UNIVERSITY OF SOUTH ALABAMA
GY 111: Physical Geology
Lecture 9: Extrusive Igneous Rocks
Instructor: Dr. Douglas W. Haywick
Last Time
1) The chemical composition of the crust
2) Crystallization of molten rock
3) Bowen's Reaction Series
Web notes 8
Chemical Composition of the Crust
Element
Wt%
% of atoms
Oxygen
46.6
60.5
Silicon
27.7
20.5
Aluminum
8.1
6.2
Iron
5.0
1.9
Calcium
3.6
1.9
Sodium
2.8
2.5
Potassium
2.6
1.8
Magnesium
2.1
1.4
All other elements
1.5
3.3
Crystallization of Magma
http://myweb.cwpost.liu.edu/vdivener/notes/igneous.htm
Bowen’s Reaction Series
Source http://www.ltcconline.net/julian
Igneous Rock
Composition
Source: http://hyperphysics.phy-astr.gsu.edu
Composition
Formation
Temperature
Dominant
Minerals
Silica content
Ultramafic
Very high
Olivine, pyroxene
Very low (<45%)
Mafic
High
Olivine, pyroxene,
Ca-plagioclase
low
Intermediate
Medium
Na-Plagioclase,
amphibole, biotite
moderate
Felsic
Medium-low
Orthoclase, quartz,
muscovite, biotite
high (>65%)
Igneous Rock Texture
Extrusive Rocks (Rapid Cooling; non visible* crystals)
Intrusive Rocks (slow cooling; 100 % visible crystals)
*with a hand lens
Igneous Rock Texture
Igneous Rock Texture
Today’s Agenda
1) Pyro-what? (air fall volcanic rocks)
2) Felsic and Intermediate Extrusive Rocks
3) Mafic Extrusive Rocks
Web notes 9
Pyroclastic Igneous Rocks
Pyroclastic Igneous Rocks
Pyroclastic: Pyro means “fire”. Clastic means particles;
both are of Greek origin.
Pyroclastic Igneous Rocks
Pyroclastic: Pyro means “fire”. Clastic means particles;
both are of Greek origin.
Pyroclastic rocks are usually erupted from composite
volcanoes (e.g., they are produced via explosive eruptions
from viscous, “cool” lavas)
Pyroclastic Igneous Rocks
Pyroclastic: Pyro means “fire”. Clastic means particles;
both are of Greek origin.
Pyroclastic rocks are usually erupted from composite
volcanoes (e.g., they are produced via explosive eruptions
from viscous, “cool” lavas)
The eruptions that produce pyroclastic rocks include
phreatic, plinian and ultraplinian.
Pyroclastic Igneous Rocks
Pyroclastic: Pyro means “fire”. Clastic means particles;
both are of Greek origin.
Pyroclastic rocks are usually erupted from composite
volcanoes (e.g., they are produced via explosive eruptions
from viscous, “cool” lavas)
The eruptions that produce pyroclastic rocks include
phreatic, plinian and ultraplinian.
These rocks cool extremely rapidly. Textures are glassy to
aphanitic.
Pyroclastic Igneous Rocks
Pyroclastic Igneous Rocks
Volcanic ash
Pyroclastic Igneous Rocks
Volcanic ash
Pumice
Pyroclastic Igneous Rocks
Volcanic ash
Obsidian
Pyroclastic Igneous Rocks
Nuee ardents and
pyroclastic flows may
deposit volcanic breccias
or ignimbrites
http://ougseurope.org/trips/santorini/santorini/Picture04.jpg
Ignimbrite
Pyroclastic Igneous Rocks
During pyroclastic eruptions,
“volcanic gas” is produced in
copious quantities (more so
than ash and rock).
•water vapor (H20)
•carbon dioxide (CO2)
•hydrogen chloride (HCl)
•sulfur dioxide (SO2)
•nitrogen oxide (N2O)
•Hydrogen cyanide (HCN)
•argon (Ar)
•helium (He)
Other
“pyroclastic”
rocks
Volcanic Bombs
Tuff
Rhyolite
Chalk board
Felsic/Intermediate Extrusive Igneous Rocks
Felsic/Intermediate Extrusive Igneous Rocks
Rhyolite
Mt. St. Helens Lava dome
Felsic and intermediate rocks are
classified by the amount of SiO2 in
their bulk chemistry (they have
relatively high SiO2 content). We can
distinguish them on the basis of
mineral composition and color.
Andesite
Mafic Extrusive Igneous Rocks
Mafic Extrusive Igneous Rocks
Basalt
http://z.about.com/d/geology/1/0/F/W/scoria.jpg
Mafic extrusive igneous rocks have
relatively low SiO2 contents in their
bulk chemistry. Ultramafic extrusive
igneous rocks are relatively rare
today.
Scoria
Mafic Extrusive Igneous Rocks
Mafic extrusive igneous rocks
erupted under water produce a
unique structure called “pillows”.
Pillow basalt forms at divergent
plate boundaries like the MidAtlantic Ridge
Pillow Basalt
Today’s Homework
1. Study; Lecture test 1 next Tuesday
Next Time
1.
2.
Quiz 5; fill in the blanks
Intrusive igneous rocks
GY 111: Physical Geology
Lecture 9: Extrusive Igneous Rocks
Instructor: Dr. Doug Haywick
dhaywick@southalabama.edu
This is a free open access lecture, but not for commercial purposes.
For personal use only.