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Igneous Rocks

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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. 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 and pyroxene are mafic igneous rocks

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.

Feldspar and quartz are felsic igneous rocks

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

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.

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.

  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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