Continental drift -- One theory's journey Today, you'd be laughed out of a geology course for questioning whether continents move through geologic time. But 75 years ago, only sceptics believed that continents could take a hike. Talk about conventional un-wisdom: W.B. Scott, former president of the American Philosophical Society, even called the theory of continental drift "utter damned rot." (!)

What's changed? Not the actual movement of continents, but our understanding of geology itself. Let's take a look at how the theory developed, and how the evidence began to favour it.

1885: Australian geologist Edward Seuss sees similarities between plant fossils from South America, India, Australia, Africa and Antarctica, and coins "Gondwanaland" for a proposed ancient super-continent with these land masses.

1910: American physicist F.B. Taylor proposes concept of continental drift to explain formation of mountain belts.

1912-15: German meteorologist Alfred Wegener proposes theory of continental drift, based on evidence from geology, climatology and paleontology. Wegener names one of the ancient super-continents "Pangea," and draws maps showing how the continents moved to today's positions.

The main events

225 million years ago during the Permian: One continent, Pangea - the super continent - existed on earth. Pangea means all lands.

200 million years ago during the Triassic: Pangea split into Laurasia and Gondwanaland separated by the Tethys sea.

135 million years ago at the end of the Jurassic: The Atlantic Ocean began to form as Europe and Africa split from the Americas.

Evidence for continental drift, or plate tectonics:

Alfred Wegener

Evidence for continental driftMatching mountainrangesStructural evidence: Mountain belts: The Appalachian Mountains extend to mountains in Greenland, the United Kingdom (Caledonian mountains) and Norway, indicating that these land masses were once joined.

Stratigraphic (layers of rock) or lithological (rock type) evidence: Rock types on opposite sides of the Atlantic are very similar, for example, Gabon in West Africa and Brazil in South America.

Evidence for continental driftEvidence for continental drift

Matchingrock typesand agesof rocks

Continental fit: The shorelines of South America and Africa appear to fit. This was first spotted by Sir Francis Bacon in 1620. Geologists have looked at this further and identify continental fit at the edge of the continental shelf as being more important. Prof. Ballard identified the 2000 foot isobath as being where the best fit is.

Palaeoclimatic evidence: 300 million years ago glacial sediments and striations suggest that the southern continents (South America, South Africa, India, and South Australia) were joined together as Gondwana over the south pole. Now these landmasses are far apart.

Fossils or Palaeontologic evidence: Fossil remains of the plant genus Glossopteris (looks like a type of fern) occur on all five Gondwana continents. The seeds were too heavy to be carried by wind, and would have died quickly in salt water, indicating that the continents were once joined. Similarly, fossils of several reptiles occur on several continents e.g. Mesosaurus and Hystrasaurus (a non-swimming sheep size reptile). What were the arguments against this fossil evidence?You may ask why are marsupials so rare?

Sea Floor Spreading: The ocean floors are spreading away from mid-oceanic ridges, indicating that the ocean floor is moving. In 1962, Harry Hess of Princeton University proposed that seafloor spreading would explain this movement. (Mid-oceanic ridges, more than 65,000 kilometers long, are the largest mountain range on Earth).

The age of rocks: The oldest rocks on the ocean floor are younger than 220 million years, while the oldest terrestrial rocks are about 4 billion years old, indicating that the ocean floor is recycled back into the Earth.

Drilling ocean floor sediments showed that they increased in thickness away from the mid-oceanic ridges.

More recent evidence:

Paleomagnetism: British scientists find that magnetic fields recorded in rocks from Europe and North America indicate the rocks were formed in far different locations than their present positions. The pattern of continental drift recorded by rocks show Europe and North America have drifted away from each other for more than 100 million years. This movement opened the Atlantic Ocean.

Magnetic stripes: Earth's magnetic field periodically changes polarity. When the hot rock solidifies at the mid-oceanic ridges, it retains the polarity of the moment. Bands of seafloor rocks paralleling the mid-oceanic ridges carry a record of this alternating polarity. The bands appear to be the same on either side of the ridge.

Magnetic stripes on the sea-floor. Magnetic field of Earth reverses on semi-regular basis. Minerals act like compass needles and point towards magnetic north. “Hot” rocks record the direction of the magnetic field as they cool.