Plate Tectonics

• Sir Francis Bacon 1620:– “It appears that South America and

Africa fit together”

• Antonio Snyder published a sketch in 1655 showing S.A. and Africa together

• Benjamin Franklin in 1782 said– “The crust of the Earth must be a shell floating on a fluid

interior. Thus, the surface of the globe would be capable of being broken and disordered by the violent movements of the fluids on which it rested.”

Theory but no evidence

• Most scientist thought it was more coincidence than fact that the landmasses appear to “fit together.”

• Couldn’t fathom a process where these large landmasses moved, and they didn’t have any “evidence” that they had actually moved.

Drifting Continents…?

1. first conception of the revolutionary theory of plate tectonics supported by evidence was proposed by the German geophysicist-meteorologist Alfred Wegener (1880-1930)

Pangaea

• Alfred Wegener proposed the theory of continental drift in early 1900’s

• Wegener’s theorized that all the continents were once a single landmass. (Pangaea)• All continents were once joined and

began gradually moving apart, in fact they’re still moving.

220 million years ago

Wegner’s theorized that all the continents were once a single landmass.

• What was the Evidence and where did it come from?

• 1- Continental shape• 2- Fossil remains• 3- Sea Floor Depth (Global Bathymetry-

SONAR)

• 4- Magnetic Stripes –A geomagnetic reversal is a change in the Earth's magnetic field such that the positions of magnetic north and magnetic south are interchanged. The Earth's field has alternated between periods of normal polarity, in which the direction of the field was the same as the present direction, and reverse polarity, in which the field was in the opposite direction.

• 5- Global Seismic (tracking Earthquakes)

• 6 - Mountain Soil Comparison

• 1 Shape

• 2 -Fossil remains (Animals and Plants)

Land from the South, moving North

• 3 sea floor depth –WWII SONAR

• 4 magnetic stripes

• 5- Global Seismicity

• 6- Mountains -

Mountain formations were once connected

Review of Plate Tectonics

• What drives Earth processes?– gravity and density differences– external (e.g. hydrologic cycle, erosion)– internal (e.g. mantle convection)

Plate Boundaries

• divergent (e.g. mid-Atlantic ridge)• transform (e.g. San Andreas fault

zone)• convergent

• ocean-ocean (e.g. Pacific-Pacific near Marianas Islands)

• ocean-continent (e.g. Pacific-North America)• continent-continent (e.g. India-Asia)

Transform Faults

Review

• Plate Boundaries– divergent (AKA constructive)– transform– convergent (AKA destructive)

• ocean-ocean (e.g. Mariana Islands)• ocean-continent (e.g. Pacific-North America)• continent-continent (e.g. India-Asia)

...to Plate TectonicsA. What are the lines of evidence

Wegener used to support Continental Drift?

B. What is seafloor spreading? How do transform faults behave oddly at the Mid-Ocean Ridge?

C. What was the key development that led to our current theory of Plate Tectonics?