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Igneous Rocks

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Igneous Rocks

Have you seen igneous rocks in the field?

Igneous rocks are everywhere! One of the most common igneous rocks that make up the crust is granite. Many mountain ranges are made of granite. The spectacular features of Yosemite Valley, like Half Dome, are granitic. Volcanoes are made of igneous rocks, such as basalt. Igneous rocks are also found where you can't see them. Oceanic crust is mostly basalt and gabbro. The mantle is peridotite.

Igneous Rocks

Igneous rocks form when magma cools and forms crystals. Magma is melted rock. What an igneous rock looks like is determined by two things. One is the composition of the magma . The other is how fast the magma cools . The rate of cooling determines the texture of the rock.


Composition

Different igneous rocks contain minerals with different compositions.  Igneous rocks are classified into 2 categories based on their composition, that is what elements they are made of.

Mafic igneous rocks contain mafic minerals. Mafic minerals are dense and dark in color. They typically contain iron and magnesium; they are low in silica. Olivine and pyroxene are mafic minerals.

Olivine is the green mineral, pyroxene is the black mineral.

Felsic igneous rocks contain felsic minerals. They typically contain aluminum and sodium; they are high in silica. Quartz and potassium feldspar are felsic minerals. Minerals and rocks with a composition in between mafic and felsic are called intermediate.

Potassium feldspar is the pink mineral, plagioclase feldspar is the white mineral, and quartz is the gray mineral.


Are these both igneous rocks?

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?

Cooling

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 made of granite that has been raised to create mountains ( Figure   below ).

California’s Sierra Nevada is 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 ).

Textures

Different cooling rate and gas content resulted in these different textures.



Vocabulary

  • felsic : Minerals that are light in color and relatively low in density.
  • mafic : Minerals that are dark in color and relatively dense.
  • magma : Melted rock.
  • extrusive : Igneous rocks that form at Earth's surface from rapidly cooling lava.
  • i ntrusive : Igneous rocks that form inside the Earth from slowly cooling magma.
  • p luton : Igneous intrusive rock body that has cooled in the crust.
  • volcanic rock : Rock that originates in a volcano or volcanic feature.

Summary

  • Magma cools to form igneous rocks. Two factors determine what type of rock forms.
  • The composition of the magma determines if the rock is mafic, felsic, or intermediate.
  • The rate the magma cools determines the texture of the rock.

Practice

Use the resource below to answer the questions that follow.

http://video.about.com/geology/What-Are-Igneous-Rocks-.htm

  1. How do igneous rocks form?
  2. What is magma?
  3. What are plutonic rocks?
  4. How are extrusive rocks formed?
  5. How are igneous rocks classified?
  6. What is granite? Where is it used?

Review

  1. How does an igneous rock form?
  2. How do mafic and felsic minerals and rocks differ from each other?
  3. What two factors determine what type of rock a magma will form?

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