These rocks don't even look like they're the same type! They are, at least in the same way that fish and mice are both vertebrates. They both cooled from magma, but the similarities end there. Can you tell what's different?
Igneous rocks cool from magma. The appearance of the rock is created by the composition of the magma. It is also determined by the rate that the magma cools. If the magma cools deep underground, it cools slowly. If the magma cools at or very near the surface, it cools quickly. This results in two different rock types. The rock types can be told apart by the size of their crystals. The size of the crystals creates the texture of the rock.
Intrusive Igneous Rocks
Intrusive igneous rocks cool underground. Deep in the crust, magma cools slowly. Slow cooling gives crystals a chance to grow. Intrusive igneous rocks have relatively large crystals that are easy to see. Intrusive igneous rocks are also called plutonic. A pluton is an igneous rock body that forms within the crust.
Granite is the most common intrusive igneous rock. Pictured below are four types of intrusive rocks (Figure below).
(A) This granite has more plagioclase feldspar than many granites. (B) Diorite has more dark-colored minerals than granite. (C) Gabbro. (D) Peridotite contains olivine and other mafic minerals.
Geological processes have brought some igneous rocks to the surface. Pictured below is a landscape in California’s Sierra Nevada Mountains made of granite that has been raised to create mountains (Figure below).
California’s Sierra Nevada Mountains are intrusive igneous rock exposed at Earth’s surface.
Extrusive Igneous Rocks
Extrusive igneous rocks form above the surface. The lava cools quickly as it pours out onto the surface (Figure below). Extrusive igneous rocks cool much more rapidly than intrusive rocks. The rapid cooling time does not allow time for large crystals to form. So igneous extrusive rocks have smaller crystals than igneous intrusive rocks. Extrusive igneous rocks are also called volcanic rocks.
(A) Lava cools to form extrusive igneous rock. The rocks here are basalts. (B) The strange rock formations of Chiricahua National Monument in Arizona are formed of the extrusive igneous rock rhyolite.
Some extrusive igneous rocks cool so rapidly that crystals do not develop at all. These form a glass, such as obsidian. Others, such as pumice, contain holes where gas bubbles were trapped in the lava. The holes make pumice so light that it actually floats in water. The most common extrusive igneous rock is basalt. It is the rock that makes up the ocean floor. Shown below are three types of extrusive igneous rocks (Figure below).
Different cooling rate and gas content resulted in these different textures.
- Intrusive igneous rocks cool from magma slowly in the crust. They have large crystals.
- Extrusive igneous rocks cool from lava rapidly at the surface. They have small crystals.
- Texture reflects how an igneous rock formed.
- How do intrusive igneous rocks form? What is another name for these rocks?
- How do extrusive igneous rocks form? What is another name for these rocks?
- How can you use texture to determine whether an igneous rock is intrusive or extrusive?
Use the resource below to answer the questions that follow.
- How are intrusive rocks formed?
- What size are the crystals in very coarse rocks? Why are they that size?
- What are the most common coarse rocks?
- How are extrusive rocks formed?
- List the three textures for extrusive rocks.
- Describe rhyolite.
- Describe pumice.
- Explain why obsidian appears black.