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 depends on: how fast/slow magma cools Mineral Composition depends on: chemical makeup of parent magma Cooling rate in igneous rocks is determined by Fig 6.15 cooling rate, which is in turn, determined by proximity to the surface W W Norton Coarse-grained Fine-grained or “phaneritic” or “aphanitic” Fig 6.16a 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 Mm 0.5 Fig 6.14cd Stephen Marshak Fig 6.12 W W Norton Fig 6.19 Stephen Marshak Naming Igneous Rocks Basic magma types: Mafic magnesium + ferrum High in Mg, Fe Dark, dense Felsic feldspar + silica (quartz) High in Si Lighter, less dense Naming Igneous Rocks: Upper mantle plutonic igneous rocks, “peridotite” or “dunite” (Near Globe, AZ) Ultramafic: ~ 40% Silica (less than gabbro/basalt, with more Fe and Mg) Naming Igneous Rocks: Glassy Volcanic Rocks Felsic Obsidian Pumice Volcanic Glass Volcanic “Froth” 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 Products of volcanic Fig 6.02 eruptions includes more W W Norton than just lava flows! Explosive Volcanic Eruptions Violently explosove volcanic eruptions produce: These accumulate to form: Rock fragments (all sizes) Finely-fragmented ash Molten bombs 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 Pumice: Formed by Formed by quenching of lava the quenching of gas-charged lava Volcanic ash: Lithifies to form a volcanic tuff [...]... Rhyolite volcanic Zooming in: Fig 6.17a W W Norton Mineral proportions after Hamblin and Howard Naming Igneous Rocks: Upper mantle plutonic igneous rocks, “peridotite” or “dunite” (Near Globe, AZ) Ultramafic: ~ 40% Silica (less than gabbro/basalt, with more Fe and Mg) Naming Igneous Rocks: Glassy Volcanic Rocks Felsic Obsidian Pumice Volcanic Glass Volcanic “Froth” Bowen’s Reaction Series Magmas don’t...Naming igneous rocks Mafic Aphanitic Gabbro Phaneritic plutonic Basalt volcanic Zooming in: Fig 6.17a Figure 6.17 Text, page 155 Naming igneous rocks Aphanitic Diorite Phaneritic volcanic Andesite plutonic In t e r m e d i a t e Zooming in Fig 6.17a Figure 6.17 Text, page 155 Naming igneous rocks Aphanitic Granite Phaneritic plutonic Felsic Rhyolite... Kresan Products of volcanic Fig 6.02 eruptions includes more W W Norton than just lava flows! Explosive Volcanic Eruptions Violently explosove volcanic eruptions produce: These accumulate to form: Rock fragments (all sizes) Finely-fragmented ash Molten bombs Pyroclastic Volcanic Rocks Fig 6.08a W W Norton Naming Igneous Rocks Pyroclastic volcanic rocks: Composed of rock fragments ejected during eruptions