Plate Tectonics Paradigm Flashcards
What does paradigm mean?
Theory
What is a divergent plate boundary?
Plates moving apart
What is a convergent plate boundary?
Plates moving together
What are the 4 previous theories of the plate tectonics paradigm?
- geosyncline model (contraction theory)
- continental drift
- passive plate tectonics
- active plate tectonics
What is the geosyncline early theory of the tectonics paradigm?
- scientists noticed changes in topography (mountains etc) and proposed that it must be cooling over time.
If earth is cooling over time, it must be contracting. They believed that as the earth cooler, the crust/top layer would “crinkle” to create mountains
When was the geosyncline model of the tectonic paradigm valid?
Up until the late 1960s
What are the problems with the geosyncline model of the tectonics paradigm?
High points and low points are not evenly distributed
What is the continental drift early theory of tectonics paradigm?
- Wegener proposed that continents/land masses were moving. He didn’t know how they were moving.
He observed rocks and fossils that he thought formed closer together, but were found far apart.
Holmes studied radioactive decay. (1940s)
Radioactive decay created heat in earth centre.
He proposed that convection currents must exist. Thought plates were moving like rafts floating on the mantle
When was the continental drift theory of the tectonics paradigm valid?
Early 1900s.
Holmes 1940s
What is the passive plate theory of the tectonics paradigm?
- states that we now accept plates are moving but we do not know how/why they move
- After ww2, there was much more monitoring of seismic activity (tracking bombs + testing)
- because of this monitoring, they could then say that the plates are moving
What is the active plates theory of tectonics paradigm?
States there is a driving force for why they are moving
When was the active plate theory of tectonics paradigm valid?
Only really accepted in 80s and 90s
What is seafloor spreading?
The process by which new oceanic crust is formed when mid ocean ridges and spreads outwards, pushing older crust away from the ridge
What is continental drift?
Plate tectonics theory suggests that lithospheric plates have moved to their present positions. It is believed that a large southern land mass Gondwanaland existed about 250Ma made up of what are now South America, Africa, Antarctica, India and Australia As a single continent, it had the same rocks and fossils and was glaciated at the same time
How can mountain chains tell us plates are moving?
Fold mountain chains can be 100s of kms long. The map of gondwanaland shows how one precambrian fold mountain chains crosses from Africa and south America back to Africa, in a continuous belt
How do fossils tell us the plates are moving?
If Africa and South America have always been separated, they should have a different fossil record, especially for animals and plants which were terrestrial
How does glaciation tell us the plates are moving?
In both South America and Africa, there are sedimentary deposits of angular, poorly sorted and scratched pebbles (clasts) in a fine grained matrix. Glacial striations are used to trace the movements of the glaciers from one common source in central southern Africa
How does GPS measure plate movement?
GPS locators are fixed positions on earth’s surface, talking to satellites. Satellites are used to locate exact location from fixed receivers on earth’s surface
What have scientists found about movement in Iceland?
Only example of a divergent boundary on land.
Diverging at 2.5cm a year
What is geodesy?
A field of science and maths which concerns itself with the size and shape of the earth and precise location of points on its surface
What does geodesy help us to calculate?
Distance. Taking inti account the curve of the earth’s surfacem. Use a curved coordinate system
What is an ellipsoid?
A slightly squashed sphere that is smooth (sphereoid)
Over long distances, geoids are better
What is a geoid?
Shows the earth’s lumpiness
Still quite smooth. Shows some elevations and depressions to take earth’s unevenness into account
Name the 7 main plates.
North American
Pacific
South American
African
Eurasian
Indian
Australian
What happens at transform plate boundaries?
Two plates moving past each other horizontally in opposing directions.
At these boundaries, we see shallow focus earthquakes.
There is no creation or destruction of crust and no volcanoes, rift valleys, or mountain ranges
How many major earthquakes have there been at the San Andreas fault in the last 400 years?
3
What was the 1906 San andreas fault earthquake responsible for?
Biggest killer was fires from gas pipes
How long has the San andreas fault been around?
30 million years.
