In Lecture Today: Geology in the news presentation by Michael Zara. Classification of igneous rocks. Nature and classification of igneous plutons. Origins of magma and relationship to plate tectonic setting.

Naming Igneous Rocks Igneous rocks are classified by their: Texture Mineral Composition depends on: how fast/slow magma cools depends on: chemical makeup of parent magma

Fig. 6.15 W. W. Norton Cooling rate in igneous rocks is determined by which is in turn, determined by proximity to the surface. Fig. 6.15 W. W. Norton

Fig. 6.16a Coarse-grained or “phaneritic” Fine-grained or “aphanitic” 0 Mm 0.5 0 Mm 0.5 Grain size in igneous rocks is determined by cooling rate. Photod by Dr. Kent Ratajeski, Dept. Geology and Geophysics, University of Wisconsin, Madison

Fig. 6.14cd Stephen Marshak

Fig. 6.12 W. W. Norton

Fig. 6.19 Stephen Marshak

Mafic Felsic Naming Igneous Rocks High in Mg, Fe. Dark, dense Basic magma types: Mafic Felsic magnesium + ferrum High in Mg, Fe. Dark, dense feldspar + silica (quartz) High in Si. Lighter, less dense

Naming igneous rocks Basalt volcanic M a f i c Gabbro plutonic Zooming in: Basalt volcanic Aphanitic M a f i c Gabbro plutonic Phaneritic

Fig. 6.17a Figure 6.17 Text, page 155

Naming igneous rocks volcanic Andesite In t e r m e d i a t e plutonic Zooming in Andesite volcanic Aphanitic In t e r m e d i a t e Diorite plutonic Phaneritic

Fig. 6.17a Figure 6.17 Text, page 155

Naming igneous rocks volcanic Rhyolite F e l s i c plutonic Granite Zooming in: Rhyolite volcanic Aphanitic F e l s i c Granite plutonic Phaneritic

W. W. Norton. Mineral proportions after Hamblin and Howard. Fig. 6.17a W. W. Norton. Mineral proportions after Hamblin and Howard.

(less than gabbro/basalt, Naming Igneous Rocks: Ultramafic: ~ 40% Silica (less than gabbro/basalt, with more Fe and Mg). Upper mantle plutonic igneous rocks, “peridotite” or “dunite”. (Near Globe, AZ)

Obsidian Volcanic Glass F e l s i c Volcanic “Froth” Pumice Naming Igneous Rocks: Glassy Volcanic Rocks Obsidian Volcanic Glass F e l s i c Volcanic “Froth” Pumice

crystallize all at once! Bowen’s Reaction Series Magmas don’t crystallize all at once! Minerals with the highest melting temperatures come out first, followed by minerals with successively lower melting temperatures. The sequence of silicate mineral crystallization in magmas was first studied by University of Chicago geologist, N.L Bowen, in the 1920’s.

Bowen’s Reaction Series Box 6.2 Text, page 145. Fig. 6.06 W. W. Norton This diagram shows the order in which silicate minerals crystallize from magma and how that Relates to mineral content and rock type.

Basaltic lavas are very fluid and travel far from the vent to produce volcanoes with low profiles.

Basalt eruptions on land produce flows that travel great distances. Fig. 6.20a W. W. Norton Columbia River basalts

Basaltic lavas erupted under water produce “pillows” Fig. 6.21a W. W. Norton

Fig. 6.21b Stephen Marshak

Fig. 6.21c © Peter Kresan

Fig. 6.02 W. W. Norton Products of volcanic eruptions includes more than just lava flows! Fig. 6.02 W. W. Norton

Pyroclastic Volcanic Rocks Explosive Volcanic Eruptions Violently explosove volcanic eruptions produce: Rock fragments (all sizes) Finely-fragmented ash Molten bombs These accumulate to form: Pyroclastic Volcanic Rocks

Fig. 6.08a W. W. Norton

Naming Igneous Rocks Pyroclastic volcanic rocks: Composed of rock fragments ejected during eruptions. Types include: “Tuff”: Composed of ash, finely fragmented volcanic rock. Welded tuffs form when ash is so hot, it deforms plastically. Scoria: Red or black, frothy lava, denser than pumice. Volcanic Breccia: Coarse, angular rock fragments, usually in an matrix of fine to coarse ash.

Obsidian: Volcanic glass. Formed by quenching of lava. Pumice: Formed by the quenching of gas-charged lava. Obsidian: Volcanic glass. Formed by quenching of lava. Volcanic ash: Lithifies to form a volcanic tuff.