igneous rocks - intrusive
TRANSCRIPT
-
8/13/2019 Igneous Rocks - Intrusive
1/46
The Rock Cycle
A rockis a naturally formed,consolidated material usually
composed of grains of one or
more minerals
The rock cycleshows how onetype of rocky material gets
transformed into another
Representation of how rocks are
formed, broken down, and processed
in response to changing conditions Processes may involve interactions
of geosphere with hydrosphere,
atmosphere and/or biosphere
Arrows indicate possible process
paths within the cycle
-
8/13/2019 Igneous Rocks - Intrusive
2/46
Imagine the first rock and the cycles that it
has been through.
-
8/13/2019 Igneous Rocks - Intrusive
3/46
The Rock Cycle and PlateTectonics
Magmais created by melting of rock
above a subduction zone
Less dense magma rises and cools
to form igneous rock Igneous rock exposed at surface
gets weathered into sediment
Sediments transported to low areas,
buried and hardened into sedimentary rock Sedimentary rock heated and squeezed at depth to form
metamorphic rock
Metamorphic rock may heat up and melt at depth to form magma
Convergent plate boundary
-
8/13/2019 Igneous Rocks - Intrusive
4/46
Igneous Rocks
Magma is molten rock
Igneous rocks form when magma
cools and solidifiesIntrusiveigneous rocks form when
magma solidifies underground
Granite is a common example
Extrusiveigneous rocks form whenmagma solidifies at the Earths
surface (lava)
Basalt is a common example
Granite
Basalt
-
8/13/2019 Igneous Rocks - Intrusive
5/46
Igneous Rock Textures
Texture refers to the size, shape and
arrangement of grains or other constituents
within a rock
Texture of igneous rocks is primarilycontrolled by cooling rate
Extrusive igneous rockscool quickly at or
near Earths surface and are typicallyfine-
grained(most crystals 1 mm) Coarse-grained igneous rock
Fine-grained igneous rock
-
8/13/2019 Igneous Rocks - Intrusive
6/46
Fast cooling
Slow cooling large xtals
small xtals
The basics of igneous rock textures
It is a function of viscosity ofthe melt, which is controlled by
composition and temperature.
-
8/13/2019 Igneous Rocks - Intrusive
7/46Coarse grained igneous rock
-
8/13/2019 Igneous Rocks - Intrusive
8/46Fine grained igneous rock
-
8/13/2019 Igneous Rocks - Intrusive
9/46
Special Igneous Textures
Apegmatiteis an extremely coarse-grainedigneous rock (most crystals >5 cm) formed
when magma cools veryslowlyat depth
Aglassy texture contains no crystals at all,and is formed by extremely rapid cooling
Aporphyritictexture includes two distinct
crystal sizes, with the larger having formedfirst during slow cooling underground and
the small forming during more rapid cooling
at the Earths surface
Pegmatitic igneous rock
Porphyritic igneous rock
-
8/13/2019 Igneous Rocks - Intrusive
10/46
Pegmatite:
Very coarse grained igneous rock
-
8/13/2019 Igneous Rocks - Intrusive
11/46
Porphyritic igneous rock:
Big xtals in a fine grain matrix
-
8/13/2019 Igneous Rocks - Intrusive
12/46
Igneous Rock Identification
Igneous rock names are based on texture(grain size) and
mineralogic composition
Textural classification
Plutonicrocks (gabbro-diorite-granite) are coarse-grained and cooled
slowly at depth
Volcanicrocks (basalt-andesite-rhyolite) are typically fine-grained and
cooled rapidly at the Earths surface
Compositional classification
Maficrocks (gabbro-basalt) contain abundant dark-coloredferromagnesian minerals
Intermediaterocks (diorite-andesite) contain roughly equal amounts of
dark- and light-colored minerals
Felsic rocks (granite-rhyolite) contain abundant light-colored minerals
-
8/13/2019 Igneous Rocks - Intrusive
13/46
Igneous Rock Identification Igneous rock names are based on texture(grain size) and
mineralogic composition
-
8/13/2019 Igneous Rocks - Intrusive
14/46
-
8/13/2019 Igneous Rocks - Intrusive
15/46
Source: http://csmres.jmu.edu/geollab/Fichter/IgnRx/simpclass.PDF
-
8/13/2019 Igneous Rocks - Intrusive
