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How do igneous rocks form?

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Quick Answer

Igneous rocks are formed when magma crystallizes and solidifies. After this transition from liquid to solid, igneous rocks are classified as either intrusive or extrusive.

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In order for an igneous rock to be considered intrusive it must undergo the change from a liquid to a solid state inside of the Earth. Due to the high temperatures inside the Earth, this process can take thousands or even millions of years. This type of igneous rock tends to be much larger and has a coarse texture.

Extrusive rocks are usually found near volcanoes or fissures in the Earth's crust where magma has erupted. These tend to be much smaller and have a very smooth texture. Obsidian is one of the most well-known, and also one of the most common of the extrusive igneous rocks.

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Related Questions

  • Q:

    How does the cooling rate of igneous rocks affect crystal size?

    A:

    Igneous rocks that are allowed to cool more slowly form larger crystals, while igneous rocks that cool quickly form smaller crystals. It is simply a matter of time. The longer it takes for magma to cool, the more time is allowed for the crystals to form.

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  • Q:

    Where can igneous rocks be found?

    A:

    The most common type of igneous rock, basalt, also known as malfic rock, can be found on oceanic plates at divergent plate boundaries. Intermediate and felsic igneous rocks show up along continental margins, as well. Because igneous rocks form from the cooling of lava and magma, they can be found around areas where volcanic activity is or has been present.

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  • Q:

    What are examples of igneous rocks?

    A:

    Examples of igneous rocks include granite, pegmatite, diorite, gabbro, dunite and peridotite. Other examples of igneous rocks are kimberlite, rhyolite, quartz porphyry, dacite, latite, andesite, basalt, obsidian and pumice.

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  • Q:

    What are some examples of intrusive igneous rocks?

    A:

    Some examples of intrusive igneous rocks are granite, diorite, syenite, gabbro and pyroxenite. These rocks were formed over eons in magma chambers deep in the earth under high pressure and high temperatures.

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