Introduction — the Earth is a dynamic planet

• The Earth has been in constant change since it formed; the rocks of the crust do not stay in their original state.
• They are subjected from time to time to internal and external forces that make them take new positions and shapes.
• The forces released from inside the Earth cause earthquakes, the agitation of seas and oceans, the advance or retreat of their waters over the land, and the shifting and movement of the continents relative to one another.

The role of the lithosphere and the asthenosphere

• The lithosphere is made of the crust plus the rigid upper part of the mantle, and it is about 100 km thick.
• Below it lies the asthenosphere, about 350 km thick: a plastic rock material that behaves like a fluid under pressure and temperature.
• Convection-current eddies spread through the asthenosphere and help the continents float and move over it.