Continental Drift, Paleomagnetism, and Plate Tectonics
Continental Drift: Alfred Wegener
Pangaea: “all earth”
Evidence for Continental Drift (1900s)
Fit of continents
Fossil evidence
Glaciation and paleoclimate
Ancient Mt. Belts
Rejection of Continental Drift: NO MECHANISM for continent motion
Fast Forward to the 1940s and 1950s: exploration of the seafloor
Mid Ocean Ridges
Magnetic signature of some rocks (magnetic minerals)
Paleomagnetism: minerals align with magnetic field at the time they form, retain it as
long as they are not heated above the CURIE POINT
“Magnetic Stripes” on the seafloor indicate flipping of the polarity of the poles
Implies SEAFLOOR SPREADING (mechanism for continental drift)
Magnetic time scale, and Apparent Polar Wandering
Oceanic crust forms at MID OCEAN RIDGES, consumed at SUBDUCTION ZONES
(trenches)
Outer layer of the earth (LITOSPHERE) can be divided into PLATES, that move
(different than continents and oceans, plates can contain one or the other, or both)
Plates move on top of the ASTHENOSPHERE
about a dozen major plates (Pacific Plate, North American Plate for examples)
Most activity (EQ, Mt. building, volcanoes), occurs at or near plate boundaries, so we
focus on those
DIVERGENT BOUNDARIES
Mid Ocean Ridges
Rift Valleys
Shallow EQ, volcanoes, high areas relative to surrounding land due to high heat
flow
TRANSFORM BOUNDARIES
lateral (horizontal movement)
San Andreas Fault System is an example
Shallow EQ, some vertical motion, no volcanoes
CONVERGENT BOUNDARIES
Subduction zones, high mountains
Deep EQ, mountains, volcanoes if there is subduction
Features depend on the type of crust interacting (Continent-Continent, OceanicContinent, Oceanic-Oceanic)
HOT SPOTS and Mantle Plumes
Mid Plate areas of Activity (Like Hawaii)
Relative and Absolute Plate motions
Driving Force of Plate Tectonics: Convection in the Mantle of the Earth due to heat
transfer