Download CHAPTER 2 Plate Tectonics and the Sea Floor

The Rock Cycle Revisited
Chapter 11
Geology Today
Barbara W. Murck & Brian J. Skinner
N. Lindsley-Griffin, 1999
MOUNT EVEREST, HIMALAYA MTNS.
Planet-Shaping Processes
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)
N. Lindsley-Griffin, 1999
Planet-Shaping Processes
Impact craters:
flat floors
raised rims
blankets of ejecta
around them
N. Lindsley-Griffin, 1999
Barringer Meteor Crater, > 50,000 yrs old
near Flagstaff, Arizona (Fig. 11.2, p. 297)
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
N. Lindsley-Griffin, 1999
Nordlingen Cathedral, Ries Crater
Germany (Fig. 11.3, p. 298)
Planet-Shaping
Processes
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
N. Lindsley-Griffin, 1999
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
N. Lindsley-Griffin, 1999
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
Cratons are made up of
shields and platforms
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
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.
Plunging anticlines and synclines Appalachian orogen, 300 m.y. old
See Fig. 11.6, p. 301
N. Lindsley-Griffin, 1999
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
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.
N. Lindsley-Griffin; : Dolgoff 1998
Himalaya Mountains
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
India near the end of its 80 m.y. journey north:
Granitic plutons, andesite generated by ocean-continent
convergence
Marine
sediments
deposited
in Tethys sea
along active
margin of Asia
and passive
margins of
India
N. Lindsley-Griffin; Dolgoff, 1998
Himalaya Mountains
India and Asia collide, crushing the sedimentary wedges
between them and producing:
Folding and
faulting.
Regional
metamorphism.
Crustal uplift.
N. Lindsley-Griffin; Dolgoff, 1998
Himalaya Mountains
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
N. Lindsley-Griffin; Dolgoff, 1998
Isostasy
Isostasy -- the flotational balance
of lithosphere on asthenosphere
Mountains have thick roots of continental crust beneath them.
Low density helps maintain high elevations.
Fig. 11.7, p. 301
N. Lindsley-Griffin, 1999
Isostasy and
Mountains
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.
(N. Lindsley-Griffin, 1999; Source: Dolgoff 1998)
Actually, the plumb-bob swings
less than predicted, suggesting that
mountains are underlain by deep
roots of less dense rock.
Igneous 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
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.)
N. Lindsley-Griffin, 1999
Igneous
Rocks
MORB (Mid Ocean Ridge Basalt)
forms by decompression melting as
pressure decreases.
Fig. 11.9
p. 304
N. Lindsley-Griffin, 1999
Midocean
Ridges
N. Lindsley-Griffin, 1999
Pillow basalts - rounded bulbous
forms - erupt in the central rift
valleys of midocean ridges.
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. 306-307
N. Lindsley-Griffin, 1999
Ophiolites
Ophiolites are on-land rock
sequences interpreted as oceanic
crust and upper mantle.
Fig. 11.11, p. 305
N. Lindsley-Griffin, 1999
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
Convergent Margins
Wet magmas erupt
explosively to form
stratovolcanoes with
thick pyroclastic
deposits.
3 Russian stratovolcanoes
Lapilli, Fig. 6.16, p. 172
N. Lindsley-Griffin, 1999
Convergent Margins
Partial melting of
peridotite produces
andesitic magma.
(Minor basaltic magma is
also produced, especially
at ocean-ocean
subduction zones)
Diorite
Andesite
Tab. 6.1, p. 162
N. Lindsley-Griffin, 1999
Convergent
Margins
Differentiation and
fractional
crystallization
produce more
silicic rocks from
the original
andesitic magmas.
Basalts are lower in
potassium than MORB.
N. Lindsley-Griffin, 1999
Granite,
Rhyolite
Tab. 6.1, p. 162
Gabbro,
Basalt
Mantle Plumes
• 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.
N. Lindsley-Griffin, 1999
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
Fig. 11.12, p. 309
Mantle Plumes
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
Fig. 11.16, p. 312
Mantle Plumes
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
N. Lindsley-Griffin, 1999
Continental Interiors
Continental crust thickened by compression or
collision may begin to melt by wet partial melting.
Viscous granitic magma forms plutons.
Fig. 11.15, p. 311
N. Lindsley-Griffin, 1999
Metamorphism and Plate Tectonics
The type of metamorphism that occurs is controlled by
plate tectonic setting.
Fig. 11.19
p. 315
N. Lindsley-Griffin, 1999
Sedimentation
Thick 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
Sedimentation
Continental collisions produce structural basins along
mountain fronts filled with thick clastic sediments.
Fig. 11.21B
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).
Melange
Fig. 11.21C
p. 318
N. Lindsley-Griffin, 1999
Sedimentation
Near volcanic arcs the sediments are rich in volcanic
ash and eroded clasts of andesitic lava.
Volcanic ash
Fig. 11.21C
p. 318
N. Lindsley-Griffin, 1999
Appalachian
Mountains
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 Orogeny
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.
© Houghton Mifflin 1998. All rights reserved
Source: Dolgoff 1998