The Rock Cycle Revisited Chapter 11 Geology Today Barbara W. Murck & Brian J. Skinner N. Lindsley-Griffin, 1999 MOUNT EVEREST, HIMALAYA MTNS.

Earliest history of solar system = intense bombardment by meteorites and planetesimals Impacts became less common after 4.0 b.y. but continue today. Meteorite, Alberta (Fig. 11.1, p. 297) Planet-Shaping Processes N. Lindsley-Griffin, 1999

Planet-Shaping Processes Impact craters: flat floors raised rims blankets of ejecta around them Barringer Meteor Crater, > 50,000 yrs old near Flagstaff, Arizona (Fig. 11.2, p. 297) N. Lindsley-Griffin, 1999

Planet-Shaping Processes Impact craters: “Shock” metamorphism - intense heat, pressure Diamonds and high-P quartz Glass beads Iridium-rich clay layers Brecciated rocks at depth Nordlingen Cathedral, Ries Crater Germany (Fig. 11.3, p. 298) N. Lindsley-Griffin, 1999

Impacts and their side effects have affected Earth and its life forms many times in the past, and will continue to do so in the future. N. Lindsley-Griffin, 1999 Planet-Shaping Processes Leonid Meteor Shower, 1998

Earth - the Tectonic Planet Plate tectonics became the dominant planet-shaping process on Earth at least 3 billion years ago (based on ages of oldest deformed rocks). No other terrestrial planet appears to have active plate tectonics today N. Lindsley-Griffin, 1999 Popocatepetl volcano, Mexico 1998 eruption

Structure of a Continent Cratons - stable continental crust, free of deformation for at least 1 b.y. (dark brown). Surrounded by Orogens of successively younger ages (light orange, tan) N. Lindsley-Griffin, 1999

Structure of a Continent Continental shields are the old cores of continents: Precambrian granite intruding gneiss, schist, greenstone (lava) Platforms overlie shields: generally flat-lying strata, Paleozoic and younger Houghton-Mifflin, 1998; N. Lindsley-Griffin, 1999 Cratons are made up of shields and platforms

Structure of a Continent Orogens - elongate regions of crust intensely deformed and metamorphosed during continental collisions Age of folding and faulting is younger than age of rocks deformed. N. Lindsley-Griffin, 1999 Plunging anticlines and synclines - Appalachian orogen, 300 m.y. old See Fig. 11.6, p. 301

Mountain Building Orogens form by repeated collisions of oceanic terranes such as volcanic arcs, old ocean ridges, and hot spot islands. Cordilleran mountain belt consists of many small crustal fragments, each with a different history before its accretion to North America. N. Lindsley-Griffin, 1999

Mountain Building Orogens are created by subduction and related folding and faulting. Material on oceanic plate is scraped off and added to edge of continent. N. Lindsley-Griffin, 1999; Dolgoff, 1998

Mountain Building Orogens are also affected by changes in the type of plate boundary. Subduction of small ocean plates may change a convergent margin to a transform margin. N. Lindsley-Griffin, 1999; Dolgoff, 1998

N. Lindsley-Griffin; : Dolgoff 1998 Mountain Building Here, the tiny Juan de Fuca plate is being destroyed along the Cascadia trench. The subduction zone shortens while the San Andreas Fault lengthens northward.

Indian Microcontinent 80 m.y. ago, India breaks off from Africa as Pangea separates Over 80 m.y., the ocean crust subducts under Asia N. Lindsley-Griffin; Dolgoff, 1998 Himalaya Mountains

Indian microcontinent, imbedded in the ocean crust, moves north When all the ocean crust subducts under Asia, India smashes into Asia Continental crust is buoyant, and subducts only a short distance before stopping N. Lindsley-Griffin; Dolgoff, 1998 Himalaya Mountains

The partially subducted, buoyant continental crust pushes up the mountains like a beach ball pushed under water will support a human N. Lindsley-Griffin; Dolgoff, 1998 Himalaya Mountains

Granitic plutons, andesite generated by ocean-continent convergence Marine sediments deposited in Tethys sea along active margin of Asia and passive margins of India India near the end of its 80 m.y. journey north: Himalaya Mountains N. Lindsley-Griffin; Dolgoff, 1998

India and Asia collide, crushing the sedimentary wedges between them and producing: Folding and faulting. Regional metamorphism. Crustal uplift. Himalaya Mountains N. Lindsley-Griffin; Dolgoff, 1998

Today: India has been added to Asia along a suture zone marked by: Deformed oceanic lithosphere (ophiolites). Deformed Tethys sedimentary wedge Granitic batholiths Very thick continental crust Himalaya Mountains N. Lindsley-Griffin; Dolgoff, 1998

Isostasy Isostasy -- the flotational balance of lithosphere on asthenosphere N. Lindsley-Griffin, 1999 Fig. 11.7, p. 301 Mountains have thick roots of continental crust beneath them. Low density helps maintain high elevations.

How do we know that granitic roots extend down into the mantle like a ship’s keel? A plumb-bob should be gravitationally attracted toward high mountains by thick rocks piled on mantle. Actually, the plumb-bob swings less than predicted, suggesting that mountains are underlain by deep roots of less dense rock. (N. Lindsley-Griffin, 1999; Source: Dolgoff 1998) Isostasy and Mountains

Igneous Rock and Plate Tectonics Plate tectonics controls how rock melts and what is produced. Melting occurs by: Increasing T Decreasing P Adding water N. Lindsley-Griffin, Russian stratovolcanoes

N. Lindsley-Griffin, 1999 Fig. 11.8, p. 303 Dry melting temperature is pressure sensitive - the higher the pressure, the higher the temperature must be to melt the rock.

