the rock cycle revisited chapter 11 geology today barbara w. murck & brian j. skinner n....
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The Rock Cycle RevisitedThe Rock Cycle RevisitedChapter 11
Geology TodayBarbara 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 ProcessesPlanet-Shaping Processes
N. Lindsley-Griffin, 1999
Planet-Shaping ProcessesPlanet-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
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
Planet-Shaping Processes
Leonid Meteor Shower, 1998
Earth - the Tectonic Planet
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
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
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
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 BuildingMountain BuildingOrogens 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 BuildingMountain BuildingOrogens 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 BuildingMountain BuildingOrogens 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 BuildingMountain 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 MountainsHimalaya 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 MountainsHimalaya 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 MountainsHimalaya Mountains
Granitic plutons, andesite generated by ocean-continent convergence
Marine sedimentsdepositedin Tethys seaalong activemargin of Asiaand passivemargins of India
India near the end of its 80 m.y. journey north:
Himalaya MountainsHimalaya 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 MountainsHimalaya 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 MountainsHimalaya Mountains
N. Lindsley-Griffin; Dolgoff, 1998
IsostasyIsostasyIsostasy -- 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 bythick 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 MountainsIsostasy and Mountains
Igneous Rock and Plate TectonicsIgneous 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, 1999
3 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 RocksIgneous Rocks
MORB (Mid Ocean Ridge Basalt) forms by decompression melting as pressure decreases.
N. Lindsley-Griffin, 1999
Fig. 11.9
p. 304
Midocean Ridges
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 RidgesMidocean 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. 306-307
N. Lindsley-Griffin, 1999
OphiolitesOphiolites Ophiolites are on-land rock sequences interpreted as oceanic crust and upper mantle.
N. Lindsley-Griffin, 1999
Fig. 11.11, p. 305
OphiolitesOphiolites
Rock sequence:
marine sediments
pillow basalts
basaltic dikes and sills
layered gabbro
peridotite or
serpentinite
(metamorphosed
peridotite)
N. Lindsley-Griffin, 1999
Convergent MarginsConvergent 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, 1999
3 Russian stratovolcanoes
Convergent MarginsConvergent 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 MarginsConvergent 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
Convergent Margins
Granite,
Rhyolite
Tab. 6.1, p. 162
Gabbro,
BasaltBasalts 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 PlumesMantle 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 PlumesMantle Plumes
Fig. 11.12, 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 PlumesMantle Plumes
Fig. 11.16, 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 PlumesMantle Plumes
Fig. 11.17
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 InteriorsContinental Interiors
Fig. 11.15, p. 311
Metamorphism and Plate TectonicsMetamorphism and Plate Tectonics
The type of metamorphism that occurs is controlled by plate tectonic setting.
N. Lindsley-Griffin, 1999
Fig. 11.19
p. 315
SedimentationSedimentationThick sedimentary wedges form in continental rift valleys and along passive margins.
Alluvium, evaporites, beach sediments, shallow marine.
Fig. 11.21A
p. 318
N. Lindsley-Griffin, 1999
SedimentationSedimentationContinental collisions produce structural basins along mountain fronts filled with thick clastic sediments.
Fig. 11.21B
p. 318
N. Lindsley-Griffin, 1999
SedimentationSedimentationDeep-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. 11.21C
p. 318
Melange
SedimentationSedimentationNear volcanic arcs the sediments are rich in volcanic ash and eroded clasts of andesitic lava.
N. Lindsley-Griffin, 1999
Fig. 11.21C
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
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
Appalachian Mountains
© Houghton Mifflin 1998. 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 OrogenyAppalachian Orogeny
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