How Old Is the Ocean Floor?

Mapping and Dating the Least Known Part of Earth

Age of the cceanic lithosphere
A look at the age of the oceanic lithosphere, which rarely exceeds 180 million years in age. Credit: National Oceanic and Atmospheric Administration/Department of Commerce

The youngest crust of the ocean floor can be found near the seafloor spreading centers, or mid-ocean ridges. As the plates split apart, magma rises from below the Earth's surface to fill in the empty void. The magma hardens and crystallizes as it latches onto the moving plate and continues to cool over millions of years as it move farther away from the divergent boundary. Like any rock, the plates of basaltic composition become less thick and more dense as they cool.

When an old, cold and dense oceanic plate comes into contact with a thick, buoyant continental crust or younger (and thus warmer and thicker) oceanic crust, it will always subduct. In essence, oceanic plates are more susceptible to subduction as they get older. Because of this correlation between age and subduction potential, very little ocean floor is older than 125 million years and almost none of it is older than 200 million years. Therefore, seafloor dating isn't that useful for studying plate motions beyond the Cretaceous. For that, geologists date and study continental crust.   

The lone outlier (the bright splash of purple that you see north of Africa) to all of this is the Mediterranean Sea. It is the lasting remnant of an ancient ocean, the Tethys, that is shrinking as Africa and Europe collide in the Alpide orogeny. At 280 million years, it still pales in comparison to the four billion-year-old rock that can be found on the continental crust.

 

A History of Ocean Floor Mapping and Dating

The ocean floor is a mysterious place that marine geologists and oceanographers have struggled to fully grasp. In fact, scientists have mapped more of the surface of the Moon, Mars and Venus than the surface of our ocean. (You may have heard this fact before, and while true, there is a logical explanation as to why.) 

Seafloor mapping, in its earliest, most primitive form, consisted of lowering weighted lines and measuring how far the sunk. This was done mostly to determine near-shore hazards for navigation. The development of sonar in the early 20th century allowed scientists to get a clearer picture of seafloor topography. It didn't provide dates or chemical analyses of the ocean floor, but it did uncover long oceanic ridges, steep canyons and many other landforms that are indicators of plate tectonics. 

The seafloor was mapped by shipborne magnetometers in the 1950s and produced puzzling results - sequential zones of normal and reverse magnetic polarity spreading out from the oceanic ridges. Later theories showed that that this was due to the reversing nature of Earth's magnetic field.

Every so often (it has occurred over 170 times over the past 100 million years), the poles will suddenly switch. As the magma and lava cool at seafloor spreading centers, whatever magnetic field is present gets ingrained into the rock. The ocean plates spread and grow in opposite directions, so rocks that are equidistance from the center have the same magnetic polarity and age. That is, until they get subducted and recycled under less-dense oceanic or continental crust.

 

Deep ocean drilling and radiometric dating in the late 1960s gave an accurate stratigraphy and precise date of the ocean floor. From studying the oxygen isotopes of the shells of microfossils in these cores, scientists were able to begin studying the Earth's past climates in a study known as paleoclimatology