Earth ~200 million years ago

*Age of Fossils determined by radiometric dating The Geologic Time Scale Based on *Age of Fossils determined by radiometric dating Look at timeline: Breakup of Pangaea was 200-190 mya: time of dinosaurs!

The Continental Drift Hypothesis Proposed by Alfred Wegener in 1915. 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

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

Continental Drift: Evidence Animal and plant fossils of same age and type

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

Continental Drift: Evidence Glacial features of the same age on different land masses show these used to be one landmass

Continental Drift: Reactions Received well in Europe and southern hemisphere. BUT rejected in U.S. Lack of a suitable explanation prevented continental drift’s widespread acceptance Theory provided evidence for WHETHER it happened and HOW IT USED TO BE but no answer to WHAT CAUSED IT. Conflict remained unresolved

The Rise of Plate Tectonics During WW II and the Cold War U.S. Navy needed to map the seafloor to find and hide submarines. Used sonar to create maps of: oceanic ridges—submerged mountain ranges fracture zones—cracks perpendicular to ridges trenches—narrow, deep gashes abyssal plains—vast flat areas seamounts—drowned undersea islands They dredged up rocks of the seafloor: included only basalt, gabbro, and serpentinite—no continental materials.

Other interesting things they learned about the seafloor Marine geologists found that seafloor magnetism has a striped pattern completely unlike patterns on land. Mason & Raff, 1961 Black: normal polarity White: reversed polarity Both: very magnetic

Their observations led to: Hypothesis: Stripes indicate periodic reversal of the direction of Earth’s magnetic field. To test this hypothesis, scientists determined the ages of the older stripes of basalt AND the polarity of young basalts using the newly developed technique of K-Ar radiometric dating. The study supported the reversal hypothesis... Yay!

The Rise of Plate Tectonics And then (1962-1963) 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.

How magnetic reversals form at a spreading center

Bands of seismicity—chiefly at trenches and oceanic ridges

The Theory of Plate Tectonics “group authorship” in 1965-1970 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: divergent convergent transform

Tectonic Plates on Modern Earth

Divergent boundaries: mostly in oceans spreading center forming oceanic ridges

How magnetic reversals form at a spreading center

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

continent could lead to formation of How rifting of a continent could lead to formation of oceanic crust. ex: East Africa Rift ex: Red Sea ex: Atlantic Ocean

Our current theory says that Pangaea was ripped apart by such continental rifting & drifting.

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

Another subduction zone—this one with oceanic material on both sides. O-O Modern example of volcanic islands: Japan

Earthquake depth indicates subduction zones

Collision zones form where both sides of a convergent boundary consist of continental (buoyant) material. Modern example of C- C: Alps, Rockies, 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-NAmerican plate boundary is MUCH more complex than this diagram shows.

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

Two related ideas are widely accepted: 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. All of this caused by earth’s radioactive, hot, hot core making the asthenosphere move in convection currents.

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

Let’s talk about the ages of continental and oceanic crust… Based on all the knowledge we have gained so far, does continental crust ever get subducted and recycled into the earth’s mantle? Does oceanic crust get subducted and recycled into the earth’s mantle? Review the slides on convergent boundaries…

Oceanic crust was first formed 4 Oceanic crust was first formed 4.5 billion years ago and continental crust came later but… Scientists have found that continental crust is sooooo much older (up to 4 billion years old) than even the oldest oceanic crust (about 170 million years old) found so far on the earth today Scientists have not found really really old oceanic crust yet because the old stuff usually gets recycled

Most oceanic crust is recycled by subduction at convergent boundaries Oceanic crust subducts because it’s more dense New oceanic crust is formed at divergent boundaries. Continental crust never subducts because it’s less dense…the really old continental crust is still on the surface billions of years later Review the slides on convergent and divergent boundaries!!

Field Trip Briefing The California subduction zone (9’ seismic line) Subduction to transform (Atwater animation) Faults of the Bay Area (SF-SJ maps) Rock types we’ll encounter

Landslide north of Mussel Rock Occurred between 2 am and 8 am, 2/21/05