Lecture 8: Continents - Past and Future Flashcards
What is the Wilson Cycle?
Periodic opening and closing of oceans (or formation and rifting apart of supercontinents).
What is the time period of one Wilson cycle?
around 600 Ma
How many Wilson cycles do we believe modern style plate tectonics have operated for?
At least 4
What landforms existed before 2500 Ma?
No major continents - only isolated volcanic islands
How were volcanic islands brought together?
oceanic plate subduction
What happened when volcanic islands were brought together?
Accreted to form bigger landmasses (process still happens today)
What evidence do we use to reconstruct past continental configurations?
mountain belts (ancient sites of plate collisions)
Paleoclimate indicators
Fossil distributions
Paleomagnetic data
What are the 4 main supercontinents we’ve identified?
Pangea
Rodinia
Columbia
Kenorland
Which supercontinents can we reconstruct?
Only Pangea and Rodinia
What time frame did Pangea exist in?
300-200 Ma ago
What time frame did Rodinia exist in?
900-750 Ma ago
What time frame did Columbia exist in?
1800-1300 Ma ago
What time frame did Kenorland exist in?
2500-2100 Ma ago
Why do we think super continents break up?
They essentially self-destruct
Are supercontinents pulled apart by external forces?
No
Describe the process through which a supercontinent can break up
Continental collisions produce unusually thick crust.
This thick crustal layer acts as an effective insulator to mantle-derived heat.
Heat builds up beneath the thick continental crust, as well as within it (due to the high concentration of radioactive isotopes).
As the crust and underlying mantle become hotter, the rocks expand, becoming less dense and therefore more buoyant, leading to doming.
The arching of the crust causes extension and normal faulting, which in turn leads to lithospheric thinning, volcanism, and continental rifting ( may evolve into a MOR)
How long will the Wilson cycle continue for?
will continue as long the Earth’s mantle is hot and plastic enough to convect
What do current understandings of plate motions and triple junctions suggest will happen in 50 Ma?
Africa & Europe will have collided forming a huge Mediterranean mountain chain.
The Atlantic Ocean will have started to subduct.
New subduction zones will encircle Australia.
What do we believe will happen in approx 250 Ma?
2 options:
If Atlantic subducts ‘Pangea Proxima’
If Pacific subducts ‘Amasia’
What seperated Northern and Southern Britain 500 Ma ago?
A wide ocean - the lapetus
When did Northern and Southern Britain collide and why?
Subduction of the ocean resulted in continental collision - 400 Ma ago
Where di Southern and Northern Britain collide?
20 degrees south of the Equator
What was the effect of North and South Britain colliding?
Caledonian mountains (uplifted by collision Folding, faulting and granite intrusions across Scotland and N. England
Where has Britain been drifting since the collision?
North
What landscapes did Britain pass through, and what was the effect?
equatorial and subtropical landscapes
Resulting in coal deposits, coral-rich limestone, desert sandstones and evaporites
What was Britain like 100 Ma ago?
Submerged by sea levels - due to global warming
How did the white cliffs of Dover form and when?
Via chalk deposits in the Cretaceous era. Formed from calcite plates of coccolithophores, which thrived in warm, shallow seas
What is our proof that we seperated from Greenland and Northern America 60 Ma ago?
Basalt lavas and drykes in Scotland date from this episode`
When did we seperate from Greenland and North America
50 Ma ago
How far has Southern Britain travelled in 500 Ma?
approx 1200km North
What does direction of movement of one plate in relation to another depend on?
Transform fault boundaries indicate the direction of relative plate movement
Seafloor isochrons reveal the positions of divergent boundaries in earlier times
How did economic geologists use the former fit of continents?
to find mineral and oil deposits by correlating the rock formations in which these resources exist on one continent with their predrift continuations on another continent.
