The region of the earth immediately under the lithosphere is the mantle. The upper region of the mantle contains molten rocks and gases. These molten rocks containing gases are known as magma. The temperature at the bottom of the mantle is higher than at the top. The hot magma from below being lighter rises up and relatively less hot magma heavier moves down towards the centre of the earth giving rise to vast slow convection currents.
As each convection current of rocks in the mantle moves horizontally under the lithosphere, it drags the overlying plate with it. Several convection currents exist in the earth’s mantle and these are responsible for the movement of the earth’s plates. When two earth plates push each other, this may result in the formation of mountains. Thus, a plain on the earth may not always remain a plain.
The convection currents in the hot magma are also responsible for the following Earth Process.
1. Formation of plateaus
2. Eruption of Volcanoes
Let us study these earth processes one by one.
1. Formation of Plateaus
A plateau is highland with a broad more or less flat surface. They are generally higher than plains. A plateau is formed by the emergence of hot magma through the vents on the surface of the earth. Magma after coming to the surface, is known as ‘lava’. The hot magma after coming out of the surface of the earth solidifies on cooling and forms igneous rocks.
The rocks formed by the solidification of a molten material (magma) are called IGNEOUS ROCKS. When large amount of magma rises to the earth’s surface it flows out and accumulates in layer after layer lava and produces an elevated land called plateau. The west central part of India – The Deccan Plateau was formed in this way.
2. Eruption of Volcanoes
A volcano is a vent or an opening in the earth’s crust through which hot materials consisting of molten rocks and gases (magma) gush out from the interior of the earth. We have just discussed that plateau is formed when magma comes to the surface of the earth quietly through the vents in the earth’s surface. Sometimes that vent which existed in the past gets plugs and prevents the hot magma from emerging to the earth’s surface. As a result, the pressure of gases present in magma starts increasing at this point from within. And when the pressure of gases increases tremendously then the closed vent opens up suddenly. The lava along with the gases rushes out of the vent and solidifies forming a cone-shaped hill. Volcanoes are found near the vents where two plates supporting the lithosphere are joined to one another.
An earthquake is a natural vibration of the ground produced by rupturing of a large mass of rocks beneath the surface of the earth.
As already mentioned, the convection currents of hot magma cause the earth’s plates to scrape, push and glide past one another. When the rock at the joint of plates is slippery, the plates gently slide past one another. This way the internal pressure is released.
In some cases, however, the rocks at the joint of two plates of the lithosphere interlock into one another strongly. They do not move and resist pressure from within. As a result, internal pressure goes on building over hundred of years. Ultimately, the internal pressure increases so much that it exerts an intolerable strain on the joint of the two plates and breaks them apart. This releases large amount of energy, which produces shock waves, and lithosphere at that place starts shaking with the intensity of thee waves. This vibration of the ground produced by rupturing of a large mass of rocks is known as earthquake.
Earthquakes are caused due to violent volcanic eruptions. The place beneath the earth’s surface from where the earthquake originates is called the FOCUS and the point on the earth’s surface immediately above the focus is called the EPICENTRE. The effects of earthquake are greater if its focus is near the surface than when it is at a great depth. The intensity if earthquake and its time of occurrence is recorded by an instrument called SEISMOGRAPH. The intensity of earthquake depends on the amount of energy released when the plates of lithosphere give way to pressures from within. Intensity of an earthquake is measured on RICHTER SCALE named after the scientist C.F. Richter who invented it. The earthquakes which measure upto 3 on the Richter scale are mild and may not be even noticed by the people. An earthquake which measures 8 or more on Richter scale is very strong and can destroy entire cities causing a great loss to life and property.
As the plates of the lithosphere crash into one another producing mountains, causing volcanic eruptions and earthquakes, a part of the lithosphere may fall down into hot magma in the mantle of the earth. This part of the lithosphere melts and becomes a part of the mantle as magma. On the other hand, magma emerges form the vents present elsewhere on the surface of the earth and solidifies there in the form of rocks. Thus, the material of the lithosphere is constantly being recycled by processes taking place within the dynamic earth.