Igneous Rocks and Their Origin
Chapter 3
The Rock
Cycle
• A rock is composed of grains of one or more minerals
• The rock cycle shows how one type of rocky material is transformed
into another
Three Types of
Geologic Rocks
• Igneous rocks
- Formed from volcanic eruptions
- either intrusive or extrusive
• Sedimentary rocks
- Formed from deposition and
erosional processes
• Metamorphic rocks
- Deforming of rocks above from
exposure to high pressure and
temperature
The Rock Cycle and Plate Tectonics
• Magma is created by melting
above a 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 areas,
buried and hardened into sedimentary rock
• Sedimentary rock heated and squeezed at depth to form
metamorphic rock
Volcanism and Extrusive Rocks
(Chapter 4)
Mount St. Helens
Hawaii
What differences do you see in these two volcanoes ?
How are these Volcanoes Different?
Hawaii
Mt St. Helen's
Types of Volcanoes
1. Shield Volcano (Hawaiian)
- Low viscosity “runny” lava
- Shallow slopes (shield volcanoes)
- Low silica content
2. Strato Volcano (Mt. St. Helens)
* Strombolian
-Intermediate explosivity
-Can eject showers of lava
* Plinian
- High viscosity lava
- Steep slopes (stratovolcanoes)
- High silica content
Shield Volcano
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Flood Basalts
from Snake River Plates, Idaho
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Stratovolcano
Steep slopes formed by high viscosity, silica rich magmas
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How a stratovolcano is formed
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Extrusive Volcanic Rocks:
- Large volcanic bombs
- Intermediate tephra
- Small ash
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Volcanoes and Volcanic Hazards
Mount Etna, Sicily experienced a Strombolian eruption in 2002
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Lakes of Death in Cameroon
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Volcanic Gases
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Deadly eruptions
Figure 6.10
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Laki Volcano, Iceland
- Fissure eruption 1783 – 1784
- Largest lava flow in recorded history
- Winter after eruption temperature
dropped 1oC - 2.5oC.
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Volcanic Gases form Earth's Atmosphere and Oceans
* Gas clouds from volcanic eruptions help form our atmosphere
* How would this process explain the different atmospheres on other
planets and habitability ?
Volcanic Gases form Earth's Atmosphere and Oceans
Mount St. Helens
Hawaii
* Which type of volcano do you think releases more gases into the
atmosphere during an eruption ?
- Shield Volcano
- Strato Volcano
Volcanoes on Mars
* Which type of volcano do you think Olympus Mons is on Mars ?
* How would Olympus Mons contribute to it's atmosphere ?
Mars Atmosphere
Carbon dioxide
Nitrogen
Argon
Oxygen
95%
3%
1.5%
Trace amounts only
Atmospheric pressure is only 1% of Earth so does not protect
surface from Sun's radiation
What Causes Volcanic Eruptions ?
Press & Siever
Fig. 1.16
Types of Igneous Rocks
• Magma is molten rock
• Igneous rocks form when magma
cools and solidifies
– Intrusive igneous rocks form when
magma solidifies underground
Granite
• Granite is a common example
– Extrusive igneous rocks form when
magma solidifies at the Earth’s
surface (lava)
• Basalt is a common example
Basalt
Extrusive Volcanic Rocks:
- Large volcanic bombs
- Intermediate tephra
- Small ash
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How do we Know Igneous Rocks Formed at Depth?
Torres del Paine,
Chile
• Mineralogy / Chemistry ?
• Grain size (coarse vs fine grained)
• Lab experiments require high P & T to form large grains
• Outcrops: See intrusions into country rock
-Contact/chill zones, baked and metamorphosed
• Xenoliths of country rock found in igneous intrusions
Igneous Rock Textures
• Texture refers to the size, shape and
arrangement of grains
• Texture of igneous rocks is controlled by
cooling rate
• Extrusive rocks cool quickly at the Earth’s
surface and are fine-grained (small <1 mm)
• Intrusive rocks cool slowly deep beneath
Earth’s surface and are coarse-grained
(large >1 mm)
Fine-grained igneous rock
Coarse-grained igneous rock
Special Igneous Textures
• A pegmatite is a very coarse-grained rock
(crystals >5 cm) formed when magma cools
very slowly at depth
• A glassy texture contains no crystals at all,
and is formed by rapid cooling
• A porphyritic texture includes two crystal
sizes, formed first during slow cooling (large
crystal) and then forming small crystals
during rapid cooling
Pegmatitic igneous rock
Porphyritic igneous rock
Igneous Rock Identification
•
Igneous rock names are based on texture (grain size) and
composition
Textural classification
•
–
–
•
Coarse-grained: Plutonic rocks (gabbro-diorite-granite) cooled slowly
at depth
Fine-Grained: Volcanic rocks (basalt-andesite-rhyolite) cooled rapidly
at the Earth’s surface
Compositional classification
–
–
–
Mafic rocks (gabbro-basalt) contain dark-colored ferromagnesian
minerals, iron rich (50% silica)
Intermediate rocks (diorite-andesite) contain roughly equal amounts of
dark- and light-colored minerals
Felsic rocks (granite-rhyolite) light-colored minerals, silica rich (65%)
Igneous Rock
Identification
Olivine
Igneous names
are based on
1) texture and
2) mineralogic
composition
Intrusive Rock Bodies
•
Intrusive rocks exist in bodies or structures that penetrate
or cut through pre-existing country rock
•
Shallow intrusions: Dikes and sills
•
•
Form <2 km beneath Earth’s surface
Chill and solidify quickly in cool country rock
Intrusive Rock Bodies
Plutons: deep
magmatic intrusions
- Form at considerable
depth beneath the
surface when rising
blobs of magma
(diapirs) get trapped
within the crust
Pluton in Ship Rock, New Mexico
Intrusive Rock Bodies
•
Volcanic neck
–
•
Shallow intrusion formed when magma
solidifies in throat of volcano
Dike
–
•
Light-colored dikes
Tabular intrusive structure that cuts across
any layering in country rock
Sill
–
Tabular intrusive structure that parallels
layering in country rock
Basaltic sill
Sierra Nevada batholith