Igneous Rocks

Igneous rocks are formed from lava or magma. Magma is molten rock that is underground and lava is molten rock that erupts out on the surface. The two main types of igneous rocks are plutonic rocks and volcanic rocks. Plutonic rocks are formed when magma cools and solidifies underground. Volcanic rocks are formed from lava that flows on the surface of the Earth and other planets and then cools and solidifies.

The texture of an igneous rock depends on the size of the crystals in the rock. This tells us if the rock is plutonic or volcanic. When magma cools underground, it cools very slowly and when lava cools above ground, it cools quickly. When magma and lava cool, mineral crystals start to form in the molten rock. Plutonic rocks, which cool slowly underground, have large crystals because the crystals had enough time to grow to a large size. Volcanic rocks, which cool quickly above ground, have small crystals because the crystals did not have enough time to grow very large.

The type of igneous rock is also dependent on its composition (the elements that are present). There are many different compositions of magma and lava. Fortunately, most igneous rocks are one of three basic compositions:

• Felsic igneous rocks contain relatively high amounts of silicon, sodium, aluminum, potassium (Si, Na, Al, and K) and relatively low amounts of iron, magnesium, and calcium (Fe, Mg, Ca).
• Mafic rocks contain relatively low amounts of silicon, sodium, aluminum, potassium (Si, Na, Al, and K) and relatively high amounts of iron, magnesium, and calcium (Fe, Mg, Ca).
• Intermediate rocks are what their name sounds like. Their composition is in-between mafic and felsic rocks.

The different elements present in the different igneous compositions will form different minerals. Rocks with high amounts of iron (Fe) tend to form minerals that are dark in color (such as olivine and pyroxene). As result, mafic rocks tend to be dark in color and felsic rocks tend to be lighter in color. An example of a mafic rock is basalt, the black rock that forms from lava flows in places like Hawaii. An example of a felsic rock is granite, the light colored rocks that we find in places like the Sierra Nevada mountains in California.

Image: Basalt, Granite Biotite Hornblende, Rhyolite, Gabbro, and Diorite