Fine-Grained Igneous Texture

Form as magma cools and crystallizes
Rocks formed inside Earth are called plutonic or intrusive rocks
Rocks formed on the surface
Formed from lava (a material similar to magma, but without gas
Called volcanic or extrusive rocks

Crystallization of magma
Ions are arranged into orderly patterns
Crystal size is determined by the rate of cooling
Slow rate forms large crystals
Fast rate forms microscopic crystals
Very fast rate forms glass

Classification is based on the rock’s texture and mineral constituents
Texture
Size and arrangement of crystals

Types of igneous textures
Fine-grained – fast rate of cooling
Coarse-grained – slow rate of cooling
Porphyritic (two crystal sizes) – two rates of cooling
Glassy – very fast rate of cooling
Vesicular – contains hole left by gas bubbles
Pyroclastic – fragmented; produced by consolidation of volcanic fragments