Intrusive vs extrusive rock

Short answer

Intrusive igneous rock solidifies from magma within Earth. Extrusive igneous rock forms from material erupted at the surface or into the atmosphere. Intrusive rock commonly cools slowly and develops visible crystals; extrusive rock commonly cools faster and has a fine-grained or glassy matrix. The defining difference is formation setting, while crystal size is a useful clue with exceptions. 1 2

A rock found on the surface today can still be intrusive. Erosion and uplift can expose a body that originally solidified underground. Present location and formation location are different questions. 2 3

On this page

At a glance

Question	Intrusive igneous rock	Extrusive igneous rock
Where does it form?	Within Earth, from magma that solidifies below the surface.	From erupted material at the surface or in the atmosphere.
Common alternative name	Plutonic.	Volcanic.
Typical cooling pattern	Relatively slow.	Relatively rapid around eruption and emplacement.
Common texture	Interlocking crystals large enough to see.	Fine-grained or glassy groundmass, sometimes with larger crystals.
Examples	Granite, diorite, gabbro.	Rhyolite, andesite, basalt, obsidian.
Must all crystals be the same size?	No.	No.
Does today’s surface exposure establish the class?	No.	No.

Basic definitions and examples: 1. Texture exceptions and exposure: 2 3.

What each thing is

Intrusive rock

Intrusive rock forms when magma cools within Earth. It can make large bodies, such as plutons and batholiths, or thinner sheets. NPS distinguishes sheets cutting across surrounding layers, called dikes, from those lying parallel to layers, called sills. Those terms describe the body’s relationship to its surroundings. 2

Granite is a familiar intrusive rock, but intrusive does not mean granite. Gabbro and diorite belong to the same formation-setting class while differing in composition. 1 2

Extrusive rock

Extrusive rock results from volcanic eruption. It includes solidified lava flows and rock formed from erupted fragments. The category is therefore broader than a smooth sheet of lava that cooled on open ground. NPS also describes volcanic deposits from ash flows and ash falls. 2

An eruption under water still occurs at Earth’s surface. Pillow basalts are a documented example of volcanic rock formed in a subaqueous eruption. Exposure to the atmosphere is not a universal requirement. 2

Key differences

Formation setting affects cooling and texture

Magma surrounded by rock commonly cools more slowly than erupted material exposed to cooler surroundings. More time for growth generally allows larger crystals. Rapid cooling may produce tiny crystals or volcanic glass. These are tendencies that explain textures, rather than exact clocks that assign an age to every visible grain. 1 2

Composition and texture classify different features

Two rocks can have related compositions but different textures because they cooled in different settings. NPS’s classification distinguishes composition from grain size and structure. Knowing that a sample is volcanic does not, by itself, establish whether it is basalt, andesite, or rhyolite. 2

A rock can preserve more than one cooling stage

Some volcanic rocks contain large crystals that grew in a magma chamber before eruption, surrounded by a finer material that formed later. The large crystals are phenocrysts; the surrounding finer or glassy material is the groundmass. This combination is called a porphyritic texture. Large crystals inside a volcanic rock are therefore not a contradiction. 2

How to tell them apart

Begin with the geological setting and a reliable rock description. A mapped lava flow, ash deposit, or intrusive body provides information that a loose fragment may have lost. Texture adds evidence: coarse interlocking crystals often suggest slower underground cooling, while glass and fine groundmass often indicate rapid cooling. 1 2

Do not use color as the deciding test. NPS explicitly notes that obsidian can look dark despite its commonly silica-rich composition. Nor should “large crystals” automatically settle the intrusive/extrusive question, because phenocrysts occur in both settings. 2

Where they overlap

Both are igneous rocks, formed from molten material and its products. They can belong to the history of the same volcanic system: some magma remains below ground while another portion erupts. The distinction tracks where the particular rock formed, not whether its geological history has any connection to a volcano. 1

Edge cases

Fine-grained intrusive rock: NPS describes aplite as a fine-grained intrusive rock emplaced at relatively shallow depths. “Every intrusive rock has large visible crystals” is too absolute. 2

Large crystals in volcanic rock: phenocrysts may have grown underground before eruption. Classify the rock’s history as a whole rather than treating one grain size as a verdict. 2

Volcanic glass: obsidian is extrusive even though it may lack mineral crystals. Rapid solidification can produce a glassy groundmass. 1 2

Exposed intrusive bodies: granite at a present-day rock face does not become extrusive simply because erosion has uncovered it. NPS describes former magma chambers exposed after the overlying material was removed. 2

Why the distinction exists

The classification connects a rock to its formation environment. It helps explain textures, the shapes of rock bodies, and relationships between underground magma systems and erupted deposits. A useful identification combines that history with composition rather than relying on appearance alone. 1 2

Common misconceptions and examples

“Underground now means intrusive” confuses present burial with origin. “On the surface now means extrusive” fails for exposed granite. “All volcanic rocks are fine-grained throughout” fails for porphyritic examples. NPS documents granite bodies in Yosemite and pillow basalt from underwater eruptions: both make more sense when the classification follows formation history. 2

Sources

Sources consulted October 3, 2026.

US Geological Survey — What are igneous rocks?. Intrusive/extrusive definitions, cooling, textures, and examples.
National Park Service — Igneous Rocks. Phenocrysts, groundmass, aplite, glass, intrusive bodies, and volcanic deposits.
British Geological Survey — Rocks and minerals. Intrusive cooling and later surface exposure.