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 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 ___________________).
• 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 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
• ________, 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
• 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.
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.
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
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