Chapter 6 (Part II

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Chapter 6 – IGNEOUS ROCKS
How, Why &
Where Rocks
Melt
•
•
•
Begins as solid
Molecules warm &
begin vibrating =
softening
Molecules may
vibrate violently
enough to break
bonds
How, Why &
Where Rocks
Melt
• Heat & pressure inside
Earth
– Magma chamber
– Geothermal gradient
• Continental vs. ocean crust
• Composition varies melting point
• Pressure increases melting point
• Water decreases melting point
How, Why & Where
Rocks Melt
• Heat and pressure inside Earth
– Fractional melt (partial melting)
• Mix of molten & solid rock
– 1 magma body may
produce several different
igneous compositions
How, Why &Where
Rocks Melt
• Magma (Intrusive)
– Molten rock below Earth’s
surface
• Lava (Extrusive)
– Molten rock when it reaches
Earth’s surface
How, Why & Where Rocks Melt
• Magma & Lava
– Composition
• Silica content varies (~ 45-75%)
• Water vapor & carbon dioxide
– Temperature
• Temperature varies (~ 750°C –
1200°C)
– Viscosity (resistance to flow)
• Varies in ability to flow
• Influenced by silica content &
temperature
How, Why &
Where Rocks
Melt
• Tectonic setting
– Characteristics influenced by location
• Oceanic, divergent margins
– Hot, low viscosity basaltic lava
• Subduction (convergent) zones
– Cooler, viscous lavas with more silica
• Ocean hot spots
– Hot & basaltic; build giant shield volcanoes
• Continental hot spots
– Cooler & granitic; high silica lava
Cooling and Crystallization
• Crystallization
– Process where mineral grains
form & grow in cooling magma
(or lava)
– Classified based on:
1. Texture (size of mineral
crystals)
– Volcanic (extrusive) = small
grains due to rapid cooling
– Plutonic (intrusive) = large
grains due to slow cooling
2. Composition (silica content)
Rate of Cooling
• Extrusive Textures
– Glassy
• Cools too rapidly to
form crystals
• Example: obsidian
Rate of Cooling
• Extrusive Textures
– Aphanitic
• Fine grained (small crystals)
• Examples: basalt, andesite,
rhyolite
Rate of Cooling
• Extrusive Textures
– Vesicular
• Form from trapped
gas bubbles
• Examples: pumice, scoria
Rate of Cooling
• Extrusive Textures
– Pyroclastic or fragmental
• Includes rock fragments
• Example: volcanic tuff
Rate of Cooling
• Intrusive Texture
– Phaneritic
• Course grained (large
crystals); slow cooling
inside Earth
• Examples: granite, syenite,
diorite, gabbro, peridotite
Chemical
composition
• Igneous rocks
subdivided into 4
categories based on
silica content
–
–
–
–
Felsic
Intermediate
Mafic
Ultramafic
Igneous Rock Classification
Common Igneous Compositions
Composition
Type
Felsic
Intermediate
Mafic
Ultramafic
% Silica
Other
Elements
Magma
Viscosity
Temperature
crystallization
begins
>65%
Al, K, Na
High
600-800C
55-65%
Al, Ca,
Na, Fe,
Mg
45-55%
Al, Ca,
Fe, Mg
<40%
Mg, Fe,
Al, Ca
Medium
Low
Very low
800-1000C
1000-1200C
>1200C
Igneous
Rocks
Produced
Type of
Igneous
Rock
Granite
Plutonic
Rhyolite
Volcanic
Diorite
Plutonic
Andesite
Volcanic
Gabbro
Plutonic
Basalt
Volcanic
Peridotite
Plutonic
Komatiite
Volcanic
Igneous Rock Classification
Fractional
Crystallization
• Crystals separate from
liquids during
crystallization
– Bowens reaction
series
– Predictable melting
& cooling of minerals
Plutons and Plutonism
• Plutons
– Any body of intrusive igneous rock, regardless of size or shape
• Massive vs.
Tabular
• Concordant vs.
Discordant
Plutons &
Plutonism
• Batholith
– Large, irregular shaped pluton
– Massive &
Discordant
• Laccolith
– Mushroomshaped
pluton
– Massive &
Concordant
Plutons and
Plutonism
• Dikes
– Magma squeezes into cross
cutting fracture & solidifies
– Tabular & Discordant
• Sills
– Magma intrudes between 2
layers; parallel to layers
– Tabular & Concordant
Plutons &
Plutonism
• Volcanic pipe = remnant
• Volcanic neck = remnant
exposed via erosion
Economics of Igneous
•Rocks
Uncommon uses:
–
–
–
–
–
Mining
Pumice stone
Lava soap
Fingernail files
Surgical tools

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