Download magma or lava

GEOL: CHAPTER 4
Igneous Rocks and
Intrusive Igneous Activity
Learning Outcomes
• LO1: Describe the properties and behavior of
magma and lava
• LO2: Explain how magma originates and
changes
• LO3: Identify and classify igneous rocks by
their characteristics
• LO4: Recognize intrusive igneous bodies, or
plutons
• LO5: Explain how batholiths intrude into
Earth’s crust
Igneous Rocks
• Molten rock (magma or lava) that cools
and crystallizes to form minerals
• Intrusive: underground, magma, plutons
• Extrusive: above ground, lava, volcanic
eruptions
• Large parts of continents
• All of oceanic crust
Magma and Lava
• Magma: molten rock below surface
• Less dense than surroundings and
wants to rise
• Most solidifies underground: plutons
• Lava flows: when magma reaches
the surface
• Volcanic rocks = extrusive igneous
rocks
– Lava flows
– Pyroclastic materials
Composition of Magma
• Silicate rocks usually the source
• Silica is primary constituent
• Other constituents:
– Aluminum
– Calcium
– Sodium
– Iron
– Magnesium
– Potassium
Three Types of Magma
• Felsic magma
– >65% silica
– Considerable sodium, potassium, aluminum
– Little calcium, iron, magnesium
• Mafic magma
– <52% silica
– Silica poor
– Proportionally more calcium, iron, magnesium
• Intermediate magma
– Composition between felsic and mafic magma
Magma/Lava Temperatures
• Lava usually 700ºC to 1,200ºC
• Magma hotter, but can’t measure reliably
• Mafic lava nonexplosive, easier to
measure
• Felsic more explosive, harder to
measure
• New igneous rocks take years or
millennia to cool
Viscosity
• Resistance to flow
• Higher temperatures reduce viscosity
– Hotter magma/lava moves more readily
• Increased silica content increases
viscosity
– Mafic lavas flow far
– Felsic lavas don’t flow far
• Higher amounts of dissolved gases
reduce viscosity
Origination of Magma
• Can be 100-300 km deep
• Usually shallower: upper mantle and
lower crust
• Accumulates in magma chambers
• Some magma cools: plutons
• Some rises through surface: volcanic
Bowen’s Reaction Series
• Minerals crystallize from cooling magma
in a predictable sequence
• Discontinuous branch
• Continuous branch
• Crystallization occurs on both branches
simultaneously
• Continued crystallization changes the
composition of the melt
Bowen’s Reaction Series, cont.
Discontinuous branch
• Ferromagnesian silicates only
• One mineral changes to another over
specific temperature ranges
• Olivine to pyroxene to amphibole to
biotite
• Reactions often incomplete, so can
have all ferromagnesian silicates in one
rock
Bowen’s Reaction Series, cont.
Continuous branch
• Plagioclase feldspar silicates only
• Calcium-rich plagioclase crystallizes
first
• Then increasing amounts of sodium are
incorporated until all sodium and
calcium are gone
• Rapid cooling gives calcium-rich core
surrounded by zones of increasingly
rich sodium
Calcium-rich
plagioclase
Olivine
Types of
magma
Reaction
Mafic
(45–52% silica)
Pyroxene
(augite)
temperature
Decreasing
Reaction
Amphibole
(hornblende)
Intermediate
(53–65% silica)
Reaction
Biotite
mica
Sodium-rich
plagioclase
Potassium
feldspar
Felsic
(>65% silica)
Muscovite
mica
Quartz
Stepped Art
Fig. 4-3, p. 69
Magma at Spreading Ridges
• Geothermal gradient: 25ºC/km
• Lower pressure at ridges allows
melting
• Ultramafic rocks undergo partial
melting
– Release more silica-rich minerals
(Bowen’s reaction series)
– Create mafic magma
Magma at Subduction Zones
• Volcanoes and plutons near leading
edge of overriding plate
• Partial melting at depth
– Releases water from hydrous minerals
– Water rises and enhances melting
– Mafic rocks melt, creating intermediate
and felsic magma
Hot-Spot Magma
• Interior portions of plates
• Mantle plumes: rising magma from
the core-mantle boundary
• Creates volcanoes
