Endangered Planet: An Introduction to Environmental Geology
Notes 3: Plate Tectonics
Read: Chapter 3 and
Play: Use the online resource for Plate Tectonics linked
at the top of our home page (the semi-cool animated stuff that requires
Macromedia Flash player in your browser).
Plate Tectonics:
The outer layer of the earth, or lithosphere, consists of a series
of rigid plates, approximately 70-150 km thick (40-90 mi)
These lithospheric plates slide under, past and away from each other
carrying along the continents as “passengers”
Concentrated along the plate boundaries, you get earthquakes and volcanic
activity.
Earth’s Interior
Know the crust, mantle, core and their basic compositions.
Know that the lithosphere consists of the crust and the uppermost part
of the mantle (down to ~100km depth).
The Earth’s Plates and Margins
Learn the basics: where are some of the boundaries where plates are
divergent? Convergent? Can I point to any transform plate boundaries?
These plates have moved over time:
This website has a series of maps showing the locations of the continents
over the past 620 million years:
http://wrgis.wr.usgs.gov/docs/parks/pltec/scplseqai.html#reconstruct
Driving forces:
The earth’s internal heat causes density-driven convection in the mantle,
much as water convects in a pot.
Tectonic Plate Boundaries: 3 types…
Convergent margins: moving towards each other
Divergent margins: moving away from each other
Transform margins: slipping along side each other
Boundary Types: figure
Stresses found at Plate Boundaries:
Convergent: compressive stresses - forces that push things together
Divergent: tensional stresses - forces that tear things apart
Transform: shear stresses - forces causing plates to slide past each
other
Convergent Margin:
one plate subducts beneath another.
ex. Andes (of western South America), Cascades (of northwestern US),
Aleutians (of Alaska), Japan (of Japan)
Convergent margins: photo of deformed rocks, Canadian Rockies
Ductile deformation from compression - folds rock layers (like an accordion)
Convergent boundaries:
Faulting associated with compression - faults are breaks in rock bodies
along which motion (slippage) takes place.
Faulting: photo of a fault scarp (see another from Taiwan in the
frontspiece of Chapter 4)
Fault scarp… photo where an active fault that moved during a recent
earthquake caused the surface rupture (scarp, or small cliff) that runs
across this pasture.
San Andreas Fault:
Transform Fault: west side is moving NW
Sinai Peninsula:
How oceans begin to form, an example from the Red Sea region.
See also the frontspiece of Chapter 3 that describes this activity.
Divergent Margins (image of the seafloor and continents of the Atlantic
ocean)
Mid-Atlantic Ridge: The Atlantic ocean is growing
Sea-floor Spreading Along the Mid-Atlantic Ridge
Diagram showing how magnetic "stripes" on the seafloor mark the slow
growth of the Atlantic ocean floor as divergence continues.
East African Rift
Arabia escapes northward, while Somalia is trying to move to the east.
This area has many lakes, basins and volcanoes associated with rifting.
Sinai Peninsula:
How oceans begin to form, an example from the Red Sea region.
See also the frontspiece of Chapter 3 that describes this activity.
Transform plate boundary: * Plates slide laterally past each other... * No convergence, no divergence. * Lots of earthquakes, though.
San Andreas Fault:
Transform Fault: west side is moving NW
Formation of an ocean:
This figure illustrates the gradual change as a continent rifts apart
along a divergent boundary (like the East African Rift system) and gradually
becomes a growing ocean floor.
Plate Tectonic Model
All-in-one diagram showing the various boundaries and movements of
lithospheric plates.
Convergent Margin: photo of High Himalayan Mountains
Continent-Continent Collision - Himalayan Mts
Nepal: photo of deformed rocks
Continent-Continent Collision at 26,800 feet
Life at hydrothermal vents (black smokers) at mid-ocean ridges (
Deep in the ocean along divergent boundaries, the hot water coming
up at "hydrothermal" vents creates an important chemical energy source
for a diverse underground garden of life forms, including crabs, clams
and these crazy "tube worms" up to 10 meters long!
Why hydrothermal vents? Hot Water, circulating through mid-ocean ridge.
Hot spots - Lots of volcanism, but not at a plate boundary.
* In some ocean basins, there are chains of islands that become progressively
older in one direction (ex. Hawaii)
* These island chains are associated with hot spots — plumes of magma
rising into the lithosphere, coming up from deep in the core. Plates
move past over top, leaving a chain of extinct volcanoes behind one active
volcano.
* Some hot spots are beneath continental portions of plates, creating
continental volcanism (ex. Yellowstone)
Map: Hot spot traces in Pacific
Photo: Steamboat Geyser, Yellowstone N.P.
Review: Images of color topographic maps from around
the world, showing plate boundary examples:
1. Oceanic Rifting - Mid-ocean ridge - A Divergent margin, with
Ridge segments separated by transform faults
“Fracture zones” are not boundaries
but “scars” extending into adjacent plates
2. Continental rifting - A Divergent margin, with Narrow rift
valleys, faulting, volcanoes
Narrow ocean basin between continental blocks
after it gets a little older
3. Mid-ocean ridge (close-up)
4. Oceanic transform boundaries - A Transform margin
Parallel to plate spreading direction,
Perpendicular to ridge orientation
5. Oceanic transform boundaries
6. Continental transform boundaries - A Transform margin
Not as perfectly straight as the oceanic
transforms
7. Continental transform boundaries
8. Ocean-Continent Subduction - A Convergent margin
Oceanic plate subducts beneath a continental
plate
Volcanic range develops on the overriding
continental plate
Deep-sea trench at the boundary
9. Ocean-Ocean Subduction - A Convergent margin
Oceanic plate subducts beneath another
oceanic plate
"Island arc" volcanic range develops
on the overriding plate
10. Continental collision - A Convergent margin
Two continents or plates with
continental crust colliding
One attempts to subduct beneath the
other
Faulted, folded mountain belt forms
on the overriding plate
Website where you can find all these images (look at the pictures
yourself):
National
Geophysical Data Center (http://www.ngdc.noaa.gov/mgg/image/2minrelief.html)
Locations for all these images:
1. South Atlantic Ocean, near Brazil.
2 Red Sea, Africa and Arabian Peninsula
3. South Atlantic Ocean, near Brazil.
4. South Atlantic Ocean, near Brazil.
5. Indian Ocean, near the Seychelles.
6. Red Sea, Africa and Arabian Peninsula
7. West coast North America, San Andreas fault
8. South America, Andes and Peru Trench
9. Australian plate subducting beneath Indonesian island arc
10. India-Asia collision along Himalayas, Tibetan plateau behind
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link back to class home page: http://www.uky.edu/AS/Geology/howell/110