the rock cycle revisited chapter 11 geology today barbara w. murck & brian j. skinner n....

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


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.




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


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)




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



Orogens - elongate regions of crust intensely deformed and metamorphosed during continental collisions

Age of folding and faulting is younger than age of rocks deformed.


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.


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



The partially subducted, buoyant continental crust pushes up the mountains like a beach ball pushed under water will support a human



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:



India and Asia collide, crushing the sedimentary wedges between them and producing:

Folding and faulting.

Regional metamorphism.

Crustal uplift.



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



IsostasyIsostasyIsostasy -- the flotational balance of lithosphere on asthenosphere


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


3 Russian stratovolcanoes


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.


Fig. 11.8, p. 303

Decompression melting occurs when hot rock rises through mantle and pressure decreases


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.


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


OphiolitesOphiolites Ophiolites are on-land rock sequences interpreted as oceanic crust and upper mantle.


Fig. 11.11, p. 305

OphiolitesOphiolites

Rock sequence:

marine sediments

pillow basalts

basaltic dikes and sills

layered gabbro

peridotite or

serpentinite

(metamorphosed

peridotite)


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.


Wet magmas erupt explosively to form stratovolcanoes with thick pyroclastic deposits.


3 Russian stratovolcanoes


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)



Tab. 6.1, p. 162

Diorite Andesite

Differentiation and fractional crystallization produce more silicic rocks from the original andesitic magmas.


Convergent Margins

Convergent Margins

Granite,

Rhyolite

Tab. 6.1, p. 162

Gabbro,

BasaltBasalts are lower in potassium than MORB.


• 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.



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



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.



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.


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.


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


SedimentationSedimentationContinental collisions produce structural basins along mountain fronts filled with thick clastic sediments.

Fig. 11.21B

p. 318


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).


Fig. 11.21C

p. 318

Melange

SedimentationSedimentationNear volcanic arcs the sediments are rich in volcanic ash and eroded clasts of andesitic lava.


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


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



© 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

the rock cycle revisited chapter 11 geology today barbara w. murck & brian j. skinner n....

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