Lecture Fourteen Flashcards
What are the characteristics of continental crust?
35-40km thick on average.
Al, Ca and K - rich silicate (although all the other elements that make up the crust originally came from the mantle ~ granite).
~ 2.7-2.8 g/cm^3.
Buoyant - topography (although some continental crust is below sea level).
Base of the crust is called the MOHO.
Continents occupy 25% of the Earth’s surface.
Continental crust occupies 35% of the Earth’s surface.
Mountain belts on continents are wider than mountain belts in the ocean.
What is isostasy?
Mountains have a lot of mass, and a gravitational pull.
But not as much pull as they should if the base of the crust was flat under the mountain.
Mountain ranges have less mass than expected because they have a deep root of low density rust that displaces the mantle.
Higher the mountain, the deeper the root - because of the principle of isostasy.
Thick crust has a higher elevation and a deeper root than thin crust.
Denser crust (oceanic) floats at a lower level than less dense continental crust.
Each extra 1km of mountain at the surface must have an extra 6km of root.
What is the MOHO?
Mohorovicic discontinuity is a seismic boundary that separates crust from mantle.
Describe the surface geography of continental crust.
Cratons (flat bits):
Shields = continental crust exposed at surface.
Stable platforms = continental crust buried by sediment.
Mountain belts (pointy bits).
Average chemical composition - granitic or felsic.
Common minerals:
Quarts.
Orthoclase (K) feldspar.
Plagioclase (Na/Ca) feldspar.
RIck in Si, K and Na.
What is the crust composition?
Continental crust = more felsic (has more silicates) than oceanic crust (mafic).
Continental crust is less dense and oceanic crust.
Continental crust is weaker than oceanic crust (under pressure it will break more easily).
What evidence is there for subduction?
Earthquakes from subducting slab get deeper.
Overriding plate has volcanic arc above subduction zone.
What is ocean-ocean convergence?
Plate A subducts below plate B.
Plate A heats up as is descends.
Water is driven from the subducting minerals and oceanic crust of plate A into the overlaying lithospheric mantle of plate B.
The water causes the overlying mantle to melt.
Water + Basaltic mantle + oceanic crust = slightly granitic magma.
Magma rises up to form a volcanic island are = new continental crust.
Volcanic island arcs represent juvenile continental crust.
Mantle beneath mid-ocean rides is dry and produces basaltic (mafic) ocean crust when it melts.
Mantle above subduction zone wet because of the water release from the subducting slab, and produces granitic (felsic) continental crust when it melts.
What is an ocean-continent convergence?
Water is released from the subducting plate.
Overlaying mantle melts.
Magma rises to form new granitic (felsic) crust.
The oceanic plate will always subduct under the continental plate.
As magma is rising up though granitic crust, the volcanic arc is more granitic than ocean-ocean volcanoes.
New granitic (felsic) crust is added to the continental margin (continent grows).
A mountain belt is formed at he continental margin.
What is continent-continent convergence?
No new crust is formed at continent-continent convergence margins because there is no subduction. But these margins weld old pieces of continental curst together. This process is called accretion.
What is accretion?
Large continents are formed by collision of smaller continental fragments at continental margins.
What is rifting?
Accretion at convergent margins means that continents get bigger with time.
But continents also break up.
This continent stretches and thins (extends) to form a rift valley at the surface, and hot asthenosphere rises from below (mantle upwelling) causing volcanism.
The continent ruptures and new oceanic crust is formed at an ocean ridge separating the two new plates.
Not sure is crustal extension causes mantle upwelling, or the other way around.
What are transform margins?
Continental curst is also split and rearranged along transform margins, sometimes called dispersion.
Put simply, what is the life cycle of continents?
Continental crust forms above subduction zones.
grows by accretion at convergent margins.
Is split and rearranged at rifts and transforms.
What are Wilson Cycles?
Continents break up into smaller fragments which then accrete and grow to form supercontinents, which then break up again.
This cycles can take up to 300 million years to complete.
NOTE: Continental crust is not destroyed during subduction, it can be eroded, but those sediments are uplifted during subduction to form new continental crust.
This theory is not totally correct though.
How has the volume of continental crust increase with time?
Model A:
Continental crust forms more rapidly than it did in the past.
Model B:
Continental crust forms more slowly now than it did in the past.
Model C:
Growth rate is now similar to that in the past (most popular view).
