I. Factors controlling
melting
A. Heat
Temperature within Earth
increases with depth:
geothermal gradient
So rocks at depth will melt.
The top of Asthenosphere is
~1% liquid, lithospheric
plates “float” on it.
Geothermal gradient
I. Factors controlling melting
B. Pressure:
High pressure holds the ions and
atoms in a crystalline solid closer
together
At higher pressures it harder to
vibrate and break bonds.
Increase pressure (i.e. down in
the Earth) the temperature
needed to melt the rocks also
goes up.
Decompression Melting: If the
pressure on rock is somehow
reduced or removed, as happens
when tectonic plates rift and
diverge
It’s melting point drops below it’s
current temperature and it melts.
I. Factors controlling melting
Decompression melting
Decompression melting
I. Factors controlling melting
C. Water Content:
Water (even small amounts
<1%) lowers the melting
point of rocks
Water tugs at ions and
weakens (loosens) bonds,
so lower temperatures will
vibrate the bonds more and
break them
How this works at subduction
zones to create melt dewatering of slab (due to
increase in temp and
pressure)
Melting oceanic crust (basalt) forms rocks
for continental crust (andesite)
Experiments show: melting
temp of basalt lowered
100C with just 0.1 % water.
II. Crystallization of magma
solidification
A. The Basics
1.Eventually every magma cools and
solidifies—over range of ~200C.
2. Lava will crystallize into individual
minerals (i.e. freeze salt water, you
don’t get frozen saltwater, you get ice
and salt (e.g., form separate crystals
because separate minerals)
3. Melting Temp = Crystallization Temp
a. Minerals that melted first, solidify last
(i.e. quartz and orthoclase/K-spar).
b. Minerals that melted last, will be the
first to crystallize during cooling
(olivine, pyx)
II. Crystallization of Magma
solidification
A. The Basics
3. Melting Temp = Crystallization
Temp (con’t)
a. Minerals that melted first, solidify
last (e.g., quartz and orthoclase/Kspar).
b. Minerals that melted last, will be
the first to crystallize during cooling
(olivine, pyx)
4. As magma cools, crystals
progressively form.
So a partially cooled body of magma
contains solid crystals of minerals
that crystallize at higher
temperatures, and liquid containing
the atoms and ions of minerals that
will not crystallize until the
temperature is lowered further.
B. Bowen’s Reaction Series: During cooling, minerals crystallize in a
systematic fashion based on their melting points
Bowen’s reaction series:
Highest temperature minerals first to lowest temperature minerals.
Bowen’s reaction series
Discontinuous Series:
Each successive type differs in both
composition and internal
structure
Continuous Series:
As it cools, the Ca is in the solid
plagioclase crystals and Na is in
the melt.
Na starts going into the crystal more—
plagioclase becomes zoned.
As minerals crystallize, the
composition of the remaining melt
changes - Fe, Mg, Ca taken out,
and it becomes more Na, K, Si
rich
Highest temperature minerals first to
lowest temperature minerals.
Rock Name Color
Composition
Minerals
Temp
Occurrence
Rhyolite
~70%wt SiO2
quartz
~800 C
continents
Light
low in Mg, Fe
feldspar
amphibole
mica
Andesite
Salt-pepper
~60%wt SiO2
quartz
~1000 C
continent-
feldspar
ocean
amphibole
boundary
pyroxene
Basalt
Dark
~52%wt SiO2
hi Mg ,Fe
feldspar
~1200 C
olivine
pyroxene
boundary
oceanocean
III. Processes that change the composition of the magma
A. Fractional Crystallization &
crystal settling
Crystals form (more dense) sink
to bottom of magma chamber
and can no longer react with
the remaining melt so the
minerals that form will be
different (no Fe, Mg, or Ca).
Rocks that form differ from each
other and from the original
magma composition.
This creates several chemically
diverse igneous rocks
III. Processes that change the composition of the magma.
A. Fractional Crystallization & crystal settling
III. Processes that change the composition of the magma.
Magma and early forming crystals (minerals)
III. Processes that change the composition of the magma
B. Assimilation
Incorporation of foreign
material into the magma
Pieces of wall rock may break
off the wall and fall into the
magma and wholly or
partially melt—changes
composition of the magma
Xenolith: piece of foreign rock
that doesn’t melt
(temperature not great
enough) and just gets
carried along for the ride
It stays a solid the whole time
III. Processes that change the composition of the magma
B. Assimilation
Incorporation of foreign
material into the magma
Pieces of wall rock may break
off the wall and fall into the
magma and wholly or
partially melt—changes
composition of the magma
Xenolith: piece of foreign rock
that doesn’t melt
(temperature not great
enough) and just gets
carried along for the ride
It stays a solid the whole time
III. Processes that change the composition of the magma
C. Magma Mixing
Two different magma bodies may mix and create a new magma—mix by
convection
Sometimes you can see two distinct magmas in the erupted rocks
Summary
Processes that change the composition of the magma
Processes that change the composition of the magma
A. Fractional crystallization and crystal settling
B. Assimilation
C. Magma Mixing
Factors controlling melting of rocks
A. Temperature
B. Pressure
C. Water content
Partially melt peridotite  basalt magma
Partially melt basalt  andesite magma
Partially melt andesite  rhyolite magma