Moved 350 miles past each other since it formed the “strike slip fault”
Where do most people live in california in relation to the San andreas fault?
By the fault in places like LA and San Francisco
What was the 1769 San andreas fault earthquake responsible for? (and its magnitude?)
8 richtor scale
250 mile long gash ripped
Bell Tower collapsed
Many after shocks
1 death, little damage –> not enough stuff to damage
What was the 1906 San andreas earthquake responsible for?
It was felt in oregon
Fires ripped through San Francisco
Water pipes ruined = no fire fighting
3000 died. 28,000 buildings destroyed.
Fire burned for 5 days
What is the average time frame between each major earthquake on the San andreas fault?
110-210 years
What is predicted about the next big earthquake at the San andreas fault?
Average 8.3 richtor scale as the highest.
To be massive, but not the largest
Likely to be 7.8 magnitude
Epicentre near cochella valley
How are southern californians taught to survive an earthquake?
Do not run.
Stay put, drop down.
Take cover.
Cover neck and head.
Hold on.
How long can tap water remain undrinkable after an earthquake?
Over 1.5 years
What is shear stress and how does it cause deformation (strain)?
Stress is the force acting on a rock.
Strain is the rock’s response.
Landmarks on a transform boundary will become offset
What is the coulomb stress transfer?
Stress transfer at transform plate boundaries.
At transform boundaries, some parts can move but some parts get locked.
Locked parts are where pressure builds.
Displacement=earthquake=stress release.
Sometimes, rather than being released, the energy (stress) travels along the fault –> causing secondary quakes
What is an example of the coulomb stress transfer?
Landers and big bear earthquake.
28th June 1992
Magnitude of 7.3 (Moment magnitude)
San andreas fault.
What is the most well known example of a divergent plate boundary?
Mid Atlantic ridge
How does a rift Valley form?
At a new divergent boundary.
Movement apart that is new and still on land. Crust stretches and thins, but eventually it may fracture or fault. Central blocks of land will drop down or subside. Creates a series of normal faults.
Volcanoes present in the centre
What is, and explain an example of a new divergent boundary?
We can see new divergence and a rift valley to of north of the East African Rift Valley. The fault has had greater divergence. Central land has fallen below sea level and left a small thin sea in the centre (red sea)
What is meant by the ocean core complex?
A.k.a megamullions
Associated with MORs. Dome-like structures with ridges perpendicular to the MOR
can be 10s or 100s of km wide
We see peridotite at OCCs (from mantle)
Formed when divergence happens very slowly. Very little magma. Crust fractures with low angle fault
What is meant by a magnetic reversal of the earth?
The earth has a magnetic field generated in the very hot molten core. It is a dipole. When the North Pole is transformed into the south Pole
How often do magnetic reversals occur? And how can we tell?
In the geological record, the magnetic field has undergone numerous reversals of polarity. We can see this in the magnetic patterns found in volcanic rocks - especially ocean floors.
In the last 10 million years, there have been, on average, 4-5 per million years.
They are not predictable
Us the earth’s magnetic field reversing now, and how do we know?
The current strength of the magnetic field is not particularly low in terms of the range of values it has had over the last 50,000 years and it is nearly 800,000 years since the last reversal.
We cannot confidently day it will happen in 1500 years
How quickly do the magnetic poles reverse?
A full reversal may take about one to several thousand years to complete. This is fast for geological standards but slow for humans
Is there any possible danger to life when the magnetic poles reverse?
Almost certainly not (booklet has more nerdy ness)
How can magnetic anomalies around the mid ocean floor be used to assess the rate of seafloor spreading?
- New igneous rock form at MORs
- mafic basalt contains Iron rich minerals
- iron rich minerals align with the poles while still molten
- basalt solidifies and record the pole at the time of formation
- as we move away from the ocean floor, we see “normal polarity” at the centre and alternating “reverse polarity” basalt (symmetry)
- number of reversals recorded on ocean floor
- date igneous rocks –> radiometric dating –> assign rough dates to reversals
What is the equation of sea floor spreading?
2(Distance moved(cm) ÷ time taken(years)
