The Lithosphere / Geosphere
The Geosphere or Lithosphere is the sphere that refers to the solid part of the Earth, which includes the crust and the upper mantle.
We can distinguish two types of lithosphere: continental crust, above sea level, a thick layer of around 200 km, and oceanic crust, below sea level, denser but thinner than the continental crust.
Continental Drift and Tectonic Plates
The Continental Drift theory was one of the first theories that tried to describe the continents movements across time. Developed by geologist Alfred Wegener, it establishes that 200 million years ago all the continents were united into a single supercontinent called Pangea. Pangea broke up because of the internal forces of the mantle and the tectonic plates gradually moved apart.
Today, geographers do not fully accept Wegener’s theory of continental drift, but prefer the science of plate tectonics. Geographers believe that several supercontinents like Pangea had been formed and broken up over the course of the Earth’s history.
Continents rest on massive slabs of rock called tectonic plates that are constantly moving over the mantle. Changes in the mantle cause tectonic plates to slide against each other or to move apart. Thus, the plate’s boundaries are unstable.
This instability causes the creation and destruction of the different forms of relief we can see in the crust.
Internal Forces that create Relief
Internal forces and pressures from the mantle are the reason why tectonic plates move apart, slide or collide against each other. This causes changes on the Earth's relief.
The most important changes are:
Fold: deformations of the Earth’s surface where rock layers bend.
Fault: breaks in rock layers where the rock is too hard to bend.
Subduction: when one rock layer from a plate sinks under the other and its materials melt into the mantle’s magma.