As an overview and a preview to the course,
we will now discuss how two general processes
operate on and in the earth. These are the
external and the internal processes.
The external processes wear the earth down
and include erosion, rivers, waves, wind,
groundwater, glaciers. Think of mountains
when you think of these processes. The Arbuckle
Mountains in southern Oklahoma along Interstate
35, they're fairly small mountains. Compare
those with the Rocky Mountains just west of
Denver, much larger, higher mountains. Why
is there a difference? Well, the Arbuckle
Mountains are 300 million years old. There's
been a lot more time for those external processes
to wear them down. The Rocky Mountains are
maybe 20 million years old, so much less time.
So what controls these external processes?
It's called the hydrologic or the water cycle.
And what is it? Well, when rain or snow falls
on the Earth, the water, it eventually moves
to the oceans where you get evaporation. You
put moisture back in the atmosphere. That
moisture then causes precipitation back on
the land. That is the hydrologic or the water
cycle. Now, I have an assignment for you for
our next class. I'd like you to research what
actually controls the hydrologic cycle and
be prepared to discuss that in class.
Now, for the internal processes. These are
the processes that build up the Earth and
include volcanoes, earthquakes, and mountain
building. What is the control? It's something
called plate tectonics, which is a relatively
new theory. Maybe you've heard about a gentleman
named Wegener, who proposed in the early 1900s
that the continents had actually drifted across
the globe. He based that on the jigsaw puzzle
fit of the continents and proposed that at
one time all the continents were together.
Well, his theory was not well accepted, and
it wasn't until the 1950s or 60s that new
evidence was presented that convinced a lot
of geologists that plate tectonics is a valid
hypothesis, and it is now a theory. And this
theory of plate tectonics basically states
that interactions between these moving plates
on the globe cause all kinds of interesting
things, such as earthquakes, mountain building,
and volcanism. So I have another assignment
for you. What drives these tectonic processes?
I'll provide one hint--it involves heat.
