Page 279 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 9 Water Resources 243
▲Figure 9.1 Water outgassing from the crust. Outgassing of water from Earth’s crust occurs in geothermal areas such as southern Iceland west of where the Eyjafjallajökull volcano erupted in 2010. [Bobbé Christopherson.]
first time the presence of water vapour and ice during the formation of new planets in a system 1000 light- years from Earth. Such discoveries prove water to be abundant throughout the Universe. As a planet forms, water from within migrates to its surface and outgasses.
Outgassing on Earth is a continuing process in which water and water vapour emerge from layers deep within and below the crust, 25 km or more below the surface, and are released in the form of gas (Figure 9.1). In the early atmosphere, massive quantities of outgassed water vapour condensed and then fell to Earth in tor- rential rains. For water to remain on Earth’s surface, land temperatures had to drop below the boiling point of 100°C, something that occurred about 3.8 billion years ago. The lowest places across the face of Earth then began to fill with water—first forming ponds, then lakes and seas, and eventually ocean-sized bodies of water. Mas- sive flows of water washed over the landscape, carrying both dissolved and solid materials to these early seas and oceans. Outgassing of water has continued ever since and is visible in volcanic eruptions, geysers, and seepage to the surface.
Worldwide Equilibrium
sea level caused by changes in the volume of water in the oceans. Such sea-level changes caused specifically by gla- cial ice melt are glacio-eustatic factors (see Chapter 17). During cooler climatic conditions, when more water is bound up in glaciers (at high latitudes and at high eleva- tions worldwide) and in ice sheets (on Greenland and Antarctica), sea level lowers. During warmer periods, less water is stored as ice, so sea level rises. Today, sea level is rising worldwide at an accelerating pace as higher tem- peratures melt more ice and, in addition, cause ocean water to thermally expand.
Distribution of Earth’s Water Today
From a geographic point of view, ocean and land surfaces are distributed unevenly on Earth. If you examine a globe, it is obvious that most of Earth’s continental land is in the Northern Hemisphere, whereas water dominates the sur- face in the Southern Hemisphere. In fact, when you look at Earth from certain angles, it appears to have an oceanic hemisphere and a land hemisphere (Figure 9.2).
The present distribution of all of Earth’s water be- tween the liquid and frozen states and between fresh and saline, surface and underground, is shown in
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Today, water is the most common compound on the
surface of Earth. The present volume of water circulat-
ing throughout Earth’s surface systems was attained ap-
proximately 2 billion years ago, and this quantity has
remained relatively constant even though water is con-
tinuously gained and lost. Gains occur as pristine water
not previously at the surface emerges from within Earth’s
crust. Losses occur when water dissociates into hydrogen
and oxygen and the hydrogen escapes Earth’s gravity to 0° space or when it breaks down and forms new compounds
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with other elements. The net result of these water inputs and outputs is that Earth’s hydrosphere is in a steady- state equilibrium in terms of quantity.
Within this overall balance, the amount of water stored in glaciers and ice sheets varies, leading to periodic global changes in sea level (discussed further in Chapter 16). The term eustasy refers to changes in global
▲Figure 9.2 Land and water hemispheres. Two perspectives that roughly illustrate Earth’s ocean hemisphere and land hemisphere.
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