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Volcanos and Their Type

Updated on December 9, 2015


Earth may seen solid and firm, but the planet is actually made up of a number of rigid crustal plates (the lithosphere) floating on a sea of semi-molten rock (the Asthenosphere). These vast plates are relatively mobile and move around at a rate of several inches per year; roughly as fast as a fingernail grows. Volcanoes of various types erupt along the boundaries of these tectonic plates. Where the plates move apart, magma (molten rock) is able to well up in between and solidify, thereby adding crust as the plates continue to spread. Such boundaries are known as spreading margins.

Elsewhere, plates are forced beneath other plates where they heat up; the edges are then melted and destroyed. Such boundaries are known as subduction margins, and they host Earth`s most violent and dangerous volcanoes. This occurs in a continuous cycles, with Earth`s crust being formed at the spreading margins and consumed in the subduction trenches.

Rift volcanoes

Although they are the most common type of volcano, rift volcanoes lie mostly out of sight at the bottom of the oceans. Along the sea floor, they form a line of continuous eruption thousands of miles long between spreading plates. It is only in rare circumstances that these mid-oceanic rift volcanoes become visible on land-in Iceland, for instance, where they run out of the ocean and right across the middle of the island. Icelanders have harnessed their abundant heat using thermoelectric power stations to supply their energy needs. Eruptions from rift volcanoes are usually gentle, but not always.

Composite volcanoes

Composite volcanoes are found at closely spaced intervals along every subduction margin, such as around the entire rim of the Pacific Ocean. It is often possible to stand on top of one volcano and see another, or several in a line. These volcanoes are the product of the descending and seawater plate mixed together with the seawater and ocean sediments, which have also been dragged down.

Deep trenches on the sea floor running parallel to the chains of volcanic peaks mark the lines along which plates are being forced beneath others and consumed. The viscous and highly gas-charged magma tends to erupt both explosively

  1. As ash and various- sized pieces of broken rock (collectively known as tephra), and

  2. As lava flows.

Alternating tephra falls and lava flows build up a distinctive steep- sided cone known as a composite cone or stratovolcano.

Periodically these volcanoes explode with a massive venting of gas, steam, and tephra, ejecting this material high into the atmosphere as an eruption column or plume. This column collapses under its own weight and the destructive, hot debris surges down the flanks of the volcano as a deadly pyroclastic flow, a dense, swiftly moving cloud of hot ash mixed with poisonous gases. After the eruption, the summit, or even the whole cone, may have blown away, leaving a huge caldera. A new volcano then begins to slowly rebuild within the old caldera. Classic examples are Mt Fuji in Japan, Mt St Helens in the United States, and Anak Krakatau in Indonesia. All stratovolcanoes are potentially dangerous, even if they have not erupted for one or more lifetimes.

Shield Volcanoes

This type of volcano occurs above specific hot spots in the mantle, where basaltic magma is able to find its way to the surface through a weakness in the crust. Erupting, flow after flow, for one or two million years, the runny, low- viscosity lava builds up an enormous broad, flat, shield- like volcanic edifice.

So heavy are these volcanoes that they begin to slowly sink. Eventually the moving plate severs the shield volcano from its feeder pipe so the lava flows stop and it can grow no further, finally, after a few million years, the original volcano disappears from sight beneath the waves. This mechanism is responsible for the many oceanic island chains, such as the Hawaiian chain in the Pacific, with the active volcano at the head of the chain. The big island of Hawaii is a classic example of a shield volcano; it stands 5 miles (8km) tall as measured from the ocean floor. Another is Reunion, a French- governed island in the Indian Ocean. These gentle volcanoes do not offer pose a threat to people living on them. Some of the very slow- moving lava, called “aa” – flows slowly enough to give people time to move historic buildings out of the way.

mt st helen:
Mt St Helena, California 95448, USA

get directions

Mt fuji:
Mount Fuji, Kitayama, Fujinomiya, Shizuoka Prefecture 418-0112, Japan

get directions

Anak Krakatau:
Krakatoa, Anak Krakatau Island, Pulau, South Lampung Regency, Indonesia

get directions


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