Transcript PLATE TECTONICS - Oakton Community College

PLATE TECTONICS
PLATE TECTONICS
Earth made of concentric spheres
(Fig. 1.14) p 18 T-14
• 1. Inner core - rich in iron and nickel, dense
• 2. Outer core - liquid
• 3. Mantle
– Asthenosphere - nearly molten and can flow very
slowly
– Lithosphere - rigid outer layer of the earth and
floats in the asthenosphere. (Fig. 1.16) p 22 T29 S&A-22
PLATE TECTONICS
• 4. Crust
– basalt (fine grained igneous rock, volcanic)
• 5. Hydrosphere
• 6. Atmosphere
PLATE TECTONICS
Earth made of concentric spheres review
• 1. Inner core
• 2. Outer core
• 3. Mantle
– Asthenosphere
– Lithosphere
• 4. Crust
• 5. Hydrosphere
• 6. Atmosphere
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Isostasy
• Isostatic adjustment
• describes the relative elevations that
materials of different densities and
thicknesses reach at equilibrium with
gravity (1.16) p 20 S&A 23 T-20
• Play the game of isostasy before class!
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Isostasy cont
• Crustal materials float in asthenosphere
(Fig. 14) p 20
• Weight of volcano bends crust (Fig 2.27)
remember from last time.
• Glaciers also cause crust so subside (a
process by which one plate descends
beneath another plate and is ultimately
resorbed into the mantle)
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Isostasy cont
• Scandinavia and Antarctica are rising due to
the melting of the glaciers that cover them.
• Remember how the block raised when we
reduced the height (from isostasy game).
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• "Moho" - the boundary, which geologists
refer to as the Mohorovicic discontinuity,
between Earth's brittle outer crust and its
hotter, softer mantle
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MOHO exposed at the surface
located between crust and mantle (S&A 22)
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Continents
• Thick accumulations of granitic rocks
• Oldest rocks on Earth - about 3.8 billion
years
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Plate tectonics
• New crust formed at mid-ocean ridges or
spreading centers Convection currents (Fig.
2.10) p 44 (this is a very important
diagram)
• Crust and upper mantle constitute the rigid
lithosphere float on nearly molten
asthenosphere
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Plate tectonics cont
• Lithosphere broken into rigid units and move
slowly older lithospheric material is being
subducted while new lithosphere is produced
along the ridges and rises.
• Trenches plates converge
• Plates move past each other along transform faults
(Fig. 2.23) p 59
• Plate movements shape ocean basins T-30 (Fig.
2.13 b) p 48
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Plate boundaries (Fig. 2.14 a. b. c.) p50
• 1. Divergent boundary - Midocean ridges plates form and move away from each other
(Fig. 2.14 a)
• 2. Convergent boundary - Trenches - plates
move toward each other and are destroyed
as they are drawn down into the mantle
(subduction) (Fig. 2.14 b, 2.20, 2.21, and
2.22) p 56-58
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Plate boundaries cont.
• 3. Fracture zones - plates slide past each
other (NO earthquakes felt)
• 4. Transform faults - plates slide past each
other (Earthquakes felt) (Fig. 2.14 c)
• Transform faults and fracture zones (Fig.
3.17) p 95 (T 38)
• San Andreas Fault (Fig. 2.23) p 59
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Continental Rifting Fig 2.17 p 52
• a. upwarping
• b. rift valley (fig 2.18) p 54 read and
understand
• c. linear sea
• d. mid-ocean ridge
• e. Table 2.1 p 51
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• Pillow lava along Juan de Fuca Ridge. Photo courtesy of
Submarine Ring of Fire 2002 Exploration, NOAA-OE.
PLATE TECTONICS
• START HERE FOR WEDNESDAY
• If you are interested in a 9th edition of our
book.
• [email protected].
PLATE TECTONICS
I want you to print out an article on hot spots
• Go to google and look up hot spots.
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Hot spots
• plumes of magma that rise from deep within
the mantle erupt (Fig. 2.24) p 62 T 54
• Plates moving across hot spots cause chains
of volcanic islands Hawaiian Islands (Fig.
2.25) p 62
• Many occur near midocean ridges. Seen
today in Iceland. (Fig2.26) p 63 formation
of sea mounts and table mounts (guyots)
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Plates moving across hot spots cause chains of
volcanic islands Hawaiian Islands
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Hot spots cont.
