CH. 3 – IGNEOUS ROCKS Ignis = “fire”
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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