........................................ The Earth's Lithosphere

Introduction. According to the chemical composition of Earth, it is divided into three layers called crust, mantle and core. We live on the surface called crust, therefore, this is the layer we would want to know better. Moreover, due to technical difficulties, the mantle had not been reached yet.

Before the year 1910, volcanoes, earthquakes, folds, faults and the shape of the continents were not connected by facts and left without any explications of their origin. But after 1910, thanks to the "Theory of continental drift" proposed by German geologist Alfred Wegener (1880-1930), all of those facts have been connected and explicated. This theory was a starting point for the current "Theory of Plate Tectonics" which argues that, according to its state, in the Earth, there is a solid lawyer called lithosphere, which is fragmented into different plates that move over the grassy zones of the mantle because of constantly running internal magma, and the collisions between those plates are causing folds, faults, earthquakes, etc. Once more it has been the great effort of scientists that allowed us to learn the reality and gave us ability to determine what has to be done to prevent natural disasters and avoid possible problems.

1 . The Earth's Lithosphere. The lithosphere is a solid superficial layer of the planet. It is consisted of the crust and for the mantle solid part, named residual mantle. From the type of mantle that is contained, the two types of lithosphere are differentiated:

• Oceanic Llithosphere. Is formed by an oceanic crust and residual mantle. It composes the oceans floor and has an average thickness of 65 km, although in the large mountain ridges that are found at the bottom of the oceans, called oceanic ridge, the thickness is only 7km.

• Continental Lithosphere. Is formed by a continental crust and residual mantle. It composes the continents. The average thickness is 120 km.

The lithosphere is floating on a soft layer called asthenosphere. The lithosphere is divided into large fragments called lithospheric plates or tectonic plates, which move together separating or colliding. The collisions that occur are the reason that earthquakes, volcanoes, folds and faults happen.

2 . Asthenosphere. Traditionally this layer has been defined as a pasty layer (fluid) mantle that reaches up to 250 km depth and is placed between the lithosphere and the rest of the mantle, which both are solid layers. In recent decades it has been found that there is no fluid layer in many places under the lithosphere. Therefore, some authors believe that the asthenosphere does not exist after all, and that it is the mantle that presents the plastic capability and everything is driven by the heat from the core of earth and for this reason there is the movement of tectonic plates.

3. The tectonic plates. Are different fragments in which the lithosphere is divides. Currently there are seven major tectonic plates and seven small tectonic plates. Their names are:

According to its constitution there are two types of lithospheric plates that are:

• Oceanic Plates. Are those made entirely by the oceanic lithosphere. For example, the Pacific Plate.

• Mixed Plates. Are those that have a part of oceanic lithosphere and a part of continental lithosphere. For example, the African Plate that contains the oceanic lithosphere until the middle of the Atlantic Ocean and the continental lithosphere in Africa.

4 . Types of interaction between plates. The tectonic plates are rigid pieces which move by floating on a surface of plastic mantle, traditionally called asthenosphere. They move because of currents of hot magma that comes from the base of the mantle. When those currents reach the bottom of the lithosphere they cool down, and driven by the continuously coming materials, they return to the fund originating different circular currents called convection currents. Such currents may break a plate and then pull the two fragments apart, or also may make them collide against one another.

The main types of contacts between plates are:

• Separation of plates. It occurs when a stream of magma reaches a lithospheric plate, fragments it and continues to separate those fragments for millions of years. An example is the Atlantic ridge, a great mountain range that lays in the middle of the Atlantic Ocean, where it continues to grow both American plates to the west and the African and Eurasian plates to the east. Because of this the Atlantic Ocean get wider about 3 cm each year.

• Collision of the oceanic lithosphere with the continental lithosphere. It is characterized by the oceanic lithosphere being inserted under the continental lithosphere (subduction), since the continental lithosphere is thicker and less dense. An example is the Pacific coast of South America.

• Collision of two continental lithospheres. As the continental lithospheres are less dense than the mantle they do not sink, which causes that sediments accumulated between them fold and form immense ridges. An example is in the Himalayas, where the India was formed because of collision with the southern edge of ancient Asia.

5 .Dynamics of the lithosphere. The differences between the elevations and depressions of a solid surface are called reliefs. The relief of the lithosphere is changing continuously. There can be differentiated processes constructive and destructives.

• The constructive processes of relief. Are the ones that cause elevations or depressions. They are caused by internal geological factors, which are the internal forces that deform lithosphere. They are provoked by the heat produced by convection currents that move the plates, and the collisions originate the folds, faults, joints, earthquakes and volcanoes.

• The destructive processes of relief. Are the ones that cause the attrition of the elevations and filling of the depressions. They are caused by external geological factors, which are the atmosphere, wind, floods, rivers, sea, ice and living things. The destruction of the relief occurs in following steps: weathering (wear on-site), erosion (wear associated with transport), transport and sedimentation.