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Environmental Geology – Terra firma?
Terra firma?
Reading: This week: Ch 3Next week: Ch 4
Internal processes:
Environmental Geology – Terra firma?
Toward a unifying theoryPlate tectonics:
Plate tectonics: Recognizes that rigid plates are moving across the Earth’s surface.
A unifying theory: A single theory that explains a large body of evidence.
Why did it come so late (1960s)?
The idea was around long before substantial evidence
Environmental Geology – Terra firma?
A continental jigsaw
1620: Francis BaconAfrica and South America fit together
1655: Antonio SniderPublished a sketch
Mid 1800’s: First proposed that continents did fit together
Toward plate tectonics:
Environmental Geology – Terra firma?
Glacial striationsToward plate tectonics:
Glacial evidence in the tropics!
Also, coal deposits in Antarctica
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Environmental Geology – Terra firma?
1912: Alfred Wegener – Continental Drift
1. Similar fossils: N America and EuropeS America and Africa
2. Similar rock formations across boundaries
Conclusion: There used to be a supercontinent - Pangaea
Flora and faunaToward plate tectonics:
MesosaurusGlossopteris
Environmental Geology – Terra firma?
Mantle convection?Toward plate tectonics:
1928: Arthur Holmes
Proposed that thermal convection in the mantle could be the driving force for plate tectonics.
However, this was a speculative idea. There was no observational evidence supporting mantle convection.
Environmental Geology – Terra firma?
SkepticsToward plate tectonics:
Wegener’s ideas were only discussed for a decade and then dropped.
• The coastlines don’t match perfectly
• Were the rock formations really the same?
Alternative: land bridges between the continents could explain the fossil record
Environmental Geology – Terra firma?
BathymetryToward plate tectonics:
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Environmental Geology – Terra firma?
Magnetic stripesToward plate tectonics:
QUESTION
Environmental Geology – Terra firma?
The Earth’s magnet
…and magnetic rocks
Environmental Geology – Terra firma?
Magnetic volcanoes
Environmental Geology – Terra firma?
Seafloor spreading?Toward plate tectonics:
QUESTION
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Environmental Geology – Terra firma?
Seafloor ageToward plate tectonics:
Environmental Geology – Terra firma?
Identifying the platesToward plate tectonics:
Environmental Geology – Terra firma?
Subduction at the trenchesToward plate tectonics:
Environmental Geology – Terra firma?
Subduction at the trenchesToward plate tectonics:
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Environmental Geology – Terra firma?
Identifying the platesToward plate tectonics:
Environmental Geology – Terra firma?
Reconstructing plate motion
Plate tectonics:
The magnetic tape recorder:
Given the oceanic record we can wind back the plates to see how the plates have been moving.
This shows us how Pangaea broke up.
Environmental Geology – Terra firma?
Continental magnetizationPlate tectonics:
Environmental Geology – Terra firma?
Plate motion further back in timePlate tectonics:
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Environmental Geology – Terra firma?
The formation of PangaeaPlate tectonics:
When we add the continental plate motion recoded we can extend plate reconstruction back beyond 200Ma
i.e. prior to the break-up of Pangaea
Environmental Geology – Terra firma?
The unifying theoryPlate tectonics:
Plate tectonics: Recognizes that rigid plates are moving across the Earth’s surface.
A unifying theory: A single theory that explains a large body of evidence.
Other examples:
Physics – theory of relativityChemistry – nature of a chemical bondAstrophysics – the Big BangBiology – DNAGeology – plate tectonics
Environmental Geology – Terra firma?
Properties of geological materialsEarth’s interior:
How can plates move across the Earth’s surface?
The Earth is not a rigid solid, but a plastic solid.
Environmental Geology – Terra firma?
Stress and strainEarth’s interior:
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Environmental Geology – Terra firma?
Stress-strain diagramEarth’s interior:
Environmental Geology – Terra firma?
Brittle and ductile rocksEarth’s interior:
Brittle failure
Ductile flow
Environmental Geology – Terra firma?
An overviewEarth’s interior: