Crystallization
Melting
Burial, Heat, Pressure =
Metamorphism
Weathering
Deposition &
Lithification
Bowen’s Reaction Series
Pluton Formation
Pluton Formation
Part 1 - See Pluton Diagram on next page
1) Magma is molten or partially melted rock from
the asthenosphere (weak layer) of the upper
mantle) or from magma bodies in the lower
crust.
Pluton Formation
Plutons form and move upwards much
like the blobs of wax in a lava lamp…
Pluton Formation
3) If magma cools and hardens underground,
it forms large plutonic formations called
batholiths. These often form the core of
mountain ranges (Ex: Sierra Nevadas).
Pluton Formation
4) When magma is forced upward into cracks
in overlying rocks it forms intrusive rock
formations such as dikes and sills.
Extrusive
Rocks
Intrusive Rocks
(Plutons)
Extrusive Rocks
Intrusive
Rocks
(Plutons)
Fractional Crystallization
5) Cooling magmas crystallize (turn solid)
between 1200 o C and 600 o C.
6a) Mafic minerals have a high melting point
(M.P.) tend to crystallize first.
Olivine
Pyroxene
Ca-Spar
Fractional Crystallization
5) Cooling magmas crystallize (turn solid)
between 1200 o C and 600 o C.
6a) Ultra-Mafic and Mafic minerals have a high
melting point (M.P.) tend to crystallize first.
Fractional Crystallization
6b) Felsic minerals have a low M.P. tend to
crystallize last (as the magma cools).
Muscovite
Mica
K-Spar
Quartz
Fractional Crystallization
6b) Felsic minerals have a low M.P. tend to
crystallize last (as the magma cools).
Fractional Crystallization
7) As minerals crystallize and fall out, the
magma’s composition changes.
(Mafic magma becomes more felsic as more
and more mafic minerals drop out.)
• Felsic minerals are still
molten and continue to
rise
• Mafic minerals crystallize
and fall out
Fractional Crystallization
High temp mafic minerals
crystallize and settle out of magma.
Mafic Magma
Felsic Magma
Mafic Minerals
Remaining magma becomes more and more
felsic as mafic minerals crystallize and settle out.
Fractional Crystallization
8) Additionally, felsic
minerals from the
solid overlying rock
mix in with the
pluton’s magma,
which also tends to
make the magma
more felsic as it rises.
Fractional Crystallization & Pluton Formation
9) As a result, plutons found near the surface tend to
be more felsic in composition, while plutons which
harden deeper down tend to be more mafic:
Andesite
Basalt
Pluton
Composition
Mafic
Ultra-Mafic
Rhyolite
Intermediate
Diorite
Gabbro
Peridotite
Intrusive Rock Type
Felsic
Granite
Plutons
Rising Masses of Magma
Felsic
Ultra Mafic Magma
10)
The very deepest plutons are very low in
silica and are called ultra-mafic plutons .
Olivine
Peridotite
Intermediate Magma
11)
When a mafic pluton has become
somewhat more sialic, but still contains a
significant quantity of mafic minerals, it is
described as being intermediate in
composition.
Diorite
“Short Cuts”
12) Sometimes, magma
can find a volcanic pipe
(conduit) which
carries the magma
rapidly to the surface
(before it can harden).
This allows magma
which normally cools
deep in the crust to
reach the surface
while it is still molten.
“Short Cuts”
Basalt lava is an example of a deep-crust mafic
magma that cools and hardens at or near the surface.
(ie. Lava can
be felsic,
mafic or
intermediate)
[No ultra-mafic
lava for over 2½
billion years!!]
Bowen’s Reaction Series
1) Bowen’s Reaction Series lists the order in which
minerals crystallize out of a cooling magma as the
pluton rises.
2) The mafic minerals have a high M.P. (1200 oC) and
turn solid deep in the crust when the magma body
first started to rise.
3) The felsic minerals have a low M.P. (600 oC) and turn
solid near the surface as the magma body reaches
the end of its upward rise.
[See Diagram on the next slide.]
Bowen’s Reaction Series
Bowen’s Reaction Series
4) The minerals at the bottom of the Bowen’s Reaction
Series Chart are the first to turn solid, because they
have the highest melting point (also the freezing
point!).
Olivine
Augite
(Pyroxene)
Ca-Spar
Bowen’s Reaction Series
Bowen’s Reaction Series
5) The minerals at the top of the chart are the last to
turn solid, because they have the lowest melting
points and the magma has to really cool off before
Muscovite
they turn solid.
Mica
Quartz
K-Spar
Bowen’s Reaction Series
600 oC
Quartz
Muscovite
Mica
Discontinuous
Bowen’s Reaction
Series
Felsic
Continuous
K - Spar
Biotite
Mica
100 % Na
Na-Spar
900 oC
Amphibole
Intermediate
50 / 50
Na / Ca
Pyroxene
Mafic
1200 oC
Ca-Spar
Olivine
100 % Ca
Ultra-Mafic
Bowen’s Reaction Series
6) As a result, any given rock will
usually be made up of minerals
which:
a) crystallize at about the
same temperature
b) form at approximately the
same depth in the earth’s
crust.
c) are closest together on
the B.R.S. chart
Bowen’s Reaction Series
7) Geologists can identify a mineral in a given rock by
seeing what other minerals are present and looking
at the B.R.S. chart to see what minerals are most
commonly found together in the same rock.
