Plate Tectonics Continued
Break up of Pangea
The driving mechanism of plate tectonics is a network of convective
heat currents, generated by the hot core of the earth and which
circulate in the mantle. The heat is provided from the decay of
Uranium-238 which is an R-process Supernova element. The overall
transport of heat from the core through the mantle is quite
inefficient so it takes a long time for these convective heat
currents to become established. Hence, plate movements are something
which occurs late (i.e. now) in the geological history of the earth.
The earth's crust is actually a two-component layer. The lithosphere
is a thin layer of rock (average density of 2.7 grams per cc) and
"floats" on top of a plastic-like layer called the asthenosphere.
Plastic-like materials are weird - they deform under stress but don't
really break. A glacier is a good example of a material that moves
and flows plastically. The convective heat currents in the mantle
impinge on the asthenosphere causing deformation and subsequent
movement of the lithospheric plates.
This process can be simulated in your kitchen by putting some jello
in a bowl and putting some peebles on top of the jello. As you shake
the bottom of the bowl, the jello deforms but doesn't break and the
rocks that float on the jello collide. (apologies to real geologists
for this analogy).
As a result of plate movements, interesting things occur at plate
boundaries. In general you don't want to live near a plate boundary
as the earth is active there. About 75% of the world's population
does live near these boundaries.
There are three types of plate boundaries:
- A divergence zone --> crustal separation --> two plates
are moving apart in opposite directions
- A convergence zone --> collision of two plates. A collision
of a less dense continental plate with a more dense oceanic plate
creates a subduction zone where the denser plate dives (subducts) beneath
the less dense plate. A collision between two continental plates
results in general uplift.
- A translational zone --> here two plates slide by one another
in opposite directions. The San Andreas Fault is the most well-known
(and potentially most deadly) translational interface.
Local Manifestations of Plate Tectonics:
The Pacific Northewest is an active tectonic zone. One of the
prime hazards of active volcanoes is the heavy mudflows which
can result from the sudden melting of their heavily glaciated
slopes. Some examples of Northwest Hazards are shown below:
Some Other Internet Resources on Plate Tectonics:
overview of all of this
the Ocean Floor
The Electronic Universe Project