Volcanicity | Geomorphology | Principle Of Geography

 

Geomorphology

Principle Of Geography

Geography Complete Study Material

(Paper - I)

Volcanicity and Volcanic Landforms

The terms volcanoes, mechanism of volcanoes and vulcanicity are more or less synonym to common man but these have different connotations in geology and geography. ‘A volcano is a vent, or opening, usually circular or nearly circular in form , through which heated materials consisting of gases, water, liquid lava and fragments of rocks are ejected from the highly heated interior to the surface of the earth ’ (P.G . Worcester, 1948). ‘A volcano is essentially a fissure or vent, communicating with the interior, from which flow s o f lava, fountains o f in can descent spray or explosive bursts o f gases and volcanic ashes are erupted at the surface.’ On the other hand, ‘the term vulcanicity covers all those processes in which molten rock material or magma rises into the crust or is poured out on its surface, there to solidify as a crystalline or semi crystalline roc k ’ (S .W . Woold ridge and R .S. Morgan, 1959). Some scientists have also used the term of vulcanism as synonym to the term o f vulcanicity. For example, P.G. Worcester (1948) has maintained that ‘vulcanism includes all phenomena connected with the movement of heated material from the interior to or towards the surface o f the earth .’ 

Types of Volcanic Eruptions

Types of volcanic eruptions depend on various factors such as chemistry of magma, temperature, viscosity, volume, presence of groundwater, and water and gas content.

Following are the different types of volcanic eruptions:

• Hydrothermal eruption: These eruptions include ash and not magma. They are driven by the heat caused in hydrothermal systems.
• Phreatic eruption: This is driven when the heat of the magma interacts with the water. These eruptions to do not include magma and only ash.
• Phreatomagmatic eruption: This eruption takes place when there is the interaction between the newly formed magma and water.
• Strombolian and Hawaiian eruption: Hawaiian eruption has 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 Phinian 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
• Lava volcanoes

Cinder Cones

• Cinder cones are circular or oval cones made up of small fragments of lava from a single vent that have been blown up. Cinder cones result from eruptions of mostly small pieces of scoria and pyroclastics that build up around the vent.
• Most cinder cones erupt only once. Cinder cones may form as flank vents on larger volcanoes, or occur on their own.

Composite Volcano

• Composite volcanoes are steep-sided volcanoes composed of many layers of volcanic rocks, usually made from high-viscosity lava, ash and rock debris. These types of volcanoes are tall conical mountains composed of lava flows and other ejecta in alternate layers, the strata that give rise to the name.
• Composite volcanoes are made of cinders, ash, and lava. Cinders and ash pile on top of each other, lava flows on top of the ash, where it cools and hardens, and then the process repeats.

Shield Volcano

• Shield volcanoes are volcanoes shaped like a bowl or shield in the middle with long gentle slopes made by basaltic lava flows. These are formed by the eruption of low-viscosity lava that can flow a great distance from a vent.
• They generally do not explode catastrophically. Since low-viscosity magma is typically low in silica, shield volcanoes are more common in oceanic than continental settings. The Hawaiian volcanic chain is a series of shield cones, and they are common in Iceland, as well.

Lava Domes

• Lava domes are formed when erupting lava is too thick to flow and makes a steep-sided mound as the lava piles up near the volcanic vent. They are built by slow eruptions of highly viscous lava.
• They are sometimes formed within the crater of a previous volcanic eruption. Like a composite volcano, they can produce violent, explosive eruptions, but their lava generally does not flow far from the originating vent.

Categories of Volcanoes

1. Active volcanoes: An active volcano is a volcano which is either erupting or is likely to erupt in the future. There are about 600 active volcanoes in the world, most of them being around the pacific “ring of fire”.
   • The Stromboli volcano erupts so much fire that it has been termed as the lighthouse of the Mediterranean.
2. Dormant volcano: A dormant volcano is one that is not currently erupting but has erupted within recordable history and is expected to erupt again in the future.
   • Mt Kilimanjaro is one such volcano.
3. Extinct volcano: Extinct volcanoes are considered dead and are not expected to ever erupt again.
• Aconcagua of Andes is a typical 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)