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].[break]
Areas of the ocean floor formed by spreading--known as abyssal plains--are covered with organized processions of abyssal hills and fracture zones running perpendicular to the mid-ocean ridges. Yet the western Pacific looks nothing like this. Its physiography is more like a muddy New England road in March. The seemingly randomly oriented chains of seamounts--undersea mountains that rise at least one kilometer from the ocean floor--and the oceanic plateaus that make up the "muddy road" of the western Pacific have no systematic age gradients across them. The only characteristic they share is that they are almost all from the mid-Cretaceous, to the extent that we even know their ages.
The first step in my investigation was to measure the changing rate of production of oceanic crust. To do this, I compiled information on the areas and ages of ocean floor and estimated the thickness of the crust. I was able to calculate this rate for the past 150 million years, nearly back to the maximum age of the worlds ocean basins.
On set of the Pulse
THE BOX on the opposite page shows several of the geologic consequences of the mid-Cretaceous superplume event. The calculation of overall crustal production [see third graph] clearly shows the mid-Cretaceous superplume. This histogram of the world-total rate of crust production shows the plumes sudden onset 120 million to 125 million years ago, when formation of ocean crust nearly doubled in about five million years. Crustal production peaked soon after the onset of the pulse, tapering off more or less linearly over the next 70 million to 80 million years. Then, 30 million to 40 million years ago, it returned to values nearly the same as those before the episode. The mid-Cretaceous superplume in ocean crustal production stands out on a global scale. Yet the mere existence of the pulse does not indicate the reason for it.
I thought the key to the puzzle might lie in the development of oceanic plateaus and undersea mountain chains. During the mid-Cretaceous epoch, the rate of production of these formations jumped at the same time as the overall rate did, with a similar impulsive onset and a long succeeding taper back to normal values. Although the maximum amplitude at the height of this pulse was less than that for the world-total curve, the relative increase was much larger. Whereas total output of oceanic crust initially doubled, plateau and seamount production increased by a factor of five. So whatever produced the superplume episode also had the strongest effect on plateau and mountain-chain generation.
What causes these undersea plateaus and seamount chains? Independently, other investigators have converged on the notion that they result from plumes of material from deep in Earths mantle that have been overheated and thus rise buoyantly because of their reduced density [see "Large Igneous Provinces," by Millard F. Coffin and Olav Eldholm; Scientific American, October 1993]. In particular, oceanic plateaus result from the initial massive, rapid eruptions caused by these rising plumes. Such upwellings occasionally occur on the continents where we can study them directly. Exotically named regions such as the Paran Basalts of Brazil, the Deccan Traps of western India and the Siberian Traps of northern Russia consist of vast fields of basalt flows, several hundred kilometers across and one or two kilometers thick [see "Volcanism at Rifts," by Robert S. White and Dan P. McKenzie; Scientific American, July 1989]. The oceanic plateaus are similar to their continental cousins, but they are even larger. For instance, the largest of the oceanic plateaus (the Ontong-Java Plateau of the western Pacific) is estimated to be 25 times bigger than the largest continental one (the Deccan Traps).
Seamount chains trail away from oceanic plateaus and result from material behind and below the head of the rising plume material. Because the plumes are relatively fixed, and the overlying tectonic plates drift horizontally on the surface of the planet, the subsequent eruptions in the mountain chains record the motions of the plates. Thus, these seamount chains should be oldest next to their parent oceanic plateau and trace a path of younger and younger seamounts that ends in an active volcano if the "chimney pipe" to the deep mantle is still alive. The best known of these seamount chains is the Hawaiian Islands, which extends underwater far to the northwest of the islands themselves. Its rising plume system exists today below the island of Hawaii, where volcanic eruptions continue to rumble. The islands and seamounts become successively older to the northwest as they are rafted away on the Pacific plate, which moves in a northwesterly direction over a fixed plume location.[break]
Once I realized that the features of the ocean crust most affected by the mid-Cretaceous volcanic activity--seamount chains and plateaus--were each formed by plumes of mantle material, it was a small logical step to suppose that the entire anomaly resulted from plume activity on a much larger than normal scale. Because I live in a superlative-prone society, I named this a "superplume episode." The initial pulse of the superplume reached Earths surface around 120 million years ago; the intense volcanic activity started suddenly and continued through the mid-Cretaceous, lasting tens of millions of years, gradually tapering off after that.
THE SUPERPLUME EPISODE was most likely caused by the upwelling of one or perhaps several enormous plumes that ascended through the easily deformed mantle, spread out at the base of Earth's more rigid outer shell, known as the lithosphere, and erupted onto the ocean floor. Although the Pacific was most strongly affected, evidence of the superplume event is also present in the Indian, South Atlantic and Caribbean oceans. The area of the Pacific involved may have been several thousand kilometers across, in sharp contrast with the size of regions affected by today's plume activity, which are usually one tenth that size in area.
I suspect the overheated plumes rise from the very base of the mantle and affect the process that causes reversals of Earths magnetic field in the underlying outer core. There is a general inverse correlation between the production rate of crust formed by plumes and the frequency of reversals of Earth's magnetic field. For example, during periods of intense plume activity, including during the mid-Cretaceous, almost no magnetic reversals take place. Conversely, as is the case today, when plume activity is low, magnetic reversals occur at record pace.
How Earth's magnetic field actually reverses its polarity is a mystery [see "Probing the Geodynamo," on page 28]. Peter Olson of Johns Hopkins University and I think the correlation between crustal formation and magnetic field reversals may provide a clue to understanding how the reversals take place and to determining the source of the mantle plume material. We believe an increase in the "boiling rate" of the core somehow causes magnetic reversals to become more infrequent. Additionally, the connection may reveal information about the advent of the next superplume.
Boiling iron within the outer core is the likeliest source of Earth's magnetic field. Such molten iron is an excellent electrical conductor, and the convective motion of the iron and its associated electrical field almost certainly generates Earth's magnetic field. The heat given off by the molten iron percolates through the core-mantle boundary, the lid to this boiling pot, by the process of conduction. The heat becomes trapped just above the boundary in the lowermost 100 to 200 kilometers of solid silicate rock of the mantle. This process continues until enough excess heat accumulates. Then the buoyancy of the overheated, less dense lower mantle overcomes the viscosity of the overlying cooler, more dense mantle rock. Huge plumes of mantle material rise nearly 3,000 kilometers through the mantle and eventually trigger volcanic eruptions at the surface. Ascending material removes heat from the lowermost mantle, allowing the outer core to boil even more vigorously than before.
THE MOST RECENT of these major overturns erupted 120 million to 125 million years ago as the mid-Cretaceous superplume episode. Much of the material that surfaced at this time left the muddy-road effect seen today on the western Pacific seafloor. An episode that nearly doubles the world-total rate of oceanic crustal production in a short period must have staggering geologic consequences. The mid-Cretaceous was characterized by several profound anomalies resulting from the superplume.
First and probably least controversial is the rise in worldwide sea level to an elevation 250 meters or so higher than it is today. Assuming that the total amount of seawater in the planet's oceans is constant, a rise in the level of the sea surface is simply a reflection of a corresponding rise in the level of the seafloor. Ocean above newly formed crust is abnormally shallow because the crust and underlying lithosphere are still relatively warm, less dense and therefore expanded. As the two cool, they contract, allowing the seafloor to deepen. This phenomenon of expansion and contraction explains why oceanic ridges, where new crust is being formed, are raised above the older, deeper crust found on the flanks. If an abnormal amount of new crust is formed rapidly--as it was at the beginning of the mid-Cretaceous pulse--then the average seafloor level will be elevated, and the sea surface will rise accordingly. In the mid-Cretaceous, rising sea levels drowned much of what is dry land today; for example, my birthplace in Iowa was then at the bottom of the ocean. When the water receded, it left deposits of limestone and chalk, including the famous White Cliffs of Dover in England.[break]
Earth's surface temperature also increased as a result of the superplume episode. When molten lava erupts, it releases certain chemicals, including carbon dioxide. Higher amounts of carbon dioxide in the mid-Cretaceous atmosphere led to a natural greenhouse effect that raised global temperatures by roughly 10 degrees Celsius. Studying the effects of elevated carbon dioxide levels during this period could reveal possible scenarios for Earth's climate in the future. Massive burning of fossil fuels and large-scale deforestation continue to increase the level of carbon dioxide in the modern atmosphere.
