P5+VolcanoES


 * What is a volcano?**

A volcano is a vent through which molten rock known as magma rises from underground to the earths surface.A volcano is active if it is erupting lava, generating seismic activity or releasing gas. If a volcano has been dormant for more than 10,000 years, it is termed extinct. The form of a volcano points to the type and size of its eruption.
 * How are volcanoes formed?**

Most volcanoes occur where two plates meet. When two plates move apart causing a gap, hot molten rock - called lava - rises up between them. This type of volcano occurs on the ocean floor and is mostly invisible. If the amount of magma is large enough, it rises above the surface of the ocean and an island is created.If two plates collide and one plate is forced beneath the other plate, the friction makes the first melt and magma rises up. Only a few volcanoes on earth are formed like this, but their eruptions are the most violent and dangerous ones. Sometimes volcanoes also form in the middle of the plates which are called hotspots. These are places that are connected by channels to the hot mantle of the earth.
 * Shield volcano**

Shield volcanoes are almost completely basalt. When magma is very hot and flowing, gases can escape and eruptions are gentle with considerable amounts of magma reaching the surface to form lava flows. Shield volcanoes have a broad, flattened dome-like shape created by layers of runny lava flowing over its surface and cooling down. Because the lava flows easily, it can move down gradual slopes over large distances from the volcanic vents


 * Composite volcano** (also called **strato volcanoes**),

Alternating layers of rock fragments and lava form these volcanoes. This is why they are called composite volcanoes. They are also known as strato-volcanoes. Composite volcanoes generally erupt in an explosive way. When very viscous magma rises to the surface, it usually clogs the crater pipe, and gas in the crater pipe gets locked up. This cause increase in pressure resulting in an explosive eruption. Although strato-volcanoes are usually large and conical, we have more shapes of them: concave, pyramidal, convex-concave, helmet-shaped, collapse caldera, nested, multiple summits, elongated along a fissure.


 * Caldera volcano**

They form when huge amounts of magma erupts out of sub-surface magma chambers. The removal of magma leaves a void below the surface and the top collapses in to form the caldera. The resulting basin-shaped depression is roughly circular and is usually several kilometers or more in diameter. The lava erupted from caldera volcanoes is very viscous and generally the coolest with temperatures ranging from 650°C to 800°C and is called rhyolitic magma. Although caldera volcanoes are uncommon, they are the most dangerous. Volcanic hazards from this type of eruption include tsunami from caldera collapse, large pyroclastic surges and widespread ash fall.


 * Spatter Volcanoes**

When hot erupting lava contains just enough explosive gas to prevent the formation of a lava flow, but not enough to shatter it into small fragments the lava is torn by expanding gases into fluid hot clots, ranging in size from 1cm to 50cm across, called spatter. When the spatter falls back to Earth the clots weld themselves together and solidify forming steep-sided accumulations. These accumulations focused on an individual vent are called spatter cones. In fact all volcanoes could be complex volcanoes since all of them are made up of multiple flows, ash layers**,** domes cones, etc. in varying amounts. However, when we call a volcano a complex volcano it is because we mean the "system" of those volcanoes is not "simple". Caldera complexes for instance have often got a large caldera with many subsidiary vents and deposits, some of which could be considered "volcanoes" in their own right. Frankly speaking, a volcano that consists of a complex of two or more vents is reckoned as a compound or complex volcano.
 * Complex Volcanoes** (also called **compound volcanoes**).