Extrusive vs. Intrusive — What's the Difference?
Edited by Tayyaba Rehman — By Maham Liaqat — Updated on April 3, 2024
Extrusive rocks form from lava that cools and solidifies on the Earth's surface, while intrusive rocks crystallize from magma within the Earth.
Difference Between Extrusive and Intrusive
Table of Contents
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Key Differences
Extrusive igneous rocks, also known as volcanic rocks, cool quickly on the Earth's surface, leading to fine-grained textures, whereas intrusive igneous rocks, or plutonic rocks, cool slowly beneath the surface, resulting in coarse-grained textures. This difference in cooling rates significantly influences their physical characteristics.
The rapid cooling of extrusive rocks often prevents the formation of large crystals, making these rocks appear smooth or with very small crystals, while the slow cooling of intrusive rocks allows large crystals to form, giving these rocks a visibly grainy texture. This contrast highlights the impact of the environment on rock formation.
Extrusive rocks, such as basalt and obsidian, are typically formed from lava flows or volcanic eruptions, which exposes them directly to the atmosphere or water, whereas intrusive rocks, like granite and gabbro, crystallize from magma that does not reach the surface, remaining encapsulated within the Earth's crust. This origin difference reflects their respective formation processes.
Due to their formation on or near the surface, extrusive rocks often contain voids or bubbles (vesicles) left by escaping gases, whereas intrusive rocks are generally denser with a more uniform composition because gases had more time to escape before the rock solidified. This structural difference affects their porosity and density.
In geological studies, extrusive rocks provide important information about volcanic activity and surface conditions, while intrusive rocks offer insights into the Earth's interior processes and the composition of the crust. The study of these rocks, therefore, contributes to our understanding of geological history and tectonic activities.
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Comparison Chart
Cooling Rate
Rapid
Slow
Grain Size
Fine-grained or glassy
Coarse-grained
Formation Location
On the Earth's surface
Beneath the Earth's surface
Common Examples
Basalt, Andesite, Rhyolite
Granite, Diorite, Gabbro
Porosity
Higher, due to vesicles
Lower, more dense
Compare with Definitions
Extrusive
Rocks that solidify from lava on the Earth's surface.
Basalt forms from rapidly cooling lava flows.
Intrusive
Offers insights into the Earth's interior processes.
Studying intrusive rocks helps geologists understand magma's movement and cooling within the crust.
Extrusive
Often associated with volcanic eruptions.
Pumice is formed from frothy lava, making it light enough to float on water.
Intrusive
Characterized by coarse-grained textures.
Gabbro's large crystals result from the slow cooling of magma.
Extrusive
May contain vesicles from gas bubbles.
Scoria, a type of extrusive rock, contains numerous vesicles.
Intrusive
Not directly associated with volcanic eruptions.
Diorite forms deep within the Earth, away from the immediate effects of volcanism.
Extrusive
Characterized by fine-grained or glassy textures.
Obsidian is known for its smooth, glassy appearance due to rapid cooling.
Intrusive
Typically denser and more uniform.
Intrusive rocks like granite are often used in construction due to their density and strength.
Extrusive
Provides clues about surface volcanic activity.
The study of extrusive rocks helps geologists understand past volcanic eruptions.
Intrusive
Rocks that crystallize from magma underground.
Granite forms when magma cools slowly beneath the Earth's surface.
Extrusive
Derived from magma poured out or ejected at the earth's surface. Used of igneous rocks.
Intrusive
Causing disruption or annoyance through being unwelcome or uninvited
That was an intrusive question
Tourist attractions that are environmentally intrusive
Extrusive
Jutting out; extruding.
Intrusive
(of a sound) pronounced between words or syllables to facilitate pronunciation, such as an r in saw a film.
Extrusive
(geology) Of rocks: forced, while in a plastic or molten state, on to the Earth's surface to lie atop existing rocks.
Intrusive
Relating to or formed by intrusion.
