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Chapter 5 : Rocks from molten
Rocks liquids
from molten liquids
Including adaptions from Dupre and
Copeland (2004)
Concepts you should know for the exam
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Texture – grain size of intrusive (e.g., granite) and extrusive rocks
(e.g., basalt)
Relative cooling rates of intrusive and extrusive igneous rocks
Rocks from lavas vs. pyroclastic rocks
Mafic, intermediate and felsic rocks
Know table! (Temperature, Silica, Na, K, Fe, Mg, Ca content)
Melting point as a function of pressure, water content and composition
Partial melting of magma-generating rocks
Fractional crystallization
Shapes of magmatic bodies – sills, dykes, batholiths
Hot spots and volcanoes (mafic composition)
Volcanoes at convergent margins ( mafic to felsic composition)
Volcanoes at divergent margins (mafic composition)
Lavas, welded tuff, vesicular basalt, volcanic bombs, pyroclastic flows,
volcanic cloud
Relation between the shape of volcano, chemistry, viscosity of lavas
(shield volcano, cinder cone volcano,composite volcano)
Dangerous pyroclastic flows cause almost 30% fatalities natural
disasters vs. tsunamis (~20%)
Lecture Outline
1. Where do magmas form?
2.How do igneous rocks differ from one
another?
3. How do magmas form?
4. Magmatic differentiation
5. Forms of magmatic intrusion
6. Igneous activity and plate tectonics
Lecture Outline
1. Where do magmas form?
2.How do igneous rocks differ from one
another?
3. How do magmas form?
4. Magmatic differentiation
5. Forms of magmatic intrusion
6. Igneous activity and plate tectonics
Divergent Plate Boundary
Usually start within continents—
grows to become ocean basin
2. How do igneous rocks differ
from one another?
Texture – size of crystals
Coarse-grained rocks
Fine-grained rocks
Mixed texture rocks
1. How do igneous rocks differ
from one another?
Texture is related to rate of cooling.
Intrusive igneous rocks
Extrusive igneous rocks
Andesite - granite near the
surface
Hand sample-Santiago de Chile
Microscope
slide
Giant’s Causeway,
Ireland
Genetic Classification of
Igneous Rocks
• Intrusive: crystallized from
slowly cooling magma intruded
within the Earth’s crust; e.g.
granite, gabbro
Genetic Classification of
Igneous Rocks
• Extrusive: crystallized from
rapidly cooling magma extruded on
the surface of the Earth as lava or
erupted as pyroclastic material.
Extrusive Igneous Rocks
Include:
• rocks formed from the cooling of
lavas
• rocks formed by the cooling of
pyroclastic material, i.e. fragmented
pieces of magma and material erupted
into the air
Last Days of Pompeii-Karl Briullov-Copyright © 1999. George Mitrevski.
Auburn University
Mt St Helens
crater 08-780, 60 mph,
800 deg Fpyroclastic
flow- USGS
Composition and Classification
of Igneous Rocks
• Chemistry: e.g. % SiO2
• Mineralogy: e.g.
–
–
–
–
Felsic (Feldspar and Silica)
Intermediate
Mafic (Magnesium and Ferric)
Ultramafic
Two basic compositional groups:
Felsic igneous rocks
Mafic igneous rocks
Why last figure is so important
• The color and mineral distribution
indicate an increasing density and
melting temperature. Darker igneous
rocks generally weigh more and are
formed at higher temperatures and
pressures. This reflects the densitystratification of the whole Earth!
Felsic Igneous Rocks:
-Igneous rocks rich in minerals
high in silica. They include:
Granite
Rhyolite
Intermediate Igneous
Rocks:
-Igneous rocks in between in
composition between felsic and mafic
igneous rocks. They include:
Granodiorite
Dacite
Diorite
Andesite
Mafic Igneous rocks
-very low silica content, and consist
primarily of mafic minerals. The most
common ultramafic rock is:
Peridotite
What controls the melting
temperatures of minerals?
