2. EXTERNAL AND INTERNAL GEOLOGICAL PROCESSES

Talking book 

The Earth's surface has been subjected to numerous changes and phenomena known as geological processes. Cliff erosion, a volcanic eruption, or sedimentation at a mouth of a river are examples of these geological processes. The geological agents responsible for these processes are: waves, the internal heat of the Earth, or rivers.

Geological agents and processes are classified as internal and external.

• Internal geological agents and processes are drive by the heat that is stored in the Earth's interior. They usually occur far from the surface. The main internal geological agent is the movement of the lithospheric plates. The most evident and catastrophic events caused by this movement are Earthquakes and volcanic eruptions. Others are the movement of the continents, the opening of new seas and the closing of old ones, or the formation of mountain ranges.

It can be said that internal geological processes build the Earth's relief.

• External geological agents and processes affect the Earth's surface. They are powered by solar energy. External processes shape the relief created by internal processes. External agents carry out this process: water, ice, wind, atmosphere and human beings. They include all the changes that alter or wear down the rocks and deposit materials resulting from erosion.

Internal agents and processes tend to lift and build the Earth's relief from the interior. External agents and processes tend to destroy and shape the Earth's relief.

Although processes such as volcanic eruptions and earthquakes affect the Earth's surface, they clearly originate in the Earth's interior. Therefore, they are internal processes.

2.1. The engine of internal and external geological processes

Talking book 

Both internal and external geological agents need a source of energy to drive them so that processes can take place.

2.1.1. The Earth's internal heat

Talking book 

The internal heat stored in the Earth is responsible for our planet being geologically active. The origin of this heat comes from two sources: most of it has been stored since the beginning of the Earth's formation resulting from the collision of the planetesimals. The rest is generated by the disintegration of radioactive elements.

This internal heat generates convection currents in the mantle, which behaves like an extremely thick liquid. When the materials in the interior heat up, they expand, become less dense and rise. As they cool, they descend and continue the cycle.

The average gradient on the continents is 30º C per km, although the values are higher in active volcanic areas and lower in old cratons.

2.1.2. Solar energy

Talking book 

External agents that shape the relief are driven by solar energy that arrives at the Earth's surface. This energy creates differences in temperature in air masses, causing winds. It also powers the water cycle as it permits the evaporation of large quantities of water from the oceans that fall on the continents in the form of rain or snow.

2.1.3. Gravity

Talking book 

Gravity also has an important role in both external and internal geological processes. It contributes to the formation of convection currents in the Earth's interior, in which cold, dense materials sink and are replaced by hotter and lighter materials that rise. On the surface, gravity causes water and ice accumulated on the continents to move towards the sea, forming rivers and glaciers which erode and shape rocks along their courses. In addition, gravitational pull is responsible for one of the movements of the sea: the tides.