Extrusive
(geology) Rock which has been formed through volcanic extrusion.
Intrusive
Intruding or tending to intrude.
Extrusive
Forced out at the surface; as, extrusive rocks; - contrasted with intrusive.
Intrusive
(Geology) Of or relating to igneous rock that is forced while molten into cracks or between other layers of rock.
Extrusive
Of rock material; forced out while molten through cracks in the earth's surface
Intrusive
(Linguistics) Epenthetic.
Intrusive
Tending to intrude; doing that which is not welcome; interrupting or disturbing; entering without permission or welcome.
Did it ever cross your mind that he might find all those questions you ask intrusive?
Intrusive
(geology) Of rocks: forced, while in a plastic or molten state, into the cavities or between the cracks or layers of other rocks.
Intrusive
(linguistics) epenthetic
Intrusive
(geology) An igneous rock that is forced, while molten, into cracks or between other layers of rock
Intrusive
Apt to intrude; characterized by intrusion; entering without right or welcome.
Intrusive
Tending to intrude (especially upon privacy);
She felt her presence there was intrusive
Intrusive
Of rock material; forced while molten into cracks between layers of other rock
Intrusive
Thrusting inward;
An intrusive arm of the sea
Common Curiosities
Why are extrusive rocks important to study?
They provide valuable information about volcanic activity and the Earth's surface environment at the time of their formation.
Can the same type of magma produce both extrusive and intrusive rocks?
Yes, depending on where it cools and solidifies; for example, rhyolite (extrusive) and granite (intrusive) can originate from the same magma.
Why are intrusive rocks generally stronger than extrusive rocks?
Intrusive rocks are denser and have a more uniform composition due to their slow cooling, which contributes to their strength.
Are all volcanic rocks extrusive?
Yes, volcanic rocks are considered extrusive because they form from lava that erupts onto the surface.
Do all extrusive rocks have a glassy texture?
Not all; while some, like obsidian, are glassy, others may be fine-grained but not glassy.
What determines the texture of igneous rocks?
The cooling rate of the magma or lava determines the rock's texture; rapid cooling results in fine-grained textures, while slow cooling produces coarse-grained textures.
How do geologists differentiate between extrusive and intrusive rocks in the field?
By examining the rock's texture, grain size, and formation location, among other characteristics.
Can intrusive rocks be found on the Earth's surface?
Yes, intrusive rocks can be exposed at the surface due to erosion of overlying materials.
Can extrusive rocks have large crystals?
It's rare because extrusive rocks cool quickly, but phenocrysts (larger crystals) can form in a finer matrix if cooling rates vary.
What role do gases play in the formation of extrusive rocks?
Gases escaping from lava can create vesicles (bubbles) in extrusive rocks, affecting their porosity and texture.
Can intrusive rocks form anywhere beneath the Earth's surface?
Yes, but their formation depth can vary widely, from shallow depths to deep within the Earth's crust.
Why might an intrusive rock contain larger crystals than an extrusive rock from the same volcanic event?
Because it cooled more slowly underground, allowing time for larger crystals to form.
What is the significance of studying intrusive rock formations?
They offer insights into the processes occurring within the Earth's crust and mantle.
What is a key indicator of an intrusive rock's formation depth?
The size of its crystals; larger crystals typically indicate slower cooling at greater depths.
How does the composition of magma affect rock type?
The chemical composition of magma can influence whether a rock is felsic (rich in silica) or mafic (rich in magnesium and iron), affecting its color and density.
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Written by
Maham Liaqat
Edited by
Tayyaba RehmanTayyaba Rehman is a distinguished writer, currently serving as a primary contributor to askdifference.com. As a researcher in semantics and etymology, Tayyaba's passion for the complexity of languages and their distinctions has found a perfect home on the platform. Tayyaba delves into the intricacies of language, distinguishing between commonly confused words and phrases, thereby providing clarity for readers worldwide.
