Melting Weathering

Deposition &LithificationBurial, Heat, Pressure =

Metamorphism

CrystallizationThe Rock Cycle

THE ROCK CYCLE

Bowen’s Reaction Series

Pluton Formation

Pluton FormationPart 1 - See Pluton Diagram on next page1) 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 FormationPlutons form and move upwards much

like the blobs of wax in a lava lamp…

Pluton Formation3) 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 Formation4) 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)

Intrusive Rocks

(Plutons)

Extrusive Rocks

Fractional Crystallization5) 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 Crystallization5) 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 Crystallization6b) Felsic minerals have a low M.P. tend to crystallize last (as the magma cools).

QuartzK-Spar

Muscovite Mica

Fractional Crystallization6b) Felsic minerals have a low M.P. tend to crystallize last (as the magma cools).

Fractional Crystallization7) 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

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.

Fractional Crystallization8) 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

Andesite Rhyolite

Ultra-MaficMafic

FelsicPluton Composition

Intrusive Rock Type

Intermediate

PeridotiteGabbro

DioriteGranite

PlutonsRising 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

Felsic

Ultra Mafic Magma10) The very deepest plutons are very low

in silica and are called ultra-mafic

plutons .

Olivine Peridotite

Intermediate Magma11) 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 Series1) 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 Series4) 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 Series5) 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

MuscoviteMica

K-Spar

Bowen’s Reaction Series

Bowen’s Reaction

Series

Olivine Ca-Spar

Pyroxene

Amphibole

Biotite Mica Na-Spar

K - Spar

MuscoviteMica

Quartz

1200 oC

900 oC

600 oC

Felsic

Intermediate

Mafic

Ultra-Mafic100 % Ca

100 % Na

50 / 50Na / Ca

ContinuousDiscontinuous

Bowen’s Reaction Series6) 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 Series7) 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.”)

GraniteGabbro

Bowen’s Reaction SeriesSide Bar Question: Granite contains:1) Pink K-Spar Crystals 2) Clear Quartz Crystals3) 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

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

Bowen’s Reaction SeriesHow 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 SeriesHornblende 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.)

Bowen’s Reaction

Series

Olivine Ca-Spar

Pyroxene

Amphibole

Biotite Mica Na-Spar

K - Spar

MuscoviteMica

Quartz

1200 oC

900 oC

600 oC

Sialic

Intermediate

Mafic

Ultra-Mafic100 % Ca

100 % Na

50 / 50Na / Ca

Silicate Structure

Felsic

Silicate Structures

Independent Tetrahedra NO O’s shared

Single Chain2 O’s shared

Double Chain2 or 3 O’s shared

Silicate Structures

Sheet Structure3 O’s shared

Framework Structure4 O’s shared

Bowen’s Reaction Series

Bowen’s Reaction SeriesSide Bar Question 2: Describe the trendin 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 sharedSheet Structure = 3 shared3-D Framework = all 4 shared

Bowen’s Reaction SeriesThe 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.

Bowen’s Reaction

Series

Olivine Ca-Spar

Pyroxene

Amphibole

Biotite Mica Na-Spar

K - Spar

MuscoviteMica

Quartz

1200 oC

900 oC

600 oC

Sialic

Intermediate

Mafic

Ultra-Mafic100 % Ca

100 % Na

50 / 50Na / Ca

Silicate Structure

Granitecontainsthese minerals:

Bowen’s Reaction Series

Bowen’s Reaction

Series

Olivine Ca-Spar

Pyroxene

Amphibole

Biotite Mica Na-Spar

K - Spar

MuscoviteMica

Quartz

1200 oC

900 oC

600 oC

Sialic

Intermediate

Mafic

Ultra-Mafic100 % Ca

100 % Na

50 / 50Na / Ca

Silicate Structure

Dioritecontainsthese minerals:

Bowen’s Reaction Series

Bowen’s Reaction

Series

Olivine Ca-Spar

Pyroxene

Amphibole

Biotite Mica Na-Spar

K - Spar

MuscoviteMica

Quartz

1200 oC

900 oC

600 oC

Sialic

Intermediate

Mafic

Ultra-Mafic100 % Ca

100 % Na

50 / 50Na / Ca

Silicate Structure

Gabbrocontainsthese minerals:

Bowen’s Reaction Series

Bowen’s Reaction

Series

Olivine Ca-Spar

Pyroxene

Amphibole

Biotite Mica Na-Spar

K - Spar

MuscoviteMica

Quartz

1200 oC

900 oC

600 oC

Sialic

Intermediate

Mafic

Ultra-Mafic100 % Ca

100 % Na

50 / 50Na / Ca

Silicate Structure

Peridotite containsthese 3 minerals:

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