• Others beneath the continent - gisers in
Yellowstone National Park
• Flood Basalts - from volcanic activity that
produces widespread gently sloping
surfaces. Commonly surround volcanic
islands
PLATE TECTONICS
Earth’s magnetic field
• Fig 2.7 and 2.9 p 40 and 43
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Earth's magnetic field (Fig. 2.12) p 46
T 28
• Changes orientation at irregular intervals, as
of today we do not know why.
• Minerals record the orientation of Earth's
magnetic field at the time when the rocks
cooled ~ 100,000 yrs (Fig. 2.11) p 45
• Measure with magnetometers T 48
PLATE TECTONICS
Earth's magnetic field cont.
• Form bands with same orientation - like
tape recorder T-26
• Matthews and Vine saw the magnetized
rocks and decided that the rocks were
younger in the center of the ridge, older at
edges
– Permits determining age of ocean floor
PLATE TECTONICS
Neat stuff on the earths magnetic
field
• http://science.nasa.gov/headlines/y2003/29d
ec_magneticfield.htm
• www.pbs.org/wgbh/nova/magnetic/reversal
s.html
– This shows an interactive. I expect you to do
this
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Hydrothermal circulation p 93
• Sea water circulates through hot, newly
formed rocks (Fig 3.17 a) p 93
• 1. cooling them
• 2. removes metals and deposits them in ocean floor
vents
• 3. ocean water circulates through newly formed
crust every 5 - 10 million years. (remember the
ocean has been around a very long time)
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• Volcanic eruptions occur more frequently
on rapidly spreading segment than on slow
spreading ones.
• A hydrothermal vent is a geyser on the
seafloor.
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Three types of hydrothermal
vents p 94
• black smokers
• white smokers
• cooler discharge
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black smokers
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White smoker
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Hydrothermal Vents cont.
• Three types of hydrothermal vents
– 1. most spectacular are black smokers.
• a. They discharge superheated waters (300 - 400oC)
at high rates much like a fire hose.
• b. Black because of chem. rx. 2o to those that occur
in the water forming sulfur-bearing minerals.
• c. form large fragile chimney like mounds up to 10
meters high made of porous silica, native sulfur, and
sulfur-bearing minerals.
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Hydrothermal Vents cont.
• d. Color the mounds with yellows and blacks (like
Halloween decorations) Read Recovery of Black
Smokers p 90-91
• e. Tube worms: Very fast growing and reach sizes
up to 3 m (10 ft) other animals include sea
anemone, clams, crabs, fish and bacteria Fig 15.25
p 477
• f. Temperature fluctuations are common occurring
in days to seconds.
• g. Micro-organisms
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Tube worms T 134
PLATE TECTONICS
Hydrothermal Vents cont.
– 2. White smokers
• a, are not as hot (200-330 oC) are also common
• b. circulating fluids have mixed with cold ocean
waters
• c. milky discharge thus the name
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Hydrothermal Vents cont.
– 3. Cooler discharge (cold seep) (5 - 25oC)
• a. water flows out through cracks and fissures in the
ocean floor.
• b. cold seep waters are about the same
temperature as the surrounding waters
• c. the discharge water is clear
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Hydrothermal Vents cont.
Hydrothermal circulation continues for millions of years as
the rocks cool.
• Eventually, fractures fill with mineral deposits and fluids
no longer pass through.
• Sediments accumulate on the ocean floor.
• Where do the minerals come from?
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All three vents support abundant growths of bottom-dwelling
organisms.
• Chemosynthesis: the process by which certain
microbes create energy by mediating chemical
reactions
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Continental Margins
• (steep slopes that descend to the sea floor)
p 100 - 102
• One of the most outstanding features of the
continental slopes are submarine canyons.
(Fig 3.9 p 83) Submarine canyons are steep
sided and V-shaped in cross section with
tributaries similar to those of river-cut
canyons.
