the mid-cretaceous superplume episode

At one oÕclock in the morning on December 13, 1989, I was awak-ened in my bunk on board the

scientiÞc drillship JOIDES Resolution bythe sounds of celebration in the adjoin-ing cabin. Since I had to relieve thewatch at four anyway, I stumbled nextdoor to join the party. The paleontolo-gists in our expedition had just report-ed to my coÐchief scientist, Yves Lance-lot, now at the University of Aix-Mar-seilles, that microfossils of the Jurassicperiod had been recovered from thehole in the ßoor of the western PaciÞcOcean that we were drilling more thanthree miles below us. Two days later thedrill reached the volcanic basementÑoceanic crust of Middle Jurassic age,about 165 million years old. A 20-yearmystery was solved. At last, we hadhard evidence of the worldÕs oldestdeep-sea sediments and volcanic rocksthat are still in place from eons ago.

In succeeding days I reßected on whythe quest had taken so long. My col-leagues Clement G. Chase of the Univer-sity of Arizona, Walter C. Pitman III of

Lamont-Doherty Geological (Earth) Ob-servatory, Thomas W. C. Hilde of TexasA&M University and I had Þrst consid-ered the problem in the 1970s. The tar-get was not a small one. We had predict-ed from geophysical data that an area inthe western PaciÞc the size of the con-tinental U.S. should be Jurassic in age,somewhere between 145 and 200 mil-lion years old. But whenever we dredgedor drilled in this area, we almost invari-ably recovered rocks called basalts,formed by volcanic eruptions during themid-Cretaceous, generally ranging in agefrom 80 to 120 million years and no old-er. The Þrst such basalt samples weredredged from the Mid-PaciÞc Mountainsin 1950 by an early expedition of theScripps Institution of Oceanography.Until the JOIDES discovery, however,geologists had not made much prog-ress in answering the questions con-cerning the origin of the seemingly everpresent mid-Cretaceous basalts or thepossible existence of underlying Juras-sic material.

The 1989 discovery provided somequalitative answers. The older sedimentsand oceanic crust were buried duringthe mid-Cretaceous epoch by what wenow refer to as a ÒsuperplumeÓ of vol-canic material. Finally, our geophysicalmusings of the early 1970s could besupported with facts: the Jurassic ex-isted in the western PaciÞc. We hadsamples of it locked away on board theJOIDES Resolution.

Because I am a geophysicist, I try todescribe the earth and its processesquantitatively. I wanted to determinethe size of the mid-Cretaceous super-plume of the western PaciÞc, hoping tolearn something of its origins. But say-ing that and doing it are two diÝerentthings. What do you measure, and howdo you measure it? I did not even knowwhat ÒnormalÓ was, so how could I de-scribe the ÒanomalousÓ mid-Cretaceous

superplume episode? The problem hadto be expanded beyond the time andspace framework of the mid-Cretaceouswestern PaciÞc. I decided to examinethe rate of formation of oceanic crustÑmainly volcanic rocks such as basaltsthat make up the solid basement under-neath the seafloorÑfor all the oceanbasins over their entire histories. Thenthe mid-Cretaceous anomaly, what-ever it was, would stand outagainst the background.Clues to the timing ofthe next superplumemight also appear.

At the time ofthe mid-Cre-taceous,

82 SCIENTIFIC AMERICAN February 1995

The Mid-Cretaceous Superplume Episode

The earth has an erratic “heartbeat” that can release vast amounts of heat from deep within the planet. The latest

“pulse” of the earth occurred 120 million years ago

by Roger L. Larson

ROGER L. LARSON Þrst became ac-quainted with the oceans when he leftIowa State University with a bachelorÕsdegree in geology. He headed west in anew 1965 Ford Mustang and Þve yearslater earned a Ph.D. in oceanographyfrom the Scripps Institution of Oceanog-raphy at the University of California, SanDiego. He became interested in the tec-tonic history of the western PaciÞc in1971 as a research associate at the La-mont-Doherty Geological (Earth) Obser-vatory of Columbia University. That in-terest has continued during LarsonÕswork as a professor of oceanography atthe University of Rhode Island. He hasserved as chief or coÐchief scientist on atotal of 13 oceanographic expeditions tothe region. He still drives the Mustang.

