Plate Tectonics Continued

Break up of Pangea

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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:

• Tectonic Details
• Pacific Northwest Details
• Find the Fault lines
• Mt. Adams
• Mt. Rainier
• Mt. Lassen
• Mt. St. Helens

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:

• Volcanoes all in a Line
• Recent Lava Intrusions
• Basalt Flows in the Pacific Northwest
• Mudflows from Mt. Rainier

Some Other Internet Resources on Plate Tectonics:

Another overview of all of this

Imaging the Ocean Floor

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