Planet Earth: Geosphere

Geosphere: Definition

Geosphere is defined as the solid part of the planet. It includes not only the superficial rocks, but also all the materials inside the Earth.

The main components of the geosphere are minerals and rocks, although in some layers or regions the rocks can be molten. Liquid or semiliquid materials, such as magma, are also components of the geosphere, but are not proper minerals or rocks.

The most rigid part of the geosphere is located in the upper layers and is called lithosphere.

Geosphere: Layers.

The geosphere is made of three concentric layers. These layers have different properties and composition. The boundaries between these layers and sub-layers, called discontinuities, are also very important.

Earth's Layers

Geosphere layers:

• Crust.
• Continental Crust.
• Oceanic Crust.
• Mantle.
• Upper Mantle.
• Lower Mantle.
• Core.
• Outer Core.
• Inner Core. 

Crust

The crust is the outermost and thinnest layer of the planet. It is less than 30km thick on average and occupies less than 1% of the Earth total volume. The most abundant components of this layer are silicates.

Earth’s crust can be divided into two different parts: continental crust and oceanic crust.

The continental crust is the part of the crust that forms the continents. It includes the emerged surface of the planet and a shallow part of the submerged crust, adjacent to the continents, called continental shelf.

It is thicker than the oceanic crust, it is 30km to 50km thick, and it is thicker under the mountains than under the coast. We can say that mountains have a sort of roots made of crust that penetrate deeper into the Mantle. It is, besides, older than the oceanic crust.

The typical components are the sodium potassium aluminium silicate rocks, such as granite. Due to this, it is called FELSIC.

The oceanic crust is the part of the crust that forms the sea floor. It is thinner than the continental crust, it is from 5km to 10km thick on average. It is younger than the continental crust, merely because the oceanic crust is constantly being generated by the intense volcanic activity that occurs in the middle of the large oceans, in volcanic chains called ridges. 

Oceanic crust, besides, is destroyed in some deep regions of the oceans called oceanic trenches.

Summing up, new oceanic crust is constantly being formed in the ridges of the middle of the ocean and, at the same time, old oceanic crust is being destroyed in the oceanic trenches, that are adjacent to some continental shelves. 

The typical components of this part of the crust are the iron magnesium silicate rocks, such as basalt. Due to this, it is called MAFIC.

The boundary that separates the crust and the Mantle is called Mohorovicic discontinuity.

Mantle

The mantle is the intermediate layer, between the crust and the core. It is a very thick layer, about 2900km thick and constitutes about 84% of the Earth's total volume.

It is mainly made of solid rocks, although in some parts, above all in the upper part of the layer, the rocks have plastic properties and are not totally rigid. The most abundant components are silicates.

The mantle's temperature depends on the depth, rising from around 400°C in the upper boundary to 4000°C at the lower boundary. The pressure, logically, also rises depending on the depth due to the weight of the overlying rocks. This pressure is called lithostatic pressure. Summing up, the deeper the rocks are, the higher the temperature and pressure on those rocks. Rocks have different properties according to these two physical parameters.

The mantle has two different parts, called upper and lower mantle.

The upper mantle is adjacent to the crust and it is between 300km to 500km thick. It can also be divided into two parts. The rocks of the upper part, just beneath the crust, are rigid. The group of rigid rocks of the upper mantle and the crust form a rigid structure called lithosphere. The Earth's surface is made of pieces, like a jigsaw, called tectonic plates. Tectonic plates are solid and rigid structures made from the rocks of the crust and the upper part of the mantle.

In the lower part of the upper mantle, under the lithosphere, there is a region where rocks are partially molten. This region is called asthenosphere. The tectonic plates are, in some way, floating on the asthenosphere.

The lower mantle is just beneath this plastic region. It is very thick, about 2000km, and it is made of solid and rigid rocks. Its temperature is very high, above all at the lowest part, but rocks are not molten due to the enormous lithostatic pressure.

The boundary that separates the lower mantle of the core is called Gutenberg discontinuity.

Core

The core is the inner layer of the planet. It is around 3500km thick and extremely hot, with temperatures over 4000°C. In fact, the earth's core is hotter than the Sun surface. It is also extremely dense. The main chemical components are nickel and iron.

The core has two parts, the outer and the inner core.

The outer core is between the mantle and the inner core. It is 2200km thick. And the extremely high temperature, from 4000°C in the outer part to 6000°C near the inner core, melts the rocks, so this layer is liquid.

The inner core is the deepest layer of the Earth. Although its temperature is even higher than in the outer core, over 5000°C, the extremely high lithospheric pressure solidifies the rocks, so this layer is solid.

The movement of the nickel and iron fluid of the outer core around the solid nickel and iron of the inner core causes a dynamo effect, that is responsible for the magnetic field of the planet.

Earth’s Surface

As we studied, the Earth’s surface is made of large pieces called tectonic plates. Tectonic plates are like pieces of lithosphere than can move or crunch one another. Different plates are separated by different kind of boundaries. The most important ones are constructive boundaries, destructive boundaries and transforming boundaries.

Constructive boundaries are borders where new crust is formed. The most important ones are the oceanic ridges, volcanic ridges in the middle of the ocean where the volcanic activity creates new crust, causing the expansion of the oceanic floor.

Destructive boundaries are borders where the crust is destroyed. The crust passes to interior layers of the Earth in a process called subduction. The most important destructive boundaries are in the oceanic trenches.

Transforming boundaries are borders where crust is neither formed nor destroyed. They are faults that separate different tectonic plates. The friction between different plates can cause geological phenomena, such as earthquakes.

Continental features

When two tectonic plates collide some typical continental structures, like mountain ranges, are formed, due to the deformation of the crust in the zones where plates crush. The Andes and the Himalayas are two examples.

The oldest parts of the continental crust tend to be flat due to the effect of erosion, forming continental plains. Some deserts, such as The Sahara, are examples of great plains.

The part of the continental crust that is submerged under the sea is called the continental shelf. It is an extension of the continental crust, near the coastline, covered by water although it is always shallow, less than 200m deep. The continental shelves finish in the continental slopes, where the sea floor descends abruptly from 200m to more than 4000m on average.

Oceanic features

The friction of the tectonic plates in the subduction zones causes many geological phenomena. The friction tends to increase the temperature of the rocks, melting them and transforming them into magma. When this magma reaches the surface, it forms volcanoes. When the subduction is under the sea, the magma forms volcanic islands and archipelagos. Japan is a good example.

The oceanic trenches associated to the subduction zones are the deepest parts of the ocean.

As we studied, in the middle of the oceans there are constructive boundaries called oceanic ridges. They are long submarine chains of mountains with high volcanic activity. They are responsible for the formation of oceanic crust.

Due to this process, the sea floor tends to be flat. Abyssal plains are the largest plains of the planet. They are large extensions of submerged flat land, under more than 4000m of water on average.