PLATE TECTONICS
Continental Margins two types
(Fig 3.7) p 82
• 1. Active continental margins lie along
edge of plates (Convergent)
– Contain many active volcanoes, frequent
earthquakes, young mountains
– Common along Pacific margins, called Pacifictype margins are frequently narrow
• 2. Passive margins of continents lie in
middle of plates
PLATE TECTONICS
ACTIVE MARGINS
PLATE TECTONICS
Earthquakes:
• 1. common near Pacific-type margins
• 2. deep earthquakes indicate subduction - in
subduction zones, plates move as large slabs and
drag against the rocks above and below causing
earthquakes in those areas
• 3. subduction causes a drag on rocks -> deform
the rocks along the margins -> energy buildup ->
earthquakes -> energy releases
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Subducting plate is old and dense
• it sinks into the mantle as a steeply dipping
slab
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Subducting plate is young
• still warm, and relatively buoyant
• slab dips at a shallow angle
• occurs along the eastern margin of the
Pacific, where the American plate is
overriding recently formed crust
• volcanoes occur on land
• many of earthquakes
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Exotic terranes
• terranes have a history distinct from adjoining crustal
fragments are welded onto continents during subduction
(A&S-41)
PLATE TECTONICS
Go to web for animation
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PASSIVE MARGINS
PLATE TECTONICS
Passive margins of continents
• No earthquakes, no volcanoes. Fig 3.7 p 81 book
• Called Atlantic-type margins
• Form after continents are rifted apart and tend to
be wide
• Thick sediment deposits and old oceanic crust
– trenches do not form,
– sediment is folded into mountain ranges (Appalachians,
Alps, Himalayas, and Urals)
PLATE TECTONICS
Passive margins cont.
• Economically, they are important because
of the accumulations of oil and gas that they
often contain. Most of the world's giant oil
and gas fields occur in such deposits.
• Found on Atlantic Ocean, Antarctic Ocean.
Arctic Ocean, and Indian Ocean
PLATE TECTONICS
Mantle convection (Fig 2.10 p 44)
• Occurs in mantle, causing plate movements
• Supplies molten rock to midocean ridges,
causing volcanism
• 1. hot-spot volcanoes originate at unusually
hot areas of the core mantle boundary
• 2. the overlying mantle melts, forming
plumes of magma that rise and penetrate the
crust as volcanoes.
PLATE TECTONICS
Mantle convection cont.
• 3. these hot spots do not move with the overlying
mantle and thus remain fixed in location for tens
of millions of years
• 4. red patches - continental flood basalts and
oceanic lava plateaus T 54
• Crust cools as it ages, and grows denser
• Oldest, densest rock sinks into the mantle at the
trenches
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Formation of ocean basins (Fig 2.17 abcd)
• Formed through the breakup of continents.
Begins when a continent remains in one
location for >100 million years continent
impedes heat flow from the earth's interior.
PLATE TECTONICS
Formation of ocean basins cont
• a. underlying mantle heats, expanding and uplifting the
overlying lithosphere
• b. continents rift, forming narrow valleys (rift valley in
Africa)
• c. valleys widen into narrow ocean basin, e.g., Red Sea
midocean ridges in the middle of ocean - actively widening
• d. ocean continues to widen until the oldest crust becomes
dense enough to sink, causing the ocean to gradually
narrow
PLATE TECTONICS
Destruction of ocean basins Fig 2.20 abc p 56
•
•
•
•
•
Ocean - continent convergence
Ocean - ocean convergence
Continent - continent convergence
Mountains form when basin closes
Appalachian Mountains mark site of ancient
ocean that closed about 400 million years
ago Can see the sedimentary rocks on the
AT
PLATE TECTONICS
• START HERE ON MONDAY
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Mantle convection (Fig 2.10 p
44)
• Occurs in mantle, causing plate movements
• Supplies molten rock to midocean ridges,
causing volcanism
• 1. hot-spot volcanoes originate at unusually
hot areas of the core mantle boundary
• 2. the overlying mantle melts, forming
plumes of magma that rise and penetrate the
crust as volcanoes.
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• 3. these hot spots do not move with the
overlying mantle and thus remain fixed in
location for tens of millions of years
• 4. red patches - continental flood basalts
and oceanic lava plateaus T 54
• Crust cools as it ages, and grows denser
• Oldest, densest rock sinks into the mantle at
the trenches
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Boundaries Table 2.2 p 53
•
•
•
•
a.
b.
c.
d.
divergent - plates move apart
convergent - plates collide
transform - plates slide past one another
Study this table (It is on this exam)
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Points to remember
•
•
•
•
Continent - from granite
Ocean - from basalt
Heat flow - Fig 2.10 p 44
Table 2.1 p 51 This is the culture vulture
question
PLATE TECTONICS
Breakup of Pangaea on to present
• Began about 225 million years ago, with the
breakup of Pangaea about 180 million years
ago T 21 - 23 and 15
• North Atlantic formed first, South Atlantic
later
• Indian Ocean is the youngest basin
• Pacific is the remnant of Panthallasia,
previous cycle
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Present spreading cycle cont.
• Mediterranean is the remnant of Thetys Sea
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END Plate Tectonics
We have seen all the material that
will be covered on Exam I
PLATE TECTONICS