Plate Tectonics

What evidence did Wegener present for continental drift?

The continents fit like puzzle pieces, rock types and fossils across gaps were the same age and type 

What new evidence became available in the 1950 and 1960s in support of moving continents and
plate tectonics?

Mid ocean ridge, polar wander 

What are the three types of plate tectonic boundaries?

What are the driving forces of plate tectonics?

Give examples of these three boundaries (places on Earth)

What processes are related to plate tectonics?

Why do plate tectonics exist

the rigid crust moves over the ductile asthenosphere

What are the two types of crust?

Oceanic and continental

What are the main differences between the two types of crust?

thickness, density, age, composition

Describe the oceanic crust in 3 words

Dense, ‘cold’, young

Describe the continental crust in 3 words

Buoyant, ‘hot’, old

How is oceanic crust made?

Made by melting of the mantle at mid ocean ridges

How is continental crust made?

Continental crust is made by melting of the mantle above subduction zones

What is a subduction zone?

The geological area where one tectonic plate slides under the other due to density difference at converging boundaries

What is polar wander

The apparent movement of the Earth’s magnetic pole position across Earth’s surface throughout geological time

How is polar wander mapped

Using basaltic rocks of different ages as they “lock in” the poles once they solidify

What is paleomagnetism

measurements of the magnetic direction and inclination represented within rocks by ferromagnetic minerals

How did polar wander prove that the continents moved?

Paleomagnetism suggested that poles were in separate places at the same time, as this isn’t possible one of the rocks must have “spun” as the continent drifted to provide a false direction

Who discovered sea floor spreading in 1950

Marine Tharp

What else is a mid ocean ridge?

A divergent plate boundary

What are hot spot tracks?

linear chains of volcanic islands get progressively younger along the chain

What is a mantle hotspot?

A stationary upwelling of hot mantle rock that creates volcanoes as tectonic plates move over it, generally independent of plate motion

What does Isostasy mean

the balance between buoyant (upward) and gravitational (downward) forces

What happens when plates are not in isostasy

plate tectonics

What are continental rifts

when continental crust begins to split apart within a tectonic plate

Describe the process of continental rifting 

Begins due to mantle upwelling (convection like in a boiling pot of water)
→ Large-scale bulge in one area initiates a triple junction
→ Two of the three splits will keep moving apart to form the new plate margin
→ Initial setting that develops is a rift valley

Rifting thins the crust which has what effect in term?

reducing of lithostatic pressure and decompression melting

What is a proto-ocean

If continental rifting thins the crust enough, the land between plates will sink below sea level to form a proto-ocean

What are Mid-ocean ridges

Where oceanic crust moves apart at a tectonic plate boundary

What is a passive margin

the transition between oceanic and continental crust that is not an active plate boundary

Why is the mid ocean ridge a ridge

The new crust is relatively lighter than the old ocean crust and thus is more buoyant

What are transform boundaries?

where plates move laterally passed one another

What are transform boundaries associated with?

damaging earthquake activity

What happens in a Oceanic-Continental collision?

continental crust less dense, so oceanic crust subducts

What happens in a Oceanic-Oceanic collision

the older denser crust is subducted

What happens in a Continental-Continental collision

continental crust too buoyant to be subducted, remnant oceanic crust detaches, continental crust thickens to form a collisional mountain belt with the center marked by a ‘suture’ zone

What is a Wadati-Benioff zone

Where earthquakes occur related to the subducted slab as it interacts whilst subducting with the earth

What is a trench?

where the crustal flexure of the down-going slab results in a very deep zone that does not fill with sediment because the crust moves over geological time and scrapes sediment away

What is the accretionary wedge

Where sediments that have settled on the oceanic crust are scraped off the subducting slab, subject to high pressure and metamorphism

What is a forearc basin 

a low point between the accretionary wedge and the arc where sediments can accumulate

What is a retro-arc basin

Rocks become compressed and new mountains form that push down on the crust

What is ridge push?

as the new lithosphere cools at a mid ocean ridge

What is slab pull 

As the slab descends into the mantle, the dense slab sinks under its own weight, pulling on the rest
of the lithosphere

This entire model of tectonics can be described as …

The Wilson Cycle

How long does the Wilson Cycle take to complete?

500-700 million years

What is the composition of continental crust

felsic

What is the composition of oceanic crust

mafic

Why are continental crusts felsic

flux melting at subduction zones creates a felsic magma at a volcano

Why are oceanic crusts mafic 

composed straight from the mafic mantle melt 

How thick is continental and oceanic crust?

20-70km’s / 5-10km’s

What’s the word to describe climate evidence

Paleoclimate evidence

What was Wegner’s 4 pieces of evidence

fit of continents, fossil assemblages, rock formations and mountain chains, Paleoclimate evidence

What were the 3 new pieces of evidence

Seafloor spreading, paleomagnetism, earthquake and volcano mapping

What are the 3 driving forces of tectonics

mantle convection, ridge push, slab pull

Why is subduction zone flux melting andesitic (intermediate)?

partial flux melting of the mantle