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Aug 31 2018

How to Identify Igneous Rocks: 8 Steps (with Pictures)

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How to Identify Igneous Rocks

When you hold an igneous rock, the rock you hold in your hand is one of the oldest things in the world. Igneous rocks are formed from lava, magma or ash from a volcanic eruption or flow [1]. Igneous rocks have distinctive properties that can help you distinguish them from other types of rocks, and identify the specific type of igneous rock you have.

Steps Edit

Method One of Two:
Identifying Igneous Rocks Edit

Classify igneous rocks into two main types: intrusive or extrusive. Each of these types of rocks will have specific properties that will help you distinguish which type your igneous rock is. [2]

  • Magma is molten rock that flows beneath the earth’s surface. Igneous rocks are formed by cooling magma.
  • The location of the formation of the rock, as well as how fast the magma cools will determine the type of igneous rock.
  • Intrusive rocks form from the cooling of magma deep beneath the earths surface. Since this occurs below the earth’s surface, the magma will cool very slowly.
  • As magma cools, it will form crystals.
  • Intrusive rocks have larger crystals typically formed together to form the rock mass.
  • An example of an intrusive rock is granite.
  • When magma is above the earth’s crust, it is known as lava.
  • Extrusive rocks form through very fast cooling of lava above the Earth’s surface.
  • Extrusive rocks have very small, almost microscopic crystals. These are often described as fine-grained rocks. You usually can’t see these with the naked eye.
  • The most common type of extrusive rock is basalt.

Identify the texture type of your rock. There are 7 different texture classes for igneous rocks, each with their own unique features. [3]

  • Pegmatitic igneous rocks have very large crystals, more than 1 cm in size. These are the slowest cooling type of igneous rocks.
  • Remember, the slower a rock cools, the larger the crystals will be.
  • Phaneritic igneous rocks are composed of interlocking crystals that are smaller than crystals in pegmatitic but still visible with the naked eye.
  • Porphyritic igneous rocks have crystals of two different sizes, often with large crystals set in areas of smaller crystals.
  • Aphanitic igneous rocks have a fine-grained texture and most of their crystals are too small to see with the naked eye. You will need to use a magnifying glass to observe the crystals in aphanitic rocks.
  • Igneous rocks that form too quickly for crystals to form have what is called a glassy texture. Obsidian is the only glassy igneous rock, and can be identified by its dark color. This looks like dark black glass.
  • Vesicular igneous rocks, such as pumice, look bubbly and form before gases are able to escape as lava forms the rock. This also formed with very rapid cooling.
  • Pyroclastic igneous rock is a texture composed of volcanic fragments ranging from very fine (ash) to very coarse (tuffs and breccias). [4]

Look at the composition of your rock. Composition refers to the percentage of certain minerals in your rock. You will need a rock guide to determine what minerals are present in your rock. There are four main composition types for igneous rocks: [5]

  • Identifying the composition of your rock can be very difficult if you are not an experienced rock collector or geologist.
  • If you have any questions about how to identify a rock, contact a collector or geologist at a local college or university.
  • Felsic igneous rocks are light in color. Their mineral composition is primarily feldspars and silicates such as quartz.
  • Granite is an example of a felsic rock.
  • Felsic rocks have a low density and contain 0-15% mafic crystals. Mafic minerals are olivine, pyroxene, amphibole, and biotite.
  • Mafic igneous rocks are dark in color and consist mainly of magnesium and iron. They contain 46-85% mafic mineral crystals and are high in density.
  • Basalt is an example of a mafic rock.
  • Ultramafic igneous rocks are also dark in color and contain higher amounts of the minerals found in mafic rocks. These rocks have greater than 85% mafic mineral crystals.
  • Dunite is an example of an ultramafic rock.
  • Intermediate igneous rocks contain 15-45% mafic mineral crystals. They share minerals with both felsic and mafic rocks and are intermediate in color.
  • Diorite is an example of an intermediate rock.

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