– Hawaiian Islands
Changing Magma Composition:
Crystal Settling
• Physical separation of minerals by
crystallization and settling
• Olivine, first formed, denser than
magma, so it sinks
• Makes remaining magma less mafic,
more felsic
Changing Magma Composition:
Assimilation
• Magma reacts with country rock
• Country rock melts and changes
composition of magma
• Inclusions of incompletely melted
country rock
Changing Magma Composition:
Magma Mixing
• A volcano can erupt lavas of different
composition
• Some of these magmas mix, which
changes composition
Igneous Rock Textures
• Mineral appearance
• Size most important
– Cooling rate of magma or lava
• Shape
• Arrangement
Aphanitic Texture
• Rapid cooling
• Mineral nuclei form faster than
mineral growth
• Fine-grained
• Lava flows: extrusive
Phaneritic Texture
• Slow cooling
• Magma underground
• Mineral growth faster than nuclei
formation
• Coarse-grained
• Plutons: intrusive
Porphyritic Texture
•
•
•
•
•
Minerals of markedly different sizes
Phenocrysts = large minerals
Groundmass = small minerals
Complex cooling history
Porphyry
Glassy Texture
• Lava
• Very rapid cooling
• No ordered 3-D framework of
minerals
• Natural glass
Vesicles
• Magma can contain water vapor and
other gases
• Gasses trapped in cooling lava
• Vesicular: many small holes from
gases
Pyroclastic Texture
• Also called fragmental texture
• Explosive volcanic activity
• Consolidated ash from eruptions
Phenocrysts
Stepped Art
Fig. 4-7, p. 73
Classifying Igneous Rocks
• Texture
– Aphanitic to Phaneritic
• Composition
– Ultramafic <45% silica
– Mafic 45% to 52% silica
– Intermediate 53%-65% silica
– Felsic >65% silica
Ultramafic Rocks
•
•
•
•
•
•
<45% silica
Mostly ferromagnesian silicates
Darker minerals: dark rocks
Peridotite: mostly olivine
Pyroxenite: mostly pyroxene
Komatiites: very old lava flows
Basalt-Gabbro
• Mafic magma: 45% to 52% silica
• Basalt: aphanitic, lava flows
• Gabbro: phaneritic, lower part
oceanic crust
• Large proportion ferromagnesian
silicates
• Dark color
Andesite-Diorite
• Intermediate magma: 53%-65% silica
• Andesite: aphanitic, convergent plate
boundary volcanoes
• Diorite: phaneritic, in crust
• Plagioclase feldspar with amphibole
or biotite
Rhyolite-Granite
• Felsic magma: >65% silica
• Rhyolite: aphanitic, uncommon,
explosive eruptions
• Granite: phaneritic, most common
intrusive rock
• Potassium feldspar, sodium-rich
plagioclase, quartz
Pegmatite
• Texture, not composition
• Typically granitic composition
• Minerals at least 1 cm across
Other Extrusive Igneous Rocks
•
•
•
•
•
Volcanoes erupt fragmental material
Ash: <2 mm
Tuff
Rhyolite tuff
Welded tuff
Other Extrusive Igneous Rocks,
cont.
• Volcanic glass
• Obsidian
– Color varies
– Conchoidal fracture
• Pumice
– Vesicular, floats
• Scoria
– Vesicular
Plutons
• Magma cools below the surface
• Exposed at surface through uplift and
erosion
• Geometry
– Tabular
– Cylindrical
– Irregular
• Concordant: boundaries parallel to
country rock
• Discordant: boundaries cut across
country rock
Pluton Types
•
•
•
•
•
•
Dikes
Sills
Laccoliths
Volcanic pipes and necks
Batholiths
Stocks
Dikes, Sills, Latholiths
• Dike:
– Discordant
– Up to 100 m thick
– Intrude into fractures
• Sill:
– Concordant
– Often intrude sedimentary rocks
• Laccolith:
– Inflated sill, domed upward
Volcanic Pipes and Necks
• Volcano pipe: central conduit of
volcano
• Volcanic neck:
– Hardened magma of volcanic pipe
– Exposed through erosion
Batholiths and Stocks
•
•
•
•
•
•
Batholith: 100 km2 or larger
Stock: smaller
Mostly discordant
Usually granitic
Near convergent plate boundaries
Mineral resources
How Batholiths Intrude Crust
• Forceful injection:
– Rises slowly
– Forces aside country rock
– Some country rock fills in underneath
• Stoping:
– Rising magma detaches and engulfs
country rock