SUPERPLUMES build vast areas of ocean-ic plateaus and seamounts (right ), com-pared with the small region aÝected bynormal plumes (left ). The plumes areshown in a progressive sequence of

NO

RM

AL

PLU

ME

EVEN

T

IAN

WO

RP

OLE

Copyright 1995 Scientific American, Inc.

widespread volcanic eruptions coveredor created vast amounts of ocean ßoorvery quickly. Typically, though, seaßoorspreading generates most of the ocean-ic crust in a slower, more regular way.In this process the crust becomes oldersymmetrically away from mid-oceanridges where molten magma rises upout of the earthÕs mantle and then coolsand solidiÞes. As new magma continuesto rise, the older oceanic crust is raftedaway from the eruption center and ontothe ßanks of the ridge. Thus, any par-ticular parcel of crust is transported asif it were on one of two identical con-veyor belts moving away from the mid-ocean ridge in opposite directions [seeÒThe Mid-Ocean Ridge,Ó by Kenneth C.Macdonald and Paul J. Fox; SCIENTIFIC

AMERICAN, June 1990].Areas of the ocean ßoor

formed by spread-ingÑknown

as abyssal plainsÑare covered with or-ganized processions of abyssal hillsand fracture zones running perpendic-ular to the mid-ocean ridges. Yet thewestern PaciÞc looks nothing like this.Its physiography is more like a muddyNew England road in March. The seem-ingly randomly oriented chains of sea-mounts, taller than abyssal hills, andthe oceanic plateaus that make up theÒmuddy roadÓ of the western PaciÞchave no systematic age gradients acrossthem. The only characteristic they shareis that they are almost all from the mid-Cretaceous, to the extent that we evenknow their ages.

The Þrst step in my investigation wasto measure the changing rate of pro-duction of oceanic crust. In order to dothis, I compiled information on the ar-

eas and ages of ocean ßoorand estimated the

thickness

of the crust. I was able to calculate thisrate for the past 150 million years,nearly back to the maximum age of theworldÕs ocean basins. These calculationsof overall crustal production clearlyshow the mid-Cretaceous superplume[see illustration on page 85 ].

The Onset of the Pulse

The world-total histogram shows theplumeÕs sudden onset 120 to 125

million years ago, when formation ofocean crust doubled in about Þve mil-lion years. Crustal production peakedsoon after the onset of the pulse andthen tapered more or less linearly overthe next 70 to 80 million years. It re-turned to values nearly the same asthose before the episode 30 to 40 mil-lion years ago. The mid-Cretaceous su-perplume in ocean crustal productionstands out on a global scale. Yet themere existence of the pulse does notindicate the reason for it.

I thought the key to the puzzlemight lie in the development of

oceanic plateaus and under-sea mountain chains.

During the mid-Creta-ceous epoch, the

SCIENTIFIC AMERICAN February 1995 83

events, which occurs for both superplumes and normal plumes: birth atthe thermal boundary layer, ascent through the mantle, ßattening at thebase of the lithosphere and, Þnally, eruption at the surface. The actual ge-ography of the mid-Cretaceous superplume event in the western PaciÞcwould show the plumes in a more irregular order.

POTE

NTIA

LFU

TUR

EE

RU

PTIO

NS

ERUPTING PLATEAUS AND SEAMOUNTS

SUPERPLUME

EPISODE

INNER CORE

MANTLE LITHOSPHERETHERMAL

BOUNDARYLAYER

OUTER CORE


rate of production of these formationsjumped at the same time as the overallrate did, with a similar impulsive onsetand a long succeeding taper back tonormal values. Although the maximumamplitude at the height of this pulsewas less than that for the world-totalcurve, the relative increase was muchlarger. Whereas total output of oceaniccrust initially doubled, plateau and sea-mount production increased by a fac-tor of Þve. So whatever produced thesuperplume episode also had thestrongest eÝect on plateau and moun-tain chain generation.

