Drill Cores and Fossils: Was the Atlantic Always There?

Drilling Log: Crossing the South Atlantic

1Down the middle of the South Atlantic Ocean runs a long chain of underwater mountains called the Mid-Atlantic Ridge. Along its center, melted rock rises from below, cools and hardens into a dark volcanic rock called basalt. A research drilling ship sailed east from the ridge toward Africa and stopped at six sites, labeled A through F.

2At each site the crew lowered a drill pipe several kilometers to the seafloor. The drill cut through a layer of soft mud and then into the basalt below. Back on deck, scientists studied tiny fossils in the mud resting directly on the basalt. Because that mud began to settle soon after the basalt formed, its age tells roughly how old the basalt is. They also measured how thick the whole mud layer was. Mud on the open seafloor builds up very slowly, from tiny shells and dust sinking through the water.

3Geologist Dr. Nnamdi Eze posted the results in the table below. A second ship drilling west of the ridge, toward South America, reported a matching set of ages on that side. Ages in the table are in millions of years, so a basalt age of 5 means the rock formed about 5 million years ago.

4Evidence also comes from the land. Fossils of Mesosaurus, a reptile about 1 m long, have been found in rocks about 280 million years old in eastern South America and in southern Africa. Scientists think it lived in lakes and shallow, not very salty water, and it was too small to cross thousands of kilometers of open ocean. Fossils of Glossopteris, a seed fern whose seeds were likely too large to blow across an ocean, are found on several southern continents. Geologists have also found that rock formations of the same type and age line up along the coasts of Brazil and West Africa when the two coastlines are fitted together like puzzle pieces.