Igneous Rocks

Science, Grade 6


Table Of Contents: Igneous Rocks

1. Formation and Classification of Igneous Rocks
Igneous rocks form from cooling magma and lava. They are classified according to their mineral origin, texture and composition.
2. Origin of Igneous Rocks
The origin of igneous rocks is either intrusive or extrusive. Cooling magma below the Earth's surface forms intrusive rocks, while erupting lava forms extrusive rocks.
3. Igneous Rock Texture
The texture of an igneous rock is related to how long it takes to cool. Slow-cooling rock forms more crystals and has coarse grain. Fast-cooling rock has fine grain.
4. Igneous Rock Composition
Dark igneous rocks, referred to as mafic, have high concentrations of calcium, iron and magnesium. Light igneous rocks are described as felsic, and they have high concentrations of silica, aluminum and potassium.
5. Basalt
Basalt is a dark igneous rock with visible crystals. It is the most common extrusive rock, forming most of the ocean's crust.
6. Obsidian
Obsidian is a dark, glassy, extrusive igneous rock. It cools so quickly that it does not have visible crystals.
7. Pumice
A lightweight rock called pumice is formed when lava froths up with gases. Pumice is the only rock that can float on water.
8. Granite
Granite is a light-colored, intrusive igneous rock with visible crystals. It is found at the core of most mountains.
9. Igneous Rock Uses
Igneous rocks have a variety of uses. Pumice is used as an abrasive, and granite is used to make porcelain. Many other types of igneous rock are used in the building industry.