3-25

Today in class:
We sketched the 3 different types of convergent plate boundaries and explained/drew what happens at each:
Ocean - ocean: One plate (whichever is most dense) subducts, forming a trench. As it subducts, it melts, forming volcanoes (volcanic island arc). Example: Japan
Ocean - continent: oceanic plate is more dense, so it subducts, forming a trench. As it subducts, it melts, forming inland volcanoes. Example: Cascade mountains (Mt. St. Helens)
Continent - continent: Since both plates are of low density, neither plate subducts. Plates are deformed as they mash together, becoming thicker. Mountains are built this way. Important to note that plates become thicker atop the water and below. Thickest crust on Earth is under mountains. Example: Appalachians (not growing anymore) or Himalayas (still growing rapidly)
Discussion of answers to last night's homework and how the time scales work out closely to support Wegener's "coastline matching" evidence.
Slides of evidence for plate tectonics. The big ones that I focused on and expect freshmen to understand and be able to apply:
1. Matching coastlines (North/South America vs. Europe/Africa)
2. Fossil record (ancient fossils of plants/animals found only in certain locations across continents which are now very distant, separated by oceans)
3. Geologic similarities between different regions (rock record similar in now distant regions, Appalachian Mountains - formed as parts of other mountains formed, most recently some in Britain and Scandanavia)
4. Evidence of continental rifting (East Africa Rift Valley) show that continents can split apart.
5. Mid Ocean Ridges and Seafloor Spreading - discovered due to military - submarine needs, there exists a huge mid ocean ridge up and down the Atlantic ocean, growing every year.
6. Hot Spots - plate movement and new volcanoes in the "same" location relative to the mantle below.
7. Magnetic evidence - reversals and pole movement stored in rocks formed during times of altered magnetic fields provide evidence of time scales and rates/directions of movement.

HOMEWORK!
READ p. 373-378.
P. 379 Due Monday