Download Igneous Rocks

Lecture Outlines
Physical Geology, 15/e
Plummer, Carlson & Hammersley
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Igneous Rocks, Intrusive
Activity, and the Origin of
Igneous Rocks
Physical Geology 15/e, Chapter 3
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The Rock Cycle
Rock – a naturally formed, consolidated
material usually composed of grains of one or
more minerals
Rock cycle – shows how one type of
rocky material gets transformed into another
• Representation of how rocks are formed,
broken down, and processed in response to
changing conditions
• Processes may involve interactions of
geosphere with hydrosphere, atmosphere
and/or biosphere
• Arrows indicate possible process paths
within the cycle
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The Rock Cycle and Plate Tectonics
Convergent Boundary – magma is
Convergent plate boundary
created by melting of rock at a convergent
boundary/subduction zone.
• less dense magma rises and cools to form
igneous rock
• igneous rock exposed at surface gets
weathered into sediment
• sediments transported to low-lying areas,
buried and hardened into sedimentary rock
• sedimentary rock heated and squeezed at
depth to form metamorphic rock
• metamorphic rock may heat up and melt at
depth to form magma
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Igneous Rocks
Igneous rocks form when
magma cools and solidifies
• intrusive igneous rocks form
when magma solidifies
underground
• extrusive igneous rocks form
when magma solidifies at the
Earth’s surface (lava)
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Classification of Igneous Rocks
• Texture – the rock’s appearance with respect to
the size, shape and arrangement of grains or other
constituents. Crystal size is determined by the rate
of cooling of the magma.
• Intrusive – formed deep underground and
typically cools slowly
• Extrusive – formed at or near the Earth’s
surface and cools quickly.
• Chemical Composition – mineral content
indicates origin and evolution of the magma
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Igneous Rock Textures
Crystalline Textures
•Fine Grained or Aphanitic – crystals
are too small to see easily with the
naked eye. Magma cooled quickly at
or near the surface
Fine-grained (Aphanitic) igneous rock
•Coarse Grained or Phaneritic crystals are large enough to see with
the naked eye. Magma cooled slowly.
Coarse-grained (Phaneritic) igneous rock
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Igneous Rocks Textures
Crystalline Textures (cont.)
• Pegmatitic– extremely coarse–grained
(most crystals >5 cm), formed when magma
cools very slowly at depth
Pegmatitic Texture
• Porphyritic – includes two distinct crystal
sizes, with the larger phenocrysts having
formed first during slow cooling underground
and the smaller groundmass forming during
more rapid cooling at the Earth’s surface
Porphyritic Texture
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Igneous Rocks Textures
Obsidian - Glassy Texture
Pumice – Vesicular Texture
Glassy Textures
•Glassy – contains no crystals at all, and is
formed by extremely rapid cooling of the
magma.
Textures Due to Trapped Gas
•Vesicular – contains cavities (vesicles) in
extrusive rocks resulting from gas bubbles that
were in the lava. Scoria and Pumice are
examples.
Fragmental Texture
•Pyroclastic – consolidated pyroclastic debris
such as ash, pumice or crystalline rock. Tuff and
Volcanic Breccia are examples
Tuff – Pyroclastic Texture
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Chemistry of Igneous Rocks
Rock chemistry, particularly silica (SiO2) content, determines
mineral content and general color.
• Felsic (silicic) rocks – >65% silica by weight, and contain lightcolored minerals that are abundant in silica, aluminum, sodium
and potassium. Rhyolite and Granite are examples.
• Intermediate rocks – silica contents between 55% and 65% by
weight. Diorite and Andesite are examples.
• Mafic rocks – silica content between 45% and 55% by weight,
contain dark-colored minerals that are abundant in iron,
magnesium and calcium. Gabbro and Basalt are examples.
• Ultramafic rocks – <45% silica, by weight, and composed almost
entirely of dark-colored (black/green) ferromagnesian minerals.
Peridotite and Komatite are examples.
