Three Rock Types:
•  Igneous = ‘from fire’
•  Sedimentary = ‘settled
from water’
•  Metamorphic = ‘change
shape’
There are three ways to make a mineral
Crystallize from melt
Precipitate from water
Product of reaction between other minerals
Thus, there are three basic rock-types roughly corresponding
to these three processes
Notice: rocks are products of Earth processes and thus they
record events in Earth history
Given the definition of a mineral, which of the following is
a mineral?
A.  Chert - amorphous SiO2
B.  Modern shell fragments
of crystalline CaCO3
C.  Coal
D.  Petroleum (oil)
E.  Salt
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Rock Cycle
•  Earth processes
‘transfer’ rocks
from one type to
another
–  Erosion
–  Burial
–  Melting
•  More later...
Let’s begin with igneous rocks. These are rocks formed from
minerals that crystallize from a melt.
Where does melting occur?
Each setting represents a different melting process...
Two ways to melt the asthenosphere
•  Decompressive melting
–  Asthenosphere rises faster than it looses heat
•  as seafloor spreading thins lithosphere, or •  in a rising hotspot
–  Hot shallow asthenosphere crosses the melting
curve (solidus)
–  These melts rise through the lithosphere to
erupt at Mid-ocean ridges or hot-spot
volcanoes
•  Hydration (or Flux) Melting
–  Volatile fluids are boiled and squeezed from
the subducting lithosphere
–  Addition of fluids to the asthenosphere lowers
the melting temperature of the asthenosphere
below the ambient temperature (‘Solidus with
water’ is lower than the ‘geotherm’)
–  These melts rise through the lithosphere and
erupt as subduction zone volcanoes
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Igneous Rocks are produced in Magmatic Systems
•  Magma chamber
–  Supplied by melt
–  Stores & processes
Magma
–  Magma = melt +
crystals
–  Rocks derived from
depth = Intrusive
•  Volcanic system
–  conduit to surface
–  Lava = magma
extruded onto surface
–  Rocks from volcano =
Extrusive
How does magma rise through crust?
Given that igneous rocks crystallize from melts during
cooling, what does the size of the mineral crystals in a rock
indicate?
A.  Small crystals indicate
rapid cooling.
B.  Small crystals indicate
slow cooling
C.  Large crystals indicate
rapid cooling
Intrusive vs. Extrusive Rocks
•  Intrusive rocks cool deep
within the Earth –  Cool slowly = time for
crystals to grow
–  Large visible crystals
(Phaneritic texture)
•  Extrusive rocks cool on
surface –  Cool quickly (freeze)- little
time for crystals to grow
–  Glass (frozen liquid) or
very small (not visible)
crystals.
–  Phenocryst = large crystal
imbedded in fine matrix.
These were once growing in
the magma chamber and
were erupted with the lava.
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Intrusive vs. Extrusive: Texture
Extrusive = frozen magma
(crystals + liquid)
Intrusive
Felsic vs. Mafic
Granite
Gabbro
Basalt
Rhyolite
Sill vs. Dike (horizontal vs. vertical tabular intrusions)
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Volcanoes: Shape & Size (why are they different? Later in class)
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Review Questions
•  At which types of plate boundaries are volcanoes found?
•  Which melting mechanism results from the addition of
water? At which plate boundary does this type of melting
occur, and why?
•  What determines the size of crystals in an igneous rock?
•  Why do intrusive rocks have large crystal sizes?
•  How are basalts different from (1) gabbro, (2) granite, and
(3) rhyolite?
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