chapter 6 – igneous rocks. how, why & where rocks melt begins as solid molecules warm &...
TRANSCRIPT
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
• Heat and pressure inside Earth– Fractional melt (partial
melting)• Mix of molten & solid rock
– 1 magma body mayproduce several different igneous compositions
How, Why & Where Rocks Melt
How, Why &Where Rocks Melt• Magma (Intrusive)
– Molten rock below Earth’s surface
• Lava (Extrusive)– Molten rock when it
reaches Earth’s surface
• 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
How, Why & Where Rocks Melt
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 % Silica Other
ElementsMagma
Viscosity
Temperature crystallization
begins
Igneous Rocks
Produced
Type of Igneous
Rock
Felsic >65% Al, K, Na High 600-800CGranite Plutonic
Rhyolite Volcanic
Intermediate 55-65%Al, Ca, Na, Fe,
MgMedium 800-1000C
Diorite Plutonic
Andesite Volcanic
Mafic 45-55% Al, Ca, Fe, Mg Low 1000-1200C
Gabbro Plutonic
Basalt Volcanic
Ultramafic <40% Mg, Fe, Al, Ca Very low >1200C
Peridotite Plutonic
Komatiite Volcanic
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– Mushroom-
shaped 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