§ 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 Earths surface.
§ Cools slowly
§ Coarse-grained large crystals
§ Granite and many gemstone rocks are intrusive rocks.