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How are igneous rocks formed from magma or lava?
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Earth sciences
Geologist using a rock hammer to sample active pahoehoe lava for geochemical analysis on the Kilauea volcano, Hawaii, June 26, 2009.
Igneous rocks are formed through the cooling and solidification of molten or partially molten material known as magma. The process of formation depends on where the cooling occurs:1
- Intrusive (Plutonic) Rocks: When magma is forced into older rocks deep within the Earth’s crust, it cools slowly due to high surrounding temperatures and pressures. This slow cooling allows minerals to grow into large, visible grains, resulting in a phaneritic texture.213
- Extrusive (Volcanic) Rocks: When magma erupts at the surface as lava, it is exposed to much cooler temperatures and chills rapidly. This quick cooling prevents large mineral growth, resulting in an aphanitic (fine-grained) or glassy texture. Extrusive rocks can form from lava flows or from pyroclastic materials ejected during volcanic explosions.13
Some intrusive rocks, known as hypabyssal, form near the surface where faster cooling leads to finer textures. Regardless of the environment, the resulting rock's characteristics are determined by the magma's chemical composition, temperature, and cooling rate.14