GEL topic #11: Convergent and Transform tectonics

oceanic and continental crust

• broad categories of crust

• dominant rock composing them are igneous rocks that form by the solidification of a molten fluid like magma or lava

• float above denser rock of mantle

oceanic crust

• 7-10km thick

• composed of silica rock dominated by elements like Si and O along with smaller amounts of Fe and Mg

• created by solidification of molten rock along mid-ocean ridges and is then transported laterally by seafloor spreading

• iron and magnesium in rock makes it dense (average 2.9 g/cc density)

continental crust

• 30-70 km thick

• formed by variety of processes that tend to incorporate lighter elements into rock such as potassium, sodium and aluminum

• most granite (formed by solidification of magma deep underground)

• average density of 2.7 g/cc

why do ocean basins exist?

denser (iron rich) oceanic crust sinks deeper into underlying mantle, relative to continental crust

convergent plate boundaries

plate boundary where two plates converge to produce linear mountain belts

• primary force is compression

• lithosphere is destroyed along convergent margins

compression

squeezing and deformation caused by the squeezing motion of the two plates toward each other

deformation

the bending and breaking of rock, commonly associated with tectonic compression and uplift of rocks to create mountain ranges

three types of convergent boundaries

oceanic/continental, oceanic/oceanic and continent/continent

oceanic/continental convergence

plate composed of oceanic lithosphere flex/bends and dives beneath a continental part of a plate, lithosphere material is returned to mantle

• produce chains of volcanoes, deep sea trench and powerful earthquakes

subduction

process where oceanic lithosphere descends down into the mantle

deep ocean trench

process of subduction creates arcuate contact between two plates

• bathymetric expression of a subduction zone and represent the actual plate boundary

• represents actual plate boundary

how are volcanoes formed from subduction?

• water-saturated sediment of the seafloor is dragged down along the broad surface of the subduction zone

• as subduction plate reaches certain depths where the temperature and pressures are just right, the water is released into overlying rock of asthenosphere

• inclusion of water causes mantle rock to melt and create magma because water interferes with the atomic structure

• buoyant molten rock (magma) rises toward the surface where it pools within bodies called magma chambers

• volcanoes align roughly parallel to convergent margin forming continental volcanic arc

magma chamber

chamber supplies magma to volcanoes on the surface within mountain belt

• gives off heat to surrounding rock and can cool/solidify to become granite

continental volcanic arc

volcanoes align roughly parallel to the convergent margin, forming linear mountain chain

how do convergent boundaries affect mountain ranges?

they are uplifted along convergent boundaries as compressional tectonic stress causes rock to either break along faults, rotate into tilted orientations or bend into folds (examples of deformation)

deformation

the process that changes the shape, size, or volume of the Earth's crust

• results in uplift of rocks into higher elevations

• ex. rock breaks along faults, rotates into titles orientations or bend into folds

what causes rocks along continental side to deform?

compression of two gigantic plates against one another

what forms elongates mountain chains?

as the rocks are compressed over millions of years the magma intrudes into these deformed rocks, forming magma chambers that may cool and solidify in place, adding gigantic rock to core of the mountains

• chain of volcanoes rise above deformed rocks creating highest peak in range

cascade range

related to subduction along Pacific Northwest cost where small oceanic plates subduct beneath the less dense North American continental plate margin

cascade subduction zone

produces linear chain of active volcanoes like Lassen peak, Mt. Shasta, Mt. hood

• hazards: large-magnitude earthquakes, active volcanism and potential for tsunami

• rate of subduction is faster than rate of seafloor spreading along mid-ocean ridges so ridges slowly migrate toward the subduction zone and are eventually consumed

inclined zone of seismicity

• characteristic of subduction zones

• earthquakes ranging in depth from near surface are common along dense subducting plate as it grinds downward against the over-riding plate

• commonly largest in magnitude

ring of fire

alignment of earthquake epicenters and volcanoes circumscribing the Pacific Ocean

• related to subduction zones

Examples of subduction-related volcanic mountain chains of Ring of Fire

• Andes Mountains of South America

• Central American cascades

• cascades of Pacific Northwest

• alaskan volcanoes

• Aleutian island chain - Japan and Philippines

• Indonesian Island Chain

oceanic-oceanic convergence

two plates composed of oceanic lithosphere may converge with older, “colder” and denser plate subjecting beneath younger, “warmer” less dense plate

volcanic island arcs

rising of magma of oceanic/oceanic convergence supplies the raw material for the creation of linear chains of volcanic islands

when does island arc volcanism occur?

when magma is generated long the subduction zone buoyantly reach the overlying seafloor and erupts, with the underwater volcano eventually building above sea level through time as a volcanic island

what convergence dominates the eastern pacific along the americas?

oceanic-continental convergence

what convergence dominates western pacific from Aleutian to New Zealnd

oceanic-oceanic convergence

age of oldest rocks on earth?

4 billion

continental/continental convergence

third type of convergent margin exists where continental plates converge with other continental plates

• ex: Himalayas/Tibetan Plateau, European Alps

example of continent/continent collision zone

the Himalayas

• continental rock on India (part of Indian-Australian plate) has been moving continually northward for the last several tens of millions of years, ultimately wedging beneath the Eurasian plate, raising the Himalayas and Tibetan Plateau

what alleviates continental accumulation of compressional tectonic stress?

large earthquakes that happen along the entire collision zone

which plate will subduct in continental/continental convergence

neither - both plates along the convergence margin are composed of relatively low-density continental rock so they won’t subduct

collision zones

continental/continental convergence boundaries where no subduction is involved

thrust faulting

when high mountains are created along continent/continent collision zones by detachment of thick slivers of rock and stacking them like shingles one atop the other

• compressional stress squeezes them together, layering them on top and against one another, creating high elevations

what is rate of mountain building

very slowly because plate movements happen very slowly

what creates extreme topography in landscape?

when mountains are attacked by rain, snow, global ice and river incision

why are earthquakes necessary for mountain building?

thrust sheets move laterally during earthquakes

• they reflect abrupt motion along thrust faults in the region → motion along thrust faults raises mountains during shaking

how do mountain impact the environment?

• influence world’s wind and weather systems → influence distribution of climate zones

transform faults

type of plate boundary where two slide horizontally past each other along major fault surfaces cutting across continents

• lithosphere neither created nor destroyed along these plate boundaries

• actively slipping part of a fracture zone between two ridge segments and are capable of generating earthquakes

shear

primary force at transform boundaries

• blocks of rock on either side of the fault move in opposite directions, sliding laterally past one another

fault

planar fracture along which movement has occurred, offsetting massive blocks of rock and surface features on opposites sides of fault

• extend downward into crust

• cut through rock of the crust

fault line

the fault we see on landscape where fault plane intersects the surface

rupture

abrupt unlocking of one fault block against an adjacent fault block on either side of the fault plane

• may trigger earthquakes

San Andreas Fault

continental transform plate boundary in California separating the North American plates from Pacific plate

• connects a narrow and actively spreading ocean basin in the south with convergent boundary in the north

what are mid-ocean ridges segmented by?

fracture zones extending perpendicular to the ridge axis