Plate Tectonics
I.	The Idea of Drifting Continents. Essence of idea is simple:
	A.	Rigid lithospheric plates move atop a molten layer, the asthenosphere.
	B.	New crustal material is added to the lithospheric plates at spreading centers, zones of volcanic activity where the crust is being pulled apart.
	C.	Old crustal material is reincorporated into the mantle where lithospheric plates collide. The denser plate is subducted and melts as it is plunged into the asthenosphere.
	D.	Idea was first suggested by Alfred Wegener in 1912.
		1.	He suggested that the continents previously fit together, but have rafted apart.
		2.	His ideas were scoffed by the scientific community as fanciful.
II.	Accumulating evidence in support of plate tectonics
	A.	The continents fit together pretty well. E.g. The bulges of South America and Africa converge nicely if matched up correctly.
	B.	South America, Africa, Australia share some of the same types and sequential layers of sedimentary rocks.
		1.	They show distinctive glacial scouring marks on rocks of the same age, suggesting that formerly the continents were united and covered by a glacier when they were near the South Pole some 250 million yr ago.
		2.	They possess many of the same types of rare fossils.
	C.	In 1950s and 1960s, better exploration of the sea-floor and satellite mapping allowed the topography of the ocean floor to be revealed. It shows features like mid-ocean ridges (sites of spreading centers), and deep arcing trenches (sites of subduction zones).
	D.	In 1960s and 1970s, geologists discovered that the magnetic field of the earth periodically reverses itself (about every 500,000 yr).
		1.	Mapping of magnetic alignment of iron-bearing minerals in oceanic crust showed matching sets of rocks on each side of spreading centers, as revealed by sequences of magnetic reversals.
		2.	Younger basaltic rocks were close to the spreading center and they aged progressively farther away from the center on both sides.
	E.	The repercussions of plate tectonics and continental drift
		1.	Continents that were once connected are now separated (e.g. the Atlas Mts of Morocco, Appalachians in North America, and the Scottish Highlands in Europe are derived from the same mountain building sequence about 250,000 million yr ago).
		2.	Areas that were once at low latitudes may now be at much higher latitudes, and vice versa. E.g. Coal seams (from ancient tropical swamps) buried beneath the Antarctic ice cap. Evidence of continental glaciation in tropical So. America and Africa.
		3.	Huge landmasses, like Pangaea, were once united. This would considerably alter basic patterns of oceanic and atmospheric circulation.
III.	History of the breakup of Pangaea (see overheads)
	A.	First, No and So Hemispheric landmasses separated (Laurasia and Gondwanaland).
	B.	Gondwanaland stayed together for a long time, before rafting apart over 200 million yr ago.
	C.	Atlantic Ocean has been formed over the past 150 million yr or so.
	D.	North and South America have only been connected by the Isthmus of Panama for a geologically short 5-10 million years.
IV.	Modern lithospheric plates
	A.	There are seven major and numerous minor lithospheric plates that fit together to form the modern earth's crust. The seven major plates correspond to major continents or ocean basins, but the match is not perfect:
		1.	Pacific plate
		2.	North American plate
		3.	South American plate
		4.	Eurasian plate
		5.	African plate
		6.	Indo-Australian plate
		7.	Antarctic plate
	B.	Most of the plate boundaries are in the ocean (either mid-ocean ridges or subduction zones in ocean basins or along continental margins). We will note a few prominent exceptions later.
V.	Mechanisms of plate movement
	A.	Plates move at the very slow rate of about 1-4 inches per year. Comparable to the rate of fingernail growth!
	B.	The process that drives plate movements is not fully understood, but it is clear that it involves geothermal energy and convective currents in the mantle. Presumably, plumes of molten material from deep in the mantle rise to the asthenosphere and initiate plate movement along zones of sea-floor spreading.
VI.	Types of plate boundaries
	A.	Spreading centers. Sites of diverging lithospheric plates.
		1.	Tensional force is presumably induced by convective currents of molten material in the mantle. Results in plates being pulled apart, or separated along a rupture.
		2.	Plastic/molten material rises from the asthenosphere at spreading centers and adheres to the ends of diverging plates to form new basaltic rocks, young oceanic crust.
				The result is sea-floor spreading, which forms a long, linear string of primarily underwater volcanoes that form mid-ocean ridges. Iceland is a recently formed, volcanic island that sits atop the Mid-Atlantic Ridge.
