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"To the dull mind all nature is leaden. To the illumined mind the whole world burns and sparkles with light." -- Ralph Waldo Emerson

Minerals and Rocks
Igneous Rocks Table of Contents:

See some Igneous rocks

Go to USGS

Go to the Volcano World Igneous Rock Lesson...

 

Formed from cooled magma or lava

- most common type

- old name: volcanic

Magma

From 60 to 200 km below surface, at temperatures of 1,400° C, rock may be molten, i.e., liquid

Because:
  • The pressure from above is tremendous
  • Radioactive decay causes heat


 Lava

  • Is what we call magma that reaches the surface, usually via volcano.

     

Intrusive igneous rocks

  • Are those igneous rocks that are formed by the cooling of magma beneath the earth's surface.

 Slower cooling forms relatively larger crystals

  • They are exposed on the surface only after erosion, or being pushed up by forming forces.

Examples: granite, gabbro, diorite

Extrusive igneous rocks

Are those igneous rocks that are formed by the cooling of magma above the earth's surface

    - Lava
  •  Faster cooling forms relatively smaller crystals (or no crystals, in the case of obsidian, etc.)
  • Examples: obsidian, basalt, rhyolite, pumice and what is commonly called lava
    •

    CLASSIFICATION OF IGNEOUS ROCKS

    Type of Magma

    Is based on the actual minerals that make up the magma that was the source for the rock.        [CLICK to see a Table]

     

    Basaltic

    Dense, heavy, dark-colored rich in iron and magnesium

    Andesitic

    In-between, various combinations

    Granitic

    Light-colored, lower density, silicon and oxygen rich - associated with violent volcanic eruptions