An excess amount of organic carbon and inorganic carbonate was also deposited during the mid-Cretaceous. The enhanced deposition is related to the elevations in sea level and air temperature, which, we have seen, resulted from the superplume episode. Tiny plants and animals, known as phytoplankton and zooplankton, make their living floating at shallow levels in the ocean where light can penetrate. Plankton apparently thrived during the mid-Cretaceous in the abnormally warm oceans that accompanied the natural warming of the atmosphere. Typically, when these organisms die, their bodies sink in the deep sea and quickly dissolve because of the extreme pressure of the overlying seawater. But during the mid-Cretaceous, many of the dead organisms fell instead on the drowned continents. The carbon from the exoskeletons did not dissolve in the shallow waters but was preserved. Some of it formed the White Cliffs, and some was buried more deeply and eventually turned to oil. The resulting oil constitutes up to 50 percent of the world's oil supply. Ironically, this outcome of the mid-Cretaceous greenhouse event may have created the fuel for the next greenhouse episode.
Other geologic anomalies associated with the mid-Cretaceous superplume include the placement of a very large percentage of Earth's diamond deposits. Diamonds are made of pure carbon atoms, squashed into the tightest, densest conceivable packing order by pressures that exist at least 200 to 300 kilometers below Earth's surface. Most diamonds are ancient even on geologic timescales, having formed more than a billion years ago, but according to Stephen E. Haggerty of the University of Massachusetts at Amherst, many of them were brought to the surface during the mid-Cretaceous. They were transported up volcanic structures called kimberlite diamond pipes (after a mining area in Kimberley, South Africa) that extend deep down into the crust and presumably into the upper mantle. The diamonds were probably torn loose from their sources within the mantle by rising plumes and brought up in their solid, original state.
The formation of most of the mountain ranges that edge the western coasts of North and South America was strongly controlled by the superplume episode. The Sierra Nevada Mountains of western North America and the Andes Mountains of western South America were created during the mid-Cretaceous by increased subduction of Pacific crust underneath western North and South America. Subduction occurs close to the continents when the oceanic lithosphere is thrust below the adjacent landmass and recycled into the mantle below. Remember that, because of the proximity of the erupting plumes, rates of seafloor spreading in the Pacific increased dramatically. What comes up must go down if Earth's diameter remains constant, so as production of ocean floor increased, so did subduction rates. Abnormally large amounts of oceanic crust were thrust deep under the western coastlines of North and South America. As the crust and accompanying oceanic sediments sank several hundred kilometers below Earth's surface, water within the sediments and crust lowered the melting temperatures of the solid material, causing it to become semiliquid as temperatures and pressures rose. Some of the adjacent continental crust also melted from frictional heating. This molten rock combination rose back to near the surface as its density lessened and then solidified to form the granite cores of the mountain ranges that are the spine of the western coast of the Americas.[break]
The Next Pulse
OUR PLANET has clearly settled down from the effects of the mid-Cretaceous superplume event--the most recent superplume in Earth's history. But when the next superplume eruption will occur is a matter of speculation. We can get an idea of the possibility of modern superplumes by studying the time it takes earthquake waves to pass through the planet. This technique is similar to a CT scan used to obtain three-dimensional x-ray views of patients. Generally, if the earthquake waves arrive later than expected, then somewhere along their path they have passed through a part of Earth that is warmer than normal for that depth, and the warmer area has caused the waves to slow down. Conversely, if the waves arrive sooner than expected, then they have passed through a colder-than-normal area and speeded up. Warmer-than-normal areas in the deep mantle are often interpreted as mantle plumes.
Two large (more than 1,000 kilometers in all directions) warm anomalies in the deep mantle stand out, suggesting that superplumes are now forming deep within Earth. One of them is under French Polynesia in the South Pacific, the other under South Africa and the southeastern Atlantic. These have been called the South Pacific and African superplumes, although they lack the classic "mushroom cloud" appearance of a rising superplume. I suspect that the modern South Pacific superplume is the nearly exhausted remains of the mid-Cretaceous superplume, whereas the budding African superplume appears to be stalled in mid-ascent as a result of its chemically dense core.
I doubt that either of these present-day superplumes will erupt and reproduce the effects of the mid-Cretaceous, but I could be wrong--and the next superplume eruptions may be only a few million years away. I have no doubt that there will be more superplume eruptions in Earths future, but no one can predict when the next one will occur.
ROGER L. LARSON first became acquainted with the oceans when he left Iowa State University with a bachelors degree in geology. He headed west and five years later earned a Ph.D. in oceanography from the Scripps Institution of Oceanography at the University of California, San Diego. He became interested in the tectonic history of the western Pacific in 1971 as a research associate at the Lamont-Doherty Earth Observatory of Columbia University. That interest has continued during Larsons work as professor of oceanography at the University of Rhode Island. He has served as chief or cochief scientist on a total of 15 oceanographic expeditions to the region.