A hotspot created the island of Iceland and its characteristic volcanic landscape. Hitting the Hotspots Hotspots are rela- tively small regions on the earth where New Studies Reveal Critical Interactions unusually hot rocks rise from deep inside Between Hotspots and Mid-Ocean Ridges the mantle layer. Jian Lin Associate Scientist, Geology & Geophysics Department

he great volcanic mid-ocean ridge system hotspots may play a critical role in shaping the stretches continuously around the globe for seafloor—acting in some cases as strategically T 60,000 kilometers, nearly all of it hidden positioned supply stations that fuel the lengthy beneath the world’s oceans. In some places, how- mid-ocean ridges with magma. ever, mid-ocean ridge volcanoes are so massive that Studies of ridge-hotspot interactions received a they emerge above sea level to create some of the major boost in 1995 when the US Navy declassified most spectacular islands on our planet. Iceland, the gravity data from its Geosat satellite, which flew Azores, and the Galápagos are examples of these from 1985 to 1990. The satellite recorded in unprec- “hotspot” islands—so named because they are edented detail the height of the ocean surface. With believed to form above small regions scattered accuracy within 5 centimeters, it revealed small around the earth where unusually hot rocks rise bumps and dips created by the gravitational pull of from deep inside the mantle layer. dense underwater mountains and valleys. Research- But hotspots may not be such isolated phenom- ers often use precise gravity measurements to probe ena. Exciting advances in satellite oceanography, unseen materials below the ocean floor. In places seismology, geochemistry, and geodynamics, along where the seafloor contour has been well-charted by with a treasure trove of new declassified data, are ship surveys, we can employ modeling to remove revealing that hotspots appear to have important the gravitational effects of seawater and the sea- and far-reaching impacts on a surprisingly large floor. The leftover signal, called the Bouguer percentage of the global ridge system. Some 44 anomaly, reveals information about the rocks be- hotspots have been identified around the globe, and neath the ridges and hotspots. Using this tech- a large number of them are integrally connected to nique, we have detected unusually thick, hot crust the ridge system (see map at right). Indeed, these and mantle rocks beneath virtually all major

34 • Vol. 41, No. 2 • 1998 hotspots located near ridges, including Iceland and By measuring the precise travel time of seismic By measuring the Azores in the northern Atlantic Ocean, Tristan waves passing through the mantle rocks under precise travel times of seismic waves de Cunha in the southern Atlantic Ocean, the Iceland, researchers have recently identified a sur- passing through Galápagos and Easter Islands in the Pacific Ocean, prisingly narrow cylindrical “root” of anomalously mantle rocks beneath and the Marion and Bouvet hotspots in the hot rocks extending to at least 400 kilometers Iceland, scientists southwest Indian Ocean. beneath Iceland (see figure at have recently identi- Beneath the earth’s thin left). Below 400 kilometers, fied a surprisingly narrow and deep outer skin (called the lithos- seismic data cannot be easily cylindrical “root” of phere), mantle rocks creep ➤ gleaned, but more indirect anomalously hot plastically in a layer known as seismic evidence suggests that rocks, extending to a the asthenosphere, where this narrow root, which is depth of at least 400 temperatures stay near the about the width of Iceland, kilometers, and probably farther. rocks’ melting point. Below the 400 kilometers may be underlain by a region of From: Wolfe, Bjarnason, mid-ocean ridges, mantle rocks anomalously hot mantle as VanDecar, and Solomon, 1996. rise to fill the gap between two deep as 660 kilometers. Theo- separating plates. As they as- retical geodynamic models show cend, some mantle rocks liquefy that such a narrow hotspot root to form basaltic magmas, or would generate a vigorous

melts, and the buoyant magmas ➤ source of heat, providing a huge float to the top of the mantle to form volume of lava to build and feed oceanic crust. Iceland’s volcanic landscape. Even By studying the chemical composition of rocks more dramatically, this heat source dredged from the ocean floor, researchers have would also cause mantle rocks to migrate laterally A map of the world’s determined that most melts are probably produced in the asthenosphere hundreds of kilometers out major hotspots shows at depths of 20 to 80 kilometers and at tempera- from Iceland (see figure on page 36). that many of them are

