Notes on Basic Geology
Notes created & information organization based on the book:
The Dynamic Earth - an introduction to physical geology"
Brian Skinner & Stephen C. Porter   (further book information here)
also look at for additional resource information
Plate Tectonics - Page 4

Plate Tectonics - So What Moves Us?

We have no large x-ray machine that can show us directly what is happening inside the earth. We can only measure physical phenomena on or near the surface. We have been able to send space probes multiple millions of miles with near pin point accuracy. We have brought back images of moons from planets we can barely see (Pluto). (Trips to the far edges of our solar system and beyond.)

We can look at rock and landscape that has been revealed by uplift, but we can only postulate at what is happening a mere 100 km (62 miles) below us. We know certain physical principles to be true and make educated guesses as to the mechanisms occurring a few miles below.

We know two things:

1.) for the lithosphere (thus continents) to move it requires energy
2.) the only
source large enough must come from the Earth's own internal heat

We can measure the physical properties of rock in the laboratory, and know that at the pressure and temperatures in the asthenosphere rock can act like a fluid if placed under stress. This means that it can and does move vast amounts of heat energy using the convection principle. (convection: heat movement with mass movement)

If hot material is rising then it must be replaced by cool material sinking. (Otherwise the earth would be like a giant inflating balloon.) Convection is essential to keep the asthenosphere mobile or the plates on top could not move.

3 Forces

1.) the pushing apart at the central ridge. (it pushes the plates sideways)
(the problem: there is normal faulting along the ridge, and this cannot be caused by compression which is the result of a "pushing" process.)

2.) the lithosphere might be pulled apart. The sinking far ends that are subducting might be pulling the middle apart. (the problem: the lithosphere is too thin and weak not to be highly deformed in the center if it were being pulled that hard.)

3.) the plate is much thinner in the center and thicker along the edges. This is because it rises somewhat in the center due to heat expansion and cools toward the edges, becoming more dense. Thus there is a gradient between the center and edges and it automatically flows down hill.

The final answer may involve all three processes or something yet to be discovered, it is an active area of work and continues.

Building some land to live on

The oldest crust found in the ocean dates from about 150 million years ago. In terms of being able to measure older events, we have to turn our attention to the continents. Since they have to large degree been riding on the surface of the lithosphere a large portion of the crust has been around for a long time. The central features of the crust have been out of the loop (the tectonic loop) so to speak.

They have taken the brunt of the action and have stored a good deal of information about their past existence. In a sense they are like computer storage media, they have recorded much of their past history and continue to bring it along.