continental drift + plate tectonics

key idea 1

Where do convection currents exist?

What do these do?

• Asthenosphere

• Caused by vast amounts of heat generated in mantle.

• Semi-molten asthenosphere flows carrying with it the solid lithosphere and crust.

properties of the continental crust?

• Thickness: 30-70km

• Density: 2.6-2.7

• Mineral composition: mainly granitic, silicon, aluminium

properties of the oceanic crust?

• Thickness: 5-10km

• Density: 3.0

• Mineral composition: mainly basaltic, silicon and magnesium

properties of the mantle?

• Thickness: To a depth of 2900km

• Density: 5.6 at core

• Mineral composition: rich in magnesium and iron

role of convecion currents?

• heat from core creates currents in the mantle

• hot magma rises, cools + sinks → drive plate movement

• slab pull: subducting plate pulls rest of the plate

• ridge push: new crust at mid ocean ridges pushes plates apart

Alfred Wegener's idea?

• continental drift: continents used to fit together (Pangea) and moved apart overtime

• plate tectonics: movement of plates on semi-fluid aesthenosphere

What is paleomagnetism?

• magnetic minerals in basalt align with earth’s magnetic field

• earth’s magnetic field flips

• symmetrical magnetic stripes of either ridges

What is sea-floor spreading?

• harry hess proposed it

• occurs at mid ocean ridges → new oceanic crust is formed, pushes old crust away

what is age of sea floor rocks?

• rocks are youngest near mid-ocean ridges, older further away

• confirmed by radiometric dating techniques

what is evidence from ancient glaciations?

• deposits found in tropical regions suggest past glaciations

• e.g glacial till in S. America, Africa, India + Australia = connected near south pole

what is fossil records?

• identical fossils found on different continents

• mesosaurus: freshwater reptile - s. america + africa

• glossopteris: plant found in s.america, africa, antarctica + india

most reliable evidence?

• paleomagnetism: symmetrical stripes, reversed magnetic polarity match earth’s magnetic reversal timeline

• age of oceanic rocks supports this

least reliable evidence?

• fit of the continents: subjective observation → can be influenced by erosion, sea level change.

What are the 3 types of plate boundary?

• Divergent (constructive)

• Convergent (destructive)

• Conservative/tranform

what is a divergent boundary?

• 2 plates move apart

• mid-ocean ridges + rift valleys

• processes: sea floor spreading, volcanicsm, shallow earthquakes

what is a convergent plate boundary?

• 2 plate move towards each other which leads to:

  • oceanic-continental

  • oceanic-oceanic

  • continental-continental

what is a conservative boundary?

• 2 plates slide past each other

• transform faults + linear valleys

oceanic-continental (destructive)

• oceanic subducts under continental due to its higher density

• features: deep ocean trenches, fold mountains

oceanic-oceanic (destructive)

• 1 oceanic plate subducts under the other

• features: deep ocean trenches, island arcs

continental-continental (collision)

• 2 plates collide → no subduction due to low density → compressed + uplifted crust

• features: fold mountains, plateaus

What is a mid-ocean ridge?

• Very long chains of mountains on the sea floor.

• At intervals, ridges are broken by transform faults.

• Vary in shape depending on rate of spreading

How does the spreading of plates cause earthquakes?

• Transform faults

• Small, shallow-focus earthquakes occur along their lengths as they slip.

What are transform faults?

Large-scale faults in the crust at right angles to a mid-ocean ridge, which range from tens to hundreds of km.

Describe the sequence of formation of a rift valley

• (Underwater) Rising magma creates dome/bulge.

• Plates continue to spread.

• Brittle rocks fracture and fault.

• Dome subsides (sinks) forming a steep-sided valley.

• E.g. African rift valley (African plate splitting apart).

Rift zones on land

• Continental crust must be thin for rifting to occur.

• E.g. Red Sea northwards to Turkey.

• Crust has been stretched, causing faulting and forming a sunken valley known as a graben.

• As rift widened, magma erupted to surface.

• Eventually rift valley sank below sea level, forming the present-day Red Sea.