Lecture 2

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Published on 30 Oct 2010
Earth Sciences
GLG205 t 2
May 13
Plate tectonics
- Earth is divided into mechanical layers
o Outer layer is rigid lithosphere
o Floats atop plastic asthenosphere
- If we divide the earth by composition: crust t mantle t core
- If we divide by -: lithosphere - asthenosphere
Theory of plate tectonics
- Z[o]Z}Z]]À]]v}o~o]lZZoo}(lPPZu}Ào]À}
one another, and relative to the underlying asthenosphere
- As plates move, their interiors remain relatively intact, while their boundaries undergo
o Mid-ocean ridges (MOR) t spreading/diverging
E.g. Mid-Atlantic ridge moving north American plate and Eurasian plate apart
Atlantic growing wider
o Subduction t sinking of the crust
o Transform faults t sliding
- If you live at the boundaries of the plate, there will be a lot of earthquake activity
o E.g. Toronto vs. Vancouver that is at the edge of pacific plate
Plate tectonics
- Plate tectonic forces give rise to the formation of the crust, affect continental positions, and
produce changes in global topography (e.g. valleys, mountains, water bodies)
- The slow inexorable wearing down of the land surface of the earth would lead to its demise and
reduction in height to sea level in less than 20 million years owing to weathering and erosion, if
it were not for the return of the materials to the continents via plate tectonic forces
Evidence for plate tectonics
- Continents were connected
o Matching coastlines
E.g. south America matches the coastline of Africa
o Similar rocks, glacial (µ^}_}v
E.g. rocks on south America found to be same as rocks on African coastline
o Same fossils
E.g. fossils found in south America and Africa
- Plate boundaries marked by:
o Earthquakes
dZÇv[v}u]]µVu}vt edges of the plates
o Volcanoes
E.g. ring of fire around the pacific plate
Many volcanoes on the edges of the plates
o Mountain belts
E.g. alps and Himalayas
Located near the edges of plates
New mountains are still growing because they are still being pushed
Three types of plate boundaries
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- Divergent
o Plates moving apart
- Convergent
o Plates coming together
o Form mountains
- Transform
o Lots of friction from sliding causes earthquakes
Divergent plate boundary:
- Two plates spreading apart
- Mid-oceanic ridge (in ocean)
o Hot magma coming out of the surface and pushes lithosphere apart
o Newest crust is right in the middle of the ridge
o Older crust as you move away from the ridge
o E.g. Iceland t MOR exposed on land
- Continental rift (in land)
o E.g. African rift valley
o Magma coming out breaks apart land
o Water fills in and ocean forms
*know how to sketch divergence
Convergent plate boundary: two plates coming together
- Subduction: one plate pushes the denser crust beneath another
o Lots of earthquakes in subduction zone from the friction of the sliding
o Magma is less dense than the rocks surrounding it so it is more buoyant
- Three kinds of convergent boundaries:
o Ocean-ocean
E.g. japan
Two oceanic plate is coming together
The denser one gets subducted under the other
Magma is less dense so it comes out of the crust and forms a chain of volcanic
islands (volcanic island arc)
o Ocean continent
E.g. south America
Ocean gets subducted under the continent
Magma is less dense and comes out of crust forming continental volcanic arc
o Continent-continent
E.g. himalayas
Continents are very buoyant and refuse to go down, so it crumples
Subduction only going on when the oceans are still bringing the continents
When they meet, they crumple and form-
- *draw features of the different convergent boundaries (e.g. volcanic arcs)
Transform boundary: two plates sliding past one another
- E.g. san andreas fault
- Kinds of transform faults
o Active transform fault
Connects crests of mid-ocean ridges, while fossil transform faults / fracture
Stretch away from mid-ocean ridges
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