The surface of the Earth is not even. There are high places called mountains, and high flat places called plateaus. There are low places called valleys and canyons and relatively flat areas called plains. For the most part, moving air and water from the sky and seas damages rocks in high places and breaks them into small pieces. The air and water then move these pieces to lower places.
We have already seen that the solid thin lithosphere rests on plastic asthenosphere. The lithosphere is of two types, Oceanic and Continental. Oceanic lithosphere is associated with oceanic crust and exists in the ocean basins. Oceanic lithosphere is typically about 50–100 km thick. Continental lithosphere is associated with continental crust. Continental lithosphere has a range in thickness from about 40 km to perhaps 200 km, of which about 40 km is the crust.
The lithosphere is divided into tectonic plates, which move gradually relative to one another. Plate tectonics is the theory developed to explain large scale motions of the Earth’s lithosphere. This theory builds on older ideas of continental drift and seafloor spreading. Exactly how this happens is still not understood and the driving forces moving the plates continue to be active subjects of research.
Although solid, the asthenosphere can flow like a liquid on long time scales. Large convection currents in the asthenosphere transfer heat to the surface, where plumes of less dense magma break apart the plates at the spreading centres and cause the movement of plates. The deeper mantle below the asthenosphere is more rigid again due to extremely high pressure.
The lithosphere is divided into eight major and many minor plates. These lithospheric plates ride on the asthenosphere at one of three types of plate boundaries convergent boundaries, divergent boundaries or transform fault boundaries. Tectonic plates can create mountains, earthquakes, volcanoes, mid-oceanic ridges and oceanic trenches, depending on which way the plates are moving.
A convergent boundary is where two or more tectonic plates collide with each other causing massive earth movements. The Himalayas were formed by such a collision. Earthquakes and volcanoes are common near convergent boundaries. This is because of pressure, friction, and plate material melting in the mantle.
There can be two types of subduction in a convergent plate boundary. When Oceanic crust moves under, a deep ocean trench forms at the coast and an arc of mountainous volcanoes form inland as seen along the western edge of the Americas. But, when continental crust moves under, the edge of the continental plate folds into a huge mountain range. Behind it is a high plateau. The Himalayas and the Tibetan plateau are a perfect example of this. Folds can be upfolds(anticlines) or downfolds(synclines).
A divergent boundary, also known as a constructive boundary, is two or more plates that move apart from each other because of plate tectonics. When they move apart either water or magma fills the space. If it is magma, when it has cooled it then creates a new plate which then creates new land.
A fault is a fracture, or break, in the Earth’s crust (lithosphere). Some faults are active. Here, sections of rock move past each other. This sometimes makes earthquakes. Faulting occurs when shear stress on a rock overcomes the forces which hold it together. The fracture itself is called a fault plane.
An earthquake (or quakes, tremors) is shaking caused by sudden movements of rocks in the Earth’s crust. They can be extremely violent. Earthquakes are usually quite brief but may repeat. They are the result of a sudden release of energy in the Earth’s crust. This creates seismic waves, which are waves of energy that travel through the Earth. The study of earthquakes is called seismology. Seismology studies the frequency, type and size of earthquakes over a period of time. The magnitude of an earthquake and the intensity of shaking is usually reported on the Richter scale. On the scale, 3 or less is scarcely noticeable, and magnitude 7 (or more) causes damage over a wide area. Earthquakes are caused by tectonic movements in the Earth’s crust. The main cause is that when tectonic plates collide, one rides over the other, causing orogeny (mountain building), earthquakes and volcanoes.
An earthquake under the ocean can cause a tsunami. This can cause just as much death and destruction as the earthquake itself. Landslides can happen, too. Earthquakes are part of the Earth’s rock cycle. Tsunami or a chain of fast moving waves in the ocean caused by powerful earthquakes is a very serious challenge for people’s safety and for earthquake engineering. Those waves can inundate coastal areas, destroy houses and even swept away whole towns.
A volcano is a mountain where lava (hot, liquid rock) comes from a magma chamber under the ground. Most volcanoes have a volcanic crater at the top. The magma which reaches the earth’s surface is called lava and the path through which it comes out is called a vent. When a volcano is active, materials including lava, steam, gaseous sulphur compounds, ash and broken rock pieces come out of it. When there is enough pressure, the volcano erupts. Some volcanic eruptions blow off the top of the volcano. The magma comes out, sometimes quickly and sometimes slowly. Some eruptions come out at a side instead of the top.
The lava and pyroclastic material (clouds of ash, lava fragments and vapour) that comes out from volcanoes can make many different kinds of land shapes. There are different types of volcanoes based on the type of lava and manner in which it flows.OR Volcanoes are classified on the basis of nature of eruption and the form developed at the surface.
Shield volcanoes are the largest of all the volcanoes on the earth. They built out of layers of lava from continual eruptions (without explosions). These volcanoes are formed by fluid low-silica mafic lava. Because the lava is so fluid, it spreads out, often over a wide area. Shield volcanoes do not grow to a great height, and the layers of lava spread out to give the volcano gently sloping sides. Shield volcanoes can produce huge areas of basalt, which is usually what lava is when cooled. The base of the volcano increases in size over successive eruptions, where solidified lava spreads out and accumulates. Some of the world’s largest volcanoes are shield volcanoes.
A cinder cone volcano is a tall, conical volcano. Unlike shield volcanoes, cinder-cone volcanoes have a steep profile. The lava that flows from stratovolcanoes cools and hardens before spreading far as it has high viscosity. The magma forming this lava is often felsic, with high-to-intermediate levels of silica, and less mafic magma. Big felsic lava flows are uncommon but have travelled as far as 15 km.
A series of successive eruptions of different types of lava gives rise to composite volcanoes. These volcanoes are characterised by eruptions of cooler and more viscous lavas than basalt. There will be alternate layers of lava, cinder and ash.
Volcanoes are commonly seen at convergent plate boundaries. When two plates meet, one of them (usually the oceanic plate) goes under the continental plate by the process of subduction. Afterwards, it melts and makes magma (inside the magma chamber), and the pressure builds up until the magma bursts through the Earth’s crust.
The second way is when a tectonic plate moves over a hotspot in the Earth’s crust. The hot spot works its way through the crust until it breaks through.
As volcanoes and earthquakes are results of tectonic movements, they are seen concentrated around plates and there are areas where they are very common like ‘Pacific ring of fire’ around the Pacific Ocean.
Orogeny is the process of mountain-building. It takes place when two tectonic plates come together. Mountains develop while a continental plate is crumpled and thickened and involve a great range of geological processes. This is called folding. Mountains can also be formed when faults occur. A fractured area of crust may dip down leading to the elevated area forming the mountain. Volcanism can also lead to mountain formation by deposition of magma.