There is a popular
oversimplification regarding tectonic forces and the resulting
effects on surface topography. We think of all the action
taking place on top of a stable foundation when in fact things
are a little more complicated. If we think of two plates
colliding, we tend to compare it to having two loose rugs on a
slippery floor and then pushing them toward each other. One
rug might slide under the other, it might drop down into a
mysterious black hole, or you might just have crumples and
folds in both rugs.
Unfortunately, the earth’s crust is not as simple
as the above examples. We are not resting on a stable surface.
Instead the crust is akin to so much floating scum on the
mantle, which underlies the crust. The mantle is a very
viscous fluid. In many ways it will react to forces the same
way water does although it will take millions (or billions, or
whatever) times longer to do so.
If you add weight to an object floating in water,
then the object sinks lower into the water. For example, if
you fill a coal barge with coal, the barge sinks lower. If the
Mississippi River adds silt to southern Louisiana then New
Orleans will sink lower toward the mantle (Also lower relative
to the Gulf of Mexico. There is a hint here regarding long
term real estate investments.). Conversely, if you remove
weight by melting a large icecap that has previously depressed
the earth’s surface, then that area of the crust will slowly
rise back up again. If erosion removes sedimentary layers from
the top of an existing mountain range, then the remaining
portion of the earth’s crust will float higher. Thus,
metamorphic and even deeper basement layers, which were
formerly much lower in the crust, will gradually float upwards
where they will form mountains that are almost as high as
those that existed before erosion took place.
Let’s perform an experiment with the coal barge.
Assume a team of divers brings in balsa wood from an external
source and places a 5-foot thick section under the barge.
Assume balsa density equals 0.2 and water density equals 1.0.
We know that the balsa wood will lift the barge higher out of
the water (similar to a life jacket lifting the head of a
hapless swimmer), but let’s do a little arithmetic.
The total thickness of the barge/balsa complex
has increased by 5 feet. (We have added 5 feet to the bottom
of the barge.) The weight (per square foot) that we have added
is equal to the density (0.2) times the thickness (5 feet).
(Technically, we should also multiply by the weight of water
per cubic foot, but this term cancels out – hence, for
simplicity, we have ignored it.) The barge/balsa complex must
displace this additional weight of water. Thus, the bottom of
the barge/balsa complex must extend 0.2 times 5 feet equals 1
foot deeper into the water. We thus calculate that the barge
has been lifted 4 feet higher out of the water. (5 feet
thicker minus 1 foot deeper equals 4 feet higher out of the
water.) It is interesting that this amount of lifting is
independent of any cargo that might be in the barge.
We could also calculate what would happen if we
used another variety of wood that had a higher density. For
example, if we added 5 feet of live oak, which has a density
of about 0.977, then the bottom of the barge/oak complex would
have to displace an additional 0.977 times 5 feet equals 4.885
feet of water. The barge would only be lifted 5 minus 4.885
equals 0.115 feet higher (about 1.38 inches). (Hint, don’t use
ironwood, density equals 1.077, in life preservers.)
We can apply these principles to plate tectonics.
If two plates collide, either the two pieces of surface crust
must crumple, or one of the pieces must slide under the other
(usually a combination of both). In either case, we have
thickened the lightweight crustal material. (If the subducting
plate dives steeply taking most of its material down with it,
this thickening is substantially reduced, but it is still not
entirely eliminated.) When we added lightweight wood
underneath the barge, the barge rose higher out of the water.
When we thicken the lightweight components of the earth’s
crust, we call it mountain building. In the barge example, we
also noted that adding wood to the bottom of the barge caused
the complex to extend deeper into the water as the added
weight required a greater water displacement. When we used the
higher density oak instead of the balsa wood, it caused a
deeper displacement and less lifting. This is because the
density of the oak (at 0.977) was much closer to the density
of water (at 1.0) than was the density of balsa (at 0.2).
The earth’s crust (and particularly the
lithosphere under it) has a density only slightly less than
that of the underlying mantle. Thus, when we thicken crustal
material and form mountain ranges, the situation is similar to
the oak experiment with the barge. The displacement taking
place out of sight under the crust is ten to twenty times
greater than the uplift above the surface. In order to build a
mile-high mountain range we have to increase the subsurface
displacement by ten to twenty miles. (In practice, there is
enough rigidity in crustal material and the underlying
lithosphere so that the displacement depth can be somewhat
less as long as the subsurface horizontal extent is increased
enough to equal the required weight displacement.)
As noted above, this thickening of the
lightweight crustal material is usually a result of plate
collision (convergence). In theory, we could also thicken the
crust by having a team of divers bring in balsa (or other
light weight material) from an outside source, diving down
under the crust (or the rigid portion of the lithosphere), and
nailing it in place under the area where we want to form a
mountain range. (We admit this isn’t overly practical.)
Alternately, we could push or otherwise slide the earth’s
crust over a forest of balsa wood, and as we pass over each
tree, nail pieces of the lightweight material underneath our
incipient mountain range. We will return to this alternate
“sliding” process when we discuss the theory explaining all
the local mountain building since the Laramide. For now, we
will simply note that ten million years ago, a “hot plume” in
the mantle was under the Snake River Plain in Idaho. Since
then, the earth’s crust has been slowly sliding
west-southwestward so that now the “hot plume” is under
Yellowstone National Park.
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