CH. 3 – IGNEOUS ROCKS

Ignis = “fire”

Igneous Rocks

Cooling & crystallization of magmaLava = molten rock at surface

Igneous Rocks

Classified by texture & composition

Texture = crystal size of mineralsComposition = minerals present

Texture of Igneous Rocks

Depends on magma’s cooling rate1) Cools slowly- crystals are (large, small)2) Cools quickly

- crystals are (large, small)

Crystal sizes

Texture (Crystal Size)

Two environments where magma cools:

1) Deep underground2) At surface (or shallow burial)

Types of Texture

1) Fine-grained- crystals invisible to naked eye- magma cooled _________- extrusive/volcanic

Fine-grained textures

Types of Texture

2) Vesicular- fine-grained w/ many holes in it

Types of Texture

3) Coarse-grained- crystals easily visible- magma cooled __________- intrusive/plutonic

Coarse-grained texture

Types of Texture

4) Porphyritic- two size crystals in same rock

Large crystals = phenocrystsSmall crystals = groundmass

Basalt porphyry

Porphyritic Texture

2-stage cooling process:1st stage = slow cooling at depth

__________ is formed2nd stage = rapid cooling of

remaining magma____________ is formed

Types of Texture

5) Glassy- no crystalline structure (magma

cools too fast)Ex: obsidian; pumice

Glassy Texture

Igneous Rock Classification

Composition = minerals presentMostly silica with varying amounts

of other six elements (Fig 2.16)

Crustal Composition

Igneous Compositions

Mafic- 45-50% silica (“silica-poor”)- rich in magnesium, iron (Fe), &

calcium- dark-colored: black, dark gray,

brownish-red

Igneous Compositions

Felsic (feldspar & silica)- 60-75% silica (“silica rich”)- rich in aluminum, potassium, &

sodium- light-colored: light gray, pinkish

Igneous Compositions

IntermediateFine-grained: medium grayCoarse-grained: black & white

crystals

Importance of Igneous Rocks

Compose 90% of Earth’s crustTwo types of crust:

Continental crust = _________Oceanic crust = _________

Bowen’s Reaction Series

Sequence of minerals crystallizing from magma (Fig. 3.13)

First minerals to form = _______ Last minerals to form = _______

Bowen’s Reaction Series

Bowen’s Reaction Series

As rock is heated:First minerals to melt = ______Last minerals to melt = ______

Important Concepts

A mineral is stable if the environmental conditions are similar to where it formed

Ex: temperature, pressure, water- content

Stable

Ex:Mafic minerals are stable under

____ temperaturesFelsic minerals are stable under

_____ temperatures

Important Concepts

If a mineral is put under different environmental conditions, it becomes unstable

- mineral reacts to new environment by changing

New Environmental Conditions:

If at the surface, weathering occurs

If within the crust, metamorphism occurs