A note on Pele

Pele was born of the female spirit Haumea, or Hina, who, like all other important Hawai`i gods and goddesses, descended from the supreme beings, Papa, or Earth Mother, and Wakea, Sky Father. Pele was among the first voyagers to sail to Hawai`i, pursued, legends say, by her angry older sister, Na-maka-o-kaha`i because Pele had seduced her husband. Pele landed first on Kauai`i, but every time she thrust her o`o (digging stick) into the earth to dig a pit for her home, Na-maka-o-kaha`i, goddess of water and the sea, would flood the pits. Pele moved down the chain of islands in order of their geological formation, eventually landing on the Big Island's Mauna Loa, which is considered the tallest mountain on Earth when measured from its base at the bottom of the ocean (Betty Fullard-Leo, 1999; http://www.coffeetimes.com/pele.htm).

Aspects of the Emperor and Hawaiian chains unpredicted by the plume hypothesis

1. The “bend” did not result from a change in direction of Pacific plate motion: The Emperor and Hawaiian chains differ in trend by about 60°. The two trends intersect at about 30°N, near the Mendocino fracture zone (Figures 3 & 4). It is often assumed that this resulted from a change in direction of motion of the Pacific plate at ~ 50 Ma. However, such a change in plate direction did not occur, as is shown by magnetic lineations, fracture zone orientations and plate motion reconstructions [e.g., Raymond et al., 2000]. This “nonevent” [Norton, 1995] is often attributed to the collision of India with Asia or to changes in circumpacific geometry [Gordon et al., 1978]. In any case, reorganisation of global plate movements resulting from continental collision is not expected to be a geologically instantaneous change from one stable state to another as would be required if the bend is to be explained in this way [Duncan & Richards, 1991; Lithgow-Bertelloni & Richards, 1995; Richards & Lithgow-Bertelloni, 1996]. Rapid and large changes in plate motion are not expected, given current understanding of plate forces. Convective forces can change instantaneously, but plate driving forces cannot.

In addition to the change in trend, the locus of volcanism moved south by ~ 800 km relative to the geomagnetic and biofacies reference frames while the Emperor Seamount chain formed. This conclusion is supported by data from bioclastic sediment on Emperor seamounts, which corresponds to higher latitudes than present-day Hawaii [Butt, 1980; McKenzie et al., 1980]. Prior to the bend, the hotspot migrated at ~ 7 cm/year relative to the Pacific sea floor. The rate of propagation changed at the time of the bend to ~ 9 cm/year.

The Hawaiian and Emperor chains appear thus to be separate phenomena in some sense. Their orientations might be controlled by the direction of regional stress, the fabric of the seafloor, or stresses caused by previously erupted volcanoes [Hieronymus & Bercovici, 1999]. Fluctuations also occur in the directions and volumes of individual chains and volcanoes [Clague & Dalrymple, 1987].

3. There is no Emperor/Hawaiian “plume head”: Flood-basalt provinces and oceanic plateaus are commonly thought to represent the initial stages of plume volcanism [Campbell & Griffiths, 1990; Campbell & Griffiths, 1993]. If the Hawaiian-Emperor chain began with such extensive volcanism, the plateau so produced is no longer present on the Pacific plate. Oceanic plateaus are not thought to be subductable, and thus an oceanic “plume head” for the Emperor chain would presumably have been scraped off, accreted or obducted onto the Aleutian/Kurile/Kamchatka arc. There is no evidence for such material associated with the Emperor chain although other accreted plateaus, e.g. Wrangellia in the Pacific NE, have been identified. Some alternative hypotheses, e.g., propagating cracks, do not require a large igneous province at the beginning of the chain. Many other hotspot tracks also lack a “plume head” (e.g., the Easter hotspot), and many oceanic plateaus and large igneous provinces on land lack an obvious “tail”, or “hotspot track” (e.g., the Siberian Traps).

• geographically, where the chain crosses fracture zones, e.g., the Mendocino, Murray and Molakai fracture zones [Basu & Faggart, 1996], mainly by increase in the variance,
• temporally, e.g., Mukhopadhyay et al. [2003] report variations in ³He/⁴He of up to 8 Ra during a single century in Kauai volcano, that correlate with variations in radiogenic isotope ratios. This suggests rapid changes in composition or contamination of the melt supplying the magma reservoir, and
• spatially, from volcano to volcano, such that different volcanoes do not appear to be fed by the same magma source.

9. Seismology has not detected a plume: The most promising seismic methods for detecting plume-like structures are:

• teleseismic tomography, which can image the three-dimensional wave-speed in the upper few hundred kilometers,
• receiver functions (waves converted at upper mantle discontinuities), which can reveal topography on the 410- and 650-km discontinuities, and the thickness of the transition zone, and
• the study of ScS multiples. (see Seismology & Transition zone pages)

A hot column of mantle should reveal low wave speeds wherever earthquake waves cross it, and should depress the 410 km discontinuity and elevate the 650 km discontinuity, because of the effect of temperature on phase boundaries. A large active upwelling would spread out below the lithosphere, and be dragged to the NW. Actual tomography shows extension of the low-velocity zone to the SE, toward the East Pacific Rise.

Early teleseismic tomography on the big island of Hawaii revealed little structure in the upper ~ 80 km, and poorly resolved lower velocities below this [Ellsworth, 1977]. An experiment using a network ~ 600 km long, involving digital seismic stations on several of the islands detected a low-wave-speed anomaly beneath the islands of Maui and Molokai, 250 km NW of the big island, but no low-wave-speed anomaly beneath the big island down to the maximum depth of good resolution there at ~ 150 km [Wolfe et al., 2002] (Figure 8).

In order to probe the upper mantle in detail throughout much of its thickness, a ~ 600-km-wide network including many ocean-bottom seismometers is required. Such an experiment is currently underway (see http://obsmac1.whoi.edu/~bobd/plume.html).

Tomography in the central Pacific can only provide relatively good resolution of upper-mantle wave speeds if surface waves and multiple ScS waves are used, and can resolve structures on a spatial scale of a few hundred kilometers. Cross sections through the best available model of this sort - S20RTS [Ritsema et al., 1999] are shown in Figure 9.

A strong low-wave-speed anomaly lies to the NW of the big island of Hawaii, which might be associated with that detected by Wolfe et al. [2002]. There is apparent continuity of low-wave-speed material between the surface in the region of Hawaii and the core-mantle boundary beneath a large swath of the Earth ranging from the New Hebrides and Samoa, throughout the south Pacific and north along the East Pacific Rise, depending on the line of cross section selected. These connections involve various anomaly contortions and tilts. From the standpoint of the whole-mantle plume model, the problem lies in choosing which of many candidate anomalies to advocate, rather than explaining why one is not detected, as is the case for other hotspots such as Iceland and Tristan da Cunha.

A magnitude 6.2 earthquake occurred on the big island in 1973, and generated shear (S) waves that travelled directly downwards, reflected from the boundary of the core and, upon their return, registered on a seismometer on the big island (ScS waves). Six multiple echoes were well recorded. The times of these waves indicate that the average S-wave speed of the mantle beneath the Hawaii region [Katzman et al., 1988] is somewhat higher than the average beneath the southwestern Pacific Ocean [Best et al., 1975], and that the propagation efficiency is high, contrary to expectations for regions of high temperature or partial melting. (see Julian & Foulger, 2003).

Transition zone thickness beneath the Hawaii region is estimated to be 229 km [Gu & Dziewonski, 2001]. This is ~ 13 km thinner than the global average of 242 km, but not significantly thinner than the transition zone throughout much of the central Pacific and other oceans (Figure 10) (see also Transition zone page).

