Plate Tectonics

Plate Tectonics (PT) - a concept that means that the earth's surface is divided into few large thick plates that move slowly and change in 'size'.

PT is a combination of 2 ideas - Continental drift (CD) and Sea-Floor (SF) spreading

Continental drift was noted by earlier people: Francis Bacon (1620), Antonio Snider (1858), but it was in the early 1900's that Alfred Wegener, a German meteorologist, came up with other evidence to make a strong case for continental drift (CD)

Edward Suess (1885), proposed Gondwana. Frank Taylor (1960), also had his own theory. Alexander du Toit provided enough evidence to support Wegener's hypothesis, who in 1937 published "Our Wandering Continents."

Pangea - A giant supercontinent (proof for such a continent)

1. Continental fit

2. Fossils - e.g Glassopteris

3. Rocks - tillites / rock sequence and mountain ranges

4. Glacial evidence (Paleoclimates) - Paleozoic glaciation on the continents of the southern hemisphere

5. Poles magnetism / Polar wandering

Paleomagnetism and the Revival of Continental Drift

Magnetic lines of force dip more steeply as the north magnetic pole is approached, the inclination (dip) of the magnetized crystals in a lava flow can be used to determine the distance from a flow to the pole.

Marine Magnetic Anomalies

Symmetry of the magnetic anomalies around the ridge crest - Vine and Drummond Matthews (V-M) Hypothesis also found that the anomaly matched the pattern of magnetic reversal

Rate of plate motion: The V-M allowed for

1. the measure of sea-floor spread. Range is 1 to 6 cm/yr

Fossil correlations - found in rocks of similar age. Rock matches (including radiometric dates)

Pangea first separated into 2 parts - Laurasia, Gondwanaland. Started breaking up ~ 200 million years ago

Sea-floor spreading

Harry Hess (1962), suggested that the sea floor was also moving. Because he lacked proof, he called it geopoetry.
What moves the sea floor?
- mantle convection explanation.
- New magma cause fracture.

1963 Vine and Matthews of Cambridge University, used a model to explain pattern of magnetic anomalies observed in sea floor rocks.

The Mid-Oceanic Ridge

Oceanic Trenches

Age of the Sea Floor

Plates and Plate Motion

Lithosphere - 70-125 km thick

Asthenosphere

Mid-ocenic ridges are plate boundaries, defined and located by mapping new bands of geologic activities such as a belt of earthquakes or volcanoes.

Plate tectonic theory is now used in explaining many geologic features - plate interactions of plate boundaries - Diverging, Converging, Transform

Fracture Zones and Transform Faults

Earthquakes occur along the entire length of the zone.

Within the transform fault zone

Diverging Plate Boundaries

Rift Valley

Converging Plate Boundaries

Ocean island arc

Fore-arc basin

Ocean/continent magnetic arc instead of island arc

Fore-arc magma back-arc thrust belt

Back-arc basin craton

Continental to Continental

Transform boundaries

Why is plate tectonics attractive? It explains the distribution of:

1. Volcanoes

2. Earthquake

3. Young mountain ranges

4. Mid-Oceanic ridge - rift valley, oceanic trenches

Cause of plate motion

1. Mantle convection

2. Mantle plumes form hot spot

Triple junction - failed arm = aulacogen.