Volcanic Landforms – Extrusive and Intrusive
Geography › Mountains, Volcanoes, Earthquakes and Tsunami · Topic 4.4
Volcanic landforms are of two kinds. Extrusive landforms form when lava erupts on the surface and include shield volcanoes, composite cones, cinder cones, calderas and lava plateaus such as the Deccan Traps. Intrusive landforms form when magma cools underground and include batholiths, laccoliths, lopoliths, sills and dykes, which are exposed later by erosion.
5 min read · UPSC Prelims · Geography · Mountains, Volcanoes, Earthquakes and Tsunami
At a Glance
- Extrusive Shield, composite, cinder cone, caldera, lava plateau
- Intrusive Batholith, laccolith, lopolith, sill, dyke
- Largest intrusive body Batholith
- Flat intrusive sheet Sill
- Vertical intrusive sheet Dyke
- Exam link UPSC Prelims, Geography: Mountains, Volcanoes, Earthquakes and Tsunami
Where Does It Fit?
This is the fourth topic of the category on mountains, volcanoes, earthquakes and tsunami. The previous topic explained magma and lava, and this one shows the landforms they build. The next topic covers the different styles of eruption. Previous topic: Volcanism – Causes and Lava Types. Next topic: Types of Volcanic Eruptions.
What Are Extrusive Volcanic Landforms?
In short: extrusive landforms are built on the surface by lava and ash.
| Landform | Features | Examples |
|---|---|---|
| Shield volcano | Broad, gentle dome built by very fluid basaltic lava; the largest volcanoes | Mauna Loa and Mauna Kea in Hawaii, Kilauea |
| Composite (strato) volcano | Steep cone built of alternating layers of lava and ash; explosive eruptions; acidic or intermediate lava | Mount Fuji, Mount Mayon, Vesuvius, Mount St Helens |
| Cinder cone | Small, steep cone made of ash and cinders around a vent | Paricutin in Mexico |
| Caldera | Huge basin formed when a volcano collapses after emptying its magma chamber, or is blown apart | Krakatoa, Toba in Sumatra, Crater Lake in the USA |
| Lava plateau (flood basalt) | Vast sheets of basalt from fissure eruptions | Deccan Traps in India, Columbia Plateau in the USA |
| Lava dome | Mound of thick lava piled over the vent | Mount St Helens dome |
A caldera that fills with water becomes a crater lake. The Deccan Traps cover about 500,000 sq km in Maharashtra, Gujarat, Madhya Pradesh and Karnataka, and are described in Movement of the Indian Plate. Mauna Loa in Hawaii is the largest active volcano on Earth by volume, and Mauna Kea, measured from its base on the sea floor, is taller than Everest. Barren Island in the Andaman Sea is India’s only confirmed active volcano.
How Do the Shield and Composite Volcanoes Differ?
| Feature | Shield | Composite |
|---|---|---|
| Lava | Basic, fluid | Intermediate or acidic, viscous |
| Shape | Broad with gentle slopes | Steep, conical |
| Eruption | Quiet, effusive | Explosive |
| Materials | Mostly lava | Lava layers with ash and cinders |
| Setting | Hotspots and ridges | Subduction zones |
What Are Intrusive Volcanic Landforms?
When magma cools and solidifies inside the crust without reaching the surface, it forms intrusive igneous rock bodies. Cooling is slow, so the rocks, such as granite and gabbro, have large crystals. These bodies are seen at the surface only after the overlying rock has been eroded away.
- Batholith: a huge, irregular mass of granite that forms the core of many mountain ranges and may be hundreds of km across. The Sierra Nevada and the Coast Mountains of Canada contain large batholiths.
- Laccolith: a dome-shaped body with a flat base that arches the layers above it, connected to the source by a pipe.
- Lopolith: a saucer-shaped intrusion that sags into the crust.
- Phacolith: a lens-shaped intrusion in the crest of an anticline or the trough of a syncline.
- Sill: a horizontal sheet of magma squeezed between layers of rock; a thin sill is called a sheet.
- Dyke: a vertical or steeply inclined sheet of magma cutting across the layers, often the feeder of lava flows. Dykes are common in the Deccan Traps and the Peninsular shield.
- Volcanic neck: the hardened magma plug of an eroded volcano, left standing as a rock tower.
