A shield volcano is a wide volcano with shallowly-sloping sides.
Shield volcanoes are formed by lava flows of low viscosity – lava that flows easily.
Consequently, a volcanic mountain having a broad profile is built up over time by flow after flow of relatively fluid basaltic lava issuing from vents or fissures on the surface of the volcano.
Many of the largest volcanoes on Earth are shield volcanoes.
The largest is Mauna Loa on the Big Island of Hawaii; all the volcanoes in the Hawaiian Islands are shield volcanoes.
There are also shield volcanoes, for example, in Washington, Oregon, and the Galapagos Islands.
The Piton de la Fournaise, on Reunion Island, is one of the more active shield volcanoes on earth, with one eruption per year on average..
Volcanic cones are among the simplest volcano formations.
They are built by fragments (called ejecta) thrown up (ejected) from a volcanic vent, piling up around the vent in the shape of a cone with a central crater.
Volcanic cones are of different types, depending upon the nature and size of the fragments ejected during the eruption.
Types typically differentiated are spatter cone, cinder cone, ash cone, and tuff cone..
Pyroclastic flow from volcanoes
Pyroclastic flows are a common and devastating result of some volcanic eruptions.
They are fast-moving fluidized bodies of hot gas, ash and rock (collectively known as tephra) which can travel away from the vent at up to 150 km/h.
The gas is usually at a temperature of 100-800 degrees Celsius.
The flows normally hug the ground and travel downhill under gravity, their speed depending upon the gradient of the slope and the size of the flow. Volumes range from a few hundred cubic metres to more than a thousand cubic kilometres, and the larger ones can travel for hundreds of kilometres although none on that scale have occurred for several hundred thousand years.
Most flows are around one to ten cubic kilometres and travel for several kilometres.
Flows usually consist of two parts: the basal flow hugs the ground and contains larger, coarse boulders and rock fragments, while an ash cloud rises above it because of the turbulence between the flow and the overlying air..
A stratovolcano is a tall, conical volcano composed of one layer of hardened lava, tephra, and volcanic ash.
These volcanoes are characterized by a steep profile and periodic, explosive eruptions.
The lava that flows from them is highly viscous, and cools and hardens before spreading very far.
The source magma of this rock is classified as acidic, or high in silica to intermediate (rhyolite, dacite, or andesite.
This is in contrast to less viscous basic magma that forms shield volcanoes (such as Mauna Loa in Hawaii), which have a wide base and more gently sloping profile.
Many stratovolcanoes exceed a height of 2500 m..
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