The internal energy of the Earth

The existence of geological processes makes the Earth a constantly changing planet. These geological processes are of two types:

• External geological processes. Processes erosion, transport and sedimentation are produced by external geological agents (wind, water, ice, ...) driven by energy from the sun. They generate sedimentary rocks.
• Internal geological processes. The thermal energy of the interior of the Earth is responsible for the changes in the interior of the Earth, such as that the Earth is divided into plates and that they move, which causes volcanic eruptions, earthquakes, and the appearance of mountain ranges. They generate magmatic and metamorphic rocks.

The changes that take place inside the Earth, sometimes, are slow. For example, Europe and America separate at a rate of 2.8 cm each year. Other times, internal geological processes are rapid, such as when an earthquake or volcanic eruption occurs.

The average temperature of the earth's surface is about 15º C, although it varies according to the energy that comes from the Sun (in addition to the pollutants that contribute to climate change). This solar energy heats the surface and a few centimeters deep, but if we measure the temperature at a greater depth we will see that the temperature increases as we go deeper. But this energy does not come from the outside, but the heat energy comes from the interior of the Earth that, by conduction, reaches the earth's surface.

Test: The internal energy of the Earth.

Test: Internal heat of the Earth.

Origin of the internal heat of the Earth

About 4.6 billion years ago the Earth and the Solar System were formed from a nebula that began to rotate, concentrating the particles of interstellar dust and gas, creating the Sun and the planets of the Solar System. As the particles became more concentrated, the gravitational field increased, attracting even more particles. The impacts produced by the new captured particles increased the temperature of the planet that was being formed. Some of the internal energy of the Earth comes from that original heat of formation of the Earth when the particles collide.

On the other hand, the decay of radioactive elements also contributes to the Earth's internal energy so great that it allows the outer core of the Earth to be molten .

Afterwards, the Earth cooled down and the elements were distributed according to their density. Thus, the innermost layer has denser materials, and the outermost, lighter ones.

Geothermal gradient

This variation in temperature, which increases with depth, is known as the geothermal gradient. The temperature in the earth's crust increases 1 ºC every 33 meters of depth. In deeper layers it does not follow that relationship.

By Pjvelasco (Own work) [CC BY-SA 3.0], via Wikimedia Commons

There are two mechanisms by which heat energy reaches the surface:

• Thermal conductivity. Rocks conduct heat very poorly, so it takes a long time for heat to transfer from rock to rock .
• Convection currents. Fluid materials, when heated, expand, become less dense, and rise to the earth's surface. There they cool, contract, and descend towards the core of the Earth, repeating the process.

This thermal energy causes the movement of materials and can take many forms, such as the movement of continents, rising mountain ranges, the appearance of volcanoes, earthquakes, etc.

By Surachit [GFDL or CC BY-SA 3.0], via Wikimedia Commons

Movement of the continents

The current continents were part of a large supercontinent called Pangea, but the convection movements of the mantle have caused it to move to its current position.

The theory of plate tectonics explains how Pangea broke and continents moved. According to Plate Tectonics, the lithosphere (external part of the Earth) is divided into tectonic plates that slide over the Earth's mantle. From the contact between plates the mountain chains arise and in their edges most of the volcanoes and  earthquakes that occur are concentrated .

Research activity: Tectonic plates.