Jian Lin tures of 1,150° to 1,400°C. Below a hotspot, however, The Geosat data show that in the cases of Ice- integrally connected to the global mid- geochemical evidence shows that mantle rocks may land and the Azores, a bulge of unusually thick and ocean ridge system start melting at greater depths and higher tempera- elevated crust extends from the two hotspots in (red lines). Green tures, giving rise to voluminous lavas and melts that both directions along the Mid-Atlantic Ridge. This lines indicate sub- solidify to form shallow ridges such as the region of thick, hot crust and mantle extends 1,300 duction zones, where Reykjanes Ridge near Iceland, underwater volcanic kilometers north and south of the Iceland hotspot plates plunge back into the mantle. Not plateaus such as the vast Kerguelen Plateau in the and some 1,000 kilometers north and south of the shown is the Jan southern ocean, hotspot islands such as Hawaii, and Azores hotspot. Together, the two hotspots appear Mayen hotspot north smaller submerged volcanoes called seamounts. to be feeding a huge supply of magma to nearly the of Iceland.

IcelandIceland

Bowie

Cobb YellowYellowstonestone Azores Bermuda Meteor Canary Hawaii Hoggar Cape Verde Afar Caroline Galapagos Fernando Comores Samoa Marquesas Ascension Society St. Helena Pitcairn Trinidade Reunion Easter San Felix Macdonald Juan Fernandez St. Paul E. Australia Tristan St. Paul Amsterdam Tasmantid Gough Discovery Amsterdam Tasmantid Discovery Crozet Kerguelen Marion Louisville Heard Shona Heard Bouvet

Balleny Progam Joint gen, MIT/WHOI

Jennifer Geor

OCEANUS • 35 Recently declassified 70˚N question currently under satellite gravity data investigation. reveal bulges of In the far southern unusually thick and elevated oceanic crust Atlantic, a string of (red, yellow and green hotspots—Tristan, on the map) extend- Gough, Discovery and ing hundreds of Shona—align with the kilometers from the ICELANDICELAND southern Mid-Atlantic Iceland and Azores 60˚N hotspots. The two Ridge, and may be con- hotspots may be tributing magma sup- feeding huge supplies plies that maintain the of magma to nearly ridge (see map opposite). the entire northern Just to the east of the

segment of the Mid-

E Atlantic Ridge. E Shona hotspot lies the

50˚N

G Map produced by Jennifer Georgen, G Bouvet hotspot, whose

MIT/WHOI Joint Program, with D data from of David Sandwell, D geological importance

I I Scripps Institution of Oceanogra- phy, and Walter Smith, NOAA. RR may go far beyond cre-

C C ation of the tiny, remote I I island of Bouvet. Re- 40˚N TT N searchers speculate that A LL the hotspot may play a A T AZORES -- critical role in maintain- DD ing the triple junction, 30˚N II M where the mid-ocean ridge branches that define the boundaries of 20˚N the South American, African, and Antarctic plates all intersect. Far- ther east, the promi- 10˚N nence of the Kerguelen, Crozet, and Marion hotspots suggests that 0˚ ridge-hotspot interac- tions have helped shape the seafloor of the

10˚S southern ocean. 70˚W 60˚W 50˚W 40˚W 30˚W 20˚W 10˚W 0˚ In the western Pa- cific, our studies show a -100 -80 -60 -40 -20 0 20 40 60 80 complex pattern of FAA (mGal) interaction between the entire northern segment of the Mid-Atlantic Ridge. Galápagos hotspot and the Cocos-Nazca Ridge, the These hotspots, like others, don’t seem to provide a boundary between the Cocos and Nazca plates. steady supply of magma, but rather periodic surges. About 8 million years ago, the ridge and hotspot What regulates this episodicity is an intriguing collided and the hotspot-induced melt flux was A theoretical estimated to be very geodynamic model Hotspot Along Ridge Axis (kilometers) 0 400 800 robust. But comparisons shows that a narrow Across Ridge Axis (kilometers) hotspot “root,” such 800 400 of older and younger as the one recently 1500 crust in the region reveal 1400

discovered beneath C) ° 1300 that the intensity of the Iceland, would gener- ridge-hotspot interaction ate a vigorous source 1200 has been diminishing as of heat (light colors) the Cocos-Nazca Ridge and produce a huge 1000

emperature (

volume of magma T gradually moved away that would migrate from the Galápagos Depth (kilometers) Depth 0

laterally along and hotspot. Today the two across the ridge axes 0 400 are 200 kilometers apart, and we hundreds of kilome- ters away from the theorize that the hotspot will lose its im- hotspot source. pact on the ridge when the two reach a distance