(“I.D. the mineral by the company it keeps.”)
Gabbro
Granite
Bowen’s Reaction Series
Side Bar Question: Granite contains:
1) Pink K-Spar Crystals
2) Clear Quartz Crystals
3) Black Mica Flakes
4) White Na-Spar Xtls
…and some little black chunks that look like someone
drew dots on it with a
black sharpie!!
Go to next page…
Granite
Bowen’s Reaction Series
Felsic
Which of these chunky black
minerals is found in granite?
(Which mineral belongs more with
the other 5 above?)
a) Hornblende or b) Augite??
Bowen’s Reaction Series
How do we know that the little black chunks are the
black chunky amphibole hornblende and not the
black chunky pyroxene augite???
Augite forms at much higher
temperatures and much
deeper in the crust than
K-Spar, Quartz, Na-Spar
and Muscovite & Biotite mica.
Bowen’s Reaction Series
Hornblende is closer to the other minerals on the
Bowen’s R.S. Chart, which means it forms at closer
to the same temperature and is more likely to be
found at the same depth in the crust as the other
minerals in granite.
(See # 6 in Part 2 of
your notes.)
Silicate Structure
600 oC
Quartz
Muscovite
Mica
Bowen’s Reaction
Series
Felsic
Sialic
K - Spar
Biotite
Mica
100 % Na
Na-Spar
900 oC
Amphibole
Intermediate
50 / 50
Na / Ca
Pyroxene
Mafic
Ca-Spar
Olivine
1200
oC
100 % Ca
UltraMafic
Silicate Structures
Double Chain
2 or 3 O’s shared
Single Chain
2 O’s shared
Independent Tetrahedra
NO O’s shared
Silicate Structures
Framework Structure
4 O’s shared
Sheet Structure
3 O’s shared
Bowen’s Reaction Series
Bowen’s Reaction Series
Side Bar Question 2: Describe the trend
in the degree of sharing of oxygen atoms
in the minerals of the Discontinuous
Series as you go from Independent
Tetrahedra at the bottom of the chart to
3-D Framework at the top of the chart:
Answer: The degree of sharing increases
consistently increases going up the chart:
Ind. Tetrahedra = 0 shared
Chain Structure = 2 shared
Sheet Structure = 3 shared
3-D Framework = all 4 shared
Bowen’s Reaction Series
The minerals that form at the top of the
B.R.S. chart are much more stable at
or near the surface, while the minerals
at the bottom of the B.R.S. are more
stable in the deep crust and mantle.
• Indeed, Quartz and K-Spar are not able
to form at great depths – it’s too hot.
• Olivine can be brought to the surface,
but the “cold, wet nasty” conditions
found near the surface cause olivine
to weather badly.
Silicate Structure
600 oC
Quartz
Granite
contains
these
minerals:
Biotite
Mica
Muscovite
Mica
Bowen’s Reaction
Series
Sialic
K - Spar
100 % Na
Na-Spar
900 oC
Amphibole
Intermediate
50 / 50
Na / Ca
Pyroxene
Mafic
Ca-Spar
Olivine
1200
oC
100 % Ca
UltraMafic
Bowen’s Reaction Series
Silicate Structure
600 oC
Quartz
Muscovite
Mica
Bowen’s Reaction
Series
Sialic
K - Spar
Biotite
Mica
900 oC
Amphibole
100 % Na
Diorite
contains
these
50 / 50
minerals:
Na / Ca
Na-Spar
Intermediate
Pyroxene
Mafic
Ca-Spar
Olivine
1200
oC
100 % Ca
UltraMafic
Bowen’s Reaction Series
Silicate Structure
600 oC
Quartz
Muscovite
Mica
Bowen’s Reaction
Series
Sialic
K - Spar
Biotite
Mica
900 oC
Amphibole
100 % Na
Gabbro
contains
these 50 / 50
Na / Ca
minerals:
Na-Spar
Intermediate
Pyroxene
Mafic
Ca-Spar
Olivine
1200
oC
100 % Ca
UltraMafic
Bowen’s Reaction Series
Silicate Structure
600 oC
Quartz
Muscovite
Mica
Bowen’s Reaction
Series
Sialic
K - Spar
Biotite
Mica
100 % Na
Na-Spar
900 oC
Amphibole
Intermediate
50 / 50
Na / Ca
Pyroxene
Olivine
1200
oC
Peridotite
contains
these 3
minerals:
100 % Ca
Mafic
Ca-Spar
UltraMafic
Bowen’s Reaction Series
Bowen’s Reaction Series
Follow these trends on the right
side of the BRS chart:
When Formed:
First to form  Last to Form
Resistance to Weathering:
Least stable  Most Stable
Where Formed:
Deep in Crust  Near Surface
Bowen’s Reaction Series