Transcript Igneous Rock

CHAPTER 5:
Igneous Rock
Rock of Ages granite quarry, Barre, Vermont
What
important
natural
resources are
found in
igneous rock?
Stawamus Chief, Squamish BC
Mafic dyke
Massive granite
(~5 m across)
Steven Earle
Igneous Rock
• Igneous rock is formed when molten, or partially molten, rock
solidifies.
Igneous Rocks (two types)
• Extrusive
– Lava and Pyroclastic Debris
– Extruded at surface or at very
– shallow levels and cooled quickly
Basalt is
Extrusive
• Intrusive
– Magma crystallized slowly within
the crust.
– No exposure to the cool
atmosphere.
– Plutonic – intrusive igneous rock
cooled slowly at great depth within
crust or mantle.
Granite is
Intrusive
What is the key textural difference between these two rocks?
Why Study Igneous Rocks?
• All rocks on Earth have evolved from the first igneous rocks
through Igneous Evolution
• Provide information about the composition of the mantle
• Absolute age can be obtained through Radiometric Dating
• Features are characteristic of a specific tectonic environment
• Host important mineral deposits (metal ores)
• Numerous other critical economic uses
• Millions of people are endangered by volcanoes
Decompression melting
• Convection brings hot rock
to shallower depths
• Rifting lowers pressure
• Lower pressure causes a
lower melting point
• Igneous rock is formed
through a process of
crystallization and magma
differentiation
Why does a lower pressure lower the melting point?
Partial Melting
• Partial Melting
– Silica-rich compounds melt before
other compounds.
• Magma Differentiation
– Four types:
•
•
•
•
Magma mixing
Crystal settling
Wall-rock assimilation
Magma migration
Bowen’s Reaction Series
• Describes the order and process of magma crystallization
Bowen’s Reaction
Series Animation
How Are Igneous Rocks Named?
• Texture
– Texture depends on crystal size
– Phaneritic:
• Slow cooling (or in the presence of water), larger crystals
• Coarse texture
• Intrusive or Plutonic
– Aphanitic:
• Rapid cooling , smaller crystals
• Finer texture
• Extrusive or Volcanic
– Other textures are Glassy, Pyroclastic, Vesicular, and Porphyritic
• Composition
– Minerals at the TOP of Bowen’s
• Dark in color
• Mafic to Ultramafic
• Iron and Magnesium are dark.
– Minerals at the BOTTOM of Bowen’s
• Light in color
• Felsic
• Sodium plagioclase, potassium plagioclase, and quartz are light
– Intermediate composition between these
How is Texture Estimated?
• Texture is estimated using
visual grain size
• (depends on crystallization
history)
What has to happen for a porphyritic texture to form in an igneous rock?
How is Composition Estimated?
• Composition is estimated using visual color
– Can be misleading
Mafic vs. Felsic
• Mafic minerals crystallize early and felsic minerals crystallize
late in magma
Minerals at the TOP of Bowen’s Reaction Series:
•Dark in color
•Mafic to ultramafic
•Iron- and magnesium-rich and relatively heavy
Minerals at the BOTTOM of Bowen’s Reaction Series:
• Light in color
• Intermediate to Felsic
• Iron- and magnesium-poor and relatively light in weight
Bowen’s Reaction Series
What does olivine react with to produce pyroxene?
What is the significance of the word “reaction” in the Bowen Reaction
Series?
Igneous Rock Naming
• Igneous rocks are named on the basis of their texture and
composition
Igneous Rock Naming
(continued)
• The igneous rock diagram shows the range of mineral proportions
of the various types of igneous rocks
What are the approximate mineral proportions (in %) of
the rocks indicated by the two dashed lines?
Classification System for Igneous Rocks
• The classification system for igneous rocks is based on the
following:
– Intrusive and extrusive igneous rocks with the same chemical
composition contain the same minerals
– Silica content decreases as iron and magnesium content
increases
– Potassium and sodium content decreases as silica content
decreases
Seven Common Types of Igneous Rock
Felsic Composition
1) Granite
intrusive
2) Rhyolite
extrusive
Seven Common Types of Igneous Rock
Intermediate
Composition
3) Diorite
intrusive
4) Andesite
extrusive
Seven Common Types of Igneous Rock
Mafic Composition
5) Gabbro
intrusive
6) Basalt
extrusive
Seven Common Types of Igneous Rock
Ultramafic Composition
7) Peridotite
intrusive
Igneous Evolution
• All rocks on Earth have evolved from the first igneous rocks
• All Igneous Rocks Result from Magma Differentiation
Tectonic Processes
• Igneous rock evolves as a product of tectonic processes and is a
major component of Earth’s crust
What is the mechanism that leads to partial melting above a subduction zone?
Identify the magma source for each type of volcano. How is the magma
source responsible for the rock composition?
Basalt Formation (Spreading Centers)
• High heat flow creates Hydrothermal (hot water) Vents (black smokers)
• Oceanic crust consists of layers of igneous rock
– Gabbro crystallizes at the base
– Sheeted dikes of basalt extend above the gabbro
– Pillow basalt, glassy fragments and metallic sulfide deposits (at black smokers)
form at the sea floor
Basalt Formation (Hot Spots)
• Intraplate sites of active Plutonism
and Volcanism
• Large Igneous Provinces of Flood
Basalts form above mantle hot
spots
• Plumes of anomalously hot
mantle rock underly active hot
spots. Mantle plumes, which are
solid rock, not magma, rise slowly
through the mantle (but faster
than the rate of mantle
convection) due to their positive
buoyancy with respect to the
surrounding rock.
• Basalt is also present at many
subduction-related volcanoes, but
typically only in minor amounts.
Modern sea-floor pillow basalt
40 mya pillow basalt on
Vancouver Island
Steven Earle
Igneous Intrusions
• Igneous intrusions occur in a variety of sizes and shapes.
Subduction Zones and Spreading Centre
• Subduction zones and spreading centers are areas in which
magma is formed as a result of partial melting, plutonism, and
volcanism.
What igneous environment is not represented in this image?
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