LC Introduction to Plate Tectonics

Plate tectonics is the difficult sounding name for a very simple idea. The idea is that the rocky crust of the earth is made of different plates.  These plates are separated by fault lines called plate boundaries.

There are actually two parts to the idea.  One is called Continental Drift (discovered by Alfred Wagener) and the other is called Sea Floor Spreading (discovered by Harry Hess).

 Alfred Wagener

Wagener was a meteorologist who studies geology as a hobby.  In 1912 he wrote a book in which he said that

*200 million years ago there was one landmass which he called Pangaea (a Greek word meaning ‘whole earth’)

*Pangaea split up into two super continents – one called Laurasia and the other, Gondwanaland.

*about 65 million years ago the continents reached their present locations.

 Wageners Evidence

*Wagener said that the continents looked like the fitted together.  If you cut them out from a map, you can re-arrange them so that they almost fit together just like a jigsaw puzzle(please don’t cut up an atlas – trace the map on paper instead and cut out the continents from it).

 * Wagener looked at rocks and mountains on the east of North America and found they were identical to rocks and mountains in Northern Africa.  This could only have happened if these two continents were once joined together and since then split apart.

 *Wagener looked at fossils of a dinosaur called Mesosaurus which were found in South Africa.  The EXACT same fossils were found in South America.  Mesosaurus could not swim so the only way its fossil could be found on separate continents was if South America and South Africa were once joined together.


….Wageners idea was turned down by geologists who said he ought to leave geology to the professionals. He died in 1915 and so didn’t live to promote his idea or see it accepted.

 Sea floor Spreading

Sea floor spreading is an idea that the sea floor is getting wider (in some places). During World War II Harry Hess began using sonar to map parts of the ocean floor.  He discovered the ocean floor was not flat but had mountains instead. These mountains were ridges – long meandering mountains that ran the length of ocean. He also discovered that the ridge had fault lines or rifts running down the centre of the ridges. An example of such a ridge is the Mid-Atlantic Ridge Rift. More investigation helped scientists discover that these  ridge-rifts sometimes opened.  When they did, magma poured out and hardened into new rock. This meant that the rifts were cracks in the earth’s crust. When they tested the age of rock on the ocean floor, they discovered that rocks near the ridge were younger than rocks further away.  This could only mean that the older rock formed in the past at the ridge and had since moved away.  The sea floor was spreading outward away from the ridge!  Scientists called these ridges ‘constructive plate boundaries’ or ‘separating plate boundaries’.  The Mid-Atlantic Ridge is one example of this.

 A puzzle

Scientists were a bit puzzled by the discovery of sea floor spreading.  If the Atlantic Ocean was getting wider, that would mean the earth was getting bigger. But since the earth is not getting bigger the only conclusion was that the ocean floor was being destroyed somewhere else.

What scientist discovered was that at some locations, for example, South America, some ocean floor was being pulled under the continents into the mantle. Scientists called these locations ‘destructive plate boundaries’ because oceanic plate was being destroyed. These locations are also called ‘colliding plate bounaries’ because two plate collide.


The discovery of plate tectonics was very important because for the first time, scientists could explain how the formation of rocks, mountains, volcanoes and the occurrence of earthquakes were all linked together.