Transcript Plate Tectonics

Introduction to Geophysics and
Planetary Physics
(6) Plate Tectonics
Geophysics
114
Geophysics
115
Plate Tectonics – Plate Boundaries
Eurasian
Plate
North
American
Plate
Eurasian
Plate
African
Plate
Pacific
Plate
South
American
Plate
IndoAustralian
Plate
Antarctic
Plate
The most important lithospheric plates and their boundaries (Press & Siever).
Geophysics
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Plate Tectonics – Moho & Co
Lithosphere
(rigid)
Asthenosphere
(plastic)
Schematic
representation of
the convection
in the Earth’s
mantle. Source:
P & S, 1994
(all).
Continental
Crust
Oceanic
Crust
Mantle
Upper Mantle
Core
The Lithosphere is rigid and brittle, it
comprises the Earth’s Crust and the
uppermost layer of the Mantle, floating on
the plastic, partly molten part of the
mantle, which is called Asthenosphere.
At Mid-Ocean Ridges new oceanic crust is
continuously built in the form of Sheeted
Dikes und Pillow Lava. The upper part of
the magma chamber solidifies as Gabbro.
The Mohorovičič-Discontinuity (short form:
Moho) is the boundary between crust and
mantle.
Ocean
Mid-Ocean Ridge
Sediments
Oceanic
Crust
Gabbro
Hypsographische
Kurve
derChamber
Erdoberfläche
Moho
Magma
Peridotite
Kumulative Verteilung der Flächenanteile
bestimmter
Mantle
Höhenstufen (bezogen auf den Meeresspiegel).
Geophysics
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Mid-Ocean Ridges
Source: NOAA
The mid-ocean ridges form a world-encompassing mountain range with 60000 km length.
Geophysics
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Mid-Ocean Ridges
In Iceland (Thingvellir) the Mid-Atlantic Ridge can be observed above sea level. The Eurasian
Plate is left of the Rift, the North-American Plate is to the right (UF) .
Geophysics
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Black Smokers & Co
„Black Smokers“ (Hydrothermal
Vents) – Sulfur and heavy metals
precipitate from sea water, which
has been heated up to over 400 °C.
3 m long Tube Worms are part of
a symbiotic community, which is
based on Chemosynthesis by
sulfur bacteria.
New oceanic crust is built in the
form of pillow lava.
Geophysics
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Black Smokers & Co
Hydrothermal Vents – like the Sully Vent in
the NE Pacific (Univ. of Washington), can
host enormous masses of Anomuran Crabs
(Kiwa sp., A.D. Rogers), deep-sea shrimps
(Rimicaris Hybisae, C. German) – or the
“Yeti Lobster” (Kiwa Hirsuta, A. Fifis).
Plate Tectonics – Schematic
D & D, 1996
At mid-ocean ridges new (basaltic) oceanic crust is formed, which then descents
in Subduction Zones to become part of the mantle again.
Geophysics
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Geophysics
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Paleomagnetism
Geographic
Pole
Mid-Ocean Ridge
Geomagnetic
Pole
Gilbert
(reverse)
Schematic representation of
the field lines of the
geomagnetic field (Dipole).
The axis is tilted 11° with
respect to the rotation axis:
tan I  2 tan  m
Gauss
(normal)
I = Inclination
m= magnetic latitude
Matuyama
(reverse)
Brunhes
(normal)
After P & S, 1994
normal
revers
Thermoremanence: When magma cools below the Curie Point, the ambient
magnetic field (and its direction) is “frozen in”.
Geophysics
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Seafloor-Age
The Seafloor-Spreading
can be illustrated with
Isochrones, connecting
points with the same age
on the ocean floor.
The
colored
bands
correspond to geologic
times, during which the
ocean floor formed.
Oceanic
crust
is
considerably
younger
than continental crust.
There is virtually no seafloor older than 200
Million Years.
Source: P & S, 1994
Geophysics
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Subduction 1
Divergent Plate Boundary
Convergent Plate Boundary
Deep-sea trench
Mid-ocean Ridge
Lithosphere
VolcanicMagmatic Zone
Mountain
Building
Deep-sea trench
Continental
Crust
Oceanic Crust
Ocean–Continent Collision
Asthenosphere
Earthquake Foci
Magma
Formation
Divergent and convergent plate boundaries. After P & S, 1994.
Deep-sea trench
Island Arc
Ocean–Ocean Collision
Island Arcs – Java
Geophysics
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The Indonesian islands Sumatra, Java, Bali, Lombok … are a prime example for an Island Arc. Volcanoes line
up – here we can see Semeru (back) and Bromo (left). Picture: D. Walton.
Island Arcs – Aleutians
Geophysics
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The Aleutians are another “textbook example” for island arcs. Here the Cleveland Volcano erupts, as seen from
the International Space Station – ISS (J. N. Williams).
Geophysics
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Island Arcs
The Alaska-Peninsula (left: NASA) can be considered as a part of the Aleutians which is connected to the
continent. Japan is also a prime example for an island arc (right: ESA).
Geophysics
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Mountain Building
The (snow covered) Andes (left: NASA) are a prime
example for mounting building by Ocean –
Continent Collision. The Himalaya is the result of
(still ongoing) Continent – Continent Collision.
Geophysics
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Subduction 2
Continent – Continent Collision: This process usually starts with the subduction of oceanic crust, which was
present between the converging continents. Sediments on the shrinking oceanic plate become part of the emerging
mountain range, parts of the oceanic crusts, which are incorporated into the continental crust are called
Ophiolites. The result is termed Tectonic Melange. When the continents actually start colliding only the mantle
material can be further subducted, the crust roughly doubles in thickness. From P & S, 1994.
Geophysics
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Terranes
Microcontinents, island arcs and island chains that are present
on the subducted oceanic, will be incorporated into the
continent as Terranes. The geology of the Pacific Cordillera in
North America is particularly complicated since about half of
the Northern “Paleo-Pacific” has been subducted underneath.
Pictures: P & S, 1994.
Geophysics
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Transform Faults
Transform Faults are characterized by lateral movement
of crustal blocks. The most famous example is the San
Andreas Fault in California. Left: the region called
„Elkhorn Scarp“ (Elk  Elch). The lateral movement is
nicely illustrated (above) by the offset in the course of
Wallace Creek (currently ~100 m). Aerial photographs:
David Lynch
Geophysics
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The Ocean Floor
Transform Fault
Axis of the East Pacific Ridge
Transform Faults are very common (but usually nit visible) on the ocean floor near the
mid-ocean ridges. Map by Heezen & Tharp, 1997.
Geophysics
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Ocean Forming
Rift Valley
Oceanic Crust
Continental Shelf
Continental Slope
Continental Crust
Passive Margin
Continental Rise
Plate Boundary
Mid-Ocean Ridge
Passive Margin
Formation of a Rift Valley and building of a new ocean with passive continental
margins (like the “young“ Atlantic). After P & S, 1994.
Geophysics
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Ocean Forming
In the Afar Triangle we find fresh cracks in the Earth crust (left: T. Wright, right: E. Baker). The region is
below sea level (Danakil-Depression) but does not have a connection to the Read Sea (yet).
Plate Tectonics – Earth History
Drifting continents, formation and breakup of Pangaea.
Geophysics
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Plate Tectonics – Earth History
Geophysics
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The emergence of the Réunion-Hotspot was responsible for the fast movement of India during the
Tertiary (Credit: R.D. Müller, Nature 2011).
Plate Tectonics – Alternatives
It could also have been like this … (Creadit: C. Heiner & S. Brune).
Geophysics
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Geophysics
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Hot Spots and Mantle Plumes
Active
Volcano
Guyot
Hot Spot – Ascending Magma
Hot Spots and volcanic Island Chains
Movement of the
Lithosphere-Plate
Geophysics
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Island Chains
The Hawaiian Islands
are
the
textbook
example for an Island
Chain, formed by a Hot
Spot. Active Volcanoes
can currently only be
found on „Big Island“
and Maui – and on
Loihi, which is still well
below sea level.
Kauai
is
already
heavily eroded, hosting
the „Grand Canyon of
the Pacific”. Beyond the
Island of Midway only
Seamounts are left.
Source: T&T, 1999.
Geophysics
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Hot Spots in the Pacific Ocean
Emperor Seamounts
Hawaii Islands
Hawaii Hot Spot
Galapagos
Tuamoto Archipelago
Pitcairn
Easter Island
Cook Islands
Source: SdW
Hot Spots in the Pacific and volcanic island chains, formed by the movement of the pacific
plate over the (more or less stationary) hot spots (“Textbook example” = Hawaii).
Geophysics
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From Volcano to Atoll
If a Hot Spot Island is formed in the right climate region, it will soon be surrounded by a
fringing coral reef, which will be transformed into a barrier-reef when the volcano starts
submerging. The final state is an Atoll with a central Lagoon. Source: T & T, 1999.
Geophysics
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From Volcano to Atoll
An once again with the German terms. The explanation for the formation of atolls was found by
Charles Darwin. Source: P & S, 1994
Geophysics
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Hot Spots und their Traces
SOurce: NASA
Volcanic Islands – Galapagos (top left)
Subsiding Volcanoes with fringing and barrier
reefs and Atolls: Tahaa+Raiatea, Bora Bora,
and Tupai, French Polynesia (left)
Atolls with completely submerged volcanoes –
Maldives (above).
Geophysics
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Hot Spots und their Traces
Bikini – Atoll (NASA).
Geophysics
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Hot Spots und their Traces
Hawaii – a textbook „Hot Spot“,
currently under Hawaii – „Big Island“,
but also creating a new island – Loihi
(currently –969 m, as of 2014).
Source: SdW
Walvis Hot Spot – actively involved
in forming the South Atlantic, currently
under Tristan da Cunha, Traces: Rio
Grande Rise in the West, Walvis
Ridge in the East.
Geophysics
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Flood Basalts
Source: SdW
Enormous Flood Basalts (Large Igneous Provinces) are deposited when the „Mushroom Head“ of a Mantle
Plume penetrates the crust (fortunately not in historic times).
Geophysics
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Deccan Flood Basalts
Deccan Traps, India.
Geophysics
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Fissure Eruptions at Hot Spots
Fissure eruption, Piton de la Fournaise (La Réunion)
Fissure eruptions in
Hawaii: Puu Oo (top
right) Kilauea (left
and right) as (very
modest) analogs for
the formation of
flood basalts in the
past.
Geophysics
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Hot Spots and Shield Volcanoes
Maun Loa in Hawaii (above, in the background; right:
satellite image, NASA) is the largest Shield Volcano
on Earth, nourished by a Hot Spot. Together with the
slightly higher (but less voluminous) Mauna Kea
(above, foreground), it rises more than 9000 m from
the sea floor.
Geophysics
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Mighty Shield Volcanoes
Quelle: NASA
Even bigger is Olympus Mons on Mars – it rises more
than 22 km above its surrounding (NASA).
Geophysics
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Hot Spot - Volcanism
A Hot Spot can also be found
under Iceland – right on the midatlantic ridge. Because of the
basaltic lava involved, eruptions
are usually quiet – like the fissure
eruption from Eyjafjalla in March
2010 (Reuters).
In April 2010, however, the eruption happened underneath the glacier Eyjafjallajökull – and the combination
lava + water is very explosive – the ascending magma was fragmented into tiny particles – volcanic ash.
(Picture: Marco Fulle). Charge separation in the eruption column can lead to spectacular lightnings.
Hot Spot – Volcanism
Geophysics
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Eyjafjallajökull was clearly responsible for the ash cloud, creating
havoc in European air traffic (left, NASA) – but not for the
spectacular aurora above (Reuters) – and also not for the modest
weather in Austria in May 2010. Only considerably larger eruptions
can have an impact on.