How are Volcanoes Formed?

What is a volcano?

A volcano is a vent through which molten rock known as magma rises from underground to the Earth's surface. Magma erupting from a volcano is called lava and it is the material, which builds up the cone surrounding the vent.

A volcano is active if it is erupting lava, generating seismic activity or releasing gas. A volcano is dormant if it has not erupted for a very long time but could erupt again in the future. If a volcano has been dormant for more than 10,000 years, it is termed extinct.

Volcanoes can be different in appearance with some having perfect cone shapes while others are deep depressions filled with water. The form of a volcano points to the type and size of its eruption, which is controlled by the characteristics, and composition of magma. The size, style and frequency of eruptions can differ greatly but all these elements contribute to the shape of a volcano.

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.

Three main types of volcano are given below which are formed in distinct ways: -

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

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.

Other types of volcanoes are mid-ocean ridges, monogenetic fields and flood basalts making in all 6 types.