Plate Tectonics

• Continental Drift
• Seafloor Spreading
• Plate Tectonics Theory
• Rodinia

That the continents might have moved in the past was proposed as early as 1596 by the Dutch map maker Abraham Ortelius in his work Thesaurus Geographicus.

Alfred Wegener proposed in 1912 the Theory of Continental Drift:

- states that all the continents comprised one giant land mass 200 million years ago,

- that they then began to break apart and drift toward their present locations

- and are still drifting.

• He named the parent continent Pangea (all land).
• Not well received at the time.
• Proof began to be gathered, and it was generally accepted after his death in 1930.

Go to theVolcano World lesson on Continental Drift to learn more basics.

Click Here for animations of continental movement

Clues that provided proof

• Fossils

  - such as:

  1. reptile Mesosaurus,
  2. fern Glossopteris
  Go see a Distribution diagram, and read about it.

  Certain fossils were found on widely dispersed continents.  Wegener theorized that the particular ones he was referring to could not have traveled across the current ocean expanse.

• Climate

  Fossil plants that require warm climates have been found in arctic-circle lands.

Glaciation leaves behind evidence such as scratches on rock and U-shaped valleys.   Wegener found glacial evidence that seemed impossible to acount for given the current distribution of the continents:   Glaciation

Check out the evidence from Polar dinosaurs in Australia

• Geology

  Rock Sequences show geological similarities that would be highly coincidental unless formation took place in the same location, at the same time.

After World War II, Sonar technology began to be used for mapping the ocean floors.

Besides being more complex than previously thought, it was also discovered that there were mid-ocean ridges that extended through many of the oceans, in some cases right down the middle.

In some places, the mid-ocean ridges are on the surface, such as Iceland and in Africa, known as the East African Rift Zone.

Harry Hess of Princeton University proposed the hypothesis that the sea-floor was spreading apart because of magma rising from below.

Coupled with the mapping, drilling cores into the sea floor to obtain rock samples produced evidence of successive layers spreading out from a central ocean ridge.

- The farther the sample was from the center, the older it was.

Additionally, the magnetic orientation of the rock samples was noted.

- Adjacent samples showed reversed polarization, indicating the Earth's magnetic poles had repeatedly switched directions!

View Animations from WGBH...

Take a good look at the "Ring of Fire"

• Now, we have the combined evidence of continental drift and sea-floor spreading which leads to the theory of the movement of:

Tectonic Plates

Read This Dynamic Earth from USGS - The Story of Plate Tectonics
 

• Not just continents, but ocean basins with them, are floating on the Earth's mantle.  [Inside the Earth], [Earth's Interior & Plate Tectonics]

  Mechanisms:

  USGS diagram of the 4 Plate Boundary types:

  • Divergent boundaries -- where new crust is generated as the plates pull away from each other.
  • Convergent boundaries -- where crust is destroyed as one plate dives under another.
  • Transform boundaries -- where crust is neither produced nor destroyed as the plates slide horizontally past each other.
  • Plate boundary zones -- broad belts in which boundaries are not well defined and the effects of plate interaction are
    unclear.

  Also:

  • Hotspots, Like the Hawaiin Islands
  • Island Arcs, like the Aleutians.

• [Convection cells in the mantle???],[The action at Plate Boundaries]
• See the Ring of Fire
• MAJOR TECTONIC PLATES OF THE WORLD

The oldest theorized continental grouping. 

- 3 times older than Pangea (750 million years)

Evidence from:

• Warm-water animal fossils found in sedimentary rocks in Antarctica
• Along with trace fossils of worm burrows,
• links East Antarctica with North America
• Implies the two were a single landmass as part of Rodinia
• Broke apart 540 million years ago.

The Formation of Pangaea

Maps