Lecture 4: Solid Earth Dynmics Flashcards
What mechanism did Alfred Wegner not have that would have been crucial to the continental drift theory?
That oceanic crusts are denser than continental which allows the latte to glide over the former
What evidence did Alfred Wegner use for his theory?
South America and West Africa appeared to fit together like a jigsaw which was supported by the geological patterns on both continents
What were the two geological patterns which matched up between South America and Africa?
Cratons: an ancient deep part of the crust
Orogenic Uplift: Essentially a mountain range
What evidence do we have for the explanation of the matching up between South America and Africa?
The fossil record - some animals must have been able to migrate between the continents and so they must have been able to swim (?) or walk between them
When could the sea floor start to be mapped and using what technology?
In the 1960s sonar pulses could be send through the ocean
What was found out about the ocean bathymetry in the early 1960s using the sonar technology?
there was the existence of of ridges/mountain ranges in which the sea floor’s elevation was higher
What is the name give today to the mountain ridge in the Atlantic that was found in the 1960s?
Mid-Atlantic ridge
What research helped to improve the understanding of the mid-atlantic ridge and its geological significance?
Palaeomagnetism studies
Give the brief explanation of sea floor spreading?
Known changing polarity of earth - studies found alternating metal orientation - Iron deposits orientated themselves with the earth’s polarity and this was then preserved in record - pattern back to ridge paired with the other side which suggested there was rock and metal emerging from ridge
What other evidence was found on the Atlantic sea floor which also supported sea floor spreading?
Accumulated sediment depth increased with distance from ridge - those places further from ridge will have had more time to accumulate sediment compared to those at the ridge edges
What subsequent problem was identified after the source of crust production was found (MAR)?
There must be somewhere else where the crust is being destroyed because the earth is not getting bigger
If these huge crusts are being destroyed at some point then what must the destruction process be like? What would you consequently expect to find somewhere else?
Turbulent and not smooth - there must be earthquakes/seismic activity somewhere
What evidence was there to support the idea of seismicity and plate destruction?
Seismic activity appeared to organise itself along certain lines that suggested they were the boundaries where plates were being destroyed.
What did the presence of sites of destruction and construction suggest?
That there are plates that make up the earth’s surface
What was a divergent plate boundary?
Crust being torn apart from eachother
What is an example of an oceanic/oceanic divergent boundary?
MAR
What is an example of a continental/continental boundary?
East African Rift Valley
What Islands has the MAR created?
Iceland and Surtsey
What is a convergent plate boundary?
Crust being forced together
What is an example of an oceanic/continental convergent boundary?
San Andreas Fault
What is an oceanic/oceanic convergent boundary example?
Caribbean
What is an example of a continental/continental boundary?
Himalayan Mountain Range
How are different volcanoes created on an convergent boundary that involves an oceanic plate?
The oceanic plate contains a lot of organic matter, sediments and other volatile materials that is less dense to the surrounding mantle when it gets sub ducted. It wants to escape so rises up through the crust.
What sort of volcanoes can be created on an O/O convergent boundary?
Island Arc
Where will the volcano form on an O/C convergent boundary?
It will probably form underneath and then above the continental crust
What happens when there is a C/C convergent boundary?
It will force the land to squeeze thereby forcing it down and up. 80-85% of this squeeze will be forced down and around 15-20% will be forced up.
How do you define a young mountain range?
<200mya
Where are young mountain ranges located?
At convergent plate boundaries
What is the name given to a plate boundary that has become relatively inactive?
Passive margins
What are cratons?
These are particularly old parts of the continental crust that are places of early life
What is an example of a craton and how old is it?
In Northwestern Australia, that is around 3Ga
What does mantle flow like?
A glacier that does not simplistically flow up as a single plume and then down via two “cells” - it is more random and sporadic
What is a Rayleigh cell?
A convection cell which better illustrates how the mantel cells operate
What direction does the pacific plate move in?
NW
What islands are created via different hot spots under the pacific plate?
Hawaii
Cobb Seamount
Austral Seamount Chain
What happens when hot spots are located underneath continental crust?
It ruptures the crust creating a volcano that are much bigger in size and geographical coverage compared to those under oceanic crust.
What is the name given to the volcanoes that form under continental crust?
Large Igneous Provinces
How can we tell areas which are subducting?
By looking at areas with faster flow velocities to the areas surrounding them
What is the Wilson Cycle?
This represents how the continent may change over time from separation to reattachment across the surface of the earth
Outline the stages of the Wilson cycle
All united - divergence between continents - continents reunite on other side of earth
What is the estimate time it takes for the Wilson Cycle to complete?
500mya
How many possible supercontinents might there have been in the earth’s history?
~8-10?
What are two other supercontinents other than Pangea?
Nuna and Rodina
When is the next supercontinent expected to form from now?
250mya
