Notes from specification
1. WHAT IS THE EVIDENCE FOR CONTINENTAL DRIFT AND PLATE TECTONICS?
1.a. There is a variety of evidence for the theories of continental drift and plate tectonics
THEORIES OF CONTINENTAL DRIFT AND PLATE TECTONICS INCLUDING:
- The basic structure of the Earth including the lithosphere, asthenosphere and the role of
convection currents
Crust:
- Continental crust: 35 km deep - solid rock - silicon and aluminium (Sial)
- Oceanic crust: 5-15 km deep - solid rock - silicon and magnesium (Sima)
Mantle:
- Lithosphere: rigid layer between asthenosphere and the crust - 100km to crust
- Asthenosphere: semi-molten - can flow under pressure - 100-300 km down -
convection currents exist here due to vast amounts of energy and heat
- Mesosphere: 300 - 2900 km - solid material
Core:
- Inner core: solid - approx 5000℃
- Outer core: liquid - approx 4500℃
Core and mantle - separated at approx 2900 km
Mohorovicic discontinuity (Moho) - separates crust and mantle
Convection currents: Heated rock rises in the current as it is less dense, semi-molten rock in
plastic flow moves across the top of the mantle taking the plate with it, the rock then cools
and sinks back down
CONTINENTAL DRIFT & THEORY OF PLATE TECTONICS
- Alfred Wegener’s theory of continental drift
- Proposed that 250 million years ago, the supercontinent Pangea split up into 2 large
landmasses
- Subsequent splitting to form the 7 continents that exist today
Geological evidence:
- Continental fit: some continents look like the used to fit together eg. South America
and Africa - least convincing evidence as imprecise fit and could be alternative
explanations
- Glacial evidence: Carboniferous period - striations
- Mountain chains aligning & rock sequences
, Biological evidence:
- Fossil evidence:
- Divergence: Fossils have been recovered in different countries from the
same time period and species - Mesosaurus and Cynognathus both in South
America and Africa
Similar fossil brachiopods found in Australian and Indian limestones
- Convergence: Before the land bridge was formed between N. and South
America there were 29 families in S. America different from those in N.
America - now both have 22 families in common
- Evidence for sea-floor spreading; paleomagnetism; the age of seafloor rocks
Paleomagnetism: most compelling evidence
- Involves measuring the orientation of iron minerals in rocks either side of mid-ocean
ridge
- As the magma rises out of rift valleys, it cools and locks in the iron bearing minerals
- Every 400,000- 500,000 years, polarity flips and Earth’s magnetic field points to the
dominant pole (currently North)
- The changes are mimicked by the alignment of magnetic minerals such as magnetite
within the oceanic crust
- Results in symmetrical bands either side of the ridge - regularity, can accurately
calculate movement
Sea floor spreading - aligns with paleomagnetism
- New younger oceanic crust is formed at mid ocean ridges which pushes older crust
away - like a ‘conveyor belt’
Age of seafloor rocks
- As the distance from the mid ocean ridge increases - the age of the rock also
increases
- Oceanic crust is approx 200 million years old, continental crust can be up to 4.5
billion years old
1.b. There are distinctive features and processes at plate boundaries
EARTH’S CRUSTAL FEATURES AND PROCESSES, INCLUDING:
- The global pattern of plates and plate boundaries
- Most earthquakes (particularly high magnitude ones) were along specific narrow
bands (plate boundaries) which suggests that the crust is broken up into different
tectonic plates
- 75% of volcanoes are located along the Pacific ‘Ring of Fire’ - correlational evidence
- Eg. converging Eurasian and Indian plates
1. WHAT IS THE EVIDENCE FOR CONTINENTAL DRIFT AND PLATE TECTONICS?
1.a. There is a variety of evidence for the theories of continental drift and plate tectonics
THEORIES OF CONTINENTAL DRIFT AND PLATE TECTONICS INCLUDING:
- The basic structure of the Earth including the lithosphere, asthenosphere and the role of
convection currents
Crust:
- Continental crust: 35 km deep - solid rock - silicon and aluminium (Sial)
- Oceanic crust: 5-15 km deep - solid rock - silicon and magnesium (Sima)
Mantle:
- Lithosphere: rigid layer between asthenosphere and the crust - 100km to crust
- Asthenosphere: semi-molten - can flow under pressure - 100-300 km down -
convection currents exist here due to vast amounts of energy and heat
- Mesosphere: 300 - 2900 km - solid material
Core:
- Inner core: solid - approx 5000℃
- Outer core: liquid - approx 4500℃
Core and mantle - separated at approx 2900 km
Mohorovicic discontinuity (Moho) - separates crust and mantle
Convection currents: Heated rock rises in the current as it is less dense, semi-molten rock in
plastic flow moves across the top of the mantle taking the plate with it, the rock then cools
and sinks back down
CONTINENTAL DRIFT & THEORY OF PLATE TECTONICS
- Alfred Wegener’s theory of continental drift
- Proposed that 250 million years ago, the supercontinent Pangea split up into 2 large
landmasses
- Subsequent splitting to form the 7 continents that exist today
Geological evidence:
- Continental fit: some continents look like the used to fit together eg. South America
and Africa - least convincing evidence as imprecise fit and could be alternative
explanations
- Glacial evidence: Carboniferous period - striations
- Mountain chains aligning & rock sequences
, Biological evidence:
- Fossil evidence:
- Divergence: Fossils have been recovered in different countries from the
same time period and species - Mesosaurus and Cynognathus both in South
America and Africa
Similar fossil brachiopods found in Australian and Indian limestones
- Convergence: Before the land bridge was formed between N. and South
America there were 29 families in S. America different from those in N.
America - now both have 22 families in common
- Evidence for sea-floor spreading; paleomagnetism; the age of seafloor rocks
Paleomagnetism: most compelling evidence
- Involves measuring the orientation of iron minerals in rocks either side of mid-ocean
ridge
- As the magma rises out of rift valleys, it cools and locks in the iron bearing minerals
- Every 400,000- 500,000 years, polarity flips and Earth’s magnetic field points to the
dominant pole (currently North)
- The changes are mimicked by the alignment of magnetic minerals such as magnetite
within the oceanic crust
- Results in symmetrical bands either side of the ridge - regularity, can accurately
calculate movement
Sea floor spreading - aligns with paleomagnetism
- New younger oceanic crust is formed at mid ocean ridges which pushes older crust
away - like a ‘conveyor belt’
Age of seafloor rocks
- As the distance from the mid ocean ridge increases - the age of the rock also
increases
- Oceanic crust is approx 200 million years old, continental crust can be up to 4.5
billion years old
1.b. There are distinctive features and processes at plate boundaries
EARTH’S CRUSTAL FEATURES AND PROCESSES, INCLUDING:
- The global pattern of plates and plate boundaries
- Most earthquakes (particularly high magnitude ones) were along specific narrow
bands (plate boundaries) which suggests that the crust is broken up into different
tectonic plates
- 75% of volcanoes are located along the Pacific ‘Ring of Fire’ - correlational evidence
- Eg. converging Eurasian and Indian plates
