Lecture 8: Continents - Past and Future

Question 1

What is the Wilson Cycle?

Answer 1

Periodic opening and closing of oceans (or formation and rifting apart of supercontinents).

Question 2

What is the time period of one Wilson cycle?

Answer 2

around 600 Ma

Question 3

How many Wilson cycles do we believe modern style plate tectonics have operated for?

Answer 3

At least 4

Question 4

What landforms existed before 2500 Ma?

Answer 4

No major continents - only isolated volcanic islands

Question 5

How were volcanic islands brought together?

Answer 5

oceanic plate subduction

Question 6

What happened when volcanic islands were brought together?

Answer 6

Accreted to form bigger landmasses (process still happens today)

Question 7

What evidence do we use to reconstruct past continental configurations?

Answer 7

mountain belts (ancient sites of plate collisions)
Paleoclimate indicators
Fossil distributions
Paleomagnetic data

Question 8

What are the 4 main supercontinents we’ve identified?

Answer 8

Pangea
Rodinia
Columbia
Kenorland

Question 9

Which supercontinents can we reconstruct?

Answer 9

Only Pangea and Rodinia

Question 10

What time frame did Pangea exist in?

Answer 10

300-200 Ma ago

Question 11

What time frame did Rodinia exist in?

Answer 11

900-750 Ma ago

Question 12

What time frame did Columbia exist in?

Answer 12

1800-1300 Ma ago

Question 13

What time frame did Kenorland exist in?

Answer 13

2500-2100 Ma ago

Question 14

Why do we think super continents break up?

Answer 14

They essentially self-destruct

Question 15

Are supercontinents pulled apart by external forces?

Answer 15

No

Question 16

Describe the process through which a supercontinent can break up

Answer 16

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)

Question 17

How long will the Wilson cycle continue for?

Answer 17

will continue as long the Earth’s mantle is hot and plastic enough to convect

Question 18

What do current understandings of plate motions and triple junctions suggest will happen in 50 Ma?

Answer 18

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.

Question 19

What do we believe will happen in approx 250 Ma?

Answer 19

2 options:
If Atlantic subducts ‘Pangea Proxima’
If Pacific subducts ‘Amasia’

Question 20

What seperated Northern and Southern Britain 500 Ma ago?

Answer 20

A wide ocean - the lapetus

Question 21

When did Northern and Southern Britain collide and why?

Answer 21

Subduction of the ocean resulted in continental collision - 400 Ma ago

Question 22

Where di Southern and Northern Britain collide?

Answer 22

20 degrees south of the Equator

Question 23

What was the effect of North and South Britain colliding?

Answer 23

Caledonian mountains (uplifted by collision
Folding, faulting and granite intrusions across Scotland and N. England

Question 24

Where has Britain been drifting since the collision?

Answer 24

North

Question 25

What landscapes did Britain pass through, and what was the effect?

Answer 25

equatorial and subtropical landscapes

Resulting in coal deposits, coral-rich limestone, desert sandstones and evaporites

Question 26

What was Britain like 100 Ma ago?

Answer 26

Submerged by sea levels - due to global warming

Question 27

How did the white cliffs of Dover form and when?

Answer 27

Via chalk deposits in the Cretaceous era. Formed from calcite plates of coccolithophores, which thrived in warm, shallow seas

Question 28

What is our proof that we seperated from Greenland and Northern America 60 Ma ago?

Answer 28

Basalt lavas and drykes in Scotland date from this episode`

Question 29

When did we seperate from Greenland and North America

Answer 29

50 Ma ago

Question 30

How far has Southern Britain travelled in 500 Ma?

Answer 30

approx 1200km North

Question 31

What does direction of movement of one plate in relation to another depend on?

Answer 31

Transform fault boundaries indicate the direction of relative plate movement
Seafloor isochrons reveal the positions of divergent boundaries in earlier times

Question 32

How did economic geologists use the former fit of continents?

Answer 32

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.