What causes these undersea plateausand seamount chains? Independently,other investigators have converged onthe notion that they result from plumesof material from deep in the earthÕsmantle that have been overheated andthus rise buoyantly because of their re-duced density [see ÒLarge Igneous Prov-inces,Ó by Millard F. CoÛn and Olav Eld-

holm; SCIENTIFIC AMERICAN, October1993]. In particular, oceanic plateausresult from the initial massive, rapideruptions caused by these rising plumes.Such upwellings occasionally occur onthe continents where we can study themdirectly. Exotically named regions suchas the Paran� Basalts of Brazil, the Dec-can Traps of western India and the Si-berian Traps of northern Russia consistof vast Þelds of basalt ßows, severalhundred kilometers across and one ortwo kilometers thick [see ÒVolcanismat Rifts,Ó by Robert S. White and Dan P. McKenzie; SCIENTIFIC AMERICAN, July1989]. The oceanic plateaus are featuressimilar to their continental cousins, butthey are even larger. For instance, thelargest of the oceanic plateaus (the On-tong-Java Plateau of the western Pacif-ic) is estimated to be 25 times biggerthan the largest continental one (theDeccan Traps).

Seamount chains trail away from

oceanic plateaus and result from mate-rial behind and below the head of therising plume material. Because theplumes are relatively Þxed, and theoverlying tectonic plates drift horizon-tally on the surface of the planet, thesubsequent eruptions in the mountainchains record the motions of the plates.Thus, these seamount chains should beoldest next to their parent oceanic pla-teau and trace a path of younger andyounger seamounts that ends in an ac-tive volcano if the Òchimney pipeÓ tothe deep mantle is still alive. The bestknown of these seamount chains is theHawaiian Islands, which extends under-water far to the northwest of the is-lands themselves. Its rising plume sys-tem exists today below the island ofHawaii, where volcanic eruptions con-tinue to rumble. The islands and sea-mounts become successively older tothe northwest as they are rafted awayon the PaciÞc plate, which moves in anorthwesterly direction over a Þxedplume location.

Once I realized that the features ofthe ocean crust most aÝected by themid-Cretaceous volcanic activityÑsea-mount chains and plateausÑwere eachformed by plumes of mantle material, itwas a small logical step to suppose thatthe entire anomaly resulted from plumeactivity on a much larger than normalscale. Because I live in a superlative-prone society, I named this a Òsuper-plume episode.Ó The initial pulse of thesuperplume reached the earthÕs surfacearound 120 million years ago; the in-tense volcanic activity started suddenlyand continued through the mid-Creta-ceous, lasting tens of millions of years,gradually tapering oÝ after that.

Overheated Plumes

The superplume episode was mostlikely caused by the upwelling of one

or perhaps several enormous plumesthat ascended through the easily de-formed mantle, spread out at the baseof the earthÕs more rigid outer shell,known as the lithosphere, and eruptedonto the ocean ßoor. Although the Pa-ciÞc was most strongly aÝected, evi-dence of the superplume event is alsopresent in the Indian, South Atlanticand Caribbean oceans. The area of thePacific involved may have been severalthousand kilometers across, in sharpcontrast with the size of regions aÝect-ed by todayÕs plume activity, which areusually one tenth the size in area.

I suspect the overheated plumes risefrom the very base of the mantle andaÝect the process that causes reversalsof the earthÕs magnetic Þeld in the un-derlying outer core. There is a general


ÒMUDDY ROADÓ of the western PaciÞc seaßoor results from the intense volcanicactivity of the mid-Cretaceous superplume event, which produced randomly ori-ented plateaus and undersea mountain chains. The ocean ßoor of the easternPaciÞc, in contrast, shows the smooth, lineated physiography that is characteristicof crust formed by seaßoor spreading. (From World Ocean Floor Map, by Bruce C.Heezen and Marie Tharp, 1977.)

©19

77 B

Y M

AR

IE T

HA

RP.

RE

PR

OD

UC

ED

BY

PE

RM

ISS

ION

OF

MA

RIE

TH

AR

P.


inverse correlation between the produc-tion rate of crust formed by plumesand the frequency of reversals of theearthÕs magnetic Þeld. For example, dur-ing periods of intense plume activity,including during the mid-Cretaceous, al-most no magnetic reversals take place.Conversely, as is the case today, whenplume activity is low, magnetic reversalsoccur at record pace. How the earthÕsmagnetic Þeld actually reverses its po-larity is a mystery. Peter L. Olson ofJohns Hopkins University and I thinkthe correlation between crustal forma-tion and magnetic-Þeld reversals mayprovide a clue to understanding howthe reversals take place and to deter-mining the source of the mantle plumematerial. We believe an increase in theÒboiling rateÓ of the core somehow caus-es magnetic reversals to become moreinfrequent. Additionally, the connectionmay reveal information about the ad-vent of the next superplume.

Boiling iron within the outer core isalmost certainly the source of the earthÕsmagnetic Þeld. Such molten iron is anexcellent electrical conductor, and theconvective motion of the iron and itsassociated electrical Þeld generates theearthÕs magnetic Þeld. The heat givenoÝ by the molten iron percolatesthrough the core-mantle boundary, thelid to this boiling pot, by the process ofconduction. The heat becomes trappedjust above the boundary in the lower-most 100 to 200 kilometers of solid sil-icate rock of the mantle. This processcontinues until enough excess heat ac-cumulates. Then the buoyancy of theoverheated, less dense lower mantleovercomes the viscosity of the overly-ing cooler, more dense mantle rock.Huge plumes of mantle material risenearly 3,000 kilometers through themantle and eventually trigger volcaniceruptions at the surface. Ascending ma-terial removes heat from the lowermostmantle, allowing the outer core to boileven more vigorously than before.

Global EÝects

The most recent of these major over-turns erupted 120 to 125 million

years ago as the mid-Cretaceous super-plume episode. Much of the materialthat surfaced at this time left the Òmud-dy roadÓ eÝect seen today on the west-ern PaciÞc seaßoor. Such an episode thatdoubles the world-total rate of oceaniccrustal production in a short periodmust have staggering geologic conse-quences. The mid-Cretaceous was char-acterized by several profound anoma-lies resulting from the superplume.

First and probably least controversialis the rise in worldwide sea level to an

elevation 250 meters or so higher thanit is today. Assuming that the totalamount of seawater in the planetÕsoceans is constant, a rise in the level ofthe sea surface is simply a reßection ofa corresponding rise in the level of theseaßoor. Ocean above newly formedcrust is abnormally shallow because thecrust and underlying lithosphere arestill relatively warm, less dense andtherefore expanded. As the two cool,they contract, allowing the seaßoor todeepen. This phenomenon of expansionand contraction explains why oceanicridges, where new crust is being formed,are raised above the older, deeper crustfound on the ßanks. If an abnormalamount of new crust is formed rapid-lyÑas it was at the beginning of themid-Cretaceous pulseÑthen the aver-age seaßoor level will be elevated, andthe sea surface will rise accordingly. Inthe mid-Cretaceous, rising sea levelsdrowned much of what is dry land to-day; for example, my birthplace in Iowa

was then at the bottom of the ocean.When the water receded, it left depositsof limestone and chalk, including thefamous White CliÝs of Dover in England.

