Volcanoes

Vulcanicity & Associated Landforms

• A volcano is a vent or fissure in the Earth’s crust that allows molten rocks, gases, ashes, dust particles and debris to escape to the surface.
• It is called Active Volcano (Mount St. Helena – U.S.A., Mount Merapi – Indonesia, Barren Island – Andaman Islands (India)) if the materials mentioned are being released or have been released in the recent past. Otherwise, it is referred to as a Dormant Volcano (e.g., Vesuvius in Italy, Chimborazo in Ecuador, Kilimanjaro in Tanzania, Mount Kenya in Kenya, and Narcondam in India).There are some Extinct/ancient volcanoes as well, which have recorded history of eruption such as Popa in Myanmar & Fujiyama in Japan.
• The mantle contains a zone called asthenosphere from which the molten rock materials discover their route to the surface.
• The matter in the upper mantle portion is called magma, and when it starts moving towards the crust, or it reaches the surface, it is called lava.
• The matter that reaches the ground includes lava flows, pyroclastic debris, volcanic bombs, ash and dust and gases such as nitrogen compounds, Sulphur compounds and minor amounts of chlorine, hydrogen, and argon.
• Erupted lava and other debris can flow up to 200 mph, destroying everything in their path.

Types of Volcanoes

Volcanoes are categorized according to the type of eruption and the shape formed on the surface.

1. Composite Volcano: These volcanoes are characterized by eruptions of cooler and viscous lavas than basalt. These volcanoes often result in explosive eruptions.In addition to lava, significant amounts of pyroclastic material and ash are ejected and settle on the ground.This material accumulates near the vent openings leading to the formation of layers, and this makes the mounts appear as composite volcanoes.

1. Shield Volcano: Apart from basalt flows, the shield volcanoes are the largest of all volcanoes on the Earth. E.g. Hawaiian volcanoes. They are made up of basalt (a type of lava that is very fluid when erupted), and that’s why these volcanoes are not steep. They become explosive when water enters the volcanic vent.Upcoming lava surges ahead in the form of a fountain and bursts the cone at the top of the vent and develops into a cinder cone.

1. Flood Basalt Provinces: These volcanoes release highly fluid lava that spreads over vast distances. Certain regions of the Earth are blanketed by extensive basalt lava flows covering thousands of square kilometers, such as the Deccan Traps.

1. Mid-Ocean Ridge Volcanoes: These volcanoes appear in the oceanic areas. There is a system of mid-oceanic ridges more than 70,000 km long that stretches through all the ocean basins. The central portion of this ridge undergoes frequent eruptions.

Types of Lava

1. Acidic/Andesitic/Composite/Strato-volcanic:These lava have a high percentage of silica, are light coloured and are of low density. Therefore, they are highly viscous with a high melting point. They move slowly and typically do not travel far before solidifying.The sudden solidifying of lava in the vent blocks the flow of outpouring lava, resulting in loud explosions & throwing out many volcanic bombs or pyroclasts.
2. Basic/Basaltic/Shield: These lava have a low percentage of silica but are rich in iron and magnesium and are highly fluid so that it flows out the volcanic vent quietly (Not very explosive).They cover extensive areas, spreading out in thin layers over long distances before solidifying.The resultant volcano is mildly sloping with a wide diameter and forms a flattened shield or dome.

Volcanic Landforms

Intrusive Landforms

Lava released during volcanic eruptions cools and solidifies to form igneous rocks. This cooling can occur either upon reaching the surface or while the lava is still within the crust. Based on where the lava cools, igneous rocks are classified as volcanic rocks (solidifying at the surface) and plutonic rocks (solidifying within the crust). Lava that solidifies within the crust takes on various forms, such as:

Batholiths: They are large bodies of magmatic material that cools in the deeper depth of crust and develops in the form of large domes. They appear on the surface after the denudational processes remove the overlying material. They are the cooled portion of magmatic chambers.

Lacoliths: These are dome-shaped intrusive bodies with a level base and connected by a pipe-like conduit from below.The Karnataka plateau is known for its dome-shaped hills composed of granite rocks.

Lapolith: The lava while moving upwards tends to move in a horizontal direction wherever it finds a weak plane. It gets rested in different forms. When it develops into saucer-shaped, concave to the sky body, then it is called lapolith.

Phacoliths: A wavy mass of intrusive rocks at times is found at the base of synclines or top of the anticline in the folded igneous rocks. Such wavy materials have a specific conduit to source underneath. They are called phacoliths.

Sill: The near-horizontal bodies of intrusive rocks are called sill or sheet.

Dykes:When lava solidifies nearly vertically to the ground, it cools in place, forming a wall-like structure known as a dyke.

Extrusive Landforms

Fissure Vent: It is a linear volcanic vent through which lava comes out, usually without explosive activity.The vent is typically only a few meters wide but can extend for several kilometers in length.

Conical Vent: A conical vent is a confined cylindrical vent through which magma gushes out violently.

Lava Plateaus: A very thin magma gets out through cracks and fissures in the Earth’s surface and flows after intervals for a long time, spreading over a large area, finally producing a layered, wave-like, flat surface. E.g., Siberian Traps, Deccan Traps, Icelandic Shield and Canadian Shield.

Crater: A crater is an inverted cone-shaped outlet through which the magma comes out. When the volcano is inactive, the crater appears as a bowl-shaped depression. When water from the rain or melted snow gets collected in the crater, it becomes a crater lake.

E.g., Lake Toba – Indonesia, Largest Volcanic crater lake in the world, Lake Titicaca – On the border of Bolivia and Peru. World’s highest navigable lake is also known as Honeymoon lake, Lonar lake – Buldhana – Maharashtra it is an example of Meteor crater lake.

Caldera: They are the most explosive of the earth volcanoes. They are so volatile that when they erupt, they tend to collapse on themselves rather than building any tall structure. These collapsed depressions are called Calderas. Their explosiveness suggests that the magma chamber supplying the lava is not only huge but is also in the vicinity.

Lava Dome: A lava dome or volcanic dome is a common feature in volcanic regions.It is a circular, dome-shaped protrusion formed by the slow extrusion of thick, viscous lava from a volcano.

Important Volcanoes of the World

Recent volcanic eruptions:

Semeru Volcano:

• Situated on the eastern part of Java Island in Indonesia, Semeru is the highest volcano in Java and among the most active in the region.
• Recently erupted in December 2021 and is frequently active.
• Indonesia, home to the largest number of active volcanoes in the world, frequently experiences seismic activity due to its location on the Pacific Ring of Fire.
• The Semeru volcano is part of the island arcs created by the subduction of the Indo-Australian plate beneath the Sunda Plate (a section of the Eurasian Plate), with the corresponding trench known as the Sunda Trench or Java Trench.

Tonga Volcanic Eruption:

• Recently, a volcanic eruption occurred in the southern Pacific Island nation of Tonga.
• The eruption triggered tsunami waves across the Pacific Ocean.
• Tonga Islands are situated along the Ring of Fire, a region characterized by intense volcanic and seismic activity encircling the Pacific Ocean basin.