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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 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

http://myweb.cwpost.liu.edu/vdivener/notes/igneous.htm Bowen’s Reaction Series

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

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

Composition Formation Dominant Silica content Temperature

Ultramafic Very high Olivine, Very low (<45%)

Mafic High Olivine, pyroxene, low Ca-plagioclase

Intermediate Medium Na-Plagioclase, moderate amphibole, biotite

Felsic Medium-low Orthoclase, , high (>65%) muscovite, biotite 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) 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” )

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 or ignimbrites

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

Ignimbrite Pyroclastic Igneous Rocks

During pyroclastic eruptions, “volcanic ” is produced in copious quantities (more so than ash and rock).

• (H20) • (CO2) • (HCl)

• dioxide (SO2) • (N2O) •Hydrogen cyanide (HCN) • (Ar) • (He) Other “pyroclastic” rocks

Volcanic Bombs

Tuff

Rhyolite Chalk board Felsic/Intermediate Extrusive Igneous Rocks Felsic/Intermediate Extrusive Igneous Rocks

Rhyolite Mt. St. Helens 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 composition and color. 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 Pillow basalt forms at divergent plate boundaries like the Mid- Atlantic Ridge 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.