The earthÕs surface temperature alsoincreased as a result of the superplumeepisode. When molten lava erupts, it re-leases certain chemicals, including car-bon dioxide. Higher amounts of carbondioxide in the mid-Cretaceous atmo-sphere led to a natural greenhouse ef-fect that raised global temperatures byroughly 10 degrees Celsius. Studyingthe eÝects of elevated carbon dioxidelevels during this period could revealpossible scenarios for the earthÕs cli-mate in the future. Massive burning offossil fuels and large-scale deforestationcontinue to increase the level of carbondioxide in the modern atmosphere.

An excess amount of organic carbonand inorganic carbonate was also de-posited during the mid-Cretaceous. Theenhanced deposition is related to theelevations in sea level and air tempera-

SCIENTIFIC AMERICAN February 1995 85

GEOLOGIC CONSEQUENCES of the mid-Cretaceous superplume event include ris-ing surface temperature and sea level. The superplume itself can be seen by the in-crease in the world-total rate of oceanic crust production; it is particularly evidentin the rate of formation of oceanic plateaus and seamount chains. Additionally, re-versals of the earthÕs magnetic Þeld ceased during the superplume. At present, re-versals occur frequently, indicating that plume activity is low.

IAN

WO

RP

OLE

300

200

100

0

40

30

20

10

0

30

20

10

0

MILLIONS OF YEARS AGO150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0

150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0

150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0

150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0

NORMAL FIELD

DATA SET BDATA SET A

OCEANIC PLATEAUS AND SEAMOUNT CHAINS

REVERSED FIELD

HIGH-LATITUDE SURFACE TEMPERATURE (DEGREES CELSIUS)

SEA LEVEL (METERS)

OCEANIC CRUST PRODUCTION (MILLIONS OF CUBIC KILOMETERS PER MILLION YEARS)

MAGNETIC REVERSALS

MID-CRETACEOUS

WORLD TOTAL


ture, which, we have seen, result-ed from the superplume episode.Tiny plants and animals, knownas phytoplankton and zooplank-ton, make their living ßoating atshallow levels in the ocean wherelight can penetrate. Plankton ap-parently thrived during the mid-Cretaceous in the abnormallywarm oceans that accompaniedthe natural warming of the atmo-sphere. Normally, when these or-ganisms die, their bodies sink inthe deep sea and quickly dissolvebecause of the extreme pressureof the overlying seawater. Butduring the mid-Cretaceous, manyof the dead organisms fell insteadon the drowned continents. Thecarbon from the skeletons didnot dissolve in the shallow watersbut was preserved. Some of itformed the White CliÝs, and somewas buried more deeply andeventually turned to oil. The re-sulting oil constitutes up to 50percent of the worldÕs oil supply.Ironically, this outcome of themid-Cretaceous greenhouse eventmay have created the fuel for the nextgreenhouse episode.

Other geologic anomalies associatedwith the mid-Cretaceous superplumeinclude the placement of a very largepercentage of the earthÕs diamond de-posits. Diamonds are made of pure car-bon atoms, squashed into the tightest,densest conceivable packing order bypressures that exist at least 200 to 300kilometers below the earthÕs surface.Most diamonds are ancient even on ge-ologic timescales, having formed morethan a billion years ago, but, accordingto Stephen E. Haggerty of the Universi-ty of Massachusetts at Amherst, manyof them were brought to the surfaceduring the mid-Cretaceous. They weretransported up volcanic structurescalled kimberlite diamond pipes (aftera mining area in Kimberley, South Afri-ca) that extend deep down into thecrust and presumably into the uppermantle. The diamonds were probablytorn loose from their sources within themantle by rising plumes and broughtup in their solid, original state.

The formation of most of the moun-tain ranges that edge the western coastsof North and South America was strong-ly controlled by the superplume epi-sode. The Sierra Nevada Mountains ofwestern North America and the AndesMountains of western South Americawere created during the mid-Cretaceousby increased subduction of PaciÞc crustunderneath western North and SouthAmerica. Subduction occurs close to thecontinents when the oceanic lithosphere

is thrust below the adjacent landmassand recycled into the mantle below. Re-member that because of the proximityof the erupting plumes rates of seaßoorspreading in the PaciÞc increased dra-matically. What comes up must go downif the earthÕs diameter remains constant,so as production of ocean ßoor in-creased, so did subduction rates. Abnor-mally large amounts of oceanic crustwere thrust deep under the westerncoastlines of North and South America.As the crust and accompanying oceanicsediments sank several hundred kilo-meters below the earthÕs surface, theminerals with the lowest melting pointsbecame semiliquid as temperatures andpressures rose. Some of the adjacentcontinental crust also melted from fric-tional heating. This molten rock combi-nation rose back to near the surface asits density lessened and then solidiÞedto form the granite cores of the moun-tain ranges that are the spine of thewest coast of the Americas.

The Next Pulse

Much of the earthÕs geologic historyis controlled by events that have

their true origins deep within the planet,some 3,000 kilometers below our feet.The story presented here hints at onlya few of the dynamic processes and in-teractions that cause material from thecore-mantle boundary to rise in sporad-ic pulses and aÝect the surface environ-ment. The most recent of these burstsdramatically altered the terrestrial cli-

mate, surface structure, and fos-sil-fuel and mineral supply.

We are only now beginning toconsider the evidence for previ-ous superplume episodes, and anargument rages concerning therate of the earthÕs past Òheart-beat.Ó Our planet has clearly set-tled down from the eÝects of themost recent superplume event,but when the next episode willoccur is a matter of speculation.During the past 40 million years,creation of oceanic plateaus andseamount chains has proceededat a much slower pace. Sea levelhas dropped to a near-record low.Because we are now living at apause in the middle of an ice age,global temperature today couldbe described more reasonably asthat of an Òice houseÓ instead ofa greenhouse. As expected duringperiods of little plume activity, theearthÕs magnetic Þeld is reversingmore frequently than ever, and alarge temperature anomaly existsat the base of the mantle, indicat-ing a slow boiling rate in the core

below. Current estimates are that tem-perature increases by 1,000 to 1,500degrees C through the lowermost 100to 200 kilometers of the mantle.

It has been 120 million years since thelast superplume event, and it is merelya matter of time before the next one.Yet we cannot predict when it will hap-pen. As scientists, we are in a situationsomewhat similar to that of a farmertrying to forecast the coming of springwhen the groundhog does not see itsshadow on the second of February. Wecan only say that the next superplumeepisode is Òjust around the corner.Ó


FURTHER READING

THE MID-CRETACEOUS SUPER PLUME,CARBON DIOXIDE, AND GLOBAL WARM-ING. Ken Caldeira and Michael R. Rampi-no in Geophysical Research Letters, Vol.18, No. 6, pages 987Ð990; June 1991.

A SUPERPLUME IN THE MANTLE. K. G. Coxin Nature, Vol. 352, No. 6336, pages564Ð565; August 15, 1991.

SUPERPLUMES AND SUPERCHRONS. MikeFuller and Robin Weeks in Nature, Vol.356, No. 6364, pages 16Ð17; March 5,1992.

THE MESOZOIC PACIFIC: GEOLOGY, TEC-TONICS, AND VOLCANISM. Edited by Mal-colm S. Pringle, William W. Sager, Wil-liam V. Sliter and Seth Stein. AmericanGeophysical Union, 1993.

SUPERKIMBERLITES: A GEODYNAMIC DIA-MOND WINDOW TO THE EARTHÕS CORE.Stephen E. Haggerty in Earth and Plane-tary Science Letters, Vol. 122, No. 1Ð2,pages 57Ð69; March 1994.

KIMBERLITE DIAMONDS, such as these from WestAfrica, were transported to the earthÕs surface dur-ing the mid-Cretaceous superplume episode.

SA

M O

GD

EN


the mid-cretaceous superplume episode

Documents