LETTERSPUBLISHED ONLINE: 26 JUNE 2011 | DOI: 10.1038/NGEO1187

Spatial and temporal variability in Hawaiianhotspot volcanism induced bysmall-scale convectionMaxim D. Ballmer1,2*, Garrett Ito1, Jeroen van Hunen3 and Paul J. Tackley2

Volcanism far from plate boundaries is often attributed to anunderlying mantle plume1–6. However, enigmatic observationsof Hawaiian volcanism, such as variations in the volumeof erupted volcanic material through time7,8, a geographicalasymmetry in the geochemistry of the lavas9–18 and secondaryvolcanism that occurs far away from the hotspot15–20, cannotbe explained by the classical mantle plume concept. Here wepresent a numerical model of mantle plume upwelling beneathHawaii. We find that small-scale convection in the ambientmantle can erode the base of the lithosphere, creating awashboard topography on the underside of the plate. As theplate migrates over the upwelling plume, the plume interactswith alternating thicker and thinner sections of lithosphereto generate temporal variations in the flux of eruptedvolcanic material. The pre-existing washboard topography alsocauses the plume to spread and melt asymmetrically. In oursimulations, this asymmetry in mantle flow generates anasymmetry in the chemistry of the erupted lavas. Finally, amore vigorous type of small-scale convection develops withinthe spreading plume, generating localized zones of upwellingwell away from the hotspot. The associated magmatism is fedby chemically distinct material originating from the edges ofthe plume conduit. Our results show that shallow processeshave an important influence on the character of volcanism fedby deep-rooted mantle plumes.

Classical plumes are typically described as purely thermallydriven, narrow upwellings rising through the entire mantle andbeing deflected into a thin ‘pancake’ beneath the overridingplate1. Such an upwelling dynamically generates an elongated,parabolically shaped swelling of seafloor topography2–4. Associ-ated ‘hotspot’ volcanism is localized and stationary, thereforeentailing an age-progressive island chain. This classical theoryhas indeed successfully predicted first-order observations at manyhotspot chains, Hawaii being among the most prominent andbest studied examples.

A set of enigmatic observations ofHawaiian volcanism, however,are not explained by the above idealized description. First, averagevolcanic flux as documented along the Hawaii–Emperor chainhas varied by a factor of >2 over typical timescales of ∼15Myr(refs 7,8). Mechanisms involving intrinsic variations in buoyancyflux or tilt of the rising plume stem have been proposed as anexplanation5–8, but not yet tested. Second, the origin of the bilateralasymmetry in lava geochemistry, as documented by compositionaldistinctions between the southern (‘Loa’) and northern (‘Kea’)volcano sub-chains (Fig. 1a), is not well understood. One set

1School of Ocean and Earth Sciences and Technology, University of Hawaii, Honolulu, Hawaii 96822, USA, 2Institute of Geophysics, ETH Zürich, 8092Zürich, Switzerland, 3Department of Earth Sciences, Durham University, Durham, DH1 3LE, UK. *e-mail: ballmer@hawaii.edu.

Ni'ihauO'ahu

Maui Arch

Kaua'i

Lana'i

Recent volcanismKea trend

Loa trend

Rejuvenated stage

shieldstage

Bathymetry (m)

Haw

aiian

South ArchVolcanic Field

North ArchVolcanic Fields

Hawaiian hotspotat 90 Myr

Hawai'iMoloka'i

Fracture Zone

15° N

20° N

25° N

160 ° W 155° W

¬3,000¬6,500

a

b

onset of SSC typically at ~70 Myr

Figure 1 | Overview and concept. a, Geographic overview and bathymetryof the Hawaiian Islands. Shield volcanoes are marked with triangles andarch volcanic fields with strong acoustic reflectivity19,20 are shaded. Theshallow seafloor surrounding the islands is referred to as the Hawaiian arch(black dashed). b, Conceptual illustration of small-scale convection (SSC)interacting with the Hawaiian plume. Undulations on the base of thelithosphere (washboard pattern; dashed yellow line) were created by SSCin the ambient mantle.

of interpretations invokes some form of compositional zoningin the upwelling plume stem9–11. Other studies emphasize thatif the mantle is a fine-scale mixture of different lithologicalcomponents, spatial variations in pressure and temperature overthe hotspot melting zone can create geographical patterns ofmagma composition that differ from those for an isochemicalsource12. Finally, widespread secondary volcanism17–20 occurring

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