GEOL 104 Dinosaurs: A Natural History
Fall Semester 2006
Plate Tectonics and the Earth Engine
Great geological discovery of the 20th Century: Plate Tectonics
Some early indication that continents may have moved in past:
- Matching coastlines between continents or other landmasses
- Distribution of fossil (and modern) plants and animals
- Matching mountain belts in different landmasses
- Evidence of ancient ice ages that didn't line up
Alfred Wegener proposed model of continental drift (in 1915):
- All continents unified into a single supercontinent (Pangaea), and all oceans a
single ocean (Panthalassa) during the Triassic
- Some force broke up Pangaea into northern (Laurasia) and southern
(Gondwana) smaller supercontinents, which drifted away from each other
- Wegener's model suggested that the continents somehow skidded over the oceanic rocks
However, big flaws in continental drift:
- What mechanism drives the drift?
- How can the continents glide over the ocean basins? (Physically impossible)
Idea was considered unlikely by many geologists.
In 1940s through 1960s, new information. Fossil, radiometric, and magnetic dates showed that
ocean basins were not ancient, but that they got younger closer to mid-ocean ridges, where
new rock was forming: called "sea-floor spreading".
In 1960s, the models of continental drift and sea-floor spreading were combined to form
- Earth's surface is comprised of numerous rigid lithospheric plates
- Plates themselves carry thick continental and/or thin oceanic crustal rock
- Energy source: heat from core of Earth forms convenction cells in mantle; drag plates
along above them
- Since plates are mostly rigid (although they can be broken or crushed or pulled), most
motion occurs at the boundaries
of plates (see also here
for more details):
- New material generated at divergent boundaries (mid-ocean ridges in sea,
rift valleys on land)
- Plates slide past each other at transform boundaries
- Plates come together at convergent boundaries, whose type depends on whether or
not one of the plates is oceanic crust:
- Oceanic crust is lost under other ocean crust or continents at subduction zones
(site of deep-sea trenches, earthquakes, volcanoes, etc.)
- Two continental masses meet at collisional boundaries: after the collision, they
will fuse ("suture") together
Heat from Earth's core moves plates, forming mountain ranges at subduction and collision
boundaries. Weather erodes uplifting mountains, wind and water and ice transports sediment
to depositional environments. Over time, material becomes buried.
Plates wander over Earth's surface, so continents move from tropics to poles or back. Also,
action of mid-ocean ridges causes sea levels to rise up (flooding continents) or lower
(draining continents). (Current situation is very low sea level).
Big change from the 1960s-1970s model: now recognize there are LOTS of little plates
(terranes) rather than just a few big plates.
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Last modified: 14 July 2006