Chapter 4: Plate tectonics The evidence for the theory of plate tectonics has been accumulating for 400 years. It is only since the 1960’s that this theory has been generally accepted. IT STARTED WITH MAPS

4.1 Continental Drift Alfred Wegener’s observations: 1. The continents fit together like puzzle pieces 1912

Fossils 2. Fossil specimens from South America match fossil specimens in Africa

Rocks & Minerals 3. Minerals, Rocks, and mountain ranges match between continents on either side of the Atlantic Current hypothesis of the time was “land bridges”.

Glacial debris 4. Glacial debris found in South America and Africa

Fossils 5. Coal and other fossil evidence of tropical climate was found in North America, Europe, and Siberia.

Continental Drift In 1912, he proposed his hypothesis called Continental Drift Alfred Wegener named his super continent “Pangaea” He named the surrounding super ocean “Panthalassa”

Alfred Wegener’s ideas were not accepted because he did not have a geologic mechanism! He died in Greenland, years after his death, that mechanism was found!

Mechanism for Continental Drift 1947 – Expedition to study the Mid-Atlantic Ridge Evidence : The rocks on the ocean floor were very young compared to continental rocks! None of the rocks were more than 150 million years old Continental rocks are as old as 4 billion years WWII in the Pacific, he tracked the sea bottom using “echo sounder”, a.k.a. sonar Hess proposed a hypothesis... Mid-Atlantic Ridge valley is a rift in the Earth’s crust where magma from the mantle comes through. He reasoned the ocean floor was moving away from both sides of the ridge, being replaced by rising magma. Princeton

Seafloor Spreading Harry Hess reasoned that that the ocean floor moved away from the ridge, and was replaced by magma. Robert Dietz named this process “Seafloor Spreading” These ideas are both hypotheses. Rock samples were very young compared to continental rocks! None more than 150 million years old. Continental rocks have been found that are 4 billion years old.

Paleomagnetism Magnetic fields on the ocean bottom are mirror images 1965 – two groups of scientists, working independently, discovered the reversal in rocks on land, and the other on the ocean floor. Here was the scientific evidence that Wegener needed to verify his hypothesis of continental drift.

Continental Drift million years ago: