gy 111: physical geology · the eruptions that produce pyroclastic rocks include phreatic, plinian...

GY 111: Physical Geology

UNIVERSITY OF SOUTH ALABAMA

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

Source http://www.ltcconline.net/julian

Bowen’s Reaction Series

Igneous Rock Composition

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%)

Source: http://hyperphysics.phy-astr.gsu.edu

Igneous Rock Texture

Extrusive Rocks (Rapid Cooling; non visible* crystals) Intrusive Rocks (slow cooling; 100 % visible crystals) *with a hand lens

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: 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.


Volcanic ash


Pumice

Volcanic ash


Volcanic ash

Obsidian


Ignimbrite

Nuee ardents and pyroclastic flows may deposit volcanic breccias or ignimbrites

http://ougseurope.org/trips/santorini/santorini/Picture04.jpg


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

Tuff

Rhyolite

Volcanic Bombs

Chalk board

Felsic/Intermediate Extrusive Igneous Rocks

Felsic/Intermediate Extrusive Igneous Rocks

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.

Mt. St. Helens Lava dome Rhyolite

Andesite

Mafic Extrusive Igneous Rocks


Mafic extrusive igneous rocks have relatively low SiO2 contents in their bulk chemistry. Ultramafic extrusive igneous rocks are relatively rare today.

Basalt

Scoria

http://z.about.com/d/geology/1/0/F/W/scoria.jpg


Mafic extrusive igneous rocks erupted under water produce a unique structure called “pillows”. Pillow basalt forms at divergent plate boundaries like the Mid-Atlantic Ridge

Pillow Basalt

Today’s Homework 1. Study; Lecture test 1 next Tuesday

Next Time 1. Quiz 5; fill in the blanks

2. Intrusive igneous rocks

GY 111: Physical Geology

Lecture 9: Extrusive Igneous Rocks

Instructor: Dr. Doug Haywick

[email protected]

This is a free open access lecture, but not for commercial purposes. For personal use only.

gy 111: physical geology · the eruptions that produce pyroclastic rocks include phreatic, plinian...

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