Welcome Geology 101 Ch 3 b Igneous Rocks Dr Ken Sprenke www'mines'uidaho'edusprenke

1
WelcomeGeology 101Ch 3 bIgneous RocksDr Ken
Sprenkewww.mines.uidaho.edu/sprenke
2
Granite -an igneous rock -an intrusive rock -from
felsic magma...
3
Granite -light colored (felsic) -large crystals
(phaneritic)...
4
Granite -from cool, viscous, high silica
magma -contains mostly nonferromagnesian
silicates.
5
The next words of wisdom are going to intrude on
the screen just like granite intruded at
Yosemite. Mantle
Vast mountain ranges are often cored by
granite. The High Sierra and the Idaho batholith
are the largest granitic intrusions in North
America. The granite formed deep in the crust
and was later uplifted to form the mountains..
6
Lava flows are extrusive...
The basalt this geologist is sampling is not
intrusive.
7
Granite forms deep in the crust from magma
intrusion..
8
HOT
COLD
Minerals crystallize from magma at different
temperatures..
9
As magma cools, ferromagnesian silicates
crystallize first, nonferromagnesian minerals
crystallize last...
10
Water lowers the melting temperature of rock.
When ocean plates are subducted, andesitic
magmas are formed...
11
Andesitic magmas have high viscosity. They
explode if extruded..
12
A viscous andesitic magma extrudes and explodes
(May, 1980)..
13
The rover on Mars discovered that these rocks are
andesitic. Some think this means that Mars once
had plate tectonics. Andesites on Earth are
formed in subduction zones. But the great
volcanoes on Mars do not appear to be explosive.
14
(No Transcript)
15
Intermediate (andesitic) magmas can also be
formed by magma mixing when two different magma
chambers collide.
16
A intrusive igneous magma chamber. The heavy
ferromagnesian silicate crystals settle to
bottom. Inclusions of country rock are
assimilated into the magma....
17
The mafic (dark) rocks in this light (felsic)
granite are inclusions of the country rock into
which the granite intruded.
18

• Because cooling of the magma takes place at a
  different rate, the crystals that form exhibit
  different properties.
• Fast cooling on the surface results in aphanitic
  texture (crystals cannot be distinguished with
  the naked eye), or obsidian (volcanic glass).
• Slow cooling at depth in the earth results in
  fewer much larger crystals, gives rise to
  phaneritic texture..

19
Aphanitic
Phaneritic..
20
What texture does this piece of fine-grained
basalt have? Aphanitic..
21
What texture does this piece of volcanic obsidian
have? Glassy
22
What texture does this piece of coarse-grained
granite have? Phaneritic
23
Porphyritic texture develops when slow cooling is
followed by rapid cooling. Phenocrysts
larger crystals, matrix or groundmass smaller
crystals.
24
This igneous rock has large crystals in a
fine-grained matrix. What is its texture?
Porphyritic
25
This piece of granite has huge phenocrysts. What
is its texture? Porphyritic.
26
What is this glassy extrusive volcanic rock
? Obsidian
27
This extrusive igneous rock is full of holes
caused by gas expansion. The holes are called
vesicles. The texture is called vesicular.
28
This felsic extrusive igneous rock is so full of
holes that it floats like a sponge PUMICE.
29
This welded volcanic ash is an igneous rock with
a pyroclastic texture. TUFF..
30
Lets summarize. Magmas can be felsic (light,
cool, very viscous, high silica,
non-ferromagnesian) intermediate (andesitic,
viscous) mafic (dark, hot, fluid, heavy, low
silica, ferromagnesian) Common textures
are aphanitic (fine-grained) phaneritic
(course-grained) Now we will use these
properties to classify igneous rocks...
31
The chart above summarizes how igneous rocks are
classified.
32
This coarse-grained rock is mostly olivine. What
is it? Peridotite (from the mantle)
33
This vesicular rock is fine-grained and rich
in ferromagnesian minerals including some
olivine. What is it? basalt
34
This rock is coarse-grained and has some olivine
and lots of pyroxene. What is it? gabbro
35
This rock has big grains of plag feldspar,
hornblende, and even a little quartz. What is
it? Diorite
36
This is a light-colored fine-grained igneous
rock. What is it? Rhyolite
37
This coarse-grained light-colored igneous rock
has only minor amounts of dark minerals, but some
quartz and lots of orthoclase feldspar. What is
it? GRANITE
38
This rock has huge crystals of quartz, feldspar,
and biotite. What is it? Granite (pegmatite)
39
Tourmaline is a semiprecous gem found in
pegmatites...
40
PLUTONS Intrusive igneous bodies
41

• Plutons
• Igneous rocks cooled at depth. Name comes from
  Greek god of the underworld - Pluto.
• Dikes - a small tabular intrusion that cuts
  across layers of pre-existing rock.Sills - a
  tabular intrusion that intruded parallel to
  layers of pre-existing rock. Lacoliths - similar
  to a sill, except the layers above are bent
  upward by the intrusion. Volcanic Necks - a
  former volcanic conduit that has been exposed by
  erosion of the volcano. Batholiths - a large
  intrusion usually of granitic or dioritic
  composition. Stocks - a somewhat smaller
  intrusion usually intruded off of a larger
  batholith.

42
This pluton cuts across rock layers in the
country rock.What is it? DIKE
43
A batholith is a pluton that is at least 100
square kilometers. Yosemite is in the Sierra
Nevada batholith...
44
Beautiful Butte, Montana. A copper deposits on
the edge of the Boulder batholith. Mineral
deposits are often associated with plutons.
45
Shiprock is an old volcanic neck but what is the
pluton on the left? A dike.
46
A volcanic neck is a volcanic pipe that has been
exposed by erosion.
47

• Methods of intrusion
• Melting - crystallization
• Stoping - xenoliths
• Injection

48
If a batholith forcefully intrudes, it deforms
the country rock. If a batholith is emplaced by
stoping, the country rock is not deformed..
49
Why do we see intrusive igneous rocks at the
surface of the Earth? Answer - They are exposed
by erosion which has removed all of the material
above the intrusion...
50
Read and master Chapter 3 of the textbook.