Hydrosphere
Earth is sometimes called the blue planet or, as we saw in Figure 1.12B—"The Blue Marble." Water more than anything else
makes Earth unique. The hydrosphere is a
dynamic mass of water that is continually on the move, evaporating from the oceans to the atmosphere, precipitating back to the land, and running back to the ocean again. The global ocean is certainly the most prominent feature of the hydrosphere, blanketing nearly 71 percent of Earth's
surface to an average depth of about 3800 meters (12,500 feet). It accounts for about 97 percent of Earth's water (FIGURE 1.14). However, the hydrosphere also includes the freshwater found underground and in streams, lakes, and glaciers. Moreover, water is an important component of all living things.
Although these lajtef sources constitute just a tiny fraction of the total, they are •r-'\ much more important than their meager
percentage indicates. Streams, glaciers, and -i\ groundwater are responsible for creating ■v"  many oi our planets varied landforms, as
,| well as the freshwater that is so vital to life !•.' \ on land.
Groundwater (spring)
figure 1.14 Distribution of Earth's water. The oceans dearly dominate. When we consider only the nonocean component, ice sheets and glaciers represent nearly 85 percent of Earth's fresh water. Groundwater accounts for just over 14 percent. When only liquid freshwater is considered, the significance of groundwater is obyious. (Glacier photo by Bemhard Edmaier/Photo Researchers, Inc.: stresm photo by B. j. Tarbvck: and Groundwater photo by Michael Collier)
Atmosphere
Earth is surrounded by a life-giving^a^oj^ envelope called the atmosphere. When we watch a high-flying jet plane cross the sky, it seems that the atmosphere extends upward for a great distance. However, when compared to the thickness (radius) of the solid Earth (about 6400 kilometers or 4000 miles), the atmosphere is a very shallow layer. One-half lies below an altitude of 5.6 kilometers (3.5 miles), and 90 percent occurs within just 16 kilometers (10 miles) of Earth's surface (figure 1.15). Despite its modest dimensions, this thin blanket of air
is an integral part of the planet, ft not only provides the air that we breathe but also
acts to protect us from the Suns intense^ heat and dangerous ultraviolet radiation. The energy exchanges that continually occur between the atmosphere and the surface and between the atmosphere and space produce the effects we call weather and climate.
If, like the Moon, Earth had no atmosphere, our planet would not only be lifeless but many of the processes and interactions that make the surface such a dynamic place could not operate. Without weathering and erosion, the face of our planet might more closely .resemble the lunar surface, which has not changed appreciably in nearly 3 billion years.
FIGURE 1.15 This unique image of Earth's atmosphere merging with the emptiness of space resembles an abstract painting. It was taken over western China in June 2007 by a Space Shuttle crew member. The thin silvery streaks (called noctifucent clouds) high in the blue area are at a beiqht of about 80 kilometers (50 miles). The atmosphere at this altitude is very thin. Air pressure here is less than a thousandth of that at sea level. The thin reddish ?one in the lower portion of the image is the densest part of the atmosphere. It is here, in a layer called the troposphere, that practically all weather and cloud formation occur. Ninety percent of Earth's atmosphere occurs within just 16 kilometers (10 miles) of the surface. (NASA)
Biosphere
The biosphere includes all life on Earth. Ocean life is concentrated in thersunlit surface waters of the sea (FIGURE 1.16). Most life on land is also concentrated near the surface, with tree roots and.burrowing animals reaching a few meters underground and flying insects and birds reaching a kilometer or so above Earth. A surprising variety of life forms are also adapted to extreme environments. Por example, on the ocean floor where pressures are extreme and no light penetrates, there are places where
vents spew hot, mineral-rich fluids that support communities of exotic life forms. On land, some bacteria thrive in rocks as deep as 4 kilometers (2.5 miles) and in boiling hot springs. Moreover, air currents can carry microorganisms many kilometers into the atmosphere. But even when we consider these extremes, life still must be thought of as being confined to a narrow band very near Earth's surface,
Plants and animals depend on the physical environment for the basics of life, However, organisms do not just respond to their physical environment. Indeed, the biosphere powerfully influences the other three spheres. Without life, the makeup and nature of the geosphere, hydrosphere, and atmosphere would be very different.
FIGURE 1.16 The hydrosphere is home to a significant portion of Earth's biosphere. Modern coral reefs are unique and complex examples and are home to about 25 percent of all marine species. Because of this diversity, they are sometimes referred to as the ocean equivalent of rain forests. (Photo by Darryl Leniufo'age lootstock)
Geosphere
Beneath the atmosphere and the oceans is the solid Earth, or geosphere. The geosphere extends from the surface to the center of the planet, a depth of 6400 kilometers, making it
by far the largest of Earth's four spheres. Much of our study of the solid Earth focuses on the more accessible surface features. Fortunately, many of these features represent the outward expressions ol the dynamic behavior of Earth's interior. By examining the most Prominent surface features and their global extent, we can obtain clues to the dynamic processes that have shaped our planet. A first look at the structure of Earth's interior and at the major surface features of the geosphere will come later in the chapter.
Soil, the thin veneer of material at Earth's surface that supports the growth of plants, may be thought of as part of all four spheres. The solid portion is a mixture of weathered rock debris (geosphere) and
organic matter from cjecayed plant and animal life (biosphere). The decomposed and disintegrated rock debris is the product of weathering processes that require air (atmosphere) and water (hydrosphere). Air and water also occupy the open spaces between the solid particles^and are considered important soil components.
an essential resource t^S? """fcnat we often^ake for granted. Soil is not a ^^3t§^]|>lf in9 entity, but it contains a great deal of wfSK^'Jfle Moreover, this complex medium >- s^pif'-i^iipports nearly all plant life, which in turn ^ ST " J Upports animal life. The pie chart shows
• ^£>;    ■fefl0 composition (by volume) of a slirii'VS r1' 'n 900C' condition for
JSl* tf. fjltant growth. Although . tSfrX.tfTpercentages vary,
1 each Soi' 'S aESf** k composed of
^L^i  mineral and •Stifnfofi1) organic matter,
'wKwBfSteJi   water' ar,d sir.
• i-?a/ai\b*   (photo by Colin
1^' ^lÉtľ
š*' -\V
5% organic matter
/é
■A.
>4