36 • Vol. 41, No. 2 • 1998 of about 500 kilometers apart. Emerging geophysical and geochemical data indicate significant dissimilarities among hotspots and tell us that the inter- actions between individual hotspots and ridges have their own peculiar dynamics. While hotspots such as Iceland and Hawaii may have their origin deep in the mantle, others may simply reflect large concentra- tions of unusual chemical properties in the earth’s shallow mantle. While Iceland’s impact on the ridge extends up to 1,300 kilometers north and south, the Marion hotspot’s effect on the Southwest Indian Ridge diminishes after only 300 kilometers.

Magma eruption rates vary among differ- Jian Lin ent hotspots, and each hotspot has its own distin- data coverage—should make the coming decade a Author Jian Lin guishable geochemical signature, which provides most exciting time to study the fascinating geologi- captured this image of Iceland’s volcanic clues to understand how melting occurs at each cal phenomenon of ridge-hotspot interactions. landscape during a hotspot. Iceland and the Azores, for example, both The National Science Foundation supported the research research expedition. show an elevated ratio of stronium 87/stronium 86 described. The author is indebted to MIT/WHOI Joint Program isotopes in the basaltic rocks they produce, com- graduates Garrett Ito (now at University of Hawaii), Javier pared with basalts produced in Mid-Atlantic Ridge Escartin (now at CNRS, Paris, France), and current student regions farther south. However, basalts generated by Jennifer Georgen for sharing exciting time at sea and ashore in studying mid-ocean ridges and oceanic hotspots. Satellite gravity map the Azones hotspot do not exhibit the same el- of the western Indian evated helium 3/helium 4 isotopic ratios that the Jian Lin joined WHOI in 1988 after graduate study at Brown and southern Atlantic Icelandic basalts do. University and research on earthquakes at the US Geological Ocean basins, as Rapidly progressing techniques in geochemistry, Survey in Menlo Park, California. He divides his travel time revealed by satellite between going to sea and investigating quakes in California. His as well as in broadband seismology, autonomous gravity data. A string latest expedition was on board the French ship L’Atalante to the of hotspots (Tristan, underwater geophysics, and geodynamic model- Mid-Atlantic Ridge south of the Azores hotspot, where spectacu- Gough, Discovery, ing—combined with the new global geophysical lar new underwater volcanoes were discovered. and Shona) aligned with the southern 20˚S CIRCIR Mid-Atlantic Ridge AFRICA (MAR) may be con- tributing magma supplies that main- 30˚S tain the ridge. The SEIRSEIR Bouvet hotspot may ALVIS RIDGE play a critical role in W CAPE maintaining the MADAGASCAR R. TRISTAN RISE MADAGASCAR R. triple junction, where three mid-ocean 40˚S AGULHAS GOUGH AGULHAS ridge branches and MAR GOUGH PLATEAU AGULHAS RIDGE the three boundaries METEORMETEOR RISERISE (white lines) separat- DISCOVERY DEL ing the South Ameri- CANO CROZET can, Antarctic, and MARION PLATEAU African plates all 50˚S KERGUELENKERGUELEN intersect. Farther SWIR PLATEAUPLATEAU SHONA CHAIN SHAKASHAKA RIDGERIDGE east, the Kerguelen, CONRAD RISE Crozet, and Marion SHONA hotspots may have BOUVET extensively shaped AAR the seafloor of the southern ocean. 60˚S SEIR is the Southeast Indian Ridge, CIR is the Central Indian Ridge, and SWIR is the South- west Indian Ridge. 20˚W 10˚W 0˚ 10˚E 20˚E 30˚E 40˚E 50˚E 60˚E 70˚E 80˚E Map produced by Jennifer Georgen, MIT/WHOI Joint Program, with data from David Sandwell, Scripps -100 -80 -60 -40 -20 0 20 40 60 80 Institution of Oceanography, and FAA (mGal) Walter Smith, NOAA.

OCEANUS • 37