Plate Tectonics II - science.smith.edu€¦ · Hotspots – a whole ‘nother story intraplate...
Transcript of Plate Tectonics II - science.smith.edu€¦ · Hotspots – a whole ‘nother story intraplate...
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Plate Tectonics II
Boundary characteristics and Driving Forces
How and why do the plates move?
An In-depth Look at Earthquakes� at divergent boundaries – shallow only, usually weak� at translational boundaries – shallow only, often strong� at convergent boundaries – often strong
� continent-continent – shallow and intermediate� subduction zones – shallow, intermediate, and deep, in that
order, moving away from trench toward overriding plate
Faults at divergent boundaries
� normal faulting in central rift valley and between blocks� Tension pulls apart basalt
blocks� Blocks in rift valley fall down
relative to others� Volcanism in rift valley creates
seamounts
� transform faulting on both sides of rift valley� Rift valley not continuous but is
punctuated by transform faults� Two plates slide past each
other at transform faults� Fracture zones are inactive
extensions of transform faults
Earthquakes at divergent bdry’s
� Shallow focus, low magnitude quakes � most occur at transform faults� some occur in central rift valley
Plate #1 Plate #2
X = prone to earthquakes
Fracture Zone(inactive seismicity)
Fracture Zone(inactive seismicity)
Central Rift Valley (new oceanic crust is formed here)
Transform Fault(active seismicity)
Plate #1 Plate #2
Plate #1
X X X X X X X X X
X X X X
Transform Faults have strike-slip motion created by two blocks sliding past one another; these faults are prone to earthquake activity due to the frictional stresses that build up along the faults.
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Faults at subduction zones� Reverse faulting in
subduction zones at deep-sea trench� ocean-ocean: more dense
plate subducts under less dense plate; get island arc
� ocean-continent: ocean crust subducts under continental crust; get volcanic mountain chain
� Accretionary wedge at trench� Composed of sediment
scraped off down-going plate� Can be uplifted eventually if
subduction leads to continent-continent collision
Subduction zone examplesOceanic-continental Convergent Boundary
broad coastal plain& continental shelf
(“passive” continentalmargin)
narrow shelf dropping offquickly to the trench(“active” cont. margin)
Peru-Chile Trench
Andes Mtns.(volcanic mtn. range)
ocean-continent
Subduction zone examples
notice the island arcs andvolcanic mountain ranges(e.g., Japan) landward of
the trenches
Mariana Islands(volcanic island arc)
Mariana Trench
1991 eruption of Mt. Pinatubo
ocean-ocean
Earthquakes at subduction zones
� Earthquake focus can be shallow, intermediate, or deep� Often very high magnitude� Can actually see downgoing plate by location of earthquake foci
� Benioff Zone – intermediate and deep earthquakes occur in this inclined zone, tilted away from trench toward volcanic arc,extends downward up to 700 km
x
active magmatic arc (volcanic island arc)
partial melting of subducting plate near base of lithosphere
sea level
rising bodies of magma
trench
oceanic lithosphere
oceanic crust
x
xx
xx
x
x
xx
x
xx
xx
x
xxx
x x = earthquake focix
x
x
f ore-arc basinbackarc basin
accretionary prism
oceanic lithosphere Benioff Zone
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Continent-continent collisions
� thrust faulting at continent-continent collisions� a sort of extreme form of reverse faulting� major mountain building regions
� another site of strong earth-quakes, shallow to deep foci
� India-Asia is classic example
deformed magmatic arc & suture zone
(massive mountain chain)
oceanic crust
x
xx
xx
x
x
xx
x
xx
xx
x
xxx
xx x = earthquake focix
x
x
x
xxx
xx
oceanic lithosphere
continental lithosphere
continental lithosphere
continental crust
continental crust
xx
x x
old magmatic
arc
deep-sea sediments & oceanic crust
squeezed between the two continents
Himalaya &Tibetan Plateau
continent-continentcollision
Faults at translational bdry’s
� transform faults, or “strike-slip” faults exist where one plate slides past another
� ocean crust neither created nor destroyed at these boundaries
� San Andreas Fault through California is the classic example� other long transform faults on northern and southern edges
of the Scotia Plate and the Caribbean Plate
Earthquakes at translational bdry’s� Shallow focus but can be strong …
Pacific Northwest – 3-in-1
� The Pacific Northwest combines all 3 plate boundary types in one region …� Divergent: Juan de Fuca Ridge� Convergent: Cascadia Subduction Zone� Translational: Mendocino Fault (northwest
extension of San Andreas Fault)
Mt. Rainer
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The Wilson Cycle (J. Tuzo Wilson)Wilson Cycle refers to the sequence of events leading to the formation, expansion, contracting and eventual elimination of ocean basins.
� Stages in basin history are:� Embryonic - rift valley forms as
continent begins to split.� Juvenile - sea floor basalts
begin forming as continental fragments diverge.
� Mature - broad ocean basin widens, trenches eventually develop and subduction begins.
� Declining - subductioneliminates much of sea floor and oceanic ridge.
� Terminal - last of the sea floor is eliminated and continents collide forming a continental mountain chain.
Hotspots – a whole ‘nother story� intraplate volcanism (within a plate) like Hawaii, or located on a
spreading ridge like Iceland� linear chains of islands, seamounts, or ridges form
� due to plate moving over stationary hot spot � hot spots are surface expressions of magma plumes rooted deep
in the mantle� over time, plate continues to move over hot spot, resulting in linear
chain of volcanoes� as plate moves, volcanoes move off hotspot, become seamounts
What drives this plate motion?oceanic
crustoceanic
crustcontinental
crustmagmatic arc(volc. mtn. chain)
landward of trench
Asthenosphereupper Mantle (ductile)
200
400
600
800
1000
km
Mantle (rigid)
spreading center
spreading center
Lithospheresea level
subductionzone
trench
Pushing force
Pulling force
Convection in the asthenosphereand/or lower in the mantle partly drives movement of the plates. In addition, the leading edges of suducting plates are pulled down by gravity, while plates at spreading ridges are pushed apart.
Convection can basically be defined as a process in which hot, less dense material rises (such as magma that feeds a spreading ridge), and cold, more dense material sinks (such as old oceanic lithosphere that is being subductedinto the mantle).
Convection connections� There are competing hypotheses as to how and where
mantle convection occurs.� The layered mantle model has two
separate zones of convection, one in the asthenosphere and the other in the lower mantle.
� According to this model, there is very little mixing between the two layers, and slabs of lithosphere either melt or pile up at the bottom of the asthenosphere.
� The whole mantle model has convective flow throughout the entire mantle.
� According to this model, subducted slabs of oceanic lithosphere sink through the 660-km boundary between the asthenosphere and lower mantle, all the way down to the core-mantle boundary, where they melt.
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Plate Tectonics Puzzler
oceanic crust
oceanic crust
continental crust
Asthenosphere (ductile)
200
400
600
800
1000
km
Mantle
Lithosphere
continental margin
continental margin
ocean-continent collision:
magmatic arc (volcanic mtn.
chain)
ocean-ocean collision:
magmatic arc (island arc)
oceanic crust
sea level
continental crust
trenchtrench
1200
1400
1600
1800
2000
2200
2400
2600
2800
3000
source of hot spot?
hot spot islandand linear chain of seamounts(see facing page)
Liquid Outer Core
spreading center
spreading center
Quick question: How manyplates are shown here?
Hint: Look for boundaries.
There are 5 plates.
4 51 2 3
Plate Tectonics summary fig.
� The figure below is a theoretical schematic of the different types of plate boundaries, rather than a representation of an actual, present-day location on Earth. Enjoy!