Igneous rocks form when hot, melted rock (magma or lava) cools and hardens. If the rock cools underground, it forms intrusive igneous rocks like granite, which have large crystals. If it cools above ground after a volcano erupts, it forms extrusive igneous rocks like basalt or pumice, which have small or no crystals. These rocks are very strong and make up most of Earth’s crust. They are used in buildings, roads, and even cleaning products like pumice stones.
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Igneous Rocks
Igneous Rocks
Igneous rocks were the first rocks to form on Earth over four billion years ago. In the beginning, the new planet was so hot that its surface was liquid lava. As Earth cooled, the lava solidified into igneous rock. Today, cooling magma and lava continue to form igneous rocks.
While lave is liquid rock found at the Earth’s surface, magma is liquid rock found underground. Magma exists in the hottest places in Earth's crust, near plate boundaries and hotspots. Hotspots are places where the mantle is exceptionally hot. Volcanoes form over the hotspots. The islands of Hawaii are examples of volcanoes formed over hot spots.
Magma under the surface of the Earth cools incredibly slowly. It can take thousands to millions of years for igneous rock to form deep within the Earth's crust. As the magma cools, crystals form. Since the magma takes so long to cool, the crystals have a long time to grow. Scientists call igneous rocks that form underground intrusive igneous rocks. Intrusive igneous rocks have a coarse-grained texture because their large crystals are easy to see. Examples of intrusive igneous rocks include granite, diorite, and pegmatite.
Lava can erupt from a volcano or ooze out of the fissures near volcanoes and plate boundaries. At the surface of the Earth, lava cools quickly. As a result, crystals do not have much time to form, so they are tiny. Sometimes, lava cools so fast that no crystals form. Obsidian is an example of an igneous rock without crystals. Scientists call igneous rocks that form at the surface of the Earth extrusive igneous rocks. Examples of extrusive igneous rocks include basalt and rhyolite.
Like sedimentary and metamorphic rocks, igneous rocks can be weathered by wind, water, plants, and animals. The sediments created from the weathering form layers that, over millions of years, become sedimentary rock. If an igneous rock is forced deep underground, the intense heat and pressure will transform it into metamorphic rock. Even farther underground, the igneous rock will melt and become magma again. The rock cycle describes how rocks change form as they move through the environment.
Igneous Rocks Videos
I like to use videos to teach new concepts because students can use visual cues to support their learning. Here are some of my favorite videos on igneous rocks.
The Geosphere
Middle School Science
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