Geoscience Reference
In-Depth Information
once more to support the growth of organisms. The key here is that the
magnitude of life we enjoy on Earth is possible because of the active
recycling of life's constituents by tectonic processes. This was first recog-
nized over two hundred years ago by James Hutton, whom we also met
in the Preface. He wrote the following in his treatise
Theory of the Earth
(1788):
The end of nature in placing an internal fire or power of heat, and
a force of irresistible expansion, in the body of this Earth, is to
consolidate the sediment collected at the bottom of the sea, and
to form thereof a mass of permanent land above the level of the
ocean for the maintainment of plants and animals.
Finally, what about energy? I will say much more about energy in the
next chapter, particularly about the types of energy needed for life,
many of which you normally wouldn't think about. On modern Earth
though, most (probably over 99%) of the energy to the biosphere ulti-
mately comes from the Sun, driving the photosynthesis of plants, algae,
and microbes (known as cyanobacteria; we will hear much more about
them in later chapters) that produce organic material and oxygen. These
products of photosynthesis are biologically recombined in Earth's great
food chains. For example, copepods in the ocean eat algae, small fish
eat the copepods, larger fish eat the small fish, and even larger fish eat
these. These fish die and are decomposed by a variety of bacteria, which
in turn are consumed by other organisms. The chain goes on and on but
it is fueled, ultimately, by the organic matter and oxygen produced by
photosynthesis. As described above, however, the organisms producing
the oxygen, and driving the biosphere, obtain their building blocks from
material recycled through plate tectonics. Thus, while the Sun supplies
the energy, the rates at which tectonics recycles basic biological compo-
nents sets the tempo.
All in all, we must agree that Earth is a pretty terrific place for life. It
sits comfortably within the habitable zone of the Sun. In addition, its
active tectonics both control the temperature of the surface environ-
ment, providing a continuous supply of liquid water, and recycle the
basic components required to fuel abundant life. As we will see in the
next chapter, the same tectonics may have also provided optimal condi-
tions for the earliest biosphere.