16/46
-
8/13/2019 Igneous Rocks - Intrusive
17/46
Quartz
PlagioclaseFeldspar
Alkali(Ca,Na)Feldspar
Plutonic Rocks
-
8/13/2019 Igneous Rocks - Intrusive
18/46
Quartz
PlagioclaseFeldspar
Alkali(Ca,Na)Feldspar
Volcanic Rocks
-
8/13/2019 Igneous Rocks - Intrusive
19/46
http://csmres.jmu.edu/geollab/Fichter/IgnRx/IgnRx.html
-
8/13/2019 Igneous Rocks - Intrusive
20/46
Igneous Rock Chemistry
Rock chemistry, particularlysilica(SiO2) content, determinesmineral content and general color of igneous rocks
Maficrocks have ~50% silica, by weight, and contain dark-colored
minerals that are abundant in iron, magnesium and calcium
Intrusive/extrusive mafic rocks - gabbro/basalt
Felsic(silicic)rocks have >65% silica, by weight, and contain light-
colored minerals that are abundant in silica, aluminum, sodium and
potassium
Intrusive/extrusive felsic rocks - granite/rhyolite
Intermediaterocks have silica contents between those of mafic and
felsic rocks
Intrusive/extrusive intermediate rocks - diorite/andesite
Ultramaficrocks have
-
8/13/2019 Igneous Rocks - Intrusive
21/46
Intrusive Rock Bodies
Intrusive rocks exist in bodies or structures that penetrate
or cut through pre-existing country rock
Intrusive bodiesare given names based on their size,
shape and relationship to country rock
Shallow intrusions:Dikes and sills
Form
-
8/13/2019 Igneous Rocks - Intrusive
22/46
Intrusive Rock Bodies
Intrusive rocks exist in bodies or structures that penetrate
or cut through pre-existing country rock
Intrusive bodiesare given names based on their size,
shape and relationship to country rock
Deep intrusions: Plutons
Form at considerable depth beneath
Earths surface when rising blobs of
magma (diapirs) get trapped within
the crust
Crystallize slowly in warm
country rock
Generally composed of
coarse-grained rocks
-
8/13/2019 Igneous Rocks - Intrusive
23/46
Intrusive Rock Bodies Volcanic neck
Shallow intrusion formed when magmasolidifies in throat of volcano
Dike
Tabular intrusive structure that cuts across
any layering in country rock
Sill
Tabular intrusive structure that parallels
layering in country rock
Pluton
Large, blob-shaped intrusive body formed
of coarse-grained igneous rock, commonly
granitic
Small plutons (exposed over 100 km2) are called batholiths
Light-colored dikes
Basaltic sill
Sierra Nevada batholith
-
8/13/2019 Igneous Rocks - Intrusive
24/46
Devils Tower; a volcanic neck, a feeder pipe
-
8/13/2019 Igneous Rocks - Intrusive
25/46
Shiprock, New Mexico; a volcanic neck
-
8/13/2019 Igneous Rocks - Intrusive
26/46
Rhumski, Cameroon; a volcanic neck
-
8/13/2019 Igneous Rocks - Intrusive
27/46
Sill; parallels layers in the country rock
-
8/13/2019 Igneous Rocks - Intrusive
28/46
Dike; cuts across layers in the country rock
-
8/13/2019 Igneous Rocks - Intrusive
29/46
Half Dome; part of the Sierra Nevada batholith
-
8/13/2019 Igneous Rocks - Intrusive
30/46
Magma formationWhy rocks melt
Heat from below
Heat upward (by
conduction and
convection) from the
very hot (>5000C) corethrough the mantle and
crust
Rate at which
temperature increaseswith increasing depth
beneath the surface is the
geothermal gradient
-
8/13/2019 Igneous Rocks - Intrusive
31/46
Heat from the mantle causes rocks at depth to melt.
-
8/13/2019 Igneous Rocks - Intrusive
32/46
Rocks can melt as they rise and decompress
Melting point of
minerals generally
increases withincreasing pressure.
Decompression
melting can occur
when hot mantle rockmoves upward and
pressure is reduced
enough to drop
melting point to the
temperature of therising rock body
-
8/13/2019 Igneous Rocks - Intrusive
33/46
Hot water under
pressure
Water becomes
increasingly reactive
at higher
temperatures
At sufficient
pressures and
temperatures, highly
reactive water vapor
can reduce themelting point of
rocks by over 200C
Insert new Fig. 3.18 here
With H2O
Without H2O
Adding water lowers the temperature at which rocks melt.
-
8/13/2019 Igneous Rocks - Intrusive
34/46
Mineral mixtures
Mixtures of
minerals, such as
quartz and
potassium feldspar,
can result in themelting of both at
temperatures
hundreds of degrees
lower than either
mineral would melton its own
Mixtures melt at lower temperatures
-
8/13/2019 Igneous Rocks - Intrusive
35/46
Summary:
Crustal rocks are heated from below
Rocks can melt during decompression
Rocks can melt because water orvolatiles are added to them
Mixing magmas can reduce the meltingtemperature
-
8/13/2019 Igneous Rocks - Intrusive
36/46
Magma Crystallization andMelting Sequence
Minerals crystallize in a predictable order (and meltin the reverse order), over a large temperature range,
as described byBowens Reaction Series
Discontinuous branch
Ferromagnesian minerals (olivine, pyroxene, amphibole,biotite) crystallize in sequence with decreasing temperature
As one mineral becomes chemically
unstable in the remaining magma,
another begins to form
Continuous branch
Plagioclase feldspar forms with a
chemical composition that evolves
(from Ca-rich to Na-rich) with
decreasing temperatureBowens Reaction Series
-
8/13/2019 Igneous Rocks - Intrusive
37/46
-
8/13/2019 Igneous Rocks - Intrusive
38/46
Six common Igneous Rocks
Volcanic
Rocks
Silica
Content
MineralsPresent (in order of
abundance)
Plutonic
Rocks
Basalt low pyroxene,olivine, feldspar,
& amphibole
Gabbro
Andesite intermediate feldspar,amphibole,
pyroxene, biotitemica
Diorite
Rhyolite high feldspar, quartz,muscovite mica,
& amphibole
Granite
Solidifying Temperature 500 C1000 C
LighterCol
or
Increasing Grain Size
-
8/13/2019 Igneous Rocks - Intrusive
39/46
Lessons from Bowens Reaction Series
Large variety of igneous rocks is produced by large
variety ofmagma compositions
Mafic magmaswill crystallize into basaltorgabbroif
early-formed minerals are not removed from the magma
Intermediate magmaswill similarly crystallize into
dioriteor andesiteif minerals are not removed
Separation of early-formed ferromagnesian minerals
from a magma body increases the silica content of the
remaining magma
Minerals melt in the reverse order of that in which they
crystallize from a magma >>>> partial melting!!!
-
8/13/2019 Igneous Rocks - Intrusive
40/46
Magma Evolution
A change in the composition of a magmabody is known as magma evolution
Magma evolution can occur by
differentiation,partial melting,
assimilation, or magma mixing Differentiationinvolves the changing of
magma composition by the removal of
denser early-formed ferromagnesian
minerals by crystal settling Partial meltingproduces magmas less
mafic than their source rocks, because
lower melting point minerals are more
felsic in composition
-
8/13/2019 Igneous Rocks - Intrusive
41/46
Magma chamber fills
Early formed mafic mineralscrystallize and settle (or areotherwise separated from theresidual melt)
The remaining melt isenriched in silica
Differentiation
M E l ti I
-
8/13/2019 Igneous Rocks - Intrusive
42/46
Magma Evolution I
Assimilationoccurs
when a hot magma
melts and
incorporates morefelsic surrounding
country rock
Xenolith
Insert new
Fig. 3.22 here
M E l ti II
-
8/13/2019 Igneous Rocks - Intrusive
43/46
Magma mixing
involves the
mixing of more
and less maficmagmas to produce
one of intermediate
composition
Magma Evolution II
-
8/13/2019 Igneous Rocks - Intrusive
44/46
Rock types and tectonic setting
Igneous Activity and
-
8/13/2019 Igneous Rocks - Intrusive
45/46
Igneous Activity andPlate Tectonics
Igneous activity occurs primarily at or
near tectonic plate boundaries
Mafic igneous rocks are commonly
formed at divergent boundaries
Increased heat flow and decreased
overburden pressure produce mafic magmas
from partial melting of the asthenosphere
Intermediate igneous rocks arecommonly formed at convergent
boundaries
Partial melting of basaltic oceanic crust
produces intermediate magmas
Igneous Activity and
-
8/13/2019 Igneous Rocks - Intrusive
46/46
Igneous Activity andPlate Tectonics
Felsic igneous rocks are
commonly formed adjacent
to convergent boundaries
Hot rising magma causes
partial melting of the granitic
continental crust
Intraplate volcanism
Rising mantle plumes can
produce localized hotspots and
volcanoes when they produce
magmas that rise through
oceanic or continental crust
Hawaii is an example