Wrap Up

Review of the main topics and ideas you encountered in Unit 4.

Plate Tectonics III: Obduction

• In subduction, denser side sinks under less-dense side.
• But continents and island arcs are too low-density to go down—”You can’t sink a continent.”
• When they run into each other, OBDUCTION results, with folding, push-together (thrust) faulting, and thickening.
• This makes the biggest mountain ranges—Appalachians (still high after 200 million years), Himalaya, etc.
• Can even push older rocks on top of younger ones.

A Little History

• Appalachians formed as proto-Atlantic closed.
• Had subduction-zone volcanoes with big eruptions, island arcs colliding with continent, etc.
• This ended when Africa and Europe hit the Americas and pushed up the Appalachians (Great Smokies, State College).
• When the push-together ended, the great, hot pile of the Appalachians spread under its own weight, with Death-Valley-type faulting.
• Thinning with spreading reduced pressure on mantle, inducing rising and melting (convection), giving Atlantic.

The Three Basic Styles

• PUSH-TOGETHER: subduction (Olympic, Crater Lake, Mt. St. Helens) or obduction (Great Smokies).
• PULL-APART: rifting/spreading/sea-floor-production (Death Valley).
• SLIDE-PAST: faulting (San Andreas).
• Can have intermediates (push-together while sliding past, or pull-apart while sliding past).
• The three types of plate boundaries, plus hot-spot activity poking up through plates, give the great majority of mountain-building, earthquakes, volcanoes, etc.

Meanwhile, Out West:

• As the Atlantic opens, Asia and Americas approach and narrow Pacific.
• Subduction under western US initially cold rock, but as the continent moved toward the Pacific spreading ridge, hotter rock was forced down, scraped along under US rather than sinking deep, and rumpled up the lithosphere to make Rockies, etc., far inland.
• Where the subduction zone reached and swallowed the ridge, rock is no longer going down under the west; the subduction zone was push-together plus slide-past, and the slide-past remains as the San Andreas Fault.
• Where and when the push-together of the subduction ended, the pile of the western US spread under its own weight, giving Death Valley faulting.
• (Things really a tad more complex than this, and some things out west aren’t explained—work for you?!—but this isn’t too far off.)

Old mountains & metamorphism

• Upper layers float on lower layers.
• When obduction collision thickens upper rocks, the mountains sticking up float on a root sticking down (like an iceberg, but bergs have 1/10 up and 9/10 down, mountains have 1/7 up and 6/7 down).
• Cut off the top of an iceberg and the bottom bobs up; erode off the top of mountains and the bottom bobs up.
• Bobbing-up of eroding mountains brings rocks to the surface that had been squeezed deep and hot.
• Heating and squeezing turns sedimentary (pieces of older rocks) or igneous (frozen from melted rock) rocks into metamorphic rocks, often pretty with ores or gems.

Tsunamis

• Undersea earthquakes, volcanoes, or landslides, or meteorite impacts, can move lots of water.
• Makes a wave (a tsunami) that is long and low in the ocean, but the wavefront slows down as it enters shallow water, and the back catches up and piles up.
• Most tsunamis tiny, but can run up on land to elevations above 1000 feet; 2004 Indian Ocean tsunami killed over 300,000 people.
• Can’t stop tsunamis, but can give real-time warnings (earthquakes, etc., make seismic waves that go faster than the tsunamis; “listen” for them with seismometers, then warn people to go inland fast).
• Can enforce zoning codes to build in safe places, and keep reefs and barrier islands healthy to break some of the tsunami energy.