Volcano

11. Individual lava flows are normally only a few feet thick, but over a l

Individual lava flows are normally only a few feet thick, but over a long period of time, repeated flows may build up a volcano. Such volcanoes are termed as :

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Shield volcano
Composite volcano
Strato-volcano
Cinder-cone volcano
This question was previously asked in
UPSC CDS-1 – 2022
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Such volcanoes are termed as Shield volcano.
The description “Individual lava flows are normally only a few feet thick, but over a long period of time, repeated flows may build up a volcano” and the implied shape resulting from such flows points to a shield volcano. Shield volcanoes are formed by effusive eruptions of low-viscosity basaltic lava that spreads out thinly and widely, building a broad, gently sloping cone resembling a warrior’s shield.
Composite volcanoes (or stratovolcanoes) are built from alternating layers of viscous lava, ash, and volcanic debris, resulting in steeper, conical shapes and often explosive eruptions. Cinder-cone volcanoes are the simplest type, formed by loose pyroclastic material (cinders) ejected from a single vent, resulting in steep slopes but relatively small size.

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12. Extrusive volcanoes are not found in which one of the following

Extrusive volcanoes are not found in which one of the following mountains?

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Alaska
Rocky
Andes
Himalayas
This question was previously asked in
UPSC CDS-1 – 2018
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The correct answer is D) Himalayas.
Extrusive volcanoes are typically associated with plate boundaries involving subduction or rifting, where magma reaches the Earth’s surface. The Himalayas were formed by the collision of the Indian and Eurasian continental plates, a process that primarily leads to intense folding, faulting, and uplift, rather than the formation of extrusive volcanoes. While intrusive igneous activity (like granite formation) occurs deep within the crust in collisional zones, molten rock rarely erupts onto the surface.
The Andes (Nazca/South American plate subduction), Alaska (part of the Pacific Ring of Fire), and historically, the Rocky Mountains region (though less active now than regions on the Ring of Fire) are associated with tectonic settings where extrusive volcanic activity has occurred or can occur. The Himalayas represent a continent-continent collision boundary, which is less conducive to surface volcanism compared to ocean-continent or ocean-ocean subduction zones or rift zones.

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13. Which one of the following statements with regard to volcanoes is not

Which one of the following statements with regard to volcanoes is not correct?

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Stratovolcanoes produce lava flows that initially follow valleys but are highly resistant to erosion.
The surrounding areas can remain highlands, lava ridges or mesas.
Hawaiian shield volcanoes are eroded by streams that form deeply carved valleys with steeply sloping heads.
The system of streams on a dissected volcano cone is not a radial drainage pattern.
This question was previously asked in
UPSC CDS-1 – 2017
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The statement “The system of streams on a dissected volcano cone is not a radial drainage pattern” is not correct.
Volcano cones, being elevated, roughly symmetrical peaks, naturally lend themselves to a radial drainage pattern where streams flow outwards from the central high point in all directions. When a volcano cone is dissected by erosion over time, the streams carving into its slopes typically maintain this radial arrangement.
Statements A, B, and C are generally correct regarding volcanic landforms and erosion. Stratovolcano lava flows can be resistant to erosion, creating distinct features. Erosion on large shield volcanoes like those in Hawaii does result in deeply carved valleys. The radial drainage pattern is characteristic of conical or dome-shaped landforms like volcanoes or isolated hills/mountains.

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