Intrusive Volcanic Landforms: Batholiths, Laccoliths, Lapoliths, Phacoliths, Sills, Dykes
Geography › Geomorphic Processes and Landform Evolution · Topic 3.6
Intrusive volcanic landforms form when magma cools and hardens below the surface, within the crust, without reaching the air. They include batholiths (huge masses of granite), laccoliths (dome-shaped masses), lapoliths (saucer-shaped masses), phacoliths (lens-shaped masses in folds), sills (horizontal sheets) and dykes (vertical or steeply inclined walls). They are exposed at the surface only after the rocks above are removed by erosion, and their slow cooling gives coarse-grained igneous rocks.
6 min read · UPSC Prelims · Geography · Geomorphic Processes and Landform Evolution
At a Glance
- Origin Magma cooled underground
- Largest form Batholith, over 100 sq km
- Horizontal sheet Sill
- Vertical wall Dyke
- Rocks formed Coarse-grained, such as granite
- Exam link UPSC Prelims, Geography: Geomorphic Processes and Landform Evolution
Where Does It Fit?
This topic follows the one on lava types and describes what happens to magma that does not erupt. It is paired with the next topic on extrusive features such as geysers and hot springs. Previous topic: Types of Lava: Andesitic (Acidic) and Basaltic (Basic) Lava. Next topic: Extrusive Volcanic Landforms: Geysers and Hot Springs.
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.
What Are the Main Intrusive Forms?
| Form | Shape and position | Example |
|---|---|---|
| Batholith | Huge, irregular, dome-like mass of granite, usually over 100 sq km, with no visible floor; the core of many mountain ranges | Coast Range Batholith (Canada), Sierra Nevada (USA) |
| Laccolith | Mushroom or dome-shaped mass with a flat base, which pushes up the layers above it | Henry Mountains, Utah (USA) |
| Lapolith | Saucer or basin-shaped mass, sagging down | Bushveld Complex, South Africa; Duluth Complex, USA |
| Phacolith | Lens-shaped body in the crest of an anticline or trough of a syncline | Folded mountain regions |
| Sill | Sheet of magma between layers of rock, parallel to the bedding; a thin sill is called a sheet | Whin Sill (England), Palisades (USA) |
| Dyke | Wall-like sheet that cuts across the layers, often vertical | Dolerite dykes across peninsular India |
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 Extrusive Volcanic Landforms: 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.
How Do Intrusive Forms Reach the Surface and Why Do They Matter?
- Exposure: erosion strips away the cover rock over millions of years, so batholiths now form the granite cores of mountains and domes.
- Dykes and plugs: where soft rock around a dyke or a volcanic neck erodes away, the hard igneous rock stands as a wall or tower, as with Devils Tower in the USA.
- Resources: many ore deposits, such as copper, gold, tin and tungsten, are formed in and near intrusions, as hot fluids give out metals. Granite and dolerite are used as building stones.
- India: the Peninsula has many granite batholiths and dolerite dykes of different ages, and the Deccan Traps are cut by dykes that fed the lava flows, as explained in Pangaea, about 300 to…">Geological History of the Earth.
The extrusive counterparts are explained in Extrusive Volcanic Landforms: Geysers and Hot Springs, and the rocks they produce in Three Families of Rocks: Igneous, Sedimentary, Metamorphic. As an analytical point, intrusive and extrusive forms are two ends of the same process, and the same magma can feed both a surface volcano and many hidden bodies below it.
Why Is This Topic Important?
- Classification Shapes and names are asked
- Resources Link to ores
- Landscape Granite hills and dykes
What Came Next?
The next topic covers the extrusive volcanic landforms, geysers and hot springs. Next topic: Extrusive Volcanic Landforms: Geysers and Hot Springs.
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
- A sill is horizontal; a dyke is vertical.
- A batholith is the largest intrusive body.
- A lapolith is saucer-shaped; a laccolith is dome-shaped.
- A phacolith lies in the crest or trough of a fold.
- Intrusive rocks are coarse-grained.
Do Not Confuse With
- Sill and dyke Parallel to layers versus cutting across them
- Laccolith and lapolith Domed upward versus sagging downward
- Batholith and laccolith Huge deep mass versus a smaller dome
Memory Hook
Sill sleeps, dyke stands; laccolith lifts, lapolith lands.
Mains Angle
Questions ask for the forms and their economic importance. These points add depth.
- Forms Shape set by the host rock
- Exposure Erosion reveals them
- Resources Ores and building stone
A Question You May Face
An original practice question, not a past paper question.
“Describe the intrusive volcanic landforms and distinguish between a sill and a dyke.”
How to Answer
- Define Intrusive forms.
- List Six forms.
- Compare Sill and dyke.
- Use Resources.
GS Relevance
Frequently Asked Questions
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 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.
What is a laccolith?
A laccolith is a dome-shaped intrusion of magma with a flat base that pushes up the rock layers above it as it forms. It is smaller than a batholith. A well-known example is the group of laccoliths forming the Henry Mountains in Utah, in the United States.
What is a phacolith?
A phacolith is a lens-shaped body of igneous rock that is found in the crest of an anticline or the trough of a syncline in folded rocks. The magma fills the space opened up by the folding, so the intrusion is shaped by the fold and lies parallel to its bedding.
PYQ Practice — Statement Analysis
1 A dyke is a horizontal intrusion of igneous rock.
A sill is horizontal; a dyke is vertical.
2 A batholith is a small extrusive cone.
It is a huge intrusive mass.
3 A sill is a horizontal sheet of igneous rock between rock layers.
A dyke cuts across them.
4 Intrusive igneous rocks are usually fine-grained.
Slow cooling underground gives coarse grains.
Past Mains Questions on This Topic
This topic comes under Geomorphic Processes and Landform Evolution. UPSC Mains questions asked on the matching syllabus topic, year-wise with marks:
Related Topics
Browse the full Geography index