Igneous Rocks

Fire’s Within

Why Should We Study Rocks?

• It’s like a good history book – it tells us Earth’s long history

Three Types of Rocks

• Igneous Rocks– Formed from

crystallization of molten magma

• Sedimentary Rocks– Sediment of pre-

existing rock

• Metamorphic Rocks– Changed rock

Igneous RocksParent Material

• Magma– Molten rock found at depth– Includes dissolved gases and crystals

• Lava– Magma that reaches Earth’s surface

• Crystallization– Formation of a crystalline solid from a liquid

“Ignis” is Latin for Fire

Igneous RockClassification

• Texture

• Composition

• Important characteristics that reveal a lot about the environment the rock formed.

Igneous RocksTexture

– Volcanic (extrusive)

– Plutonic (intrusive)

• Size, shape, and distribution of crystals

• Function of its cooling history

Igneous RocksVolcanic (extrusive)

• Rocks that form at the Earth’s surface.

• Cools quickly

Igneous Rock TextureExtrusive

• Vesicular– Air bubbles

Basalt PumiceObsidian

• Aphanitic– Fine grained

• Glassy– Glass like

Igneous RocksPlutonic (intrusive)

• Rocks that form beneath the Earth’s surface

• Cools slowly

Igneous Rock TextureIntrusive

• Phaneritic– coarse grained

Granite

Igneous Rock• How was this rock formed?

Dacite

• Two-stage cooling history• Contains both coarse- and fine-grained crystals -

Porphyritic

Igneous RocksMineral Composition

• Felsic

• Mafic

– light-colored rocks

– dark-colored rocks

– composed mostly of Si and Al

– i.e. granite

– composed mostly of Fe and Mg

– i.e. basalt

Igneous Rocks Naming Rocks

Origin of MagmaHow does solid rock melt?

• Heat – geothermal gradient (~25°C/km)

• Pressure – decompression melting

• Volatiles – addition of H2O

Magma FormationPartial Melting

Magma Evolution

• Magmatic Differentiation– Bowen’s Reaction Series– Assimilation– Crystal Settling– Magma Mixing

Magma EvolutionMagmatic Differentiation

• Formation of one or more secondary magmas from a single parent magma

Magmatic DifferentationBowen’s Reaction Series

• Geologist N.L. Bowen found that minerals tend to form in specific sequences of in igneous rocks

Magma EvolutionMagmatic Differentiation

• Assimilation– Incorporates surrounding foreign rock

• Crystal Settling– Heavier minerals settle at the bottom of the chamber

• Magma Mixing– One magma body intrudes another