How do igneous rocks form?

Igneous rocks form through the cooling and solidification of molten rock, known as magma or lava. This molten material originates from partial melts in a planet's mantle or crust due to increased temperatures, decreased pressure, or compositional changes. Depending on where the cooling takes place, these rocks develop either underground as intrusive rocks or on the surface as extrusive rocks.

What we know

• Igneous rocks are formed through the cooling and solidification of magma or lava. [1, 2, 5]
• Magma is derived from partial melts of existing rocks in a terrestrial planet's mantle or crust, typically caused by increased temperature, decreased pressure, or composition changes. [2]
• Solidification occurs below the surface as intrusive rocks or on the surface as extrusive rocks. [2, 3, 5]
• Igneous rock can form with crystallization into granular, crystalline rocks or without crystallization to form natural glasses. [2, 5]

Still open

• Some geologists use the term plutonic rock synonymously with intrusive rock, while others subdivide intrusive rocks by crystal size into coarse-grained plutonic and medium-grained subvolcanic or hypabyssal rocks.

Where to go next

• compared withWhat are sedimentary and metamorphic rocks?
• how it workedHow do intrusive and extrusive rocks differ?
• the bigger pictureWhat is the broader classification of rock types?
• what followedHow do weather and erosion create rock formations?
• a case in pointWhat are batholiths, dikes, and sills?
• an unexpected connectionCan rocks form without creating crystals?Igneous rocks do not always form crystalline structures and can instead cool into natural glasses.

Sources

• [1]igneous rock (Wikidata Q42045) · CC0 1.0
• [2]Igneous rock · CC BY-SA 4.0
• [3]Intrusive rock · CC BY-SA 4.0
• [4]List of rock types · CC BY-SA 4.0
• [5]List of rock formations · CC BY-SA 4.0