*External pressure
*and Water content
*Internal composition
(including internal
water content)
Factors Affecting Melting
of Minerals (and Rocks)
• Pressure: Increased Pressures raises
melting points
• Water Content (internal and external
to the mineral): Increased Water
Content lowers melting points
• Composition: Felsic minerals melt at
lower temperatures than mafic
minerals
Lecture Outline
1. Where do magmas form?
2.How do igneous rocks differ from one
another?
3. How do magmas form?
4. Magmatic differentiation
5. Forms of magmatic intrusion
6. Igneous activity and plate tectonics
4. How do magmas form?
When rocks melt (or partially melt).
Why do rocks melt?
When the temperature exceeds the
melting point of the rock or some
minerals within the rock.
Partial Melting
--Occurs when some of the
minerals forming a rock melt at
lower temperatures than other
minerals within the same rock
If different minerals melt at
different pressures that means
that different minerals become
solid at different temperatures
too.
What does this imply about
the internal composition of a
magma body as it cools???
Fractional
Crystallization!
Lecture Outline
1. Where do magmas form?
2.How do igneous rocks differ from one
another?
3. How do magmas form?
4. Magmatic differentiation
5. Forms of magmatic intrusion
6. Igneous activity and plate tectonics
Magmatic
Differentiation ?
What is
If, during fractional crystallization, the remaining
magma were to erupt it would be
(a) more felsic or
(b) more mafic than the original magma????
Lecture Outline
1. Where do magmas form?
2.How do igneous rocks differ from one
another?
3. How do magmas form?
4. Magmatic differentiation
5. Forms of magmatic intrusion
6. Igneous activity and plate tectonics
What do we know about the
shape of magmatic bodies?
Are they sheets? Blobs?
Spheres?
“Magmatic Pipes -101”
• Dykes are near-vertical
• Sills are horizontal and squeeze in
between other layers of rock
• Plutons are deep (km) bodies of
solidified magma. An example of a
plutonic rock is granite or gabbro.
www.jemmoore.com/flying/ shiprock.html
Shiprock, NM. It's hard to tell, mainly because I was flying at about 12,000ft when I
took this picture, but this massive piece of rock sticks up over 2,000ft from the
Shiprock, NM
• www.jemmoore.com/flying/ shiprock.html
• “Shiprock, NM. It's hard to tell, mainly because I
was flying at about 12,000ft when I took this
picture, but this massive piece of rock sticks up over
2,000ft from the surrounding plain, and is as big as a
small city. It is on Navajo land, and is a significant
spiritual site for the Navajo. The hard, volcanic
spines radiating out from the main spire are really
amazing. From this high up, you couldn't even see an
eighteen-wheeler on the ground next to Shiprock, it
would be just a speck.”
6. Igneous activity and plate tectonics
Magmatic geosystems:
Island arc plate subduction
Plate divergence
Hot-spot volcanism
Continental plate subduction
Geomagmatic systems of Earth
Geomagmatic systems of Earth
Island arc plate subduction
ISLAND ARC
PLATE SUBDUCTION
Island arc
volcano
Mafic to intermediate
intrusives (plutonism)
Mafic to intermediate
extrusives (volcanism)
Subduction zone
Oceanic
lithosphere
Plate divergent boundary
PLATE
DIVERGENCE
Mid-ocean ridge
Basaltic extrusives
Basaltic intrusives
Partial melting
Of upper mantle
Rising magma
Hot-spot volcanism
HOT-SPOT
VOLCANISM
Hot-spot volcano
Basaltic extrusives
Basaltic intrusives
Mantle plume
(hot spot)
Mantle
Continental plate subduction
CONTINENTAL
PLATE SUBDUCTION
Mafic to felsic intrusives
Mafic to felsic extrusives
Subduction
zone
Continental
margin volcano
Summary of Volcanoes
• Subduction volcanoes in continental
crustal and oceanic crust at
convergent plate margins
• Hot spot volcanoes (anywhere)
• decompression volcanoes at divergent
plate margins
END of CHAPTER 5
IGNEOUS ROCKS-rocks from
liquid melts