Defence
A large body of magma that cools and solidifies deep inside the Earth's crust, often forming a dome-shaped mass, is called a:
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Answer: Option D — Batholith
Detailed Solution
A batholith is a very large mass of intrusive igneous rock that forms when magma cools and solidifies slowly deep within the Earth's crust. Batholiths often take a large dome shape and may cover areas of more than 100 square km.
Features of batholiths:
- They are the largest type of igneous intrusion, often hundreds of kilometres long.
- They are usually made of granite or similar coarse-grained rocks, because slow cooling allows large crystals to form.
- They form the cores of many mountain ranges.
- They become exposed at the surface only after the overlying rocks are removed by long periods of erosion.
Examples:
- The Sierra Nevada batholith in California, USA.
- The Coast Mountains batholith in British Columbia, Canada.
- In India, large granite masses of the Peninsular region, such as parts of the Karnataka and Tamil Nadu plateaus, are exposed intrusions.
Intrusive and extrusive igneous rocks:
- Igneous rocks form from the cooling of molten rock.
- Extrusive (volcanic) rocks form when lava cools quickly at the surface, producing fine-grained rocks such as basalt.
- Intrusive (plutonic) rocks form when magma cools slowly underground, producing coarse-grained rocks such as granite.
Types of intrusive igneous landforms:
Batholith: very large, deep-seated, dome-shaped mass.
Laccolith: a dome-shaped intrusion with a flat base and an arched top, which pushes up the overlying rock layers. It is connected to the magma source by a pipe-like conduit.
Lopolith: a saucer-shaped intrusion, concave upward.
Phacolith: a lens-shaped intrusion found at the crest of an upfold or the bottom of a downfold.
Sill: a horizontal, sheet-like intrusion that lies parallel to the layers of rock. A thin sill is called a sheet.
Dyke: a vertical or steep, wall-like intrusion that cuts across the rock layers.
Why not the other options?
- ✗A dyke is a vertical, wall-like intrusion.
- ✗A sill is a horizontal, sheet-like intrusion.
- ✗A laccolith is a smaller, dome-shaped intrusion with a flat base.
Quick tip
Remember: Batholith = largest deep, dome-shaped granite intrusion; laccolith = dome with flat base; sill = horizontal; dyke = vertical.
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Earth: Structure, Rocks & Landforms — Preparation
This question tests your understanding of Earth: Structure, Rocks & Landforms, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers the interior of the Earth (crust, mantle and core), types of rocks, plate tectonics and continental drift, earthquakes and volcanoes, and the landforms created by rivers, glaciers, wind and waves. Questions frequently ask about the layers of the Earth and their discontinuities, the three types of rocks (igneous, sedimentary and metamorphic), the Pacific 'Ring of Fire', and which agent forms which landform, such as river meanders and oxbow lakes or glacial cirques and moraines. Alfred Wegener proposed the continental drift theory in 1912. Practise the Earth's structure and landforms questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.