Today’s Lecture:Today’s Lecture:Chaps. 3 & 4Chaps. 3 & 4Continental Drift & Seafloor SpreadingContinental Drift & Seafloor Spreading

Today’s Lecture:Today’s Lecture:Chaps. 3 & 4Chaps. 3 & 4Continental Drift & Seafloor SpreadingContinental Drift & Seafloor Spreading

1. Alfred Wegener and the hypothesis of

continental drift.

2. Evidence supporting continental drift.

3. Hypothesis of polar wandering.

4. Discoveries leading up to the hypothesis

seafloor spreading.

- Major features of the seafloor.

5. Rock magnetism - a review.

6. Tests of the seafloor spreading hypothesis.

In Lecture Today:

Historical DevelopmentHistorical DevelopmentContinental

Drift

1915Alfred Wegener published his hypothesis ofcontinental drift

He hypothesized:

existence of single “super-continent” Pangaea (“pan - G - uh”)

~ 200 million years ago Pangaea broke intosmaller pieces, which “drifted” to their present positions

200 million years ago

PE

GN

AA

GONDWANALAND

LAURASIA

A

MOVIE

Historical developmentHistorical developmentCONTINENTAL DRIFT

Evidence from rocks:

Mountain belts on one continent match up to the rocks found on another continent.

Appalachians (eastern US)

British Isles, Scandanavia

Example: Rocks of similar age & structure are found in the:

Historical developmentHistorical developmentCONTINENTAL DRIFT

Paleoclimate evidence

Gondwanaland glacial deposits are today scattered around on different continents…

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Paleoclimate evidence

…but when Pangea is reassembled, these glacial deposits all match up perfectly!

(ancient)

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Fossil Evidence

Distributions of many fossil species found on different continents only make sense in light of continental drift

Mesosaurus

LystrosaurusHow can the same speciesevolve on widely separatedcontinents???

Jurassic Aquatic Reptile, Mesosaurus

Mammal-like reptile, Lystrosaurus

Historical developmentHistorical developmentCONTINENTAL DRIFT

1924Alfred Wegener’s book translated to English, and was greeted with (to put it politely) hostile criticism! Main objections:

There was no clear drift mechanism and thus, no way to explain how continents could “plow” their way through the seafloor.

The seafloor was believed to be static, fixed and very old. Radiometric dating was not developed until the 1950’s. Most of our knowledge of the seafloor was based on original surveys by the HMS Challenger global expedition carried out during the 1850’s.

Historical DevelopmentHistorical DevelopmentCONTINENTAL DRIFT

Alfred Wegener’s continental drift idea died until 1950’s, when the field of military oceanography began to mature. New discoveries on the seafloor lead to the hypothesis of seafloor spreading proposed by American scientists, Hess and Dietz.

During the 1970’s, the hypothesis of seafloor spreading was tested using discoveries in the field of rock magnetism by two british scientists, Vine and Matthews (1963).

Remnant magnetism: The orientation of the magnetic field of the Earth is recorded by the magnetic minerals foundin igneous rocks. It is in this way that scientists learned aboutperiodic reversals of the polarity of the field.

Historical DevelopmentHistorical Development

Concept of “Polar wandering”

•Discovered in the 1950’s.

•Based on measuring the remnant magnetic field preserved in rocks

•Using the dip angle of the remnant magnetic field preserved in rocks, it was possible to infer the position of the pole at the time the rocks formed.

•These studies lead to the discovery of polar wandering, which suggested that either 1) the poles have “wandered” or 2) the continents have drifted.

CONTINENTAL DRIFT

CONTINENTAL DRIFT Rock MagnetismRock Magnetism

-Certain minerals are magnetic (e.g., magnetite, iron)-They loose magnetization when heated above Curie point (580oC for iron)-When cooled below Curie pt, magnetic grain aligns w/ Earth’s magnetic field

Basics:

The dip angle of theremnant magnetic field preserved in

a rock indicates thelatitude where

it formed!

Rock MagnetismBasics:

Discovering the nature of the seafloor: The advent of military oceanography

Iceland

Mid-Atlantic Ridge

Mapping the ocean floorMapping the ocean floor

Methods:

Sonar (echoes of sound waves)

Satellite radar

The World’s Mid-Ocean Ridges

Mid-ocean ridgesMid-ocean ridges

Longest continuous

mountain range on

Earth (70,000 km long!)

Elevated 2-3 km

above ocean basins.

Composed of young,

basaltic lavas

Peru-Chile Trench

Mid-ocean ridgesMid-ocean ridges

The shape of the seafloor:Topographic profile across the

East Pacific Rise.

Deep Ocean BasinsDeep Ocean Basins

Abyssal plains: Flat featureless areas ofthe deep seafloor covered by sediment Seamounts (conical-shaped seafloor volcanoes and Guyots (Flat-topped volcanoes formed by erosion at sea level.

Abyssal Plains30% of Earth’s

surface

Seafloor sedimentsSeafloor sediments

terrigenous

biogenous

hydrogenous

Types Derived from:

land Mineral grains from cont. rocks

water Minerals crystallize out of water

organisms Marine animal shells, skeletons

Marianas Trench

Deep Ocean TrenchesDeep Ocean Trenches

MarianasTrench in the South Pacific

Deep Ocean Trenches: Narrow, arcuate features that represent the deepest parts of seafloor. Zones of Plate Convergence which lie parallel to arcuate chains of volcanic islands.

Concept of Seafloor SpreadingConcept of Seafloor Spreading

Ocean ridges are positioned above mantle upwellings, which cause the seafloor to spread, like a conveyor belt.

Magma replaces seafloor as it moves away, creating new oceanic crust.

Deep ocean trenches mark locations where oceanic crust dives back into planet

In 1959, Princeton University Professor Harry Hess proposed:

Seafloor Spreading Hypothesis TestedSeafloor Spreading Hypothesis Tested

Magnetic “stripes” discovered on the seafloor! In 1963, Vine & Matthews connected seafloor spreading & continental drift, using magnetic field reversals recorded by lavas that had been erupted along mid-ocean ridges.

Symmetric patterns (“stripes”) on either side of spreading center (mid-ocean ridge) indicated that the seafloor wasmoving in opposing directions, away from the central ridge axis and acting as giant “tape recorder” for magnetic reversals.

Changes in width of a given stripe indicated changes in the rate of seafloor spreading.

Rocks of the seafloor could be assigned an age using radiometric dating methods and spreading rates calculated.

Next Time: Plate boundariesNext Time: Plate boundaries

Divergent boundaries

Convergent boundaries

Transform fault boundaries

Three Margin Types: