Earth Science

Earth Materials

Intrusive and Extrusive Igneous Rocks

Study Tip
Have you ever wondered how scientists come up with so much information just by looking at a rock? Rock textures can reveal a lot about the rock’s history.

Igneous Rocks

  • The rate that magma cools determines whether an igneous rock is intrusive or extrusive.
  • The cooling rate is reflected in the rock’s texture.
  • Igneous rocks make up most of the rocks on Earth.
  • Most igneous rocks are buried below the surface and covered with sedimentary rock, or buried beneath the ocean water.
Study Tip
Have you ever wondered how scientists come up with so much information just by looking at a rock? Rock textures can reveal a lot about the rock’s history.
Study Tip
The same way a person may cast a shadow over another person when they stand under the sun, planets or celestial bodies that have aligned themselves cast shadows over one another as well.

Intrusive Igneous Rocks

  • Intrusive Igneous Rock: Igneous rocks that form inside the Earth from slowly cooling magma.
  • Pluton: Igneous intrusive rock body that has cooled in the crust.
  • When magma cools within the Earth, the cooling proceeds slowly.
  • Slow cooling allows time for large crystals to form.
  • Intrusive igneous rocks have visible crystals.
Sierra Nevada
California’s Sierra Nevada is intrusive igneous rock exposed at Earth’s surface.

Extrusive Igneous Rocks

  • Extrusive Igneous Rock: Igneous rocks that form after lava cools above the surface.
  • These rocks usually form from a volcano (called volcanic rocks.)
  • Cool much more rapidly than intrusive rocks.
  • There is little time for crystals to form, so extrusive igneous rocks have tiny crystals.
Extrusive igneous
Extrusive igneous rocks have tiny crystals.

Earth Science

Rock Textures

  • Porphyritic: Igneous rock texture in which visible crystals are found in a matrix of tiny crystals.
  • Vesicular: Igneous rock texture with holes that indicate the presence of gas bubbles in the magma.
Rock Textures
Different cooling rate and gas content resulted in these different textures.
Concept Check
  • What is the difference between intrusive and extrusive?
  • What kinds of textures can rocks have?