Volcanos and Their Type
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.
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 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
As ash and various- sized pieces of broken rock (collectively known as tephra), and
As lava flows.