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