Plate Tectonics Courtesy of Dr. Richard Sedlock Department of Geology San José State University

Earth ~200 million years ago

The Geologic Time Scale Based on *Fossils *Correlation Later *Calibrated with radiometric dating

The Continental Drift Hypothesis Proposed by Alfred Wegener in Supercontinent Pangaea started to break up about 200 million years ago. Continents "drifted" to their present positions. Continents "plowed" through the ocean crust.

Continental Drift: Evidence Geographic fit of South America and Africa Fossils match across oceans Rock types and structures match across oceans Ancient glacial features

Tight fit of the continents, especially using continental shelves. Continental Drift: Evidence

Continental Drift: Evidence Fossil critters and plants

Continental Drift: Evidence Correlation of mountains with nearly identical rocks and structures

Continental Drift: Evidence Glacial features of the same age restore to a tight polar distribution.

Continental Drift: Reactions Received well in Europe and southern hemisphere. Rejected in U.S., where scientists staunchly preferred induction (incremental progress built on observation) over what they perceived as speculative deduction. Lack of a suitable mechanism crippled continental drift’s widespread acceptance. Conflict remained unresolved because seafloors were almost completely unexplored.

The Rise of Plate Tectonics WW II and the Cold War: Military Spending U.S. Navy mapped seafloor with echo sounding (sonar) to find and hide submarines. Generalized maps showed: oceanic ridges—submerged mountain ranges fracture zones—cracks perpendicular to ridges trenches—narrow, deep gashes abyssal plains—vast flat areas seamounts—drowned undersea islands Dredged rocks of the seafloor included only basalt, gabbro, and serpentinite—no continental materials.

The Rise of Plate Tectonics Black: normal polarity White: reversed polarity Both: very magnetic Marine geologists found that seafloor magnetism has a striped pattern completely unlike patterns on land. Mason & Raff, 1961

Hypothesis: Stripes indicate periodic reversal of the direction of Earth’s magnetic field. To test this hypothesis, scientists determined the eruptive ages AND the polarity of young basalts using the newly developed technique of K-Ar radiometric dating. The Rise of Plate Tectonics The study validated the reversal hypothesis...

The Rise of Plate Tectonics And then ( ) geologists realized that the patterns are SYMMETRICAL across oceanic ridges. The K-Ar dates show the youngest rocks at the ridge.

The Rise of Plate Tectonics Meanwhile, U.S. military developed new, advanced seismometers to monitor Soviet nuclear tests. By the late 1950s, seismometers had been deployed in over 40 allied countries and was recording 24 hrs/day, 365 days/year. Besides the occasional nuclear test, it recorded every moderate to large earthquake on the planet. With these high-precision data, seismologists found that activity happens in narrow bands.

Bands of seismicity— chiefly at trenches and oceanic ridges

The Theory of Plate Tectonics Earth’s outer shell is broken into thin, curved plates that move laterally atop a weaker underlying layer. Most earthquakes and volcanic eruptions happen at plate boundaries. Three types of relative motions between plates: “group authorship” in divergent convergent transform

Tectonic Plates on Modern Earth

Divergent boundaries: Chiefly at oceanic ridges (aka spreading centers)

How magnetic reversals form at a spreading center

Divergent boundaries also can rip apart (“rift”) continents

How rifting of a continent could lead to formation of oceanic lithosphere. e.g., Red Sea e.g., Atlantic Ocean e.g., East Africa Rift

Presumably, Pangea was ripped apart by such continental rifting & drifting.

Subduction zones form at convergent boundaries if at least one side has oceanic (denser) material. Modern examples: Andes, Cascades Major features: trench, biggest EQs, explosive volcanoes

Another subduction zone—this one with oceanic material on both sides. Modern example: Japan

Earthquake depth indicates subduction zones

Collison zones form where both sides of a convergent boundary consist of continental (buoyant) material. Modern example: Himalayas This probably used to be a subduction zone, but all the oceanic material was subducted.

Most transform boundaries are in the oceans. Some, like the one in California, cut continents. The PAC-NA plate boundary is MUCH more complex than this diagram shows.

Hotspots, such as the one under Hawaii, have validated plate tectonic theory.

Why do the plates move? Two related ideas are widely accepted: Slab pull: Denser, colder plate sinks at subduction zone, pulls rest of plate behind it. Mantle convection: Hotter mantle material rises beneath divergent boundaries, cooler material sinks at subduction zones. So: moving plates, EQs, & volcanic eruptions are due to Earth’s loss of internal heat.

Whole-mantle convection Two mantle convection cells Complex convection How does convection work? No one knows—but they aren’t afraid to propose models!