Hotspot or Not Spot?4•

T S O U N D S W E I R D TO S AY Gabi Laske’s science group wishes for earthquakes.

But it is the truth. Themembers of the Plume-Lithosphere Undersea MeltExperiment (PLUME), ofwhich Laske is chief scien-tist, spent a good portion of2005 hoping for some good,yet not particularly destruc-tive, quakes to provide rawdata to the grid of instru-ments they placed in thewaters around Hawaii.Laske, a Scripps Institutionof Oceanography geophysi-cist, is leading PLUMEthrough a second phase offieldwork in an endeavor tostudy the shape and originof the mantle plumebeneath Hawaii. This bur-bling cauldron of liquid rockcreated the Hawaiian islandchain and it continues to fuel

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S C R I P P S I N S T I T U T I O N O F O C E A N O G R A P H Y

The icon of the world’s most active

volcanic system, Hawaii’s Kilauea

volcano gets its energy from sources

in Earth’s mantle but the path magma

takes to the surface is unknown.

B Y R O B E R T M O N R O E

Spot?Where does Hawaii’s explosive volcanism come from?

I

6•

Left, Geophysicist Gabi

Laske, flanked in a Scripps

lab by seismometers used

in the PLUME project, uses

seismic waves to resolve

the architecture of the vol-

canic plumbing system

beneath Hawaii.

the extensive volcanism on the“Big Island” of Hawaii. It isrelentlessly building up seamountsthat will someday join the island chain.

But the prevailing answer for howforces from the mantle push throughEarth’s crust to create the chain—anexplanation that has sufficed fordecades—is more andmore seeming a littletoo pat. Until nowthere had been littlebeyond theories, com-puter models, andpatchy data to shore upor disprove what’sknown as hotspot theo-ry. PLUME promisesto provide a break-through by creatingthe first-ever high-resolution images ofdeep earth layers beneath the Hawaiianislands.

Initial results from PLUME’s firstphase already challenge the narrative ofHawaiian volcanism. In the end, theeffort could have a societal as well asscientific payoff. In finding the originand pathways of Hawaii’s volcanism,PLUME stands to help researchers bet-ter understand tectonic hazards such asundersea landslides and identify areas ofthe island chain still vulnerable to erup-tions. At another deeper level, PLUMEintends to resolve one of the most fun-damental unknowns of geology—howEarth loses its heat—via this one-of-a-kind look at one of its main releasepoints.

“This is a really important dataset,” said Hubert Staudigel, a Scrippsgeologist who studies Pacific Ocean vol-canoes but is not affiliated with the

PLUME project. “For the first time we’re looking in great detailat the root zones of this volcano system.”

But creation of a data set requires data. Thus for another ninemonths or so, while an array of submerged seismometers records away,the PLUME team will continue what it’s been doing—crunchingnumbers and quietly hoping for the Earth to move.

H O T S P O T S R E D U X

Around the world, most volcanoes are found in the places wheremost earthquakes happen, at the boundaries of tectonic plates.Not all follow the pattern though. In 1963, Canadian geophysicistJ. Tuzo Wilson came up with the hotspot theory as a way to explainwhy certain locations in the world featured volcanic activitydespite not being located near such boundary regions.

Wilson proposed that certain volcanic chains come into beingwhen tectonic plates pass over especially hot mantle regions thatlast for great periods of time in fixed locations. The aggregation ofhot rock pushes magma through the crust and up to the seafloor,an action dramatically manifested by the Hawaiian islands, theworld’s most productive volcanic system. As the Pacific Platemoves over this hotspot at a rate of seven to nine centimeters (2.8

to 3.5 inches) per year, the plume has enoughtime to pile up a sufficient amount of magmato create volcanic mountains.

In assembly line fashion, the hotspothas created a chain of peaks headingnorthwest in a neat progression, making itan ideal test site for Wilson’s theory. TheHawaiian chain includes seamounts thatnever reached the surface as well as thevolcanic peaks that make up America’s50th state. It also includes seamountsyet to break the surface like Loihi, tothe southeast of Hawaii. Scientistsanticipate Loihi breaking the surfacebetween 10,000 and 100,000 years fromnow and, no doubt, seeing the develop-ment of timeshares and golf courses soonafter. The entire Hawaiian chain restsupon a 2,000-kilometer (1,240-mile) -longundersea plateau, known as a swell. Wilsonnoted with general veracity that the fartheraway the islands were from the hotspot, theolder they were.

7•

Above, Back from the depths, ocean-bottom seismometer packages await

a trip back to Scripps after a year-long deployment on the seafloor.

TO THE BOTTOM

OF THINGS The apparent shift in the plume head led PLUME investigator

Gabi Laske to hypothesize that perhaps several fingerlike

pathways (above) rather than one main chamber feed

Hawaii’s volcanoes. If that’s true, places other than the island

of Hawaii could be susceptible to volcanic activity. Data

gathered during a 2005 deployment of seismometers (yel-

low icons) and a second round taking place now (red

icons) could lead to an answer.

