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Geology of the Pacific Northwest

Geology-208

Week 1:  PLATE TECTONICS

The Theory

Sir Francis Bacon in 1620, Benjamin Franklin in 1782, and Alfred Wegener in the early 1900s wondered about jigsaw-shaped similarities in map outlines of continent boundaries. Franklin pictured the Earth's crust as "a shell floating on a fluid interior." He postulated that "the surface of the globe would be capable of being broken and disordered by the violent movements of the fluids on which it rested." Wegener was able to put the world's broken pieces together in such a way that one supercontinent resulted. He was further convinced by fossils found in a variety of places separated by oceans, and by the characteristics of their descendents, that these places must have been joined at one time. He proposed the Plate Tectonics theory—the relocation of continental and oceanic boundaries—to explain these phenomena.

Earth's Structure

The ground upon which we live is situated above miles-thick layers of solid and liquid rock and metal:

  1. a CORE of metal (dense and heavy iron and nickel). Liquid metal surrounds solid metal.
  2. a MANTLE of rock (dense ultramafic rock). A solid lithosphere layer (divided into moving plates) hovers over a semi-liquid (gooey, peanut-butter consistency) asthenosphere layer.
  3. a CRUST of rock:
    1. ocean crust – dark colored mafic rock; dense
    2. continental crust – light to medium colored felsic rock; less dense

Where We've Been

Scientists have located twelve large and several smaller moveable crustal plates that have been changing the earth's topography and moving the boundaries of our lands and oceans several centimeters every year in specific directions. The way in which the plates (likened to rafts or barges transporting parts of the earth's lands and seas) move helps scientists understand how mountain ranges are formed, why volcanos erupt, and why earthquakes occur.

Because of movement in the earth's upper-mantle lithosphere, some scientists believe that 225 million years ago the earth was one gigantic continent called Pangaea. They believe that Pangaea split into two continents 200 million years ago. Then 135 million years ago, during the Jurassic period, those two continents split further. Oceans separated the land masses more prominently during the Cretaceous period, 65 million years ago. Earth's plates move toward, separate from, and pass by the boundaries of other plates.

Plates that move on a collision course toward other plates ( –>  <– ) are said to have convergent boundaries. The plates either collide (collision zones) and form mountain ridges, or they subduct (subduction zones) when an ocean crust is pushed beneath a land crust. When the edges of plates collide, the earth quakes and erupts. Cooled lava and ash form rock hard layers that become new mountains or add height to old mountains.

Plates moving away from other plates ( <–  –> ) have divergent boundaries. Magma fills in the gaps and creates more crust.

Plates sliding diagonally toward but passing by each other have transform boundaries.

What causes the movement of the plates? Well, the earth is a great oven of heated, radioactive, unstable atoms, and that heat must escape. The heated rocks begin to move, like recycled air, to a rolling boil. The mantle rises from the heat, and the heat circulates the mantle, causing it to move.

Where We're Headed

Africa is moving the slowest of all, so the Africa plate is used as the reference point to measure the movements of the other plates.

RESOURCES

Georgia Geoscience Online (Gore, Pamela J. W.; August 10, 2000)
Historical Perspective (USGS; May 5, 1999)
Major Tectonic Plates Of The World (USGS; June 27, 2001)
Tectonic Plate Motion (Chao, Ben; February 26, 1998)
Chernicoff, Stanley, and Fox, Hayden A., 2000, Essentials of Geology (2nd Edition), Houghton Mifflin Company, Boston, p.196

GLOSSARY



Send comments to Rob Viens (e-mail: rviens@bcc.ctc.edu) or call him at his Bellevue Community College office at (425) 564-3158. Office hours are by appointment.


© 2001-2002, Rob Viens, Bellevue Community College. All rights reserved.
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