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Igneous rock identification
Identification is based on
texture (grain size) and
mineral composition
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How Magma Forms
Heat for Melting Rock
• Most heat comes from the core by
conduction or convection through the
mantle and crust.
The Geothermal Gradient and
Partial Melting
• The geothermal gradient is the rate at
which temperature increases with
increasing depth but it is never high
enough to cause rock to melt because
melting pints of minerals generally
increase as pressure increases.
Melting will occur by a reduction in the
melting point by the presence of water.
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How Magma Forms
Decompression Melting
• Melting occurs when rising mantle rock is subject to lower
melting points as the pressure is reduced
Addition of Water (Flux Melting)
• Water becomes increasingly reactive at higher temperatures
• At sufficient pressures and temperatures, highly reactive water vapor can
reduce the melting point of rocks by over 200°C
Decompression Melting
Flux Melting
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How Magmas of Different Compositions Evolve
A large variety of igneous rocks indicate that there are a large
variety of magma compositions.
Sequence of Crystallization and Melting
Bowen’s Reaction Series - Minerals crystallize in a predictable
order, over a large temperature range.
Bowen’s Reaction Series
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How Magmas of Different Compositions Evolve
Differentiation – process by which different ingredients separate
from an originally homogenous mixture.
•Crystal Settling – changes the magma composition as the crystals
are removed from the melt as they settle downward.
Partial Melting – process by which the magma composition
varies as different minerals/rocks melt at different temperatures.
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How Magmas of Different Compositions Evolve
Assimilation – process
whereby a hot magma
composition will change
as it melts and
assimilates adjacent
rocks into the magma.
Magma Mixing –
composition of a magma
body changes as it mixes
with another magma
body.
Assimilation
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Magma Mixing
Intrusive Bodies
Intrusive rocks exist in bodies or structures that penetrate or cut
through pre-existing country rock
Intrusive bodies are given names based on their size, shape and
relationship to country rock
..
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Intrusive Bodies
Shallow Intrusive Structures
• Igneous bodies that apparently solidified near the surface of
the Earth.
• Relatively small compared to bodies formed at great depth.
• Tend to cool more rapidly than those that form at greater
depth and likely fine-grained.
Volcanic neck - Shallow intrusion formed when magma solidifies in
throat of volcano.
Shiprock, NM
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Intrusive Bodies
Shallow Intrusive Structures
• Dikes and Sills
• Dike
• Shallow, tabular intrusive structure
that cuts across any layering in
country rock
Light-colored dikes
• Sill
• Shallow, tabular intrusive structure
that parallels layering in country rock
Basaltic sill
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Intrusive Bodies
Intrusives that Crystallize at Depth
Plutons – deep, large, blob-shaped
intrusive body formed of coarse-grained
igneous rock, commonly granitic in
composition


stocks– small plutons (exposed over <100
km2)
batholiths – large plutons (exposed over
>100 km2)
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Abundance and Distribution of Plutonic Rocks
• Granite - Most abundant rock in mountain ranges and
interior lowlands of continents.
• Ancient basement plutonic rocks are exposed in just a few
places such as Grand Canyon, Arizona.
• Gabbro and basalt - the predominant rocks of the oceans.
• Andesite - the building material of young mountain ranges
• Ultramafic rocks make up the upper mantle.
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Explaining Igneous Activity & Plate Tectonics
Igneous processes at Divergent
Boundaries
•
mafic igneous rocks such as
basalt and gabbro are
commonly formed at
divergent boundaries
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Explaining Igneous Activity by Plate Tectonics
Intraplate Igneous Activity
• Rising mantle plumes can produce
localized hotspots and volcanoes
when they produce magmas that
rise through oceanic or continental
crust
• Hawaii is an example
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Explaining Igneous Activity by Plate Tectonics
Igneous Processes at
Convergent Boundaries
• The Origin of Andesite
• differentiation of mafic
magmas
• partial melting of oceanic
crust
• The Origin of Granite
• partially melted lower
continental crust
• Magmatic underplating
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End of Chapter 3
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