Volcanism

• Volcanism is the process of molten rock coming out of the Earth’s surface.
• It happens because of the Earth’s internal heat and is part of the rock cycle.
• Volcanic eruptions occur when molten lava reaches the surface of the Earth.
• Volcanoes are most commonly found in places where tectonic plates are diverging or converging.
• Many volcanoes are located underwater.

Types of Volcanic Eruptions

• Different types of volcanic eruptions depend on various factors such as chemistry of magma, temperature, viscosity, volume, presence of groundwater, and water and gas content.
• The following are the different types of volcanic eruptions:
  • Hydrothermal eruption: ash is erupted but not magma. It’s driven by the heat caused in hydrothermal systems.
  • Phreatic eruption: driven when the heat of the magma interacts with water. These eruptions do not include magma and only produce ash.
  • Phreatomagmatic eruption: occurs when newly formed magma interacts with water.
  • Strombolian and Hawaiian eruption: Hawaiian eruptions have fire fountains while the Strombolian eruption has explosions due to lava fragments.
  • Vulcanian eruption: these eruptions last for a short period of time and can reach up to a height of 20 km.
  • Subplinian and Plinian eruptions: Subplinian eruptions reach up to 20 km height, while Plinian eruptions reach up to 20-35 km.

Types of Volcanoes

• Volcanoes are grouped into four types: cinder cones, composite volcanoes, shield volcanoes, and lava domes.
• Cinder cones are circular or oval cones made up of small fragments of lava blown up from a single vent. They form from eruptions of small pieces of scoria and pyroclastics that build up around the vent. Most cinder cones erupt only once.
• Composite volcanoes are steep-sided volcanoes composed of many layers of volcanic rocks, usually made from high-viscosity lava, ash, and rock debris. They are tall conical mountains composed of lava flows and other ejecta in alternate layers. Cinders and ash pile on top of each other, and lava flows on top of the ash.
• Shield volcanoes are shaped like a bowl or shield in the middle with long gentle slopes made by basaltic lava flows. They are formed by the eruption of low-viscosity lava that can flow a great distance from a vent. They generally do not explode catastrophically.
• Lava domes are formed when erupting lava is too thick to flow and makes a steep-sided mound near the volcanic vent. They are built by slow eruptions of highly viscous lava and can sometimes form within the crater of a previous volcanic eruption. Like composite volcanoes, they can produce violent, explosive eruptions, but their lava generally does not flow far from the originating vent.

Categories of Volcanoes

• Active volcanoes:
  • These are erupting or expected to erupt in the future, and there are about 600 of them in the world.
  • Most active volcanoes are located around the Pacific “ring of fire.”
  • The Stromboli volcano is known as the lighthouse of the Mediterranean because of its frequent eruptions.
• Dormant volcanoes:
  • These are not currently erupting but have erupted within recordable history and are expected to erupt again in the future.
  • Mt Kilimanjaro is an example of a dormant volcano.
• Extinct volcanoes:
  • These are considered dead and are not expected to erupt again.
  • Aconcagua of the Andes is an example of an extinct volcano.

Volcanic Mountains

• Volcanic mountains are formed as a result of volcanic activities on the surface of the Earth. The magma inside the Earth erupts out as lava through the cracks on the Earth’s surface. This cools down repeatedly to form volcanic mountains.
• Example:
  • Mt. Kilimanjaro(Tanzania)
  • Mt. Fuji (Japan)
  • Mt. Merapi (Sumatra)
  • Mt. Mayon (Philippines)
  • Mt. Agung (Bali)
  • Mt. Cotopaxi (Ecuador)

Volcanic Landforms

Intrusive Landforms:

• Lava discharged during volcanic eruptions cools and develops into igneous rocks.
• Cooling can occur either on the surface or while still in the crust.
• Igneous rocks are categorized as plutonic or volcanic based on the location of cooling.
• Lava that cools inside the crust forms intrusive forms.
• Intrusive forms refer to diverse forms of igneous rocks that cool inside the crust.
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• Batholiths:
  • Cooled magma chambers
  • Large bodies of magmatic material
  • Formed as large domes in deeper parts of the crust
  • Appear on surface after denudation processes
  • Made of granitic material
• Laccoliths:
  • Large dome-shaped intrusive bodies
  • Have a level base and a pipe-like channel from below
  • Located at deeper depths than surface volcanic domes
  • Localized source of lava
  • Karnataka plateau has dome hills made of granite rocks.
• Lopolith:
  • Occurs when lava moves horizontally on a weak plane
  • Takes the shape of a concave saucer
  • Formed from intrusive rocks
• Phacolith:
  • Wavy mass of intrusive rocks
  • Found at the base of synclines or top of anticlines in folded igneous country
  • Has an outlet to magma cavities
• Sills:
  • Near horizontal bodies of intrusive igneous rocks
  • Thick deposits are called sills
  • Thin deposits are called sheets
• Dykes
  • Dykes are the most commonly found intrusive forms in the western Maharashtra area.
  • When the lava makes its channel through cracks and the fissures, it solidifies almost perpendicular to the ground.
  • This gets cooled in the same position to grow a wall-like structure. Such structures are known as dykes.
  • These are regarded as the feeders for the eruptions that led to the development of the Deccan traps.

Extrusive Landforms

• Magmas that reach the surface and solidify, form extrusive landforms.
• Rocks formed by either plutonic or volcanic activity are called igneous rocks.