Newspaper

Description

Volcanism

Why in News?

Mount Etna is Europe’s most active volcano and one of the largest in the world. Its recorded volcanic activity dates back to 1500 B.C. Since then, it has erupted more than 200 times.

The current eruptions at Etna have led to flight cancellations at nearby Catania airport.

The use of cars and motorbikes has also been banned for 48 hours due to high amounts of ash on the roads. Ash can be slippery and increases the risk of accidents.

What are Volcanoes?

“Volcanoes are openings, or vents where lava, tephra (small rocks), and steam erupt onto the Earth’s surface.” Volcanoes can be on land and in the ocean. They are, in part, a result of their own eruptions but also the general formation of our planet, as tectonic plates move.

Formed when molten magma in the earth’s interior escapes through the crust by vents and fissures in the crust, accompanied by gases (like Hydrogen Sulphide, Sulphur Dioxide, Hydrogen Chloride, Carbon Dioxide), steam and pyroclastic materials. Over 1300 volcanoes exist on the earth out of which about 600 are active.

Causes of Volcanism: 

• It is closely connected with crustal disturbances, particularly where there are zones of weakness due to deep faulting or mountain folding. 

• Increasing temperature with increasing depth below the earth’s crust (1° per 32 km) where the interior of the earth can be expected to be in a semi-molten state, comprising solid, liquid and gaseous materials, collectively termed as Magma.

• Origins of gases and water vapour due to heating of water underground. 

• Ascent of Magma forced by enormous volume of gases and water vapour.

• Movement and splitting of the major and minor plates of the earth – most of the active volcanoes are found along the plate margins (mid-oceanic ridges and the ocean trenches , Ben off Zone or Ring of Fire).

• The magma has gases such as carbon dioxide, sulphurated hydrogen and small proportions of nitrogen, chlorine and other volatile substances. The gases and vapour increases the mobility and explosiveness of the lavas which are emitted through the orifice or vent of a volcano during a volcanic eruption.

Mountain ranges like the Andes in South America and the Rockies in North America, as well as volcanoes, formed through the movement and collision of tectonic plates.

There are four main types of volcanoes: cinder cones, composite or stratovolcanoes, shield volcanoes and lava domes. Their type is determined by how the lava from an eruption flows and how that flow affects the volcano, and, as a result, how it affects its surrounding environment.

Some of the most active volcanoes are located in the Pacific Ring of Fire, which includes New Zealand, Southeast Asia, Japan and the western coast of the Americas. About 90% of all earthquakes worldwide strike within this region.

Volcanic landforms

The lava that is released during volcanic eruptions on cooling develops into igneous rocks. The cooling may take place either on reaching the surface or also while the lava is still in the crustal portion. Depending on the location of the cooling of the lava, igneous rocks are classified as volcanic rocks (cooling at the surface) and plutonic rocks (cooling in the crust).

Intrusive Volcanic landforms

 Batholiths: A large body of magmatic material that cools in the deeper depth of the crust develops in the form of large domes. They appear on the surface only after the denudational processes remove the overlying materials.

 Lacoliths: These are large dome-shaped intrusive bodies with a level base and connected by a pipe-like conduit from below. It resembles the surface volcanic domes of composite volcano, only these are located at deeper depths. It can be regarded as the localised source of lava that finds its way to the surface. The Karnataka plateau is spotted with domal hills of granite rocks. Most of these, now exfoliated, are examples of lacoliths or batholiths.

 Lapolith, Phacolith and Sills: As and when the lava moves upwards, a portion of the same may tend to move in a horizontal direction wherever it finds a weak plane. It may get rested in different forms. In case it develops into a saucer shape, concave to the sky body, it is called lapolith. 

A wavy mass of intrusive rocks, at times, is found at the base of synclines or at the top of anticline in folded igneous country. Such wavy materials have a definite conduit to source beneath in the form of magma chambers (subsequently developed as batholiths). These are called the phacoliths. 

The near horizontal bodies of the intrusive igneous rocks are called sill or sheet, depending on the thickness of the material. The thinner ones are called sheets while the thick horizontal deposits are called sills.

 Dykes: When the lava makes its way through cracks and the fissures developed in the land, it solidifies almost perpendicular to the ground. It gets cooled in the same position to develop a wall-like structure. Such structures are called dykes.