Plate Tectonics
Lithosphere - rocks are cold (below 1300 °C) enough that they do not significantly deform on time scales of billions of years. Lithosphere is about 100 km thick in ocean basins away from ridges. It may be even thicker under continents.
Mid-Ocean ridges - divergent boundaries. Pressure melting creates basaltic magma. Ridges are topographic highs because of greater bouyancy. Oceanic crust is about 6 km thick and consists of basalt on top and a thicker layer of gabbro below.
Subduction zones - cold, dense lithosphere is gravitationally unstable and sinks back into the interior of the earth. Subduction zones have a bathymetric trench and a volcanic arc from melting of sediments and mantle material.
Transform faults - plates slide past each other. Subsidence on either side is different producing a cliff.
Continents - more slicic and less dense material. Mean thickness is 35 km. Age of continents can be determined from radiometric age dating. This technique requires that the original amount of decay (daughter) product be known. Isotopic ratios are usually used.
Paleomagnetism
Remanent Magnetism - permanent magnetization of rocks either by cooling down below Curie temperature (e.g., basalts at mid-ocean ridge) or during deposition. Fe atoms in rocks become preferentially aligned. More rocks can not be permanently magnetized. Only ferrimagnetic minerals such as magnetite and hematite can retain remanent magnetism.
Declination - angle between geographic north and magnetic field direction.
Inclination - angle between the magnetic field direction and the horizontal
Present day field can be reasonably approximated as a magnetic dipole with the North magnetic pole located at an angle 11° from geographic North (79°N and 70°W). Current location of the Magnetic North Pole (Inclination 90°) is 73°N and 100°W).
A Paleomagnetic measurement of declination and inclination can be used to locate the magnetic pole position at the time the rock acquired its magnetization (Polar wander curves). Alternatively, if the magnetic poles have not moved, declination and inclination can be used to determine where the rock was when it acquired its magnetization.
The earth's magnetic field switches polarity presumably due to changes in the liquid outer core. Magnetic reversals can be used like a bar code to determine the age of a rock
Euler Poles - movement of a plate on a sphere can be compactly described as rotation about a pole. Euler poles (latitude, longitude, and rotation rate) have been compiled for all of the plates. The latitude and longitude of a point can be used to determine the distance between the point and the Euler pole. The rotation rate can be converted into a linear velocity using this distance from the Euler pole.
Triple Junctions - points where 3 plate boundaries come together (e.g., Cocos, Nazca, and Pacific plates is a ridge-ridge-ridge triple junction). In order for a triple junction to be stable, the three vector velocites defining relative motions between plate pairs must form a closed triangle.
Wilson Cycle - Ocean basins open and close causing continents to split apart and then crash back together again.
Hotspots - areas of apparently stationary volcanism from deep in the mantle. A hotspot produces a chain of volcanoes as a plate moves over it, which can be used to determine the direction and rate of plate motion (e.g., the bend in the Hawaiian-Emperor Chain records a major change in the direction of the Pacific plate.
Driving Mechanisms - the earth is a giant heat engine trying to transfer heat energy from its interior to the surface. Ridge push and slab pull cause the plates to move across the surface which in turn stirs the interior bring heat to the surface.
Slab pull - old, cold dense oceanic lithosphere sinks under its own weight. As the lithosphere is strong, the negative buoyancy of the slab pulls the plate after it.
Ridge push - Gravity acting on the topography of ridges tries to spread the ridge outward. This pushes the rest of the plate away from the ridge.