Evidence for Continental Drift

•  Fit of the continents
• 1620 - Francis Bacon recognizes parallelism of the Atlantic shores
• 1858 - Antonio Snider - maps depicting continental drift Discovery of Plate Tectonics Continental Drift (cont.)
• Late 1890's - Eduard Suess - postulated Gondwanaland 
• 1915 - Alfred Wegener - postulated Pangea ? Continents plow through ocean crust- criticized
• Fossils (p. 33)
• Glossopteris - fossil fern
• Mesosaurus

• Rock ages and structures 
• "Appalachians" continue into Newfoundland, Greenland & Europe
• Paleoclimatic Evidence
• ancient corals presently in northern latitudes 
• glacial sediments near equator

• Harry Hess and Robert Dietz (1960s)
• seafloor separates along ridges
• new seafloor forms from upwelling molten mantle material

• Symmetrical pattern of magnetic anomolies on either side of MOR (p. 225)

• Development of magnetic "stripes" on the sea floor (p. 225)

• Deep Sea Drilling (p. 222)
• Age of Ocean Crust

• Theories of Continental Drift and Seafloor Spreading combined into one grand unified theory - Plate Tectonics
• lithosphere broken into series of plates that "float" on asthenosphere and move relative to each other
• Continents carried with plates
• Ocean crust is subducted, continents are not

• What constitutes a single plate?
• All parts should be moving at same velocity (speed and direction) l about 20 separate plates and 8 major plates
• Largest plate: Pacific plate
• Plate thicknes: ocean: 80-100 km, continent: 100-400 km
• include oceans and continents; most major plates include continents (except Pacific plate)

Plate Tectonics (cont.)
Plate Boundaries (Fig. 2-3)

• Plate boundaries - where different plates meet
• Divergent--plates moving apart
• Convergent--plates moving toward each other
• Transform--plates slide past one another

•  Features of Divergent Plate Boundaries (Fig. 2-6, 2-7, 2-8)
• surface expression is a mid-ocean ridge (MOR)
• new crust and plate being formed
• volcanism and earthquakes
• process called Sea Floor Spreading

• Divergent
• Ocean-ocean
• Mid-Atlantic Ridge, East Pacific Rise, Red Sea, Gulf of California
• Continent-continent
• East African Rift
• Spreading Rates
• Half spreading rate = distance from ridge/age.
• For point A
• Distance from MOR/Age of crust = half spreading rate
• fastest: ~12 cm/year at East Pacific Rise
• Slowest: ~1 cm/year at Mid-Atlantic Ridge
• average: ~6 cm/year

• Convergent Plate Boundaries (Fig. 2-5) 
• Plate and ocean crust being destroyed--subduction--zone called Subduction Zone
• oldest seafloor ~200 Ma old
• Volcanism and earthquakes
• Bouyancy
• Ocean crust and plate cools with age and distance from ridge; becomes denser; subside
• More buoyant continental crust difficult to subduct.  Results in mountain building
 
• More buoyant continental crust difficult to subduct. Results in mountain building

 
• Convergent Plate Boundaries
• Ocean-ocean
• trenches--long, narrow trough (ex. Mariana Trench)
• island arcs--arcuate volcanic island chain (ex. Japan, Aleutians)

 
• Ocean-continent
• mountain belt (ex. Andes)
• trenches (ex. Peru-Chile trench)

 
• Continent-continent
• mountains--Himalayas
• Transform Boundaries
• Plates slide past one another
• Ocean-ocean 
• offset in mid-ocean ridges
• Continent-continent
• San Andreas Fault

• Plates reflect flow in mantle, but what directly drives their movement?
• Hypotheses
• Plates carried by convecting mantle
• entire mantle convecting
• upper mantle convecting
• Push-Pull
• Plates pushed by weight of ridges
• Plates pulled by lithosphere sliding “downslope”
• Hot spots from Earth's deep interior