Volcanoes

 

 

Volcano is a  place on the earth’s surface (or any other planet) where molten rock and gases are erupted. or  hill or mountain built up by the eruption of molten rock.Volcanic eruptions are caused by magma (a mixture of liquid rock, crystals, and dissolved gas) expelled onto the Earth’s surface.

Basaltic magmas tend to be fairly fluid (low viscosity), Rhyolitic magmas tend to have even higher viscosity,Viscosity is an important property in determining the eruptive behavior of magmas.

Fissure Volcano
Fissure volcanoes have no central crater at all. Instead,giant cracks open in the ground and expel vast quantitiesof lava. This lava spreads far and wide to form huge poolsthat can cover almost everything around. Whenthese pools of lava cool and solidify, the surfaceremains mostly flat. Since the source cracksare usually buried, there is often nothing”volcano-like” to see – only a flat plain.
Cinder cones
Cinder cones are simple volcanoes which have a bowl-shaped crater at the summit and steep sides. They only grow to about a

thousand feet, the size of a hill. They usually are created of eruptions from a single opening, unlike a strato-volcano or shield volcano which can erupt from many different openings. Cinder cones are typically are made of piles of lava, not ash. During the eruption, blobs (“cinders”) of lava are blown into the air and break into small fragments that fall around the opening of the volcano. The pile forms an oval-shaped smallvolcano.

Shield volcanoes
Shield volcanoes are the largest volcanoes. They have gentle, almost straight slopes and a wide, circular base.These volcanoes form when thin, running lava flows over a large area. Each time the volcano quietly erupts, thelayers of cooled lava build up.The lava that forms shield volcanoes contains a large volume of basalt. It has a very low viscosity. Basaltic lavacontains few gases, resulting in quiet, flowing eruptions. Mauna Loa, in Hawaii, is an example of a shield volcano.

Composite volcano.
A violent eruption occurs, forcing cinders and ash fromthe vent. This material settles around the vent, much like a cinder-cone volcano. This event is followed

by a quieteruption. The lava flows over the cinders. It eventually cools and hardens. After many of these alternatingeruptions, a tall cone-shaped mountain forms.
The lava that forms composite volcanoes also has a low viscosity. It contains large amounts of silica, water, andgases. These violent volcanoes can be very dangerous. Mount Etna, in Italy, and Mount Saint Helens, in the UnitedStates, are two examples of composite volcanoes.