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Igneous Rocks


§        Ignis: means ignite in Latin.

§        Comes from magma in the mantle. Lava on the surface and magma in the mantle.

§        Classified by composition and texture.

§        Composition: means what minerals it is made from.

§        Texture: What are the shape, size, arrangement and distribution of the minerals?

o       Four basic textures:

§        Glassy - Shiny, no crystals: example-obsidian

§        Fine-grained – very small crystals (need microscope to see): example - basalt

§        Coarse-grained – mid-size crystals (visible to unaided eye): example - granite

§        Porphyritic – Large crystals and small crystal (like rocky-road ice cream) – example – trachite

§        The way a rock forms influences its texture: The longer it takes to cool the larger the crystal size.

§        Extrusive Igneous rocks: form from lava – When lava cools on or near the surface. 

§        Cools quickly

§        This produces very fine or no crystals (Glassy and Fine-grained texture)

§        Basalt and Obsidian are extrusive igneous rocks. 

§        Pumice is formed at the very top of the surface and has many air bubbles because it cooled so quickly.

§        Intrusive Igneous rocks: form from magma – When magma cools deep within Earth’s surface.

§         Cools slowly

§        Coarse-grained – large crystals

§        Granite and many gemstone rocks are intrusive rocks.