In summary, the results of seismic studies of the mantle beneath the Hawaiian region to date suggest that:

Other possible constraints or coincidences

• The Hawaiian chain is bounded by a large gravitational moat and arch structure that is most strongly evident in bathymetry around its southeastern end (Figure 7).
• Some Hawaiian basalts and xenoliths have extreme ³He/⁴He ratios compared with the MORB mean. In common with other “hotspots” these also exhibit high variance and have lower ³He abundances than MORB glasses.
• The volcanoes of the Hawaiian chain, and possibly also the Emperors, feature long, nested, lateral rift zones which are rarely seen on other oceanic islands (see Samoa page).
• Hawaiian volcanoes are distributed along the paired Loa and Kea trends that comprise arrays of sinusoidal volcano loci. These trends appear to be controlled by the loads of the volcanic edifices [Hieronymus & Bercovici, 1999].
• The volcanic constructs (groups of volcanoes) along the chains are mostly oriented at a high angle to the overall trend of the volcanic chain. The orientations of the rift systems are mostly oblique to structure on the underlying seafloor such as ridge segments and transform faults.
• Hawaii is about as far away from other active volcanic chains, hotspots, plate boundaries of all types and continents as it is possible to be, very close to the center of the colossal Pacific plate.
• The oldest end of the Emperor chain is subducting almost exactly into the cusp of the Aleutian/Kurile trench – the “NW Pacific pocket”. The cause and effect of this are debated.
• The great bend occurs close to where the chain crosses the Mendocino fracture zone.

Summary

What, then, is the evidence in support of a deep mantle plume? The Hawaiian hotspot has a very regular time-progressive track. Clearly, during the last ~ 50 Myr the melt anomaly has remained approximately stationary with respect to the North American plate, beneath which the Pacific plate is subducting at the Aleutian trench. (Since the Pacific plate is moving much faster than any other plate on Earth, the Hawaiian hotspot is essentially fixed with respect to every other plate also.) The magmatic rate is currently very high, and is characterised by OIB geochemistry. This includes high maximum ³He/⁴He isotope ratios, commonly considered to be a “smoking gun” for a plume from the lower mantle.

Alternative mechanisms have been proposed for all these observations (see also Cracks & Stress page). Other Pacific volcanic chains roughly parallel to the Hawaiian chain may (e.g., the eastern end of the Louisville chain) or may not (e.g., the Line Islands) be time-progressive. High rates of melt production have been explained by fertile mantle composition, perhaps resulting from remelting subducted oceanic crust, and this would also explain OIB geochemistry. Shallow models for high ³He/⁴He isotope ratios, e.g., release of old helium stored for 100s of Myr in subducted, lower-crustal, olivine-rich cumulates, have also been proposed and not challenged to date [e.g., Anderson, 1998a; Anderson, 1998b; Meibom et al., 2003] (see Helium Fundamentals page). However, a persuasive, alternative model for the Emperor and Hawaiian volcanic chains remains to be fully quantified.

Any satisfactory theory for Hawaiian volcanism must explain (or rationalize) the:

• change in migration direction of the melting locus at the bend,
• association of the great bend with the Mendocino fracture zone,
• change in migration rate at the bend,
• apparent commencement of the volcanic chain near a ridge,
• absence of a “plume head”,
• large variations in magmatic production, and a current magmatic rate about 3 times greater than the next most productive hotspots,
• absence of a significant heat flow anomaly,
• absence of lithospheric thinning,
• absence of a strong high-temperature signal in the erupted basalts,
• production of very large volumes of magma even though the depth to the top of the melting column is exceptionally large compared with MORs,
• spatial and temporal variation in the composition of erupted lavas on a variety of scales,
• remote location of Hawaii, near the center of a very large plate,
• location of the oldest end of the chain with respect to the “Pacific pocket”,
• unique rift zones,
• paired Loa and Kea trends,
• seismic whole-mantle mantle structure that is apparently normal compared with the Pacific ocean elsewhere, and
• occurrence of a bathymetric swell (a moat and “arch”) along the eastern two-thirds of the Hawaiian chain and wrapping around its southeastern end, with alkalic basaltic volcanism occurring at some places along it.

In conclusion, Hawaii is not fully explained by any current hypothesis. It is impressive that a region of the Earth so extensively studied for so many years, by so many Earth scientists with so many techniques could remain so intransigent to full understanding. Many of the numerous features that are not yet fully understood, and the parameters of alternative hypotheses, are not currently being studied, but they offer exciting research opportunities.

News & Discussion

"Fixed" Hawaiian hotspot not fixed

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