Past Changes in Physical Geography /
10/30/02
Lecture for 10/30 and 11/1/2002
Plate Tectonics and Continental Drift
I. Continental Drift
Early theory (Taylor 1908, Wegener 1910)
Opposition
II. Resurgence of C.D. as Plate Tectonics
http://www.ucmp.berkeley.edu/geology/techist.html
III. Current Model and Processes
Midoceanic ridges - spreading
Ocean trenches -- subduction
IV. Geographic Changes
Gondwana, Euramerica and Siberia
Gondwana and Laurasia
Pangea
Split Again
VI. Early Life
Marine
Dinosaurs
Mammals
K/T boundary
Flowering Plants
====================================================
Remember to learn the Geologic Time Scale, see text p.174 or handout on GTS
Some web links:
UC Berkeley has some very good pages and animation with deatiled
text:
http://www.ucmp.berkeley.edu/geology/anim3.html
UCSB has done some fine animation of the Pacific Plate movements and
related subjects: http://www.geol.ucsb.edu/Research/TopicsInfo/Tectonics.html
and wouldn't you know it.: www.platetectonics.com
====================================================
Handout
Some pertinent conclusions of Wegener's theory on Continental Drift
1. Continental rocks, called sial, are fundamentally different, less dense, thicker, and less highly magnetized than those of the ocean floor (basaltic, called sima). The lighter sialic blocks, the continents float on a layer of viscous, fluid mantle.
2. Major landmasses of the earth were once united as a single super continent, Pangea (1920 term). Pangea broke into smaller continental plates that moved as they floated on the upper mantle. Breakup of Pangea began in the Mesozoic, but North America was still connected with Europe in the north until the late Tertiary or even the Quaternary.
3. Breakup of Pangea began as a rift valley, which gradually widened into an ocean, apparently by adding materials to the continental margins. The midoceanic ridges mark where opposite continents were once joined, and the ocean trenches formed as the blocks moved. Distribution of major earthquake centers and regions of active vulcanism and mountain building are related to the movements of these crustal plates.
4. The continental blocks have essentially retained their initial outlines, except in region of mountain building, so the manner in which the continents were joined is seen by the matchup of their present margins. When this is done similarities in the stratigraphy, fossils and reconstructed paleoclimates of now distant landmasses are evidence that those blocks were once united. These patterns are inconsistent with any explanation that assumes fixed positions off the continents and ocean basins.
5. Estimated rates of movement for certain continents range between 36 and 0.3 m yr-1 , the fastest being Greenland, which separated from Europe only 50,000 to 100,000 years ago.
6. Radioactive heating in the mantle may be a primary cause for block
movement, but other forces are probably involved. Whatever the causal processes,
they are gradual and not catastrophic.