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.
Slide 5
Pluton Formation Plutons form and move upwards much like the
blobs of wax in a lava lamp
Slide 6
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).
Slide 7
Pluton Formation 4)When magma is forced upward into cracks in
overlying rocks it forms intrusive rock formations such as dikes
and sills.
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
Slide 10
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.
Slide 11
Fractional Crystallization 6b) Felsic minerals have a low M.P.
tend to crystallize last (as the magma cools). Quartz K-Spar
Muscovite Mica
Slide 12
Fractional Crystallization 6b) Felsic minerals have a low M.P.
tend to crystallize last (as the magma cools).
Slide 13
Fractional Crystallization 7) As minerals crystallize and fall
out, the magmas 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
Slide 14
Fractional Crystallization Mafic Magma Felsic Magma High temp
mafic minerals crystallize and settle out of magma. Mafic Minerals
Remaining magma becomes more and more felsic as mafic minerals
crystallize and settle out.
Slide 15
Fractional Crystallization 8)Additionally, felsic minerals from
the solid overlying rock mix in with the plutons magma, which also
tends to make the magma more felsic as it rises.
Slide 16
Fractional Crystallization & Pluton Formation Andesite
Rhyolite Ultra-Mafic Mafic Felsic Pluton Composition Intrusive Rock
Type Intermediate Peridotite Gabbro Diorite Granite Plutons Rising
Masses of Magma 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 : Basalt
Slide 17
Felsic
Slide 18
Ultra Mafic Magma 10)The very deepest plutons are very low in
silica and are called ultra-mafic plutons. Olivine Peridotite
Slide 19
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
Slide 20
Short Cuts 12) Sometimes, magma can find a pipe (conduit) which
carries the magma rapidly to the surface (before it can harden).
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 in the crust to reach the
surface while it is still molten. This allows magma which normally
cools deep in the crust to reach the surface while it is still
molten.
Slide 21
Short Cuts lava is an example of a deep-crust mafic magma that
cools and hardens at or near the surface. Basalt lava is an example
of a deep-crust mafic magma that cools and hardens at or near the
surface. (ie. Lava can (ie. Lava can be felsic, be felsic, mafic or
mafic or intermediate) [No ultra-mafic lava for over 2 billion
years!!]
Slide 22
Bowens Reaction Series 1)Bowens 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 o C) 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 o C) and turn solid
near the surface as the magma body reaches the end of its upward
rise. [See Diagram on the next slide.]
Slide 23
Bowens Reaction Series
Slide 24
4)The minerals at the bottom of the Bowens Reaction Series
Chart are the first to turn solid, because they have the highest
melting point (also the freezing point!). OlivineAugite (Pyroxene)
Ca-Spar
Slide 25
Bowens Reaction Series
Slide 26
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 they turn solid. Quartz Muscovite
Mica K-Spar
Slide 27
Bowens Reaction Series
Slide 28
Olivine Ca-Spar Pyroxene Amphibole Biotite Mica Na-Spar K -
Spar Muscovite Mica Quartz 1200 o C 900 o C 600 o C Felsic
Intermediate Mafic Ultra-Mafic 100 % Ca 100 % Na 50 / 50 Na / Ca
Continuous Discontinuous
Slide 29
Bowens 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
earths crust. c)are closest together on the B.R.S. chart
Slide 30
Bowens 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.) Granite Gabbro
Slide 31
Bowens Reaction Series Side Bar Question: Granite contains:
1)Pink K-Spar Crystals2) Clear Quartz Crystals 3)Black Mica
Flakes4) 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
Slide 32
Bowens Reaction Series Which of these chunky black minerals is
found in granite? (Which mineral belongs more with the other 5
above?) a) Hornblende or b) Augite?? Felsic
Slide 33
Bowens 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.
Slide 34
Bowens Reaction Series Hornblende is closer to the other
minerals on the Bowens 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.)
Slide 35
Bowens Reaction Series Olivine Ca-Spar Pyroxene Amphibole
Biotite Mica Na-Spar K - Spar Muscovite Mica Quartz 1200 o C 900 o
C 600 o C Sialic Intermediate Mafic Ultra- Mafic 100 % Ca 100 % Na
50 / 50 Na / Ca Silicate Structure Felsic
Slide 36
Silicate Structures
Slide 37
Slide 38
Bowens Reaction Series
Slide 39
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
Slide 40
Bowens 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 its too hot. Olivine can be brought to the surface,
but the cold, wet nasty conditions found near the surface cause
olivine to weather badly.
Slide 41
Bowens Reaction Series Olivine Ca-Spar Pyroxene Amphibole
Biotite Mica Na-Spar K - Spar Muscovite Mica Quartz 1200 o C 900 o
C 600 o C Sialic Intermediate Mafic Ultra- Mafic 100 % Ca 100 % Na
50 / 50 Na / Ca Silicate Structure Granite contains these
minerals:
Slide 42
Bowens Reaction Series
Slide 43
Olivine Ca-Spar Pyroxene Amphibole Biotite Mica Na-Spar K -
Spar Muscovite Mica Quartz 1200 o C 900 o C 600 o C Sialic
Intermediate Mafic Ultra- Mafic 100 % Ca 100 % Na 50 / 50 Na / Ca
Silicate Structure Diorite contains these minerals:
Slide 44
Bowens Reaction Series
Slide 45
Olivine Ca-Spar Pyroxene Amphibole Biotite Mica Na-Spar K -
Spar Muscovite Mica Quartz 1200 o C 900 o C 600 o C Sialic
Intermediate Mafic Ultra- Mafic 100 % Ca 100 % Na 50 / 50 Na / Ca
Silicate Structure Gabbro contains these minerals:
Slide 46
Bowens Reaction Series
Slide 47
Olivine Ca-Spar Pyroxene Amphibole Biotite Mica Na-Spar K -
Spar Muscovite Mica Quartz 1200 o C 900 o C 600 o C Sialic
Intermediate Mafic Ultra- Mafic 100 % Ca 100 % Na 50 / 50 Na / Ca
Silicate Structure Peridotite contains these 3 minerals:
Slide 48
Bowens Reaction Series
Slide 49
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
