Hawaii, we thought we knew you
The classic Plate Tectonics story for Hawaii now appears to be mistaken. The Pacific plate did not suddenly change directions, and there is no deep, stationary plume. Other explanations fit the evidence better.
(This discussion is presented in the Plate Tectonics paradigm, referencing professional geologic journal papers. The Shock Dynamics perspective is described afterwards.)
The original idea
"The Hawaii-Emperor island and seamount chain is the most prominent morphologic feature on the seafloor, with a sharp 60° change in azimuth, called the Hawaii-Emperor bend (HEB). The HEB serves as a textbook example of the fixed hot spot hypothesis, in which changes in the azimuth of volcanic lineaments are explained by changes in plate motion, and the hot spots that created these volcanoes remain fixed beneath the moving tectonic plates."9 "Hawaii is thought to be the strongest currently active plume."13
1) Pacific plate motion
- no rapid change in direction
2) Segments en
echelon - volcanoes not in one line
3) Plume swell
(bulge) - not all there
4) Plume heat -
5) Basalt chemistry - lots of variation
"The volcanoes along the Hawaiian Islands align along two distinct geographic segments (the 'Loa' and 'Kea' trends). Geochemical studies of the lavas that make up the Hawaiian Island volcanoes... point to the puzzling feature that two mature volcanoes situated only 40 km apart (Mauna Loa and Kilauea) are remarkably dissimilar in their geochemistry. In fact, geochemical variations... along the Loa trend show gradational changes with decreasing volcano age [see below], while the... volcanoes in the Kea trend are chemically distinct from adjacent volcanoes in the Loa trend."7
6) Hotspot stability
- much motion
Plume theory seems infinitely flexible. Researchers have proposed putting plumes in motion to solve the problem. Using paleomagnetic and radiometric age data, one group found that "the Emperor Seamount trend was principally formed by the rapid motion (over 4 cm per year) of the Hawaiian hotspot plume."15
Plumes are considered to advance in stages: first, the plume head and tail rise together. Then the head flattens and is assimilated, leaving only the tail. Finally, the tail is distorted by mantle flow, and may split into separate, winding segments. Melt zones "under hotspots usually do not show a straight pillar shape, but exhibit winding images, suggesting that plumes are not fixed in the mantle but can be deflected due to the influence of mantle flow."18 Complicating the matter, "the distribution of seamounts in time and space... indicate that either the Pacific plate has undergone numerous short-term velocity changes or the path of the upwelling plume has been affected in some way."7 One researcher proposed that hotspot motion southward may be due to deep mantle flow, but upper mantle convection cell return flow may be stringing out the top of the plume in the opposite direction.3 Another proposed that individual "plumelets" rise from a single deep melt zone, each plumelet forming a seamount segment.7 In the end he wondered why the supposed change in Pacific plate motion had so little effect on underlying mantle flow, and why "the generation of a new subduction zone (such as along the Tonga-Kermadec trench at about 45 million years ago) and subsequent intrusion of slab material exerted no observable impact on flow in the underlying mantle.7
7) Tomography (like a seismic MRI) - conflicting images
"Hawaii should have the most readily resolvable conduit [tail] as it is situated away from ridge systems and is supposedly the strongest plume." Two studies in 1998 "searched for low-velocity anomalies [melt zones] in the lower mantle beneath the hotspot, but found no low-velocity anomaly which correlated with the surface expression of volcanism." So both invoked plume deflection to resolve the issue. But while one "suggested the conduit to lie to the southeast of Hawaii," the other "claimed a double conduit to the northwest of Hawaii."13 Similarly, the author of a 2004 tomographic study believes his cross-section shows the prominent melt zone beneath Hawaii connected by a thin melt zone to a moderate melt zone offset to the north and down to the core-mantle boundary (below 2700 km). However, his regional tomographic map shows the prominent melt zone thinning considerably below the transition zone (660 km), reappearing offset to the west at 1100 km depth, and disappearing below 1820 km. So the plume would be deflected by mantle flow to the south or to the east.18
