Igneous Rocks Rocks that begin as a hot fluid material called magma which cools and crystalizes to form a rock. Igneous is Latin for Fire.

Two Types of Igneous Rocks Intrusive Igneous Rocks are formed when magma does not make it to the surface and cools in side channels of volcanoes or deep in the Earth’s crust. Extrusive Igneous Rocks are formed outside of the Earth’s crust by Lava from volcanoes or cracks in the ocean floor.

Intrusive Igneous Rocks Intrusive rocks form when the magma cools inside the Earth’s crust and never makes it to the surface.

Course grain texture Intrusive Rocks cool very slowly inside the Earth’s crust, so the grains grow large and easy to see. Like cake rising in an oven. See the course grains of white, gray, and black

Extrusive Igneous rocks Form when the magma inside the Earth comes out onto the surface of the crust as lava. It can come out of volcanoes or large cracks in the Earth’s crust at the bottom of our Oceans. It cools very fast and has very fine grains.

Fine grain texture Extrusive Rocks cool much faster on top of the Earth’s crust, so the grains do not get time to grow and are very fine. You can not see individual shapes. Fine grains of Basalt rocks:

Minerals in Igneous rocks Igneous rocks are the oldest rocks on the Earth’s crust and are made up of minerals and other things. The more silica in the minerals of the rock the lighter color of the rock. Granite is light colored so it has a lot of silica, Basalt is dark so it is low in silica.

Uses for igneous rock Because igneous rock is usually very hard it was first used for tools and weapons, such as arrowheads and hammers. Granite makes a good building material and statues. Basalt makes good paving stones.