Overview of the main topics you will encounter in Module 4.
Plate Tectonics III: Obduction
In subduction, the denser side sinks under the less dense side.
But the density of continents and island arcs is too low to allow them to go down very far — ”You can’t sink a continent”, as geologists like to say.
Collisions involving continents and island arcs cause folding, push-together (thrust) faulting, and thickening of the crust and upper mantle.
We call this OBDUCTION.
Obduction generally makes the biggest mountain ranges, such as the modern Himalayas, and the Appalachians, which were much higher when they formed 200 million years ago than they are now.
This push-together process sometimes pushes older rocks on top of younger ones.
A Little History—The closing of the proto-Atlantic Ocean and opening of the Atlantic
Long ago, the Americas were separated from Europe and Africa by the proto-Atlantic Ocean (sometimes called the Iapetus Ocean).
The proto-Atlantic Ocean shrank before disappearing, as subduction made large explosive volcanoes in island arcs that sometimes collided with one of the continents.
The proto-Atlantic Ocean disappeared entirely in the great obduction that pushed up the Appalachians, including the Great Smoky Mountains and Mount Nittany near Penn State’s University Park campus.
When the push-together motion ended, the huge pile of rocks that made the Appalachians was hot and soft down deep and began to spread under its own weight, with Death-Valley-type faulting and thinning of the crust and upper mantle.
This thinning reduced pressure on the mantle beneath, which triggered convection there as hot rocks rose and melted, and thus led to the slow and still-occurring opening of the modern Atlantic Ocean.
The Three Basic Tectonic Styles
PUSH-TOGETHER: subduction (Olympic, Crater Lake, Mt. St. Helens) or obduction (Great Smokies)
Intermediates can occur, such as a little push-together while sliding past, or a little pull-apart while sliding past.
The three types of plate boundaries, plus hot-spot activity poking up through plates, create the great majority of mountain-building, earthquakes, volcanoes, etc.
Meanwhile, Out West:
As the Atlantic Ocean opens, Asia and the Americas are approaching each other, narrowing the Pacific.
Subduction under the western US started with cold rock, but as the continent moved toward the Pacific spreading ridge, more buoyant ("float-ier") rock was forced down, scraped along under the US rather than sinking deep, and rumpled up the lithosphere to make the Rockies, etc., far inland.
Where the subduction zone has reached and swallowed the spreading ridge in the Pacific, rock is no longer going down under the western US; 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 weight, giving Death Valley-type faulting at Death Valley and across much of Nevada and nearby.
(This may seem complex, but the full story is likely even more complex than this, and not all of it is fully known. This is close, though.)
Old Mountains and Metamorphism
The upper layers of the Earth float on lower layers.
When the collision of an obduction event thickens the upper, crustal rocks, the mountains sticking up into the air float on a root sticking down into the mantle (like an iceberg, but icebergs have about 1/10 sticking up and 9/10 sticking down, whereas mountains are closer to 1/7 up and 6/7 down).
If you cut or melt off the top of an iceberg, the bottom bobs up; if you erode off the top of mountains, the bottom bobs up.
Bobbing up of eroding mountains brings rocks to the surface that had been buried deeply, where they were heated and squeezed.
Heating and squeezing turn sedimentary rocks (pieces of older rocks) or igneous rocks (frozen from melted rock) into metamorphic rocks, which often are pretty and may contain valuable ores or gems.
Tsunamis
Undersea earthquakes, volcanoes, landslides, or meteorite impacts can move lots of water.
Such water motion 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 are tiny, but the biggest ones can run up on land to elevations above 1000 feet; the 2004 Indian Ocean tsunami killed over 300,000 people, and the tsunami from the Tohoku, Japan earthquake in 2011 did much of the damage in what was probably the most expensive natural disaster ever.
We know of no way to stop tsunamis, but we can give real-time warnings (earthquakes, etc., make seismic waves that go faster than tsunamis; we can “listen” for those seismic waves with seismometers, then warn people to go inland fast).
We can enact and enforce zoning codes to build only in safe places, and keep reefs and barrier islands healthy to break some of the energy of tsunamis.