Physical Geology
Plate Tectonics
Alfred Wegener proposed the idea of continental drift in 1915 to explain the shape of the continents.
- - used to form a large supercontinent called Pangea
- - strong fossil evidence that this was true
- - and the shapes of the continents fit together
- - also the structure and rock types fit together nicely
since then, there have been a wealth of other data like:
- - glaciation evidence
- - paleoclimatic eveidence
In the 1960's, continental drift was expanded to form the theory called Plate Tectonics based on two new technologies at the time:
1) mapping of the ocean floor
2) paleomagnetism
Using geophysics, maps of the ocean floor revealed a central ridge down the centers of the oceans with a 1-3 km deep valley in the center.
- - Here magma was welling up.
- - 1960: Hess proposed sea-floor spreading to account for continental drift.
- - mantle convection driving the spreading
- - magma forms new oceanic crust
Paleomagnetism:
- - Basalts are rich in Fe, become weakly magnetised by the Earth's magnetic field as they xlize and cool.
- - Becomes a fossil magnet, oriented w/respect to the Earth's magnetic field at the time of xlization
- - Records where magnetic north is on the Earth
- - magnetic north is different than geographic north
- - Rocks record that the magnetic pole has wandered "moved" over time.
- - these wandering "paths", discovered in Europe and NA are parallel to each other.
Magnetic field reverses polarity periodically (1-3 my). N becomes S, etc.
- - Can be detected in basalts on ocean floor.
- - Produces magnetic "stripes" on either side of a spreading center which are symmtrical and record the Earth's magnetic polarity at the time of the basalt's formation.
Eventually discovered that the Earth's lithosphere is divided into 10-12 large tectonic plates and that these plates move around on the mantle and interact in 3 ways:
1) Divergent plate boundaries
2) Convergent plate boundaries (subduction zones)
3) Transform boundaries
Divergent Plate Boundaries.
- where plates spread apart
- crust upwells, stretches, then pulls apart.
- basaltic magma is injected into the middle of the rift.
- - rift valleys then floored by basaltic rocks
- eventually water fills the valley and then an ocean.
- East Africa is an example of an opening rift (show map)
- Red Sea is an advanced rift
- Sometimes the rifts don't get a chance to open all the way.
- - Mississippi is an old rift zone
- - Champlain Valley
- - St Lawrence River Valley (show map)
Convergent Plate Boundaries
- Areas where plates collide.
-3 basic types:
- - cont-cont: like the Himalayas
- - cont-ocean: Like the Cascades
- - ocean-ocean: Island arcs like Japan
Transform Boundaries
- where plates slide past one another
- San Andreas Fault
- also called a Strike-Slip fault.
- moves in fits and starts, pieces at a time, not all at once
- - friction between plates stops the movement until the pressure builds up enough to break the piece, causing an earthquake.
Mantle plumes:
- Also called hot spots
- caused when the pieces of decending oceanic plate in a subduction zone reach the core and melt.
- causes large amounts of magma to rise in one spot over an extended period of time, forming a hot spot volcano, like Hawaii.
- magma rises in successive blobs as each pieces of oceanic crust melts.
Back to the selection page.
|
