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On the Seamount Trail:Underwater Volcanoes Hold Keys toPlate Movements and Earth's

Mantle Convection

Ancient remnants of volcanoes rise up from the seafloor in thethousandsthere are an estimated 50,000 in the Pacific Oceanalone. These underwater volcanoes, or seamounts, tell us aboutthe past 100-200 million years of our dynamic planet.

Continents and the seafloor are part of tectonic plates that moveover time. Most earthquakes and volcanic eruptions happen nearplate boundaries, where these plates are formed or collide.However, in 1963, J. Tuzo Wilson theorized that volcanic islandsdistant from plate boundaries form along "hotspots" in the Earth'scrust, where plumes of mantle material rise from deep in theEarth until they reach the bottom of the tectonic plates, wherethey partially melt and erupt on the seafloor. In this theory, as aplate moves over the mantle, the hotspots create a series ofvolcanic islands and seamounts. Islands farthest from thehotspot are the oldest; those nearest are the youngest. Thetheory matched the dated ages of the Hawaiian Island-Emperorseamounts.

Controversy

The Hawaiian model became widely accepted. Twenty years ago,most scientists believed that mantle plume hotspots were fixedspots on the Earth's crust. However, the reality has turned out tobe more complex, and researchers are questioning pastassumptions. Now, some scientists believe plumes might bemoving, while others think there are no mantle plumes at allthat cracks 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 past platereconstructions. These reconstructions tell us how plates aremoving or where a certain island or seamount was through time.The resulting plate motion models have important implications forother scientists, from geoscientists to biologists looking atbiodiversity.

Koppers' Research

Anthony Koppers, associate professor in marine geology andgeophysics, does not count himself as an adherent of one theoryor another. For 15 years, he has studied seamounts, includingSamoa, the Line Islands, Louisville, and the oldest seamounttrails that are located in the West Pacific. He has publishedseveral papers that suggest more of a mantle-plume explanation

and other papers suggesting more of a cracking-of-the-plateexplanation. He finds that seamount trails cannot be understoodentirely by applying a single unifying theory.

There is a very limited database of good ages for seamounts.Less than 1% of seamounts have been tested. Improvement ofanalysis techniques means that samples taken more than 10 or15 years ago are not of the same quality (precision andaccuracy) as analyses done today. Within the limited data set,Koppers says that it is possible to see evidence for both thehotspot and cracking hypotheses, and believes that the truth willend up being somewhere in the middle.

Line Islands. Koppers has finished dating work on the LineIslands in the southwest Pacific Ocean; these islands are aslong and continuous as Hawaii. However, the ages of samplescollected from the seamounts show no systematic ageprogression, as is the case for Hawaii. The new data suggestthat volcanism happened in three distinct episodes, occurringmore or less along the entire 4,000 kilometer-long chain ofseamounts. Because of that, the Line Islands remain an enigmain understanding these kinds of volcanism.

Anthony Koppers uses argon-argon geochronology

to date samples taken from underwater volcanoes.

Research aboard the 2005 cruise to date Samoan

seamounts. Samples taken from the base ofseamounts underwater were sharply different fromrocks sampled on the island surfaces.

The Louisville seamount trail. Map fromEarthRef.org database.

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Samoa. Koppers is publishing findings on his work in Samoa.Past age data of samples from the Samoan islands did not fit asimple hot-spot model prediction. However, the rocks that weresampled were taken from the surface of the islands, not from itsflanks, which reside as deep as 4-5 kilometers under water.

A 2005 research cruise did sample deep under water, at the baseof the volcanoes, and measured the ages of those rocks. Theresults were the same that would be predicted with a mantleplume and the Pacific Plate moving over it. Koppers thinks that

the island surface is a veneer of volcanic material formed muchlater, probably by a different process, for example, a crackrelated to the subduction of the Pacific Plate in the Tonga Trenchthat is located only a few hundred kilometers to the south ofSamoa.

Louisville Seamount Trail. Louisville is a seamount trail southof the Equator between 30 and 40 S and is long andcontinuous, like Hawaii. Koppers completed a six-week sitesurvey of 25 seamounts to prepare for an IODP (InternationalOcean Drilling Program) drilling leg at the Louisville seamounttrail. The team collected data on the structure of the seamountsto pinpoint appropriate drilling sites. They also dredged for rocksamples to be used for 40Ar/39Ar age dating.

At Louisville, Koppers wants to repeat an experiment done byBob Duncan of COAS on an IODP cruise to the Emperorseamounts, part of the Hawaiian trail. When Duncan measuredthe ages of the rocks and the location of their magnetic NorthPoles (as locked into the rocks at the time of formation), hediscovered a large shift in paleolatitude (position of the hotspot).If the mantle plume was fixed, there would be no such shift, butone would expect that the paleolatitudes measured for eachseamount would be similar to the current latitude of the BigIsland of Hawaii. From that observation, Duncan concluded thatthe mantle plume could not be fixed, but is moving around overtime.

Koppers is planning to also look at the magnetics in the rocks ofthe Louisville seamounts, to see if that plume is fixed or moving.If the plume is moving, he wants to compare its movement to thatof 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 has served as webmastersince its inception.

Research ship at the beginning of the site surveyexpedition of the Louisville seamount trail,January 2006.

Sample from seamount along the LouisvilleRidge.

Heavy seas during some of the sampling.

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