S C R I P P S I N S T I T U T I O N O F O C E A N O G R A P H Y

Where is the source of Hawaii’s volcanic activity? To answer

this, the PLUME project uses seismic waves to create an image

of a plume head, a large chamber in Earth’s mantle from which

volcanoes on Hawaii and the Loihi seamount draw magma.

The researchers assumed the plume head (above) would be

located near observed volcanic activity, but they have not been

able to place it there during a preliminary analysis. Rather, they

find unusual crust between Hawaii and Maui.

Maui

Hawaii

8•

Over the years, a few incon-sistencies in Wilson’s hotspot theo-ry have been noted. Some volcanicmaterial on Maui, for example, isyounger than some material onHawaii though Maui is fartheralong the chain. Still, to Laske andmany others in the geophysicalcommunity, it stood to reason thatthe feeder to which the islandchain owes its existence should bea distinguishable entity and that itshould be to the southeast ofHawaii, in rough proximity to thatisland’s visible volcanic activity.

The task of PLUME was to find it. To do so, PLUME scientists,

who come from five research insti-tut ions around the countrydeployed a network of seismome-ters around the Big Island inJanuary 2005 to create a tomo-graphic view of the plume head’sdimensions. The instrumentsreceive seismic waves that resonatethrough the mantle and crust. Thismeans of data recovery is similar tothe way a CT scan passes X-raysthrough the body to create animage of the tissue through which

the waves pass. For the kinds of imagery that

interest geoscientists, earthquakesare needed to produce the rawwave energy that would producerecognizable signals. The biggerthe shake, the better. Luckily, 2005was, as co-principal investigatorCecily Wolfe said, “a spectacularyear for earthquakes,” featuringmagnitude 6.7 and 7.2 temblorsalong the Gorda Ridge off theOregon coast.

“Those are wonderful earth-quakes because they’re offshore

9•

Opposite, Researchers

retrieve a numbered seis-

mometer and the rest of

its instrument package

during a January 2006

cruise. Left and below,

Kilauea flows have been

adding to Hawaii’s land-

mass since its most recent

eruption in 1983, eliciting

a corresponding flow of

visitors.

--

Two camps in the world of geophysics dispute the depth at which

volcanic activity originates. By being able to image depths as

great as 1,000 kilometers (620 miles) into Earth’s mantle, PLUME’s

1 million-square-kilometer (386,100-square-mile)-seismometer

array should resolve the issue. If the source turns out to be deep,

then upper and lower rings of Earth’s mantle probably produce

convection in concert with each other. If the source is shallow, it

means the two layers could function more independently of each

other, a conclusion that could diminish support for some tenets of

plate tectonics theory.

S C R I P P S I N S T I T U T I O N O F O C E A N O G R A P H Y

and they don’t hurt anyone but they happenat key azimuths,” Wolfe said.

The 35 seismometers simultaneouslyrecorded these seismic events. The relativedistance of the seismometers from a quakeepicenter determines when they receivethe signal in part but not entirely. The otherinfluencing factor is the composition of therock through which those waves are pass-ing. Waves travel slower through areas ofhot liquid rock seething with volcanic activ-ity and faster through solid, cold areaswhere there is no activity. The researchersrecovered the seismometers in January 2006after 12 months of data-gathering on theseafloor. The recorded speed of the wavesshould have produced a three-dimensionalimage of the plume head somewhere to thesoutheast of the island of Hawaii.

Instead there was nothing there.

Seismometer Array

Crust

Upper Mantle

Lower Mantle

Core–Mantle Boundary

Plume Head

Plume Conduit

10•

M I S S I N G M A G M A

Laske isn’t yet ready to unequivocal-ly report that her plume head hasgone missing. The evidence that itisn’t where it’s supposed to be is verypreliminary, she cautions, but if itholds, it must give rise to alternatetheories.

It is possible that the plumehead feeding Hawaii’s volcanoes islocated in a different place altogether.The waters between Maui and theBig Island are the most likely locale,Laske said. It is also possible thatthere is no plume head at all, that thesource of magma isn’t a dome but anetwork of harder-to-see conduitslurking below the islands like fingers.

Both hypotheses create theirown problems. The northwesterlymovement of the Hawaiian chain isnot in question, so how could a plumehead in Maui feed Loihi, which is 250kilometers (155 miles) away in thewrong direction. Any theory aboutfingers raises similar questions.

“You still have to find a plumb-ing system that has a plume here,”

Left, Laske and co-investigator John Collins of Woods Hole

Oceanographic Institution set out past Honolulu’s Diamond Head land-

mark. Above, Maui’s Haleakala volcano, quiet for 400 years and

counting. Opposite, Woods Hole’s Victor Bender, John Collins and

Ken Peal examine an OBS unit after it spent a year on the seafloor.

S C R I P P S I N S T I T U T I O N O F O C E A N O G R A P H Y

said Laske, pointing to the strait between the BigIsland and Maui, “and fingers that go out to Loihi.”

