See Inside Our Ever Changing Earth

The Mid-Cretaceous Superplume Episode [Preview]

Earth has an erratic "heartbeat" that can release vast amounts of heat from deep within the planet. The latest "pulse" occurred 120 million years ago

At one in the morning on December 13, 1989, I was awakened in my bunk onboard the scientific drillship JOIDES Resolution by sounds of celebration in the adjoining cabin. Because I had to relieve the watch at four anyway, I stumbled next door to join the party. The paleontologists in our expedition had just reported to my cochief scientist, Yves Lancelot, now at the CNRS Center of Oceanology of Marseille in France, that microfossils of the Jurassic period had been recovered from the hole in the floor of the western Pacific Ocean that we were drilling more than three miles below us. Two days later the drill reached the volcanic basement--oceanic crust of Middle Jurassic age, about 165 million years old. A 20-year mystery was solved. At last, we had hard evidence of the world's oldest deep-sea sediments and volcanic rocks still in place from eons ago.

In succeeding days I reflected on why the quest had taken so long. My colleagues Clement G. Chase of the University of Arizona, Walter C. Pitman III of Lamont-Doherty Earth Observatory, Thomas W. C. Hilde of Texas A&M University and I had first considered the problem in the 1970s. The target was not a small one. We had predicted from geophysical data that an area in the western Pacific the size of the continental U.S. should be Jurassic in age, somewhere between 145 million and 200 million years old. But whenever we dredged or drilled in this area, we almost invariably recovered rocks called basalts, formed by volcanic eruptions during the mid-Cretaceous, generally ranging in age from 80 million to 120 million years but no older. The first such basalt samples were dredged from the Mid-Pacific Mountains in 1950 by an early expedition of the Scripps Institution of Oceanography. Until the JOIDES discovery, however, geologists had not made much progress in answering the questions concerning the origin of the seemingly ever present mid-Cretaceous basalts or the possible existence of underlying Jurassic material.

The 1989 discovery provided some qualitative answers. The older Jurassic sediments and oceanic crust were buried during the mid-Cretaceous epoch by what we now refer to as a "superplume" of volcanic material. Finally, our geophysical musings of the early 1970s could be supported with facts: the Jurassic existed in the western Pacific. We had samples of it locked away onboard the JOIDES Resolution.

Because I am a geophysicist, I try to describe Earth and its processes quantitatively. I wanted to determine the size of the mid-Cretaceous superplume of the western Pacific, hoping to learn something of its origins. But saying that and doing it are two different things. What do you measure, and how do you measure it? I did not even know what "normal" was, so how could I describe the "anomalous" mid-Cretaceous superplume episode? The problem had to be expanded beyond the time and space framework of the mid-Cretaceous western Pacific. I decided to examine the rate of formation of oceanic crust--mainly volcanic rocks such as basalts that make up the solid basement underneath the seafloor--for all the ocean basins over their entire histories. Then the mid-Cretaceous anomaly, whatever it was, would stand out against the background. Clues to the timing of the next superplume might also appear.

At the time of the mid-Cretaceous, widespread volcanic eruptions covered or created vast amounts of ocean floor very quickly. Typically, though, seafloor spreading generates most of the oceanic crust in a slower, more regular way. In this process the crust becomes older symmetrically away from mid-ocean ridges where molten magma rises up out of Earths mantle and then cools and solidifies. As new magma continues to rise, the older oceanic crust is rafted away from the eruption center and onto the flanks of the ridge. Thus, any particular parcel of crust is transported as if it were on one of two identical conveyor belts moving away from the mid-ocean ridge in opposite directions [see "The Mid-Ocean Ridge," by Kenneth C. Macdonald and Paul J. Fox; Scientific American, June 1990].

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