What is Igneous Rock?

Image Credit: Shutterstock

Igneous rocks are formed either underground (intrusive) or above the ground (extrusive). The name igneous is derived from the Latin word ‘ignis’ meaning fire. Extrusive igneous rocks are formed from magma that cools and solidifies below the Earth's surface.

When this trapped magma cools down it turns into igneous rock. Igneous rocks that are found above the ground are formed when volcanoes erupt and throw lava above Earth’s surface. They are formed when this lava cools down.

Classification of Igneous Rocks

The characteristics of igneous rocks are based on their nature of formation. Their classification reveals information on formation conditions. The following are the main criteria for their classification:

• Texture
• Grain structure
• Vesicular
• Glassy

Composition of Igneous Rocks

There are many different compositions of magma and lava, hence, different igneous minerals. Their composition has a bearing on the color, rocks with high amount of iron tend to be dark in color while others are light colored. There are three basic types of igneous rock based on their silica composition; felsic, mafic and intermediate.

Felsic contains high amounts of silicon, aluminum, sodium and potassium. Common examples are granite and rhyolite. Mafic contains high amounts of iron, calcium and magnesium amd examples include gabbro and basalt. Finally intermediate igneous rock is a combination of felsic and mafic rock with examples being diorite and andesite.

References

• Igneous Rocks – Arizona State University