The Major Plates on Earth

Plate Tectonics

Plate boundaries can be seen as undersea mountain chains (Mid Ocean Ridges—diverging plate boundaries), deep sea trenches (converging boundaries), or major faults that primarily separate sections of mid-ocean ridges (transform plate boundaries).

• Lithosphere “floats” on a partially melted asthenosphere, similar to a raft floating on water. The lithosphere is in isostatic ________________ with the asthenosphere.

Isostatic Equilibrium• The lithosphere is composed of

continental and ocean crust and the uppermost solid mantle.

• The asthenosphere is composed of the upper mantle and is at or near the __________ temperature for upper mantle rock.

• When a load, such as an ice sheet, is placed on the lithosphere, it will isostatically_____________ the lithosphere relative to the asthenosphere.

• When the ice melts the lithosphere isostatically_______________.

Isostatic Equilibrium

Note that the thin (7-10 km), dense (3.0 gm/cm3) ocean lithosphere “floats” ________ than the continental lithosphere, which is less dense (2.6 gm/cm3) and thicker (40-70 km).

Isostatic Equilibrium

What Drives Plate Tectonics?• Heat escaping from _______

creates convection currents in the asthenosphere

• Where currents ______, plates are pulled __________ (divergence)

• The heat and material rising here causes a “____________,” which forces the plates ________ from each other

What Drives Plate Tectonics?• Where currents __________, plates are pushed together

(convergence), and usually one is pushed down into the asthenosphere (_________________)

• The weight of the subducting slab creates a “____________,” which helps the plate descend further.

Plate TectonicsTypes of boundaries:

• Divergent Boundaries: plates move away from each other _____________ Ridges • Convergent Boundaries: plates move towards each other ______________ zones• Transform Boundaries: plates ___________________ one another

(Faults like San Andreas)• Hot Spots: _______ at plate boundaries: ___________ areas in the middle of plates

Divergent Plate Boundaries• Rising ___________ from below pushes plates apart• New ____________ crust is formed• Eruptions are all ________

(non-explosive)• Undersea ____________ is created called a mid- ocean ridge• Process widens existing

oceans (i.e. Atlantic ocean), or creates new ones (Red Sea, Great Rift Valley in Africa)

Pillow Basalts

Divergent Plate Boundaries

• Divergent margins often first form under continental crust because of the ______________ properties.

• Eventually, the upwelling basaltic magma will completely melt through the overlying continental crust, a new ocean __________ will form, and the two segments of continental crust will __________ apart.

• The ocean floor becomes wider as the new ocean lithosphere is produced at the divergent margin (referred to as sea floor ___________________).

Divergent Plate BoundariesDivergence splits up

continents and makes new oceans

• Divergence beneath Africa

Divergent Plate Boundaries

• The Red Sea is a newly forming ocean• The rift valleys in Africa are splitting it

apart

•The East African rift zone represents a continental rift. The topography of East Africa is highest in Africa because of heating the base of the continent by upwelling basaltic magma. The Red Sea is an incipient ocean basin. Note the dark basalt flows seen along the margins of the Red Sea in the above satellite image.

•Basalt Flows

Hydrothermal Vents at Divergent Boundaries

Hydrothermal Vents

Hydrothermal DepositsHydrothermal circulation leaches __________ from rocks, then concentrates and deposits them in Volcanogenic Massive Sulfide deposits (VMS).

Ore minerals and metals found at VMS’s:Ore mineral MetalSphalerite (ZnS) zincChalcopyrite (CuFeS2) copperGalena (PbS) lead

We know that the Earth is not increasing in __________ due to the creation of new ocean lithosphere along seafloor spreading zones; thus it stands to reason that the ocean lithosphere must be consumed along convergent boundaries.

Convergent Plate Boundaries• ______________

convection currents in asthenosphere brings plates together

• _______, thinner plate is subducted beneath the other plate

• Subducted plate is partially________ and produces explosive volcanoes on upper plate

• Deep ______________ are formed by bending of down-going slab

• Very large _____________

• Ocean-Ocean lithosphere convergence (subduction).Examples: Japan and

Phillipines

• Ocean-Continent lithosphere convergence (subduction).Examples: Andes Mountains and Cascade Mountains.

• Continent-Continent lithosphere collision (no subduction).Examples: Himalaya Mountains and European Alps.

Converging Boundaries: Three types

Convergent Plate Boundaries

• ________, colder, denser ocean plate gets subducted and partially melts

• Deep trench is formed by bending of down-going plate

• Chain of volcanic islands is formed on upper plate = _________________

• All islands are approximately the same _______

• Volcanoes are ______________ due to sediments (silica and water-rich) which get subducted with ocean plate

• Volcanoes are tall and _________

Ocean-Ocean Collisions

Ocean-Ocean convergence. Formation of an island arc (Japan).

• Examples include all the volcanic islands in the Western Pacific

Ocean-Ocean Collisions

• Island chains are usually arc-shaped• Mineral-rich hydrothermal deposits may occur

What about Barbados?

• “____________________” forms when lots of _____________ on subducting plate cannot be forced down, so it piles up in front of the upper plate• Accretionary wedges are _____-volcanic (mostly scrunched up sediment)

Carribean Island Arc

• Ocean crust gets subducted and generates __________ beneath continent

• Explosive volcanoes form on _____________ plate (silica, water and gas-rich) due to subducted sediment and melted continental crust)

• Volcanoes are appr. the same ________

• Volcanoes are tall and _________

• __________ is formed off the coast

Ocean-Continent Collisions

Mt. Shasta

Ocean-Continent Collisions

• Cascades in Pacific Northwest• Andes in S. America• Hydrothermal mineral deposits often occur (gold in CA)

Puget LowlandOlympic or

Coastal Mts.

CascadesColumbia Plateau

Structure of a subduction zone. Note that the geographic features relevant to the Cascadia subduction zone are noted in red.

Ocean-Continent Collisions• What about the

Olympics and the mountains on Vancouver Island?

• Olympics are non-volcanic, and marine fossils are found in its tallest peaks

• Sedimentary layers are extremely folded

• _________________ __________________

See Slab Maps at http://earthquake.usgs.gov/research/data/slab/

See Interactive Plate Tectonics Maps at http://www.iris.edu/hq/programs/education_and_outreach/animations/interactive

• Continental collisions are _____________ by oceanic-continental convergence.

• As the continental lithosphere arrives at the subduction zone, it cannot be _______________ because of its low density.

• The “collision” causes upwarping and deformation of the ocean floor and ocean lithosphere. Even segments of the upper mantle can be squeezed to the surface along the ___________ zone (where the two continents are stitched together).

Continent-Continent Collisions

• Continental crust is too thick and not dense enough to subduct, so very tall _________________are produced

Continent-Continent Collisions

• Mountains are non-volcanic

• Himalayas are still rising to this

day• Older examples:

Alps Appalachians

What evidence do you think geologists used to determine the timing of plate motion of the Indian sub-continent relative to the collision with Eurasia?

___________________ signature preserved in the Fe-bearing minerals in igneous rocks of various ages.

European Alps have formed from the collision of the African Plate with the Eurasian Plate. The Mediterranean Sea will _________________ over time.

Formation of Appalachian Mountains resulted from the collision of the African and European Plates with North American Plate prior to ________ million years ago.

Ural Mountains

Formation of the Ural Mountains related to collision tectonics 250 to 300 Ma ago.

Transform Plate Boundaries• Transform faults occur

where tectonic plates move in _________________ directions.

• Usually this occurs between different segments of ___________ _____________

• Plates slide past each other

• No ___________ activity occurs along transform faults

• ____________are common

Fig.14–4c

Transform boundaries typically form along mid-ocean rift zones, where the ______________ differs and the rigid ocean crust is ____________ along the transform fault.

Transform Plate Boundaries

• Most transform faults are located on the ____________ floor. They commonly offset active spreading ridges, producing zig-zag plate margins, and are generally defined by ______________ earthquakes.

• The San Andreas fault zone in California is a ________ transform fault on land.

• It connects the East Pacific Rise, a divergent boundary to the south, with the South Gorda -- Juan de Fuca -- Explorer Ridge to the north.

Transform Plate Boundaries

Transform Plate BoundariesThe San Andreas fault connects two _______________ plate

boundaries, and is one of the only transform faults cutting through continental crust (most are under the ocean)

Bay Area S. Cal

Hot Spots• Not located at plate boundaries.• _______ magma source (near core-mantle

boundary?) stays ______________ as plate moves over it.

• Hot Spots under Oceanic Plates produce a chain of volcanic islands which get ____________away from the hot spot (i.e. Hawaii, Maldives, Reunion)

•

Hawaii

Hot SpotsMost volcanic islands produced by hot spots will __________ away over time, since the volcanoes become ____________ once the plate moves past the magma source.

Known Hot Spots Locations (red dots)

Hot Spots• Hot spots erupting through

ocean crust erupt only _________--Eruptions are non-__________________--Volcanoes are ___________ type (wide, non-

pointy)

Mauna Loa

Hot Spots• Hot Spots underneath ______________

crust produce flood basalts and calderas (i.e. Deccan Traps, Yellowstone)

• Hot spot can melt portions of the continental crust (silica-rich), creating a massive, explosive eruption which leaves a _____________ (big hole) behind

• Hot spots can have very active hydrothermal circulation, which will produce _________________deposits

• The stationary deep hot spot does not seem affected by ________________in

mantle, which moves the plates above it.

Deccan Traps

Yellowstone Hot Spot and Caldera

Summary: Major Features at Plate BoundariesFeature Diverging Converging Transform Hot SpotVolcanoes Non-explosive

(Basalt)Explosive(Andesite-Rhyolite) (except C-C collisions)

No Non-explosive(Basalt)

Volcano Age All similar All similar N/A Get older away from source, most are extinct

Earthquakes Many, small Fewer, but can be large (>9)

Many, some canbe large (>8)

Many, mostly small

Mountains? Mid-Ocean Ridges

Volcanic (C-C can be very tall)

No Volcanic, shield-type

Other Features Rift Valley in center of mountains

TrenchAccretionaryWedge (sometimes)

Offset features on a mapJoin segments of Mid-ocean ridges (MORs)

Prolific lavaproductionCalderas when located under continents

Future plate motions are predictable based on current plate motions.What will happen to the Isthmus of Panama? What will happen to the Mediterranean Sea? Where will western California end up? What about East Africa?