How Do Sills and Dykes Differ?
A sill lies parallel to the bedding of the rocks it intrudes, and a dyke cuts across them. Both are sheets that are thin compared with their length. Dykes act as barriers to groundwater and, because they are harder than the surrounding rock, may stand as ridges after erosion. Volcanic features also include fissure vents, geysers and hot springs, which are treated in Hotspots, Mantle Plumes, Geysers and Hot Springs. The rocks produced by cooled magma, igneous rocks, are in Igneous Rocks. As an analytical point, intrusive and extrusive features are two parts of the same system: the dykes and sills seen in India’s Deccan region are the plumbing that fed the lava flows seen at the surface. In answers, it helps to draw a simple sketch showing the magma chamber at the base, a pipe rising to a vent, a cone on top, a sill between two layers and a dyke cutting across them, and to label each part, because a labelled diagram earns more marks than a long description. The sketch also helps in separating features that are often confused, such as a crater, a caldera and a vent, and in showing at a glance which features are on the surface and which are hidden below ground.
Why Is This Topic Important?
- Classification Frequent match-type question
- Features Caldera and lava plateau
- India Deccan Traps and Barren Island
What Came Next?
The next topic describes the types of volcanic eruptions. Next topic: Types of Volcanic Eruptions.
Key People and Terms
- Shield volcano Broad dome of fluid lava
- Caldera Collapse basin of a volcano
- Batholith Huge underground granite mass
- Laccolith Dome-shaped intrusion
- Sill Horizontal intrusion
- Dyke Vertical intrusion
Exam Corner
Points to Remember
- Shield volcanoes are broad and gentle; composite are steep.
- A sill is horizontal; a dyke is vertical.
- Batholith is the largest intrusive body.
- Deccan Traps are a lava plateau.
- A caldera results from collapse or explosion.
Do Not Confuse With
- Sill and dyke Parallel versus cutting across layers
- Caldera and crater Large collapse basin versus small vent bowl
- Laccolith and lopolith Dome-shaped versus saucer-shaped
Memory Hook
Sills lie, dykes stand; batholith is the base, laccolith lifts the roof.
Mains Angle
Questions ask for features with examples. These points add depth.
- Surface Cones, calderas and plateaus
- Underground Batholith to dyke
- India Deccan Traps
A Question You May Face
An original practice question, not a past paper question.
“Describe the extrusive and intrusive landforms associated with volcanic activity.”
How to Answer
- Extrusive Cones, caldera, plateaus.
- Intrusive Batholith, sill, dyke.
- Compare Cooling and crystals.
- Examples Deccan Traps.
GS Relevance
Frequently Asked Questions
What are extrusive volcanic landforms?
Extrusive volcanic landforms are those built on the surface by lava and ash erupting from a vent or fissure. They include shield volcanoes, composite volcanoes, cinder cones, lava domes, calderas and lava plateaus such as the Deccan Traps.
What are intrusive volcanic landforms?
Intrusive landforms form when magma cools and solidifies beneath the surface instead of erupting. They include batholiths, laccoliths, lopoliths, phacoliths, sills and dykes, and they are exposed at the surface only after the overlying rock is eroded away.
What is the difference between a sill and a dyke?
A sill is a horizontal sheet of igneous rock that lies parallel to the layers it has intruded, while a dyke is a vertical or steeply inclined sheet that cuts across the layers. Dykes often act as feeders to lava flows.
What is a caldera?
A caldera is a very large basin-shaped depression formed when a volcano collapses after its magma chamber empties or is blown apart in a huge eruption. It may fill with water to form a crater lake, as at Crater Lake in the United States.
What is a batholith?
A batholith is the largest intrusive igneous body, a huge irregular mass of granite formed deep in the crust by cooling magma. It often forms the core of mountain ranges and is exposed after erosion removes the rock above it.
PYQ Practice — Statement Analysis
1 A dyke is a horizontal intrusion of igneous rock.
A sill is horizontal; a dyke is vertical.
2 Shield volcanoes are built by fluid basaltic lava.
They have gentle slopes.
3 The Deccan Traps are an example of a lava plateau.
They formed from fissure eruptions.
4 A batholith is a small extrusive cone.
It is a huge intrusive mass.
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