The rift down the middle of the Altantic Ocean.
The first clue to these puzzles came when the floors of the oceans were mapped. These turned out not to be flat, but riddled with mountains, valleys and trenches. One such mountain range runs right down the middle of the Atlantic Ocean.
The ocean floor on either side was then dated. Next to the continents it was oldest, at most 180 million years old, while at the ridge itself the rock is new. This suggests that brand new ocean floor is created at the rift and then moves out on either side over time. Further studies have shown that this does indeed happen: volcanic basalt wells up at the rift and then cools solid to form oceanic crust.
Thus it isn't the continents which are somehow ploughing through the sea, but instead the seafloor which moves as it expands out from the rift.
While this explains how South America and Africa have drifted apart, it leaves several further questions. If ocean floor is being created at ocean ridges, surely somewhere it must get destroyed? What drives the ocean floor apart at the rifts? And what happens when continents collide?
This mountain chain runs all around the Earth like the stitching of a tennis ball. It is visible on Google Earth.
Furthermore, there is no ocean crust anywhere on the planet older than about 200 million years. This contrasts starkly with continental crust, which can be billions of years old.
The mid-Atlantic ridge moves apart at 2.5cm per years [Wikipedia: Mid-Atlantic Ridge].
Evidence was found in favour of seafloor-spreading in the 1950s and 1960s by studying the magnetic field found in the ocean floor, which was "frozen in" when the basalt cooled. Since the earth's magnetic field changes over time, this gives a record of the seafloor's history.
See slideshow Ocean Floor Spreading: Rock as a Tape Recorder.
One might ask how the rift formed in the first place. This is known as "rifting" and the East African Rift Valley to Red Sea is a classic precursor, as is the Gulf of California Rift Zone.
Author: Tom Brown
Copyright: public domain
Date last modified: 11th Oct 2011
Peer-review status: Not yet peer-reviewed
mid_atlantic_rift.jpg: source: google search for "atlantic sea floor"; http://www.maps.com/map.aspx?pid=15757, copyright: unknown
earth_seafloor_crust_age-285.gif: source http://commons.wikimedia.org/wiki/File:Earth_seafloor_crust_age_1996.png: By http://www.ngdc.noaa.gov (File:Earth seafloor crust age 1996.gif) [Public domain], via Wikimedia Commons