Continental Drift and Ocean Floor Expansion Theories

Item 4: Introduction to Earth's Dynamics

The movement of the Earth's crust (lithosphere) is continuous, but until just a century ago, it was not understood. Erosion, earthquakes, landslides, and volcanoes were observed, but it was thought that the Earth was eternally unchanging. There are many theories that attempt to explain the dynamics of the land, which are discussed below.

The Theory of Continental Drift

The idea that the continents had moved in the past was revived when the first maps of the East Coast of South America were created, and it was observed that it matched the West African coast. Later, the following information was discovered:

• Prehistoric, identical continents are now widely separated.
• The same types of rocks from the same period appeared far apart on different continents.
• Ancient rocks indicate a past glacier where today there is a tropical climate.

As more data accumulated, a hypothesis was proposed to explain them. In 1912, Alfred Wegener, a German meteorologist, gathered a great deal of geological, paleontological, and climatic data. These data indicated that the continents had moved through time. He proposed the hypothesis of continental drift and synthesized it, explaining this avalanche of data.

Wegener showed that once, all the continents were joined in one supercontinent called Pangea. Pangea later broke up into several continents that drifted to their present positions. The hypothesis of continental drift explains:

• The fit of coastlines of several continents.
• The continuity of the long mountain chains, now fragmented and separated on different continents.
• The same fossil fauna and flora in distant continents.
• Glacial rocks in the tropics.

However, there is a problem in Wegener's hypothesis: it does not correctly explain why the continents moved on the oceanic crust, i.e., the drift mechanism.

The Theory of Ocean Floor Expansion

In the late 1950s and early 1960s, new data emerged from the seabed that led geologists to hypothesize oceanic expansion. This hypothesis suggested that the continents and segments of oceanic crust move together as one unit in blocks and that some kind of thermal convection cells operating in the Earth's interior was the mechanism responsible for the movement of these blocks.