Igneous rocks are formed when melted lava or magma cools and crystallizes, and their unique traits are based on this process. They are strong because their mineral grains grow together tightly as they cool, and their minerals are usually black, white, or gray. They have a texture similar to something baked in an oven, like black bread or peanut brittle.
Some of the main minerals in igneous rocks are feldspar, quartz, olivine and mica. The size of the minerals depends on the depth of the magma that formed the rock. Deeper magma cools more slowly and forms larger crystals. Rocks that cool over a few months have microscopic mineral grains and are called extrusive. Rocks that cool over thousands of years have small to medium grains and are called intrusive. Rocks that cool over millions of years have large pebble sized grains and are called plutonic.
Granite and basalt make up the majority of igneous rocks. Basalt is dark and fine-grained with minerals rich in magnesium and iron. It is either extrusive or intrusive and is the primary rock on the ocean floor. Granite is light and coarse-grained and rich in feldspar and quartz. It is plutonic and less dense than basalt. Granite is found nearly everywhere beneath the continents.
The word "igneous" comes from the Latin word "fire" and is related to the melting process that forms these rocks.