Evidence for Continental Evidence for Continental DriftDrift

Wegener (1915) Wegener (1915) proposed proposed PangaeaPangaea – – one large continent one large continent existed existed 200 million years ago200 million years ago

PanthalassaPanthalassa – one – one large oceanlarge ocean

Noted puzzle-like fit Noted puzzle-like fit of modern continentsof modern continents

Evidence for Continental Evidence for Continental DriftDrift

Matching Matching sequences of rocks sequences of rocks and mountain and mountain chainschains

Similar rocks on Similar rocks on different different continentscontinents

Evidence for Continental Evidence for Continental DriftDrift

Glacial ages and other Glacial ages and other climate evidenceclimate evidence

Evidence of glaciation Evidence of glaciation in now tropical regionsin now tropical regions

Direction of glacial flow Direction of glacial flow and rock scouringand rock scouring

Plant and animal fossils Plant and animal fossils indicate different indicate different climate than todayclimate than today

Evidence for Continental Evidence for Continental DriftDrift

Distribution of Distribution of organismsorganisms

Same fossils Same fossils found on found on continents that continents that today are widely today are widely separatedseparated

Modern Modern organisms with organisms with similar similar ancestriesancestries

Objections to Early Continental Drift Objections to Early Continental Drift ModelModel

Hostile criticism and open ridicule; Hostile criticism and open ridicule; Wegner was professional toastWegner was professional toast

Continents cannot plow through Continents cannot plow through ocean basinsocean basins

Tidal gravitational attractions too Tidal gravitational attractions too small to cause actual crustal small to cause actual crustal movementmovement

Evidence for Plate TectonicsEvidence for Plate Tectonics

• Earth’s magnetic field and Earth’s magnetic field and paleomagnetismpaleomagnetism

• Earth has magnetic polarityEarth has magnetic polarity

• North and South polaritiesNorth and South polarities

• Magnetic polarity recorded in igneous Magnetic polarity recorded in igneous rocksrocks– Magnetite in basaltMagnetite in basalt

Magnetic Polarity ReversalsMagnetic Polarity Reversals

Earth’s magnetic Earth’s magnetic polarity reverses polarity reverses periodicallyperiodically

Recorded in ancient Recorded in ancient rocksrocks

Paleomagnetism and the Paleomagnetism and the Ocean FloorOcean Floor

1955 – deep water rock mapping1955 – deep water rock mapping Magnetic anomalies Magnetic anomalies – regular pattern – regular pattern

of north-south magnetism “stripes”of north-south magnetism “stripes” Stripes were symmetrical about the Stripes were symmetrical about the

Mid Atlantic ridge suggesting that an Mid Atlantic ridge suggesting that an age sequence of the rocks was also age sequence of the rocks was also symmetricalsymmetrical

Sea Floor SpreadingSea Floor Spreading

Harry HessHarry Hess Depth recordings show sea floor Depth recordings show sea floor

featuresfeatures History of Ocean BasinsHistory of Ocean Basins

• Sea-floor spreadingSea-floor spreading• Mantle convection cells as driving Mantle convection cells as driving

mechanismmechanism

Plate Tectonic Processes: New Plate Tectonic Processes: New Rocks form at the spreading zoneRocks form at the spreading zone

Sea Floor Spreading EvidenceSea Floor Spreading Evidence Frederick Vine and Drummond Matthews Frederick Vine and Drummond Matthews

(1963) – everyone now believes in Wegner’s (1963) – everyone now believes in Wegner’s idea …idea …

Sea floor stripes record Earth’s magnetic Sea floor stripes record Earth’s magnetic polaritypolarity

Two halves of ocean floor are mirror imagesTwo halves of ocean floor are mirror images

Age of Ocean FloorAge of Ocean Floor

Late 1960s deep-sea drillingLate 1960s deep-sea drilling Radiometric dating of ocean rocksRadiometric dating of ocean rocks Symmetric pattern of age distribution about Symmetric pattern of age distribution about

mid-ocean ridgesmid-ocean ridges Oldest ocean floor only 180 million Oldest ocean floor only 180 million

years oldyears old