Igneneous Rocks

Checkpoint: Igneous Rocks

Magma is defined as molten rock above or below the ground. When it reaches the surface, it is called lava. A majority of the magma will never reach the surface and rather stays underground in a magma chamber and crystallizes and hardens into an igneous rock (Murck, B. W., Skinner, B. J., & Mackenzie, D., 2008).

Extrusive and intrusive rocks are two different types of igneous rocks. Extrusive rocks form at the surface after an eruption occurs from the lava that flows out and onto the surface of the Earth. The lava cools and solidifies so fast that it does not have time to form and therefore the mineral grains are very small and can only be seen under magnification. Intrusive rocks are formed from the trapped magma underground that will eventually crystallize. Intrusive rock does not cool at rapid speeds, allowing it to form mineral grains big enough to be seen with the naked eye.

Igneous rocks can be further classified into three groups depending on their silica content: felsic rock, mafic rock and ultramafic rock. Felsic rocks are light in color containing mostly light colored minerals. Mafic rocks are darker in color with lesser amounts of silica. Ultramafic rocks are denser than mafic rocks. Fractional crystallization happens when mineral grains separate from the melt from which they are crystallizing. This can happen when the crystals sink to the bottom, float to the top or through filter pressing. Since there is a combination of different compositions, cooling rates, and fractional crystallization, this will cause there to be many different types and forms of igneous rocks.

References

Murck, B. W., Skinner, B. J., & Mackenzie, D. (2008). Visualizing geology. Hoboken, NJ: John Wiley & Sons.

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