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Research Notes

Beneath the Earth's SurfaceBiologyClimate ChangeCoastal ZonesHuman healthOcean physicsPolar ScienceSatellite Remote SensingSolar System Rese arch

Students in the Field

Crab Pots and Ocean Obse rvingMicrobes in HydratesMovement of Arctic Water Mass esSilicon Cycle VariabilityZooplankton Discoveries

Inside CO ASClare Reimers, 2009 AGU FellowCharlie Miller, A SelectiveBiographyCOAS 50th Anniversary

On the Seamount Trail:

Underwater Volcanoes Hold Keys to

Plate Movements and Earth'sMantle Convection

Ancient remnants of volcanoes rise up from the seafloorin the thousandsthere are an estimated 50,000 in thePacific Ocean alone. These underwater volcanoes, orseamounts, tell us about the past 100-200 million yearsof our dynamic planet.

Continents and the seafloor are part of tectonic platesthat move over time. Most earthquakes and volcaniceruptions happen near plate boundaries, where theseplates are formed or collide. However, in 1963, J. TuzoWilson theorized that volcanic islands distant from plateboundaries form along "hotspots" in the Earth's crust,where plumes of mantle material rise from deep in the

Earth until they reach the bottom of the tectonicplates, where they partially melt and erupt on theseafloor. In this theory, as a plate moves over themantle, the hotspots create a series of volcanic islandsand seamounts. Islands farthest from the hotspot arethe oldest; those nearest are the youngest. The theorymatched the dated ages of the Hawaiian Island-Emperorseamounts.

Controversy

The Hawaiian model became widely accepted. Twentyyears ago, most scientists believed that mantle plumehotspots were fixed spots on the Earth's crust.However, the reality has turned out to be more complex,and researchers are questioning past assumptions. Now,some scientists believe plumes might be moving, while

others think there are no mantle plumes at allthatcracks in the oceanic crust can generate enough spacefor magma to come up and form a volcano.

Knowing whether and how mantle plumes move can tellresearchers about what is happening deep in the Earth.Seamount ages are key in helping scientists create pastplate reconstructions. These reconstructions tell us howplates are moving or where a certain island or seamountwas through time. The resulting plate motion modelshave important implications for other scientists, fromgeoscientists to biologists looking at biodiversity.

Koppers' Research

Anthony Koppers, associate professor in marine geology

and geophysics, does not count himself as an adherentof one theory or another. For 15 years, he has studiedseamounts, including Samoa, the Line Islands, Louisville,and the oldest seamount trails that are located in theWest Pacific. He has published several papers thatsuggest more of a mantle-plume explanation and otherpapers suggesting more of a cracking-of-the-plateexplanation. He finds that seamount trails cannot beunderstood entirely by applying a single unifying theory.

There is a very limited database of good ages forseamounts. Less than 1% of seamounts have beentested. Improvement of analysis techniques means thatsamples taken more than 10 or 15 years ago are not ofthe same quality (precision and accuracy) as analysesdone today. Within the limited data set, Koppers saysthat it is possible to see evidence for both the hotspotand cracking hypotheses, and believes that the truthwill end up being somewhere in the middle.

Line Islands. Koppers has finished dating work on theLine Islands in the southwest Pacific Ocean; theseislands are as long and continuous as Hawaii. However,the ages of samples collected from the seamounts show

Anthony Koppers use s a rgon-argongeochronology to date samples taken

from underwater volcanoes.

Resea rch abo ard the 2005 cruise to dateSamoan seamounts. Samples taken fromthe base of seamounts underwater weresharply different from rocks sampled onthe island surfaces.

The Lo uisville seam ount trail. Map fromEarthRef.org database.

Resea rch ship at the b eginning of the site

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no systematic age progression, as is the case forHawaii. The new data suggest that volcanism happenedin three distinct episodes, occurring more or less alongthe entire 4,000 kilometer-long chain of seamounts.Because of that, the Line Islands remain an enigma inunderstanding these kinds of volcanism.

Samoa. Koppers is publishing findings on his work inSamoa. Past age data of samples from the Samoanislands did not fit a simple hot-spot model prediction.However, the rocks that were sampled were taken fromthe surface of the islands, not from its flanks, whichreside as deep as 4-5 kilometers under water.A 2005 research cruise did sample deep under water, atthe base of the volcanoes, and measured the ages ofthose rocks. The results were the same that would bepredicted with a mantle plume and the Pacific Platemoving over it. Koppers thinks that the island surface isa veneer of volcanic material formed much later,probably by a different process, for example, a crackrelated to the subduction of the Pacific Plate in theTonga Trench that is located only a few hundredkilometers to the south of Samoa.

Louisville Seamount Trail. Louisville is a seamount trailsouth of the Equator between 30 and 40 S and islong and continuous, like Hawaii. Koppers completed asix-week site survey of 25 seamounts to prepare for an

IODP (International Ocean Drilling Program) drilling leg atthe Louisville seamount trail. The team collected dataon the st ructure of the seamounts to pinpointappropriate drilling sites. They also dredged for rocksamples to be used for 40Ar/39Ar age dating.At Louisville, Koppers wants to repeat an experimentdone by Bob Duncan of COAS on an IODP cruise to theEmperor seamounts, part of the Hawaiian trail. WhenDuncan measured the ages of the rocks and thelocation of their magnetic North Poles (as locked intothe rocks at t he time of formation), he discovered alarge shift in paleolatitude (position of the hotspot). Ifthe mantle plume was fixed, there would be no suchshift, but one would expect that the paleolatitudesmeasured for each seamount would be similar to the

current latitude of the Big Island of Hawaii. From thatobservation, Duncan concluded that the mantle plumecould not be fixed, but is moving around over time.Koppers is planning to also look at the magnetics in therocks of the Louisville seamounts, to see if that plume isfixed or moving. If the plume is moving, he wants tocompare its movement to that of the Hawaiian plume.

More information:

Anthony Koppers, personal page

Noble Gas Mass Spectrometry Lab

EarthRef.org Database, seamount catalog.Koppers created the databases over time, through

National Science Foundation grants. He hasserved as webmaster since its inception.

u v y x u vseamount trail, January 2006.

Sample from seamount along theLouisville Ridge .

Heavy seas during som e of the sam pling.

College of Oceanic and Atmospheric SciencesOregon State University104 COAS Administration BuildingCo rvallis, O R 97331-5503

Telephone: (541) 737-3504Fax: (541) 737-2064

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