Plate Tectonics

http://www.ucmp.berkeley.edu/geology/tectonics.html

Plate Tectonics Notes

Continental Drift – a theory proposed in 1912 by Alfred Wegener, a German scientist who noted that:

1. The west coast of Africa and the east coast of south America looked very similar, as if they might have once fitted together- Jigsaw Puzzle Fit

2. The fossil remains of a small reptile called Mesosaurus is only found in South Africa and Brazil

3. Some rock structures (Mountain Ranges and Folds) and rock types are found on continents separated by thousands of kilometres of ocean – Rocks in Newfoundland are the same type and age as those found in Norway, Sweden and Scotland)

Other Scientists found more evidence to support his theory –

1. Glacial deposits in tropical areas

2. Coal deposits in Antarctica

Wegener proposed the theory of continental drift which stated that the continents were once joined together in a large land mass known as Pangaea and have since broken apart and drifted away from each other.

• Pan means “All” and gaea means “world”

• Geologists believe the continents have broken apart and reformed several times over the history of the Earth

• Pangaea is the most recent supercontinent. Geologists believe it formed 250 Ma and started drifting apart 200 Ma

Scientists have since formed a theory of how these huge masses of rock move:

In addition to the evidence for continental drift, scientists noticed that:

• Earthquakes and volcanoes seem to occur in belts

• Bands of magnetism exist

• Areas in the ocean floor that have higher elevations also give off more heat

In the 1960’s a new theory was made called the Theory of Plate tectonics. We now know that the Earth’s surface consists of a number of moving pieces called plates. There are about a dozen plates. Some are moving toward each other while some are moving apart and others are sliding past each other.

The plates are made from a solid layer of rock called the Lithosphere. It is about 100 km thick. Most of the Lithosphere is made from basalt (an igneous rock). However, the continental plates have a composition more like granite. These plates are less dense than the basalt plates and so rise to the top and are embedded in the more dense basalt layer.

The Lithosphere lies on top of a layer in the mantle called the Asthenoshere. This layer is thought to be responsible for plate movement. This Asthenosphere is partially melted and flows very slowly. Movement of the plates is caused by convection currents where hot material in the mantle rises, spreads out and cools. When it cools it contracts, gets heavier and sinks again. Where material is rising, new material is added to the lithosphere pushing older material aside. Where material is cooling and sinking, the plates of the lithosphere are pulled together.