		3.	Most spreading centers occur in ocean basins. The main exception is the Arabian Plate and African Plate, which are diverging along a spreading center that is buried beneath the continental plate.
				This forms a linear string of features such as the Dead Sea in Israel/Jordan (lowest point on any landmass: -1600'), the Red Sea (which is formed by parting!), and the Rift Valley of Africa, with features such as Lake Tanganyika.
		4.	The tensional forces at spreading centers cause faulting of the crust (breaks), associated earthquakes, and volcanic activity (extrusive igneous rocks like basalt make up the floor of most oceans).
	B.	Subduction zone. Sites of converging lithospheric plates.
		1.	Since the earth remains virtually constant in diameter, addition of new material to lithospheric plates along spreading centers must be balanced by the loss of material from lithospheric plates where they collide with one another. These plate collisions result in subduction zones.
		2.	Collisions result in a compression force, pushing two plates together.
			a.	Compression produces folding (bending of rock layers), faulting (crustal breaks), associated earthquakes, and volcanic activity.
			b.	It also causes the denser of the two plates to be subducted, or plunged downward into the mantle, where the lithospheric plate is melted and minerals are recycled.
			c.	These melted crustal materials, being lower in density, will eventually rise back to the surface as molten magma (intrusive igneous rocks, like granite) or lava (volcanoes).
		3.	Most subduction zones occur as collisions between an oceanic and continental plate. These occur along continental margins, and produce:
			a.	volcanic mountain ranges, like the Cascade Range of the Pacific Northwest.
			b.	deep sea trenches immediately off shore. Note depth of trenches along Pacific Coast of the Americas.
		4.	Some subduction zones form where two oceanic plates collide. This forms:
			a.	distinctive volcanic island arcs (such as the Aleutians, Lesser Antilles).
			b.	deep sea trenches (like the Marianas Trench in the Pacific)
		5.	Rarely, two continental plates will collide along a subduction zone. Since continental plates are thick, their collision unleashes tremendous mountain building forces, in the form of folding and faulting.
				This is only occurring in one area today. India (formerly part of the So. Hemisphere landmasses) is rafting northward and colliding into the Eurasian plate. The result is the Himalayas, the highest mountains on earth!
	C.	Transform boundaries. Sites where two plates are sliding laterally with respect to one another.
		1.	Shearing, a lateral, or sideways, force is exerted on two plates, so that they fracture to produce faults and associated earthquakes.
		2.	Most transform plate boundaries occur in ocean basins, as lateral fractures that accompany spreading centers or subduction zones.
		3.	A prominent example of a transform boundary that occurs beneath continental plates is the San Andreas Fault.
			a.	The boundary between the Pacific and North American plates is not entirely off-shore. It passes up the Gulf of California and under the North American continent.
			b.	Baja California and parts of California actually lie over the Pacific Plate, which is being shifted northwestward along the San Andreas Fault, moving laterally in the opposite direction of the rest of North America (which is part of the North American plate).
			c.	This causes the major earthquakes that we associate with California.
VII.	Non-boundary volcanic activity can occur at hot spots.
	A.	A hot spot occurs where a convective plume of magma in the mantle rise toward the crust. Frequent volcanoes will form above this hot spot.
	B.	Since the lithospheric plate above the hot spot is not stationary, a linear chain of volcanic activity is formed.
		1.	Over oceanic crust, this forms basaltic volcanic island chains, such as the Hawaiian Islands (which can be traced all the way to the northwestern Pacific).
			a.	Youngest and highest volcanoes are still active over the current location of the hot spot, beneath the big island of Hawaii.
			b.	The islands of Hawaii to the northwest are older, extinct volcanoes that are gradually eroding.
			c.	Midway Island, 1000 miles from the modern hot spot, is a coral reef that remains on an old sea mount now eroded below the surface of the Pacific.
		2.	Over continents, hot spots cause volcanic plateaus and other geothermal activity. Yellowstone is a high elevation, volcanic plateau that is over a hot spot:
			a.	explains its geysers, thermal pools, and hot springs
			b.	explains its earthquake activity.
			c.	this hot spot can be traced westward across the Snake River basaltic plateau into eastern Oregon, where the older volcanic rocks are about 15 million years old.