Indonesia’s Merapi volcano erupts: Basics Explained

Indonesia’s Merapi volcano erupted on Saturday, spewing hot clouds up to seven kilometers, the country’s disaster management agency said in a statement. Located on the Pacific Ring of Fire, Indonesia has more volcanoes than any other country.

Indonesia is home to many active volcanoes owing to its location in the “Ring of Fire” or the Circum-Pacific Belt — an area along the Pacific Ocean characterized by active volcanoes and frequent earthquakes. The Ring of Fire is home to about 75 per cent of the world’s volcanoes and about 90 per cent of earthquakes also occur here.


A volcano is a vent or fissure in Earth’s crust through which lava, ash, rocks, and gases erupt. Material that flows to or reaches the ground comprises lava flows, volcanic bombs, pyroclastic debris, dust, ash, and gases. The gases may be sulfur compounds, nitrogen compounds, and trace amounts of argon, hydrogen, and chlorine.

They are generally found where tectonic plates (like Eurasian, Pacific, Somali, etc) diverge or converge. Examples- volcanoes occurring in mid-oceanic ridge and Ring of Fire. E.g.: Mount Stromboli is an active volcano and it produces so much of Gas clouds that it is called Light house of Mediterranean.

Basic: The basic magma are dark coloured like basalt, rich in iron and magnesium but poor in silica. They travels far and generates broad shield volcanoes.

Acidic: These are are light-coloured, of low density, and have a high percentage of silica and therefore it makes a familiar cone volcano shape.

Intrusive Landforms of Volcanoes

  • Sills: When an intrusion of molten magma is made horizontally along the bedding plains of sedimentary rocks, the resultant intrusions is called a sill.
  • Dykes: Intrusions when injected vertically as narrow walls igneous rocks within the sedimentary layers are termed as dykes.
  • Laccolith: It is large blister of igneous mound with a dome-shaped upper surface and a level base fed by a pip-like conduit from below.
  • Lopolith: A lopolith is another variety of igneous intrusions with a saucer shape.
  • Phacolith: It is a lenses-shaped mass of igneous rocks occupying the crest of an anticline or the bottom of a syncline and being fed by a conduit from beneath.
  • Batholith: It is a huge mass of igneous rocks, usually granite, which after removal of the overlying rocks forms a massive and resistant upland region.

Extrusive Landforms of Volcanoes

Stratovolcanoes (Composite Volcanoes)

They are made up of layers of lava flows interlayed with sand- or gravel-like volcanic rock called cinders or volcanic ash. This type of volcano has steeper slopes, show interlayering of lava flows and typically up to 50 percent pyroclastic material, which is why they are sometimes called composite volcanoes. The steep slope near the summit is due partly to thick, short viscous lava flows that do not travel far down slope from the  vent.Due to the higher viscosity of magmas erupted from these volcanoes, they are usually more explosive than shield volcanoes.

Pyroclastic flows are high-density mixtures of hot, dry rock fragments and hot gases that move away from the vent that erupted them at high speeds. They may result from the explosive eruption of molten or solid rock fragments, or both.

Shield Volcanoes

A shield volcano is characterized by gentle upper slopes (about 5 degrees) with somewhat steeper lower slopes (about 10 degrees). The shield volcanoes are almost entirely composed of relatively thin lava flows built up over a central vent. Shield volcanoes have small amounts of pyroclastic material, most of which accumulates near the eruptive vents, resulting from fire fuming events. Thus, shield volcanoes typically form from nonexplosive eruptions of low viscosity basaltic magma. Most shield volcanoes have a roughly circular or oval shape.


Both strato/composite and shield volcanoes can lose their tops following a series of tremendous explosions. The change in these volcanoes’ structure is visible as a large depression . The collapsing top forms a large depression in the landscape, which may later fill with water to form a lake. Craters are circular depressions, usually less than 1 km in diameter, that form as a result of explosions that emit gases and tephra.

The Deccan Traps from India, presently covering most of the Maharashtra plateau, are a much larger flood basalt province.

All around the Pacific Ocean, is a zone often referred to as the Pacific Ring of Fire, where most of the world’s most active and most dangerous volcanoes occur.  The Ring of Fire occurs because most of the margins of the Pacific ocean coincide with converging margins along which subduction is occurring


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