2.12 Plate tectonic theory Flashcards
What mechanism was originally believed to drive plate movement?
convection currents in the mantle
What are the key arguments against convection currents as the driver of plate motion?
- there is no correlation between plate area and the velocity of plate movement
- the plates move at a different speed to the mantle beneath them
What are the key arguments against convection currents as the driver of plate motion?
- there is no correlation between plate area and the velocity of plate movement
- the plates move at a different speed to the mantle beneath them
- the asthenosphere is too plastic to generate enough friction to pull the tectonic plates along
Which process is now thought to be the main driving force for plate motion?
slab pull
the dragging force of a subducting slab of oceanic lithosphere
What is meant by the term ridge push?
the elevated lithosphere at a mid-ocean ridge creates a pushing force down the tectonic plate
the ridge is elevated by the passive upwelling of mantle beneath it
What is the relative contribution of ridge push compared to slab pull?
slab pull exerts around twice the force applied by ridge push
Why does the oceanic lithosphere subduct into the mantle?
the oceanic lithosphere is cold and dense
What are the basic principles of the geosyncline model for explaining mountain formation?
- the continents are fixed through time
- the lateral compression of a few tens of kilometres could explain mountain building
- contraction of the Earth as it cooled (the radius would shrink) could provide the lateral compression required
What are the main criticisms of the geosyncline model and contraction theory?
- Earth has not cooled as much as previously believed (radioactive decay generates heat)
- Earth has therefore not shrunk as much as predicted - only a few tens of kilometres - but 1000s of kilometres of shrinkage would be needed to explain existing mountains
- contraction would lead to planet-wide lateral compression whereas mountains are found in linear belts
Who first proposed the theory of continental drift?
Alfred Wegener proposed continental drift theory in 1915
What evidence did Wegener provide for continental drift?
- The continents fit together (particularly when considering the 500m ocean depth contour)
- Rock types on either side of an ocean are the same (age, mineral composition, etc.) such as Precambrian cratons, Carboniferous coal deposits and glacial tillites, Permian red sandstones and evaporites
- Fold mountain chains are linear and one Precambrian mountain chain crosses from Africa to South America and back to Africa as a continous belt
- Fossils of the same species are found in different continents e.g. the Carboniferous land based reptile Mesosaurus and the plant Glossopteris are found in both Africa and South America
- Glacial deposits and striations are found in central southern Africa suggesting this area was previously much closer to the poles
Why might the shape of the continents no longer match up perfectly?
- erosion of the coastline would lead to gaps
- deposition of sediment would cause an overlap
- sea level is constantly changing
What is a tillite?
a sedimentary rock produced by glacial processes
What are evaporites?
sedimentary mineral deposits/rock produced by the evaporation of hypersaline water and precipitation of dissolved minerals e.g. halite (rock salt), gypsum and anhydrite
Why was Wegener’s theory of continental drift not accepted at the time?
He could not provide a mechanism for the continents to move
