fracture propagation mechanism on large
scales (Molnar, 2004), despite evidence suggesting that this
process likely occurs (Molnar, 2004; Slim et al., 2015; St Clair
et al., 2015). We are not yet at the stage where this feedback
can be accurately parameterized in landscape evolution
models, although such models likely would greatly benefit
from such an advance. We must identify the conditions under
which this process plays an important role in fracture generation
(Anderson, 2015), the subsurface fracture orientations
and spacings that result from predicted stresses, and the interaction
between hillslope and valley bottom fracturing and
alluviation in limiting valley incision rates.
On a more tractable note, small-scale feedbacks present exciting
opportunities that could be addressed relatively rapidly
and used to improve understanding of rock weathering in multiple
environments. Hydraulic clast wedging remains almost
entirely unstudied and there is nothing but circumstantial evidence
that it even occurs (Hancock et al., 1998). Basic foundational
investigations into this process must be made to
determine the role it plays in propagating deep and surficial
fractures (similar to macroabrasion), how it compares to
macroabrasion in preparing bedrock for eventual transport,
and how the process functions (e.g. how it depends on sediment
size distribution). Outside of channels, the impact of vegetation
on breaking rock on hillsides remains an exciting
frontier (Roering et al., 2010; Marshall et al., 2015). We lack a
detailed understanding of the processes by which vegetation
fractures rock, and the direction of potential feedbacks related
to that process.
Prominent methodological challenges
Fracture influences on geomorphic processes are difficult to
disentangle from other obviously important characteristics,
such as tensile strength (e.g. Bursztyn et al., 2015). Like other
systems with numerous variables driving a given process, confounding
variables left unaccounted for in previous research
hinder our ability to progress. Dealing with confounding variables
can be accomplished either by the use of more advanced
statistical tools (e.g. multivariate modeling, factor analysis, classification)
or by attempting to control confounding variables
(e.g. finding comparable field sites, or carefully designing experimental
conditions).
However, it is essential that investigations be grounded in a
similar conceptual model, such that all potential driving variables
can be tested or controlled for in attempting to examine
the influences of fracture geometry on a given process. We
suggest that these conceptual models be developed to integrate
knowledge from all process domains and scales to encourage
interdisciplinary use of previous work and make efficient progress
moving forward. Integrating broader ideas, such as connectivity
(e.g. Sklar et al., 2017), shows promise in enabling
multiple researchers to make progress cognizant of the complications
of the system under investigation.
Identifying and measuring the most relevant fracture sets or
types of fractures for a given process is a major challenge in relating
field measurements to erosivity and erosion rates. Sedimentary
bedding or metamorphic foliation, under varying
circumstances, can either exert only a small effect on cohesive
strength anisotropy, or can act as the dominant failure plane
allowing fracturing and block removal (Saroglou and
Tsiambaos, 2008). This causes confusion when measuring fracture
density, especially in foliated or sedimentary rocks. If field
measurement of fracture density is to be used in a predictive
manner, such as for the evaluation of spillway erosion or channel
evolution in response to flooding, it is imperative that the
most influential fracture sets are identified and measured, as
there may be some cases when measuring every discontinuity
or ignoring small discontinuities like foliation may improperly
represent the actual rock strength. For instance, a plucking
dominated channel may primarily exploit only widely spaced
and continuous fractures, while closely spaced, discontinuous
macroabrasion fractures may be widespread across a channel.
Measuring every macroabrasion-induced fracture may yield a
much higher estimate of the spacing of pluckable fractures than
is appropriate if considering plucking erosion rates. In addition,
some fractures may not be obvious to the naked eye while still
exerting a strong control on morphologic evolution (e.g.
Ortega-Becerril et al., 2016), causing obvious challenges during
field measurement.
Conclusions
The configuration and rate of change of landscapes fundamentally
depend on the weathering and erosion of bedrock. An extensive
literature indicates that physical discontinuities in the
form of fractures within the rock strongly influence bedrock
weathering and erosion. Multiple processes can initiate fractures
and many of these processes involve positive feedbacks
with fracture propagation. Regardless of the spatial and temporal
scales considered, fractures clearly influence erosion rate
and style; the shape, location, and orientation of landforms;
and feedbacks between erosion, fracture propagation, and the
spatial distribution of rock erodibility. Across hillslope, glacial,
coastal, and fluvial domains, the spacing of fractures correlates
strongly with erodibility. Similarly, the combined spacing and
orientation of certain fractures sets threshold stresses for the removal
of blocks. In doing so, fracture geometry can set the
erodibility and eventual form of the landscape, from steep hillsides
to glacially scoured valleys. Insights gained from the glacial,
hillslope, and fluvial domains are similar in terms of the
nature of the relationships between fracture geometry and erosion,
implying that knowledge can be applied across scales and
process domains.
Important gaps in understanding include: determining how
fracture geometry influences the conditions under which specific
erosional processes dominate; identifying the spatial scale
at which fractures should be measured to best characterize erosion
rates of specific processes; characterizing feedbacks between
erosive processes and fracture propagation; developing
methods to effectively incorporate confounding variables such
as climatic variability and the strength of intact rock when examining
fracture influences on geomorphic processes; and
41BEDROCK FRACTURE INFLUENCES ON GEOMORPHOLOGY
© 2018 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, Vol. 44, 27–45 (2019)
developing a widely applicable protocol for measuring relevant
fracture geometry. This synthesis provides a conceptual framework
for further investigation of fracture influences on geomorphic
process across landscapes by working to identify
relationships across domains and scales.
Acknowledgements—The authors thank Alison Duvall, José OrtegaBecerril,
and Peter Nelson for stimulating discussions that helped develop
the ideas presented in this manuscript. The authors thank Ellen
Daugherty for field assistance in related projects that generated many
of the pictures shown in the figures for this manuscript. The authors
are very grateful to Alex Beer and another anonymous reviewer for constructive
and detailed critique that helped improve the manuscript.
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