Notes On Volcanoes - CBSE Class 7 Geography
An opening in the earth’s crust that allows hot molten lava, ash and gases to escape from below the surface, is called a volcano. Lava is molten rocks or magma coming out from the magma chamber through a vent, which is located in the middle of a volcanic mountain and connected to a pool of lava under the earth’s surface. With time, the lava and ash flow out of the magma chamber, cool down, solidify and accumulate in layers forming a Volcano. The earth is composed of three layers: Crust, Mantle and Core. The mantle consists of magma, on which the lithospheric plates move. Raise in temperature and pressure force the magma to flow out through an opening in the earth’s surface, as lava in a volcanic eruption. The tectonic motion that causes earthquakes also causes volcanic eruptions. On the basis of the frequency of eruption, volcanoes are categorized into three types, i.e. Active volcanoes, Dormant volcanoes and Extinct volcanoes. Active volcanoes are those that erupt regularly i.e. a volcano that has erupted in the recent period of time. An active volcano can erupt at any time. In the basin of the Pacific Ocean lies a belt of active volcanoes. This area is called the Pacific Ring of Fire. Mauna Loa in the Hawaii Islands is the world’s largest volcano. It is about six miles tall from the sea floor to the summit. Volcanoes which have not erupted for many years, but have the potential to erupt again, are called as dormant volcanoes. While the volcanoes which have not erupted in thousands of years and show no signs of doing so in the future too are called as Extinct Volcanoes. Volcanoes further are categorized according to their shapes as well: Shield Volcanoes,   Cinder Cone Volcanoes and Composite Volcanoes. A shield volcano is very broad with a large base and is common in oceanic areas. Mauna Loa is an example of a shield volcano. A cinder cone volcano has steeply sloped sides made of layers of ash and loose rock and a composite volcano is made of strata or layers. These layers are formed when cinders and ash pile on top of each other, and lava flows on top of the pile. It cools and hardens to form a layer and during next eruption, the process of layer formation begins again.

#### Summary

An opening in the earth’s crust that allows hot molten lava, ash and gases to escape from below the surface, is called a volcano. Lava is molten rocks or magma coming out from the magma chamber through a vent, which is located in the middle of a volcanic mountain and connected to a pool of lava under the earth’s surface. With time, the lava and ash flow out of the magma chamber, cool down, solidify and accumulate in layers forming a Volcano. The earth is composed of three layers: Crust, Mantle and Core. The mantle consists of magma, on which the lithospheric plates move. Raise in temperature and pressure force the magma to flow out through an opening in the earth’s surface, as lava in a volcanic eruption. The tectonic motion that causes earthquakes also causes volcanic eruptions. On the basis of the frequency of eruption, volcanoes are categorized into three types, i.e. Active volcanoes, Dormant volcanoes and Extinct volcanoes. Active volcanoes are those that erupt regularly i.e. a volcano that has erupted in the recent period of time. An active volcano can erupt at any time. In the basin of the Pacific Ocean lies a belt of active volcanoes. This area is called the Pacific Ring of Fire. Mauna Loa in the Hawaii Islands is the world’s largest volcano. It is about six miles tall from the sea floor to the summit. Volcanoes which have not erupted for many years, but have the potential to erupt again, are called as dormant volcanoes. While the volcanoes which have not erupted in thousands of years and show no signs of doing so in the future too are called as Extinct Volcanoes. Volcanoes further are categorized according to their shapes as well: Shield Volcanoes,   Cinder Cone Volcanoes and Composite Volcanoes. A shield volcano is very broad with a large base and is common in oceanic areas. Mauna Loa is an example of a shield volcano. A cinder cone volcano has steeply sloped sides made of layers of ash and loose rock and a composite volcano is made of strata or layers. These layers are formed when cinders and ash pile on top of each other, and lava flows on top of the pile. It cools and hardens to form a layer and during next eruption, the process of layer formation begins again.

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