Tomography based on the travel-time of seismic waves has some inherent difficulties. "Seismic ray coverage is highly variable, and different types of rays are used to interrogate different depths." Tomography can show a false picture if the mantle is assumed to be uniform and it is not. Shallow heterogeneities (areas of varying density) and anisotropy (crystal alignment) can smear tomographic images when the mantle is assumed to be uniform, and most models do. Since most of the world's earthquakes occur in slabs, and many seismic stations are in their vicinity, the opportunity exists to smear this shallow anisotropy into an image of a deep slab. "Normal tomographic [images] cannot cancel out slab anisotropy, particularly in the deep mantle where seismic ray coverage is poor." Tomography shows that "in current subduction zones, descending slabs flatten out between 500 and 800 km." The supposed slabs in the deep mantle are separated, and up to 1500 km from expected locations.1
Return of an old
idea - lithosphere crack
"The obvious alternative" to plumes forming seamount chains is "extension that permits rise of partial melt from the asthenosphere" [between the mantle and crust]. Thus the en echelon segments in seamount chains are "extensional fissures", so that extension controls the propagation of the chain.6 "The starting point for the construction of any counter-model has to be an acceptance of the evidence for amphibole and phlogopite in the source of OIB." "Intraplate volcanism results predominantly from compositional, not" heat differences. "The low melting point of such minerals would make them susceptible to shear melting to generate intraplate tracks." These have been called "wetspots" as opposed to "hotspots".13 "The Hawaiian chain sits on a buoyant pad of mantle rich in magnesian olivine, and magmas must be rising in fissures, not broad plumes." "Volcanoes will form wherever this potential melt can be tapped: the problem is access to the surface, and there is no... need for unique heat sources, nor any geochemical need for deep sources of components of melts. Volcanoes are products of" "extension" and "propagating cracks".6
"The crack model... was suggested for the Hawaiian volcanic chain as far back as 1849."4 In 1973, intraplate volcanism, including Hawaii, was proposed to be caused by lithospheric stress, with intermittent eruptions due to the shallowness of the source regions. "Cessation of volcanism in the absence of changes in the stress field can be explained by exhaustion of low-melting point minerals." On the other hand, opening a melt pocket would give the impression of the arrival of plume material.14 In 1975, two authors wrote "we conclude that the trends and age correlations of volcanic loci in the Pacific accurately track and identify the evolution of states of stress in the Pacific lithosphere with time." There was "magma injection from a source in the asthenosphere into a rigid Pacific plate subjected to rotations of principal stress directions."8 A 1987 study evaluated two other hypotheses, small-scale convection and compressive buckling, and decided in favor of tension (pull apart) cracks in the Pacific plate. The shape and positions "of the en-echelon ridges suggest that they result from filling of tensional cracks in the lithosphere." "Experimental and theoretical studies... show that plastic yielding occurs... oriented 55-60° from the direction of tensile stress." "Extension opens the cracks and shear produces the en-echelon pattern." Pacific plate earthquakes "show plate-wide tension oriented NNE." However, their "data do not indicate the source of the tensile stress", and they speculated on several possibilities.17
If one or more chains of linear volcanic ridges are shown to be formed by extension rather than by fixed hotspots as previously proposed, then the application of the fixed-hotspot model to other linear volcanic chains may be questioned.11 The authors of the Gilbert Ridge/Tokelau Seamounts study mentioned above proposed "that the southwestern Pacific plate experienced two such short-term extensional phases."9
"The crack model is appealing because several first-order features of the Hawaiian and Emperor chains that are inconsistent with the plume model or require surprising coincidences may be consistent with the crack model. These include the inception of the Emperor chain on a ridge, the lack of a 'plume head' large igneous province, the ~60° change in propagation direