Plate BoundariesPlate Boundaries
Chapter 3
1. Divergent2. Convergent3. Transform
Rifting
Buckling
Shear
1. Divergent2. Convergent3. Transform
Diverging BoundariesDiverging Boundaries
Lava fountains (10 m high) spouting from eruptive fissures during the Lava fountains (10 m high) spouting from eruptive fissures during the October 1980 eruption of Krafla Volcano. (Photograph by Gudmundur October 1980 eruption of Krafla Volcano. (Photograph by Gudmundur
E. Sigvaldason, Nordic Volcanological Institute, Reykjavik, Iceland.)E. Sigvaldason, Nordic Volcanological Institute, Reykjavik, Iceland.)
Other evidenceOther evidence
Hydrothermal vents (discovered in 1977)Hydrothermal vents (discovered in 1977)– Thermal anomalies (unusually warm water) found in 1972 Thermal anomalies (unusually warm water) found in 1972
over a ridge near Galapagos Islandsover a ridge near Galapagos Islands– Sent Alvin down in 1977Sent Alvin down in 1977
Unusual organismsUnusual organisms Pillow lavas from recently extruded ocean crustPillow lavas from recently extruded ocean crust Hot water comes from seawater ventilating through Hot water comes from seawater ventilating through
the rocksthe rocks Energy for life comes from geothermal energy and Energy for life comes from geothermal energy and
chemical energy comes from sea water – chemical energy comes from sea water – chemosynthetic bacteria are base of food chainchemosynthetic bacteria are base of food chain
EvidenceEvidence
Spreading rates at transform faults Spreading rates at transform faults – Pacific plate moves NW at 8 cm/yrPacific plate moves NW at 8 cm/yr– N American plate moves W at 2 cm/yrN American plate moves W at 2 cm/yr– Indian plate moves NE at 12 cm/yrIndian plate moves NE at 12 cm/yr
Pacific Ocean is shrinking and Atlantic is Pacific Ocean is shrinking and Atlantic is growinggrowing– Atlantic opened about 200 MY ago so there Atlantic opened about 200 MY ago so there
should be no rocks older than this in the Atlanticshould be no rocks older than this in the Atlantic
Most recent episode of Seafloor spreading:
Pangaea first broke into 2 pieces
Sea opens between N and S continents and Between Africa and Antarctica
India moves North
S Atlantic opensAntarctica moving SIndia moving NAustralia separates
and moves N
50 MY in the future:1. Africa will move N and close Mediterranean Sea2. E Africa will detach (Red Sea rift zone) and move to India3. Atlantic Ocean will grow and Pacific will shrink as it is
swallowed into Aleutian trench.4. W California will travel NW with the Pacific Plate (LA will
be swallowed into the Aleutian trench in 60 MY).
RidgesRidges
Mid Atlantic RidgeMid Atlantic Ridge
Transform BoundariesTransform Boundaries
Axis of spreading isbroken up by curvatureof earth or unequalspreading, etc
Spreading cannotproceed evenly on the surface of a sphere(this would necessarilyrequire faster spreadingat the equator andslower spreading at thepoles
Converging BoundariesConverging Boundaries
Oceanic-oceanic convergence Continental-continental convergence
Oceanic-continental convergence
Cooler, more dense slab sinksMelting of subducting slab + water and CO2 + some mantle+ some continental lithosphere
Converging platesConverging plates
2 continental plates – e.g., Himalayas and Alps 2 continental plates – e.g., Himalayas and Alps – Can get marine fossils in mountains (remember Can get marine fossils in mountains (remember
continental shelf is part of the continental lithosphere)continental shelf is part of the continental lithosphere)
Continental and oceanic plates – e.g., S America Continental and oceanic plates – e.g., S America & N America& N America– Mountains and island arcsMountains and island arcs
2 oceanic plates – Aleutian and Marianas trenches2 oceanic plates – Aleutian and Marianas trenches– Older slab (denser and cooler) sinksOlder slab (denser and cooler) sinks
The convergence of the Nazca and South American Plates has The convergence of the Nazca and South American Plates has deformed and pushed up limestone strata to form towering peaks of the deformed and pushed up limestone strata to form towering peaks of the
Andes, as seen here in the Pachapaqui mining area in Peru. Andes, as seen here in the Pachapaqui mining area in Peru.
Nazca SlabNazca Slab
Island Arc FormationIsland Arc Formation
Transform faults & exotic Transform faults & exotic terranesterranes
Transform faults – more next timeTransform faults – more next time– Plates moving past each otherPlates moving past each other– Relieve pressure due to earth’s curvature, Relieve pressure due to earth’s curvature,
relative rates of spreading, etcrelative rates of spreading, etc
Micro-platesMicro-plates– Small plates with plateaus or microcontinentsSmall plates with plateaus or microcontinents– Get accreted onto larger continent (Pacific NW)Get accreted onto larger continent (Pacific NW)
N Andreas Fault
HotspotsHotspots
Mantle plumeMantle plume Direction of plate motionDirection of plate motion
Bands of weaker and stronger magnetic fields form mirror imageson either side of spreading center.
Newly formed crust records prevailingmagnetic field.
Magnetic north iscurrently about 11o offset from geographic north
Compasses pointto magnetic north
Paleomagnetism-“fossil” magnetic field is recorded as magma cools under a particularmagnetic field
Polar wandering – magnetic polehas moved
Lining up rocks in different placesBased on their paleomagnetism
Mantle plumes -P-wave velocity anomalies-High temperature (red and yellow)-East Africa rift valleys
Hot spots – surface expressions of plumesStationary spot and plate moving over it.
Seamounts along California's continental marginDavidson Seamount is one of several seamounts along the California continental margin. It is located about 120 km southwest of Monterey, CA, and rises to within 1300 m of the sea surface. It is oriented northeast-southwest and is a complex series of cones on parallel ridges without a summit caldera, not a typical conical shape
Ophiolite suitesOphiolite suites
Obducted ocean crustObducted ocean crust
Current researchCurrent research
Find rocks of anomalous ages in otherwise Find rocks of anomalous ages in otherwise consistent time sequences relative to ridgesconsistent time sequences relative to ridges
Sediments in trenches are often in smooth Sediments in trenches are often in smooth layers (not disturbed and contorted). layers (not disturbed and contorted). Episodic motion?Episodic motion?
Mechanism for magnetic reversals?Mechanism for magnetic reversals? Why don’t spreading centers move?Why don’t spreading centers move?
Take home pointsTake home points
Types of plate boundariesTypes of plate boundaries Major seismic features of plate boundariesMajor seismic features of plate boundaries Location of major features (e.g., Ring of Fire, mid-Location of major features (e.g., Ring of Fire, mid-
ocean ridges)ocean ridges) Tranform faultsTranform faults Formation and destruction of oceanic crustFormation and destruction of oceanic crust Function and patterns of trenches, earthquakes, Function and patterns of trenches, earthquakes,
mountainsmountains Lithospheric plates, hot spots, hydrothermal vents, Lithospheric plates, hot spots, hydrothermal vents,
island arcs, magnetic anomalies island arcs, magnetic anomalies
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