One possible explanation is that the plumehead is less like a narrow chimney focusing hot rockon specific areas of the surface and more like abroad pot of boiling pasta sauce, its seething bub-bles capable of popping up anywhere over awide surface. In this case the pot could encom-pass Maui and Hawaii. Another possibility is

that the kind of hot region of rock that PLUMEcould image seismically has been dragged to Mauiover millions of years by the moving crust, leavingbehind volcanic spots like the Big Island’s Kilaueavolcano as kinds of skidmarks left behind.

“In essence, one explanation could be thatwhat we see in the seismic images is the still-

cooling plumehead dragged along fromwhen Maui was where Kilauea is now. It

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is, so to speak, a snapshot into thepast and not what’s happeningtoday,” Laske said.

Both theories have otherimplications as well. One ofthose many fingers could beunder Maui just waiting to reac-tivate and punch through. It’sprobably been at least 400 yearssince Maui’s Haleakala volcanoerupted but the island’s time as avolcanically active one is almostcertainly not over.

“People have the perceptionthat volcanic activity on Mauiis totally extinct but that is amisconception,” Laske said.“Sometimes we need to becareful saying it will neverhappen again.”

A D E E P E R I S S U E

Despite the shudder itmight send throughMaui’s real estatecommunity, thehunt for themissing plumehead is but onecomponent o fPLUME. More sig-nificant breakthroughsare expected to come fromthe data collected in the field-work phase currently under way.

The seismometer arraystretches over 1 million squarekilometers (386,100 square miles)of ocean and encompasses nearlythe entire state of Hawaii. It isgeographically the largest deploy-ment of ocean-bottom seismome-

ters ever. A case can be made that it is the largest seismicarray ever assembled on land or at sea.

The wide aperture of the array allows for a corre-spondingly deep view into the mantle, with the seis-

mometers seeing 1,000 kilometers (620 miles) into theEarth. That depth will allow the PLUME team to see beyond

the boundary separating the lower mantle from the upper mantle,the two concentric rings between Earth’s core and its crust. The transitionbetween the two mantle layers is 660 kilometers (410 miles) deep.

PLUME researchers will be looking for seismic evidence that theplume stoking Hawaii’s volcanoes either originates at the greatestdepths of the lower mantle or that the plume begins at a shallowerpoint nearer to the crust.

If plumes originate at shallow points, the conclusion could changescientists’s views of how heat escapes from Earth and also contradict asupporting tenet of plate tectonics theory. Like the plates themselves, tec-tonics theory rests in part on a state-shifting notion that heat travels fromthe core to the surface through one big process of convection with the twomantle layers acting in concert. Plate tectonic theory assumes that processto be the more likely explanation of mantle dynamics because it wouldbetter explain the origin the energy that drives plates into each other and

Right, Laske oversees deploy-

ment of OBS units. Top,

Seismometers arranged for a

return to the lab.

13•

causes earthquakes.Because if the other scenario

is true and there are two distinctlayers of geological activity occur-ring in the mantle that are inde-pendent of each other, the energythat would seem to be needed todrive tectonic plates into eachother with enough force to causeearthquakes would be harder toaccount for. Instead, Earth wouldbe losing its heat through a very,very slow process of conduction,like the burners of a hot stoveheating up a cold frying pan.

Once the data are collected,Laske thinks a definitive answercould be ready within two years.Whatever i t s conc lus ions ,PLUME results will end a frac-tious debate that has sprung upbetween two groups of geophysi-cists in recent years. It will givedirection to one branch of geo-science that is waiting at a cross-roads for more data to work with.For researchers like Wolfe, wholives in Hawaii, it gives a causefor reflection as she contem-plates the ground she walks on.

“What we’re really lookingfor is the fundamental processbehind volcanism,” Wolfe said.“I have my state of Hawaii but Idon’t have a handle on what cre-ated it”

V E N T H O U G H T H E Plume-Lithosphere Undersea

Melt Experiment won a federal research grant in

2000, the project team had to wait five years to launch

field work all because of a seemingly mundane part of

the science package—batteries.

The project researchers needed ocean-botton

seismometers to record data over 12 months. That

meant finding batteries that could keep data loggers

and other components of the instrument packages

running that long. Such battery packs didn’t exist when

PLUME was greenlighted.

“It was a real problem for us until at least 2003,”

said Laske.

I n a cont inu ing game o f m in i a tu r i z a t ion ,

researchers at Scripps and other research centers

eventually succeeded in reducing the power draw

required by such instrument packages to the point

where industrial-grade batteries could provide a year’s

worth of charge. Each pack contains a brick of 12 DD-

cell lithium batteries. There are eight bricks per seis-

mometer package, each of which requires less power

than a flashlight bulb.

Wrapped in black plastic, a trio of battery packs powers a data logger.

S C R I P P S I N S T I T U T I O N O F O C E A N O G R A P H Y

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