IN OR OUT? IGNEOUS ROCKS
Igneous rocks are formed by the cooling and hardening of melted rock material. Students will learn how rate of cooling affects what type of rock forms.
Students will be introduced to igneous rocks, how they form, what the different types are and how to identify them.
Students will be able to:
30 minute setup
The term igneous comes from the Latin ignis, meaning "fire". Igneous is used to describe rocks that crystallize out of hot molten material in the Earth called magma. When magma pushes up through Earth's crust to the surface, it is called lava. Both magma and lava cool and harden to form igneous rocks.
Intrusive vs. Extrusive
Intrusive rocks come from magma. They cool slowly deep in Earth's crust. When magma cools underground, the crust acts like a blanket, insulating it, keeping it warm longer. Because the magma cools slowly, crystals of different minerals have time to grow. The molecules in the magma have time to arrange themselves into crystal formations before the magma hardens. Intrusive rocks have large crystals that can be seen with the naked eye. A common example of an intrusive igneous rock is granite.
Extrusive igneous rocks come from lava. Lava, at the surface, is exposed to air and water which causes the molten rock to cool rapidly. Solidifying rocks at the surface cool too quickly for large crystals to form. Molecules in the lava do not have time to arrange themselves to form large crystals. Extrusive rocks have crystals that are too small to see without magnification. A common example of an extrusive igneous rock is basalt. Some extrusive rocks, such as obsidian and pumice, cool so rapidly that they completely lack crystal structure and are considered a volcanic glass. Pumice is just like obsidian except it is tiny shards of glass.
The mineral proportions of the rock are what allows geologists to classify rocks chemically. Depending on the proportion of light minerals to dark minerals, igneous rocks can be broken into four main types: felsic, intermediate, mafic and ultramafic. The following list gives more information about igneous chemical categories. This information is provided for you, the teacher, to better understand igneous rocks.
What type of igneous rocks were formed? How did cooling rate affect the size of crystals formed? Do you think this process really happens in magma? In lava?
For teachers interested in rock identification, the following is a suggestion of how you can set up a rock-ID activity. For information about how to identify igneous rocks, refer to the above definitions and the United States Geologic Survey's (USGS) web site for igneous rocks, https://wrgis.wr.usgs.gov/docs/parks/rxmin/rock.html.
For pictures of common rock-forming minerals, refer to https://wrgis.wr.usgs.gov/docs/parks/rxmin/mineral.html#feldspar.htm. Using this information and the rocks contained in the Rocks and Minerals Discovery Chest, pass around the rocks to small groups and have them identify the rocks based on texture, mineral content and chemical composition.
Included National Parks and other sites:
Black Canyon of the Gunnison
Utah Science Core:
2nd Grade Standard 6 Objective 1, 2, 3
Last updated: February 24, 2015