PLATE TECTONICS. SCIENTIFIC METHOD Observations (Facts) Hypothesis (Interpretation – incompletely tested) Testing (More critical observations) SCIENTIFIC.

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Presentation transcript:

PLATE TECTONICS

SCIENTIFIC METHOD Observations (Facts) Hypothesis (Interpretation – incompletely tested) Testing (More critical observations) SCIENTIFIC THEORY (Tested and accepted interpretation). The foundations of our understanding

THEORY OF PLATE TECTONICS 1968 Continental Drift –Hypothesis first proposed by Alfred Wegener in 1915 Sea Floor Spreading –Hypothesis first proposed by Harry Hess in 1962 Plate Tectonics 1968 – combination of these two hypotheses

BASIC PROPOSITIONS Earth’s crust is divided into rigid plates that float buoyantly on the denser plastic mantle Elevation of the crust depends on density (rock type and temperature) & thickness Plates move laterally over surface generating unstable earthquake belts where they interact

Earthquake Foci

STABLE & UNSTABLE CRUST Stable Plates Granitic Crust – Continental Craton Basaltic Crust – Abyssal Plains Unstable Plate Boundaries Divergent – Rift Zones (Midocean Ridges) Convergent – Subduction Zones Shear Zones – Transform Faults

Eight Major Plates North America South America AfricaEurasiaAntarctica Consist of both granitic & basaltic crust Australia – India Two granitic continents plus basaltic crust Pacific- all basaltic crust Nazca – all basaltic crust

A World Cruise

STABLE & UNSTABLE CRUST Stable Plates Granitic Crust – Continental Craton Basaltic Crust – Abyssal Plains Unstable Plate Boundaries Divergent – Rift Zones (Midocean Ridges) Convergent – Subduction Zones Shear Zones – Transform Faults

THE BIG PICTURE

Evolution of Ocean Basins according to Dr. Chowns Baby OceanRed Sea AdolescentAtlantic MaturePacific SenileMediterranean DeadHimalayas Dead, rotted and dismembered Appalachians, Urals (Yuck!)

Three types of convergent boundary Basalt – Basalt (Ocean-Ocean) Island Arc-Trench (Japan, Marianas) Basalt-Granite (Ocean-Continent) Mountain Chain-Trench (Andes) Granite-Granite (Continent-Continent) Mountain Chain (Himalayas)

THE BIG PICTURE

Convergent Boundaries

Continental Collision

DRIVING MECHANISM? Convection Currents in Mantle

Simple Convection Currents In Mantle

PLATE TECTONICS THE SUPPORTING EVIDENCE Wegener’s original evidence for Continental Drift 1913 Evidence compiled since 1950, especially paleomagnetism

WEGENER’S EVIDENCE 1. Distribution of Mountains & Earthquakes Circumpacific Ring of Fire Alpine-Himalayan

2. Remarkable fit between continental margins (especially across Atlantic)

3. Continuation of old mountains from one continent to another (eg. Appalachians)

4. Similarities in rock layers and history between continents that were formerly contiguous (eg. North America & Europe, Africa & South America)

5. Similarity in fossils between continents ( eg. Mesosaurus)

6. Peculiarities in distribution of modern organisms

Distribution of Marsupials 200 Ma

7. Paleoclimatic Evidence (eg. Ancient Glacial Deposits)

EVIDENCE SINCE Direct measurements of plate movement by laser, satellite and GPS 2. Tracking of hot spots like Hawaii

The Hawaiian Hot Spot

Hot Spots and Island Chains Movement of plates over hot spots in mantle produces chains of volcanoes. As old volcanoes move away from hot spots they cool and subside. Eg. Hawaiian chain & Emperor seamounts

3.Tracking of continents by position relative to the magnetic poles

PALEOMAGNETISM

Modern Magnetic Pole Two modern continents with magnetic rocks aligned to magnetic poles

Modern Magnetic Pole Pole positions inferred from ancient magnetic rocks

Ancient Magnetic Pole Continents repositioned so that ancient pole matches modern pole position

4. Proof of spreading rates from record of magnetic reversals in seafloor basalts

Magnetic stripes produced by sea floor spreading

Age of the Ocean Floor

THE BIG PICTURE

Evolution of the Oceans 200 Ma

Breakup of Pangaea Ma

Modern oceans 50-0 Ma