Plate Tectonics and the Scientific Method

According to the theory of plate tectonics, the outer part of the Earth is a layer of rock about 100 kilometers thick. This shell is very thin compared to the size of the Earth.
This shell is divided into at least 25 pieces, which are called plates. These plates move very slowing on the partially molten Earth layer (asthenosphere) below them.
The lithosphere is rigid solid rock. Beneath it is a softer, hotter layer of solid rock called the asthenosphere. Because of the high temepratures the rock of the asthenosphere behaves in a plastic way. In effect, the lithosphere floats on the asthenosphere "carrying" the lithospheric plates that are constantly changing position.
In 1929, about the time Wegener's ideas began to be dismissed, Arthur Holmes elaborated on one of Wegener's many hypotheses; the idea that the mantle undergoes thermal convection.
This idea is based on the fact that as a substance is heated its density decreases and rises to the surface until it is cooled and sinks again.
This repeated heating and cooling results in a current which may be enough to cause continents to move.


Brooklyn College - Geology Department