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?

       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.