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WHAT DRIVES PLATE MOTIONS?
PAST PLATE MOTIONS AND CONTINENTAL DRIFT
Break-up of Pangea
Evolution of North America
Cycle of continental break-up and re-assembly
WHAT DRIVES PLATE MOTIONS? -- Mantle convection
Why does convection occur? -- uneven distribution of heat
sources
What are the important sources of heat in the mantle?
Ways that convection might move plates
1. "Push" at spreading centers, sliding away by gravity
2. "Drag" of friction between plate and convection current
3. "Pull" of slab as it is subducted into mantle
Is one mechanism dominant?
Pacific plate moves fast (subducted at trenches)
Atlantic plates move slower (away from mid-ocean ridges)
PAST PLATE MOTIONS AND PLATE TECTONICS
Break-up of Pangea (Wegener's original concept)
Evolution of North America
Central core assembled by 1,800 m.y.a. from 4-5 segments
Addition of marginal "terranes" by collision -- 25%
of present continent
Long-term (500 m.y.) cycle of continental rifting and assemblage
(Detailed notes start here)
Geophysicists who study this issue conclude that convection currents in the mantle is the fundamental driving force. Convection occurs because of unveven distribution of heat sources in the mantle. Hot currents rise because they are less dense than the surrounding mantle. Currents move laterally, loose heat and cool, then sink.
What are the sources of internal heat in the mantle? As we discussed earlier, the largest source is the decay of radioactive elements. So, some parts of the (deep) mantle must be more radioactive than other parts. A lesser source is "fossil heat," the heat energy trapped in the Earth during its formation.
Although convection is the fundamental driving force for plate motions, how it works in detail is somewhat controversial. There are three ways that convection could make plates move:
1. "Push" - - Plates are elevated at mid-ocean ridge spreading centers by the upward push of rising convection current. Because of this elevation, they slide by gravity away from spreading centers.
2. "Drag" - - Plates are pulled away from spreading centers by fricitional drag between the plate and underflowing current.
3. "Pull" - - The weight of the subducting part of a plate (it's cool and dense) literally pulls the rest of the plate.
Any combination of these "motions" could be in play for different plates. However, evidence from rates of plate motions suggest that "pull" may be dominant:
Plates largely surrounded by trenches (e.g., the Pacific) move fast.
Plates moving away from a mid-ocean ridge (e.g., the Atlantic) move slow
The theories of sea-floor spreading and plate tectonics gave new "life" to Alfred Wegener's concept of continental drift. In fact, Wegener's proposals for the sequence of drifting continents over the past 250 million years fit very well with our current understanding of the history of lithospheric plate motions and the age of the sea floor. So, it is perhaps fitting to reconsider Wegener's model for the break-up of Pangea, emphasizing the important events -- including the origin of some major oceans. We will consider the "state of the world" at various times in the past.
(TASA Chpt. 2 -- Continental Drift: Run through all frames; pause to describe.)
Prior to 225 m.y.a. Continents were assembled into a "supercontinent" that Wegener called "Pangea" (P). Pangea was made up of two smaller masses, Gondwanaland (G) in the southern hemisphere, and Laurasia (L) in the northern hemisphere.
180 m.y.a. G and L have nearly separated. India has drifted away from Africa and Antarctica.
135 m.y.a. G and L have separated. India continues to move north. The South Atlantic is "born" as a narrow gulf between South America and Africa.
65 m.y.a. South Atlantic continues to open. Africa moves north to form Mediterranean Sea. Australia begins to separate from Antarctica.
65 m.y.a to present India collides with Eurasia. North America and Greenland split from Eurasia to form the North Atlantic. North and South America drift together (by subduction and with volcanic activity) and are joined. Arabia rifts away from Africa.
The central "core" of North America "assembled" about 1,800 m.y.a from 4-5 smaller continental segments. The margins of our continents were added by collision with younger and smaller segments of continents and ocean crust that mostly migrated from the south. These marginal "terranes" make up about 25% of the present continent.
Geologists who study ancient continental regions suggest that there is a 500 m.y. cycle of continental assemblage and rifting:
"Super-continent" -> Heat build-up at base -> Rifting & Separation -> Reassemblage
If this is correct, then we are about half-way through the cycle that began with the break-up of Pangea.