Igneous Rock


There are two kinds of igneous rocks. If the rock forms from cooled lava on the surface of the earth, it is called extrusive igneous rock. If it forms from magma cooling while it is still beneath the surface, it is called intrusive igneous rock.

Intrusive igneous rocks are hardened magma. When they form underground, the earth acts like a blanket so the magma cools slowly, over thousands of years. When the rocks cool slowly they have time to form large crystals. These show up as grains in the rock that can be seen with the naked eye. Intrusive igneous rocks are usually medium or large grained.

Granite is an excellent example of an intrusive igneous rock. Granite is a hard rock that is often used in buildings. It will have at least some flecks of shiny clear or whiteish quartz mineral crystals. It will also have many grains of the mineral feldspar, which can be white, pink, gray, or black.

Click on this picture of quartz to take a closer look at its grain. You can click on that picture to come back here.

Extrusive igneous rocks are hardened lava. When lava is exposed to air or water, it cools very quickly and can only form small grains of crystals. Extrusive igneous rocks are usually fine-grained, which means the flecks of individual minerals are very small, often too small to see.

Obsidian is a kind of extrusive igneous rock. It is glassy and dark colored. It may look like one large crystal, but it can break in curves and true crystals can't. If forms sharp edges and it was used to make arrow and spear heads (like the ones seen at the bottoms of these pages for moving around).


Basalt is a common example of extrusive igneous rock. It is the most common rock in the earth's crust! It is grey or black but may turn reddish due to rust from minerals that contain iron. The individual mineral crystals are usually too tiny to see.


Pumice is another form of extrusive igneous rock, or hardened lava. Pumice forms when the lava cools so quickly that gas bubbles get trapped inside. This can happen when the lava is flung into the air from an volcanic explosion. So much air gets trapped in the rock that it can float!

Click on the player to see a video of pumice floating in water.

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