Chapter 4: Igneous Rocks Geology 121 Course Notes

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Classification of Igneous Rocks Pyroclasts Extrusive rocks Porphyry Intrusive rocks

Intrusive (plutonic)—Extrusive (volcanic) Pyroclasts Volcanic ash Bomb Pumice Extrusive rocks Porphyry Intrusive rocks

Intrusive (plutonic)—Extrusive (volcanic) Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Porphyry Intrusive rocks

Intrusive (plutonic)—Extrusive (volcanic) Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Felsic Basalt Rhyolite Porphyry Intrusive rocks

Intrusive (plutonic)—Extrusive (volcanic) Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Felsic Basalt Rhyolite Extrusive igneous rocks cool rapidly and are fine-grained. Porphyry Intrusive rocks

Intrusive (plutonic)—Extrusive (volcanic) Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Felsic Basalt Rhyolite Extrusive igneous rocks cool rapidly and are fine-grained. Porphyry Gabbro Granite Intrusive rocks

Intrusive (plutonic)—Extrusive (volcanic) Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Felsic Basalt Rhyolite Extrusive igneous rocks cool rapidly and are fine-grained. Porphyry Gabbro Granite Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form. Intrusive rocks

Intrusive (plutonic)—Extrusive (volcanic) Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Felsic Basalt Rhyolite Extrusive igneous rocks cool rapidly and are fine-grained. Porphyry Gabbro Granite Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form. Phenocrysts Intrusive rocks Porphyry

Intrusive (plutonic)—Extrusive (volcanic) Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Felsic Basalt Rhyolite Extrusive igneous rocks cool rapidly and are fine-grained. Porphyry Gabbro Granite Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form. Phenocrysts Intrusive rocks Some intrusive porphyritic crystals grow large, but the remaining melt cools faster, forming smaller crystals. Porphyry

The common rock forming minerals in the earth’s crust are made from just 8 elements.

NORMAN L. BOWEN (1887-1956) Queen’s, MIT and Geophysical Laboratory at the Carnegie Institute of Washington Pioneering work at the interface between geology with physical chemistry Studied the formation of igneous rocks in the laboratory April 29, 1948 Dear Professor Gilluly: Having had occasion some years ago to learn the art of lipreading I noticed yesterday when I was giving may paper that at the end of each of my sentences you said, "Horse shit". Evidently you had made special note of the word "equilibria" in the title of my paper and were from time to time reminding yourself and your neighbors of the gist of the discussion. You are, however, under a misapprehension as to the derivation of the word "equilibria". It does not come from equus = a horse and libria = things liberated or discharged, but is from quite different roots. If you will consult a chemist you will be able to learn the real significance of the word and I may add that I feel that one so highly placed in geological circles as you should make it a point to acquire some familiarity with the exact significance of common terms used in collateral sciences. Trusting that you will not resent my correction and suggestions, I am Yours sincerely, Norman L. B

BOWEN’S REACTION SERIES Magma composition Temperature Orthoclase feldspar ~600°C Muscovite mica Felsic, Rhyolitic (high silica) Quartz Biotite mica Sodium- rich Intermediate, andesitic Amphibole Plagioclase feldspar Mafic, basaltic Pyroxene Ultramafic (low silica) ~1200°C Simultaneous crystallization Olivine Calcium- rich

BOWEN’S REACTION SERIES As magma temperature decreases… …while plagioclase feldspar crystallizes, from calcium-rich sodium-rich form… …materials crystallize in an ordered series… Magma composition Temperature Orthoclase feldspar ~600°C Muscovite mica Felsic, Rhyolitic (high silica) Quartz Biotite mica Sodium- rich Intermediate, andesitic Amphibole Plagioclase feldspar Mafic, basaltic Pyroxene Simultaneous crystallization Ultramafic (low silica) ~1200°C Olivine Calcium- rich …and the composition of magma changes from ultramafic to andesitic.

Background info comparison The composition of tin-lead solder is adjusted to the ‘eutectic’ in order to achieve melting at the lowest temperature An important tin/lead solder is 63/37, called “eutectic,” meaning it goes directly from solid to liquid without a partially molten stage. This alloy melts at 361°F (183°C), the lowest melting point of any tin-lead alloy. Other eutectic solders include 62.5% tin, 36.1% lead and 1.4% silver (354°F, 179°C) 96% tin and 4% silver (430°F, 221°C); 97.5% lead and 2.5% silver (581°F, 305°C) 0.75% tin, 97.5 % lead and 1.75% silver (590°F, 310°C). (590°F, 31

Physical properties of melting and crystallization Binary eutectic

Fractional crystallization: Sandstone Basalt Mostly sodium-rich plagioclase feldspar; no olivine Calcium-rich plagioclase feldspar and pyroxene; no olivine 245–275 m (800–900 ft) Basaltic intrusion Olivine Basalt Sandstone Basalt cooled quickly at the edges of the intrusion.

Fractional crystallization: Pyroxene and plagioclase feldspar crystallize A gradient of pyroxene and feldspar is established. Plagioclase feldspar continues to crystallize. Olivine crystallizes first

Complex petrogenetic histories begin with Partial Melting: Partial melting creates a magma of a particular composition. Blobs of melt (solidified) in a rock (migmatite) Magma chamber A Crystallizing minerals Magma chamber B Partial melting of country rock Basaltic magma

…more fractional crystallization Partial melting creates a magma of a particular composition. A basaltic magma chamber breaks through. Cooling causes minerals to crystallize and settle. Mixing results in andesitic magma. Magma chamber A Magma chamber A Crystallizing minerals Crystals may accumulate on the sides and roof of the chamber due to turbulence. Magma chamber B Magma chamber B Partial melting of country rock Basaltic magma Magma mixing

Dikes cut across layers of country rock… Lava flow Ash falls and pyroclasts Country rock Volcano Volcanic neck with radiating dikes Stock Dike Sill Dike Dikes cut across layers of country rock… Sill Dike Sill Pluton Batholith …but sills run parallel to them. Batholiths are the largest forms of plutons, covering at least 100 km2.

Assimilation of country rock Rising magma wedges open overlying country rock. The magma melts the surrounding rock... …which changes the magma’s composition. Country rock The magma also breaks off xenoliths that sink into the magma.

A sill runs parallel to country rock layers A dike cuts across layers Dike

Intermediate Island arc volcanoes, Java, Indonesia ISLAND ARC PLATE SUBDUCTION Mafic to intermediate intrusives (plutonism) Mafic to intermediate extrusives (volcanism) Island arc volcano Subduction zone Oceanic lithosphere

Plate divergence boundary, Mid-Atlantic Ridge, Iceland Mafic PLATE DIVERGENCE Basatic extrusives Basaltic intrusives Mid-ocean ridge Partial melting Of upper mantle Rising magma

Volcanoes National Park, Hawaii Hot-spot volcano, Volcanoes National Park, Hawaii Mafic HOT-SPOT VOLCANISM Basatic extrusives Basaltic intrusives Hot-spot volcano Mantle plume (hot spot) Mantle

Intermediate to Felsic Continental margin volcano, Mt. Rainier, Washington Intermediate to Felsic HOT-SPOT VOLCANISM Mafic to felsic intrusives Mafic to felsic extrusives Continental margin volcano Subduction zone Oceanic crust Continental crust Continental mantle lithosphere Oceanic lithosphere

Mafic Sediment layers OPHIOLITE SUITE Deep-sea sediments Pillow basalts Pillow basalt Sheeted dike complex Thin section of gabbro Dikes Gabbro Peridotites and other ultramafic rocks Thin section of peridotite

Dikes intrude dikes to form sheeted dikes. A thin dike erupts, spilling lava in “pillows.” Dikes Dikes intruding dikes Hot mantle rises, decompresses, and melts. Dikes intrude dikes to form sheeted dikes. Pillow lava Newer, thinner sediments Older, thicker sediments Sediments are deposited on the spreading seafloor. Sheeted dikes in basalt Oceanic crust Gabbro The gabbro layer metamorphoses by contact with the magma. Moho Mantle Peridotite layer Spreading center Cold seawater Heated seawater carrying dissolved minerals Magma chamber Sheeted dikes Peridotite layer Mantle Seawater filters through the basalt layer, where it is heated. The heated seawater then rises. Dissolved minerals precipitate in the ocean. Crystals settle out of the magma, forming the peridotite layer.

Magma of intermediate composition is erupted to form arc volcanoes. Molten sediments combine with lithospheric magma. Trench Oceanic sediments Magma chamber Oceanic crust basalt Oceanic mantle lithosphere The water and molten sediments melt parts of the overlying plate. Asthenosphere Subducting oceanic crust carries sediments with it. H2O H2O H2O …causing the sedimentary rocks to melt at lower temperatures. Water remains trapped as the pressure and temperature increase. Sediment grains The trapped water is released as the temperature increases,… Water