Continental Drift: An Idea Before its Time
- ALFRED WEGENER
~ first proposed hypothesis in 1915
~ published The Origin of Continents and Oceans
- CONTINENTAL DRIFT HYPOTHESIS
~ supercontinent called Pangea began breaking apart about 200 million years ago
~ continents "drifted" to present positions
~ continents "broke" through the ocean crust
- WEGENER'S CONTINENTAL DRIFT HYPOTHESIS
EVIDENCE
~ fit of South America and Africa
~ fossils match across the seas
~ rock types and structures match
~ ancient climates
- MAIN OBJECTION TO WEGENER'S PROPOSAL
~ inability to provide a mechanism (cause)
Plate Tectonics: The New Paradigm
- MORE ENCOMPASSING THAN CONTINENTAL DRIFT
- ASSOCIATED WITH EARTH'S RIGID OUTER SHELL
~ called the lithosphere
~ consists of several plates
-- plates are moving slowly
-- largest plate is Pacific plate
-- plates mostly beneath ocean
- ASTHENOSPHERE
~ exists beneath the lithosphere
~ hotter and weaker than lithosphere
~ allows for motion of the lithosphere
- PLATE BOUNDARIES
~ all major interactions among plates occur along their boundaries
- TYPES OF PLATE BOUNDARIES
~ DIVERGENT PLATE BOUNDARIES (constructive margins-- new lithosphere/ground)
-- two plates move apart
-- mantle material upwells to create new seafloor
-- ocean ridges and seafloor spreading
-- oceanic ridges develop along well developed boundaries
-- along ridges, seafloor spreading creates new seafloor
~ CONVERGENT PLATE BOUNDARIES (destructive margins-- lithosphere is lost)
-- plates collide, an ocean trench forms, and lithosphere is subducted into the mantle