Geology Reference
In-Depth Information
Uniformi-what? Understanding the Earth
through Uniformitarianism
The idea that geologic processes we observe today have always been occurring and can
be used to explain the features of the earth has stood the test of time. In fact, now more
than ever, geologists recognize that the physical, chemical, and biological processes
that occur today must have occurred in the past as well. Even a feature as spectacular
as the Grand Canyon is created by the same simple process of erosion by water (see
Chapter 12) that creates creeks and gullies in your backyard.
However, when Hutton and Lyell proposed the concept of uniformitarianism, they as-
sumed that the rate and intensity of past processes were the same as those observed
today. The current understanding of uniformitarianism in geology no longer makes this
assumption. Modern uniformitarianism differs from the original idea in two very import-
ant ways:
Rates and intensity of processes may vary:
While the processes scientists observe
today occurred in the past, they may have occurred more quickly or more in-
tensely than they do now. For example, massive layers of volcanic rocks across
Siberia (called the
Siberian Traps
) suggest a period of very intense lava outpour-
ings, unlike anything humans have ever observed.
Catastrophes do play a role:
When uniformitarianism was first proposed, it ran
counter to the ideas of catastrophism. But modern geologists recognize that occa-
sional catastrophic events (such as volcanic eruptions and tsunamis) do play an
important role in shaping the earth's surface.
Pulling It All Together: The Theory of
Plate Tectonics
During World War I, a German scientist named Alfred Wegener suggested that the contin-
ents had once been connected and had drifted apart. His ideas about
continental drift
—
the movement of the continents — were based on fossil, rock, and stratigraphic eviden-
ce (which I discuss in detail in Chapter 8). However, he hadn't worked out all the details
- such as what force, or
mechanism
, propelled the continents. At the time, scientific un-
derstanding of the earth's crust as a continuous, solid, rigid layer did not allow for mov-
ing continents, and without a clear explanation for how they moved, Wegener's hypo-
thesis was strongly rejected by other geologists.
