Section 3

## Plate Tectonics - Lecture Notes

Gross Features of the Earth (REarth=6370 km)
(Moving from a high altitude above the surface to the center of the earth.)
Ionosphere - ~100 km (Layers of the atmosphere are usually separated by changes in temperature.)
Mesosphere - ~50 km (Ozone layer exists between the mesosphere and stratosphere)
Stratosphere - ~12 km
Troposphere - clouds and water vapor

Crust - Is the outer layer of the earth with an average thickness of about 15 km.  It has an average density of 3,000 kg/m3.  (The density of iron is 7,900 kg/m3 and marble is 2,700 kg/m3.)
Mantle - Is the largest part of the earth by volume (80%), extends between 15 km to 3,000 km in depth (going towards the center), and has an average density of 5,000 kg/m3.
Core - This part of the earth is usually divided into an inner core and an outer core.  Both parts have an abundance of nickel and iron.  The average density is 11,000 kg/m3.  The outer core is liquid (molten rock) but the inner core is considered a solid metallic material.

How do we know of this internal structure?

Most of what we know comes from examining the propagation of seismic waves (ground vibrations) and modeling the structure of the earth so that it is consistent with the seismic studies (and other experimental observations).

It is sometimes instructive to examine the earth's crust and mantle in a slightly different way.  It can be broken into a top, solid (or rigid) layer called the Lithosphere.  At a depth of about 100 km, the mantle becomes capable of flowing.  From this depth to about 500 km is called the Asthenosphere.
Note:  The Lithosphere includes the crust and the very top part of the mantle.  The Asthenosphere is part of the mantle.

Plate Tectonics

During about 1912 Alfred Wegener, a Meteorologist, proposed a theory of Continental Drift.  He suggested that the earth's crust was composed of solid "plates" that were in motion (albeit slow, ~ 6cm/yr) and about 200 million years ago all the continents were connected into a super-continent called Pangea.  This hypothesis was not well received when it was first proposed.

"Wegener's hypothesis in general is of the foot-loose type, in that it takes considerable liberty with our globe, and is less bound by restrictions or tied down by awkward, ugly facts than most of its rival theories.  Its appeal seems to lie in the fact that it plays a game in which there are few restrictive rules and no sharply drawn code of conduct." (R.T. Chamberlain - A Geological Miscellany, Princeton Press, 1982)

This theory is true (such that, it fits almost all observations and most scientist agree with it).  Plate Tectonics integrates continental drift, sea floor spreading, and seismic zones.

Plate Tectonics - Related Web Links

Plate Tectonic Animations from UC-Berkeley
On-line Book This Dynamic Earth: The Story of Plate Tectonics by W. Jacquelyne Kious and Robert I. Tilling
USGS historical perspective on Tectonic Plates
Paleomagnetism and the reversal of earth's magnetic field
Google - Search for Plate Tectonics

On-line Lecture Notes

GOOGLE search for plate tectonic lecture notes
Dr. Andy Frank's Physical Geology Plate Tectonics
Dr. Pamela Gore's Earth's interior lecture notes
Dr. Susan DeBari's Plate Tectonics lecture notes