Igneous Rocks Rocks that crystallized directly from a melt

Igneous rocks 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

Igneous rocks Crystallization of magma Fractionation: mineral separation in a magma chamber based on crystallization temperatures 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

Igneous rocks Classification is based on the rock's texture and mineral constituents Texture Size and arrangement of crystals Types 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

Classification of igneous rocks

Mafic magma can become felsic magma as it cools Mafic magma can become felsic magma as it cools! Bowens Reaction Series tells us that: As a magma chamber cools, the mafic minerals crystallize first, then felsic ones As minerals crystallize out, the gas is left behind in the felsic melt…

The main factor contributing to crystal size is rate of cooling The main factor contributing to crystal size is rate of cooling! Textures based on crystal size: Aphanitic: almost no visible crystals Porphyritic: large crystals in a matrix of smaller crystals Phaneritic: interlocking large crystals Vesicular: full of holes (vesicles) Glassy: resembles glass Pyroclastic: made up of fragments (often ash and pumice)

Intrusive (Plutonic) Mafic- roots of plutons Gabbro Intermediate- near the base of plutons Diorite Felsic- most plutons (very common!) Granite

Extrusive (volcanic) Mafic- shield volcanoes, cinder cones, ocean basalts Basalt Scoria Intermediate- composite cones Andacite Dacite (more felsic) Felsic- domes and pyroclastic deposits Rhyolite Tuff Obsidian and pumice

Fine-grained igneous texture

Course-grained igneous texture

Porphyritic igneous texture

Obsidian exhibits a glassy texture