Figure 3.1
The beginning of the rock cycle magma from the mantle produces igneous rocks near the Earth surface
Magma molten rock
Lava magma on the Earth surface
Magma and igneous rocks can be:
Extrusive on the Earth surface
Intrusive magma solidifies underground
Figure 3.2 The rock cycle a plate-tectonic example
Andes-type convergent margin
All of the major processes in the rock cycle
Coarse grained mineral grains greater
than 1 mm, can see with the
eye
magma cools slowly, enough
time to
form large crystals
Intrusive
rocks are very
good insulators
allows slow cooling
Fine grained mineral grains smaller
than 1 mm
magma cools quickly
glass forms when quenched
quickly,
such as lava flowing into
water
Extrusive
Figure 3.4 Contact
metamorphism
Country rock the pre-existing rock that magma intrudes into
Xenolith a broken-off piece of country rock that is floating in the
magma
Magma
type |
Coarse
grained |
Fine
grained |
Mafic
|
Gabbro |
Basalt |
Intermediate |
Diorite |
Andesite |
Silicic |
Granite |
Rhyolite |
Example of an
intrusive granite from a batholith:
Figure 3.5
Hand specimen note coarse texture, different mineral grains
Thin section note interlocking mineral grains
Classification Chart for Igneous Rocks Figure 3.6
Main trends in
chemical composition and mineral composition
from ultramafic (mantle) to silicic
(continental crust) igneous rocks:
increasing silica and aluminum
increasing Na, then K in the feldspars
decreasing Ca, Fe, and Mg
Granite and rhyolite silicic
composition Figure
3.7
mostly quartz and K-feldspar
highest concentrations of Si and Al
Diorite and andesite intermediate composition
little or no quartz
mostly Na-feldspar
abundant amphibole and pyroxene
little or no olivine
Olivine and gabbro (and peridotite)
mafic (and ultramafic) composition
almost entirely olivine and
Ca-feldspar, with some pyroxene
highest concentrations of Ca, Fe, Mg
*** Know intrusive and
extrusive equivalents ***
(coarse-grained and fine-grained rocks in chart above)
Porphyry (or porphyritic) extrusive igneous rock that has large crystals embedded in a fine-grained matrix; formed by slow cooling of an initial magma (to form large crystals) followed by a second injection of magma that extruded onto the Earth surface (to form the fine-grained matrix)
Figure 3.9
Most volcanism occurs in
tectonically active areas (lots of stress, and fractured rock)
Dike a thin, sheet-like or tabular intrusion that cuts across existing
rock layers
discordant Figure 3.7
Ship Rock, New Mexico Figure 3.8
Sill a thin, sheet-like or tabular intrusion that was injected between
existing rock layers
concordant
Magma diapirs and plutons Figure 3.12
A stock is less than 100 square kilometers, a batholith is larger
Figure 3.13 Forming a batholith
Sierra Nevada batholith
Figure 3.14
How magma forms
The geothermal gradient
Is about 3o C in 100 m Figure
3.15
higher thermal gradient in volcanic
regions
Factors that control melting temperatures Figure 3.16
Pressure
Water
Mixed minerals Figure
3.17
analogous to putting salt on the ice
on your sidewalk, lowers the melting point
discrete
minerals
olivine
pyroxene
amphibole
biotite mica
solid-solution
series in the feldspars
(can have varying percentages of Ca, Na, and K)
Ca-feldspar
Na-feldspar
K-feldspar
Both branches converge to
K-feldspar, quartz, and muscovite at the silicic end
Differentiation Assimilation Mixing
High-temp minerals
freeze out first Figure 3.19
accumulate at the bottom of the magma
chamber
Melted country rock is incorporated into the
magma Figure 3.20
The magma now has a different
composition (most rocks near the Earth
surface have much more silica and alumina than magma from the mantle)
Silicic magmas are thick and viscous Figure 3.21
Mafic magmas are thin and flow easily
Divergent boundaries seafloor spreading Figure
3.22
Mantle plume under continental crust Figure
3.22
Convergent margins injection of water into mantle from subducting
plate lowers the melting point of mantle rocks, producing magma Figure 3.22
Observation: many batholiths
are mostly granite, not diorite
which means that
continental crust is melting to produce the magma for the batholith
Figure 3.26
Intermediate magma produces diorite
Heating of crust produces granite
Granite batholith, Chile
Figure
3.4
Washington-Oregon coast Juan de Fuca Ridge offshore Figure 3.25
very shallow subduction angle because the
oceanic crust is still hot, and low density