1. PRE-PLATE TECTONICS HYPOTHESES

• Continental Drift (proposed in early 1900's by Alfred Wegner) landmasses were united in a single supercontinent called Pangaea (Greek for 'all land').

 
• Evidence for Continental Drift:
• The fit of shorelines, particularly S. America & Africa.
• Similar rock sequences and mountain ranges.
• Matching glacial deposits and paleoclimatic zones.
• Similar plant/animal fossils found only on Gondwana continents.
• Although evidence was strong, the hypothesis lacked a physical mechanism.
 
• Sea-floor Spreading (proposed in 1962 by H. Hess to account for continental drift).
• During the 1950's, intense oceanographic research lead to the discovery of a worldwide mid-ocean ridge and deep-sea trench system.
• Proposed that oceanic and continental crust are both part of larger plates and that oceanic crust was generated at mid-ocean ridges and moved laterally away from the ridge with time.
• Confirmed in the mid 1960's by:
• magnetic data from the Mid-Atlantic Ridge that showed parallel and symmetrical magnetic "stripes".
• Deep-Sea Drilling Project:

- radiometric age of oceanic crust and fossls in sediments.
- sediment thickness increases away from ridges.

 2. PLATE MOTIONS (Rate and Direction)

• Dating magnetic reversals in the oceanic crust:

Distance (x) to any magnetic reversal is the width of the oceanic crust produced during that time interval (t); therefore x/t = average plate velocity; use radiometric dating to date the reversals. Orientation of transform faults gives direction.
• Volcanic Intraplate Hotspots:

Age progression and trend of island chains (Hawaii) give absolute plate velocity and direction. Use radiometric dating of volcanic rocks.
• Global Positioning Satellites (GPS):

Radio signals are sent from satellite to ground stations on different plates. Measurements are repeated sometime later (ie. ~1 yr) and time difference is used to compute rate and direction of motion. Lasers can also be used for local studies.

 3. DRIVING FORCES(S) OF PLATE MOTIONS

• Ultimate energy source is the Earth's internal heat (original heat + radioactivity) and plate motions are driven by mantle convection.
• Locations of spreading ridges (upwelling) and trenches (downwelling) are determined by the convection cells. Asthenospheric flow puts a tractional force on the base of the lithospheric plates causing them to move.

 
• Types of Mantle Convection
• Whole-Mantle (entire mantle is convecting).
• Two-Layer (mantle is split into two layers (upper mantle = above 670 km; lower mantle = 670 to 2900 km). Convection in the lower mantle drives convection in the upper mantle.
• "Ridge-push" - the ridge is topographically high, thus gravity pushes the oceanic lithosphere "downhill" towards the trench.
• "Slab-pull" - subducting cold slab of oceanic lithosphere is denser than the surrounding warmer asthenosphere, thus it pulls the rest of the plate with it as it subducts. Minerals transform into denser forms