Plate TectonicsSea Floor Spreading

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• The plates are made of the crust and a small part of the upper mantle. These two parts together are called the lithosphere. This layer is about 100 km (~ 60 mi.) thick and is less dense than the material underneath. The “plastic-like” layer below the lithosphere is called the asthenosphere. The rigid plates of the lithosphere “float” and move around on the asthenosphere. You can think of the plates as large, flat stones placed on top of putty, soft clay.

Plate Boundaries

• Plates moving apart are called – divergent boundaries

• Plates moving together are called– Convergent boundaries

• Plates sliding past each other are called– Transform boundaries

Measuring Plate Movement

• Scientists are able to measure the small movements of plates – by very sensitive instruments aboard satellites in space

– and by laser rangefinders based on the surface of Earth.