Background for
Topography vs. Rock Type
Earth is known as
a rocky planet along with Mercury, Venus, and Mars. The entire planet is
a large experiment in densities. The most dense material (iron and nickel)
make up the solid inner core. The outer core and the mantle are made of
less dense material going outward toward the surface. These molten and
semi-molten rock layers have a very high percentage of iron-bearing minerals.
The crust itself is very thin compared to the other layers and its' position
as the outer layer is also determined by the density of the matter making
it up. The rocks that make up the ocean floor is more dense than the rocks
that make up the continents. Therefore the oceanic crust "floats" at a
lower level on the mantle than the less dense continental crust. This allows
the ocean basins to fill with water (which, of course, is even less dense
than any of the other materials mentioned thus far). There are three major
rock types: igneous (volcanic), sedimentary (from pieces of pre-existing
rock cemented together by natural processes), and metamorphic (rocks of
all three types that have been subjected to intense heat and/or pressure
inside the surface of the earth). These rock types, combined with the dynamic
processes of the planet (crustal plate movement, processes of erosion,
deposition, and lithification [turning to stone]) work together to give
the landscape we view in any location on the planet. Depending on the climate
of a region, certain rock types consistently form ridges (elevational highs)
or valleys (elevational lows) in the landscape. In the western U.S. the
units of limestone (a sedimentary rock) are often ridge forming units.
However, in regions east of the Mississippi River, limestone is not a ridge
former, but is generally found forming the valleys of the region. Of course
recent volcanic episodes can form topographic highs even though they are
not as resistant to erosion at a particular location simply because the
flowed over a higher pre-existing landform. The use of the images and techniques
suggested in this activity should act as a springboard to more in-depth
investigation, by either teacher or student, as interest dictates. Once
again, the inter-relationship of many variables in the real world of geology
will only allow for crude comparisons between rock type and topograhy.
This should nonetheless be informative and somewhat enjoyable.
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