N. Lindsley-Griffin, 1999 Fig. 11.8, p. 303 Decompression melting occurs when hot rock rises through mantle and pressure decreases

N. Lindsley-Griffin, 1999 Fig. 11.8, p. 303 Wet melting occurs when water is added to the dry mantle and melting temperatures decrease. Can start at depths of 25 km (16 mi.)

Igneous Rocks MORB (Mid Ocean Ridge Basalt) forms by decompression melting as pressure decreases. N. Lindsley-Griffin, 1999 Fig p. 304

Midocean Ridges Pillow basalts - rounded bulbous forms - erupt in the central rift valleys of midocean ridges. N. Lindsley-Griffin, 1999 Fig. B11.1, p. 306

Midocean Ridges Submarine hot springs form where seawater seeps into fractures, is heated by hot rock or magma, and emerges as mineral-laden plumes of hot water. Tube worms and other unusual life forms utilize this energy source. See Box, p N. Lindsley-Griffin, 1999

Ophiolites Ophiolites are on-land rock sequences interpreted as oceanic crust and upper mantle. N. Lindsley-Griffin, 1999 Fig , p. 305

Ophiolites Rock sequence: marine sediments pillow basalts basaltic dikes and sills layered gabbro peridotite or serpentinite (metamorphosed peridotite) N. Lindsley-Griffin, 1999

Convergent Margins Subduction at convergent margins drags sediments and seawater down into the mantle. Wet melting of mantle peridotite (and some ocean crust) under high pressure produces magma. N. Lindsley-Griffin, 1999

Wet magmas erupt explosively to form stratovolcanoes with thick pyroclastic deposits. N. Lindsley-Griffin, Russian stratovolcanoes Convergent Margins Lapilli, Fig. 6.16, p. 172

Partial melting of peridotite produces andesitic magma. (Minor basaltic magma is also produced, especially at ocean-ocean subduction zones) N. Lindsley-Griffin, 1999 Convergent Margins Tab. 6.1, p. 162 Diorite Andesite

Differentiation and fractional crystallization produce more silicic rocks from the original andesitic magmas. N. Lindsley-Griffin, 1999 Convergent Margins Granite, Rhyolite Tab. 6.1, p. 162 Gabbro, Basalt Basalts are lower in potassium than MORB.

N. Lindsley-Griffin, 1999 Active volcanoes on Hawaii lie over a plume of hot mantle material. Island rocks are progressively older to the NW. As the plate moves NW, each island is dragged away from the heat source and a new one forms. Fig. B4.2, p. 111 Mantle Plumes

Olympus Mons, Mars, - the largest shield volcano in the solar system. Evidence that Mars does not have active plate tectonics - to grow so large the volcano must have been over a stationary mantle plume for a very long time. N. Lindsley-Griffin, 1999 Mantle Plumes Fig , p. 309

Mantle plumes beneath continents may produce thick basaltic plateaus. Vast outpourings of very fluid basaltic lava that forms a series of thin sheets piled one on top of another. Columbia Plateau, WA N. Lindsley-Griffin, 1999 Mantle Plumes Fig , p. 312

Yellowstone National Park lies above a mantle plume. Rhyolitic tuff was erupted explosively after continental crust was melted by rising basaltic magmas. N. Lindsley-Griffin, 1999 Mantle Plumes Fig p. 313

Continental crust thickened by compression or collision may begin to melt by wet partial melting. Viscous granitic magma forms plutons. N. Lindsley-Griffin, 1999 Continental Interiors Fig , p. 311

Metamorphism and Plate Tectonics The type of metamorphism that occurs is controlled by plate tectonic setting. N. Lindsley-Griffin, 1999 Fig p. 315

Sedimentation Thick sedimentary wedges form in continental rift valleys and along passive margins. Alluvium, evaporites, beach sediments, shallow marine. Fig A p. 318 N. Lindsley-Griffin, 1999

Sedimentation Continental collisions produce structural basins along mountain fronts filled with thick clastic sediments. Fig B p. 318 N. Lindsley-Griffin, 1999

Sedimentation Deep-sea trenches at continental margins are filled with clastic turbidites. These are crushed and broken into melanges, mixed with bits of oceanic lithosphere (ophiolites) and deep ocean sediments (chalk, chert). N. Lindsley-Griffin, 1999 Fig C p. 318 Melange

Sedimentation Near volcanic arcs the sediments are rich in volcanic ash and eroded clasts of andesitic lava. N. Lindsley-Griffin, 1999 Fig C p. 318 Volcanic ash

This folded mountain belt extends along eastern North America from Labrador to southern Mexico Formed by collision and accretion in the Paleozoic Rifting of Pangea in the Triassic left it on a passive continental margin N. Lindsley-Griffin Appalachian Mountains

Collisions began 450 m.y.a. with microcontinents and island arcs About 350 m.y.a., Proto-Africa and Eurasia collided, as the Proto-Atlantic Ocean subducted and closed up Source: Dolgoff 1998; N. Lindsley-Griffin Appalachian Mountains

© Houghton Mifflin All rights reserved Today the accreted terranes of the Appalachian Mtns are on the passive margin formed when Pangaea fragmented. They have been tectonically quiet since the Jurassic, 200 m.y.a. Source: Dolgoff 1998 Appalachian Orogeny