Earth ~200 million years ago

The Geologic

Time ScaleBased on

*Fossils*Correlation

Later

*Calibrated with

radiometric dating

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.

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 ofthe

continents, especially

usingcontinental

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 atight polar

distribution.

Continental Drift: ReactionsReceived 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

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

The Rise ofPlate

Tectonics

Black: normal polarityWhite: 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 ofPlate

TectonicsAnd then (1962-1963) geologists realized that the

patterns are SYMMETRICAL across oceanic

ridges.The K-Ar dates

show the youngestrocks at the ridge.

The Rise of Plate TectonicsMeanwhile, 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 1965-1970

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 acontinent could

lead to formation of

oceanic lithosphere.

e.g., Red Sea

e.g., Atlantic Ocean

e.g., East Africa Rift

Subduction zones form at convergent boundaries if at least one side has oceanic (denser) material.Modern examples: Andes,

CascadesMajor features: trench, biggest EQs, explosive

volcanoes

Another subduction zone—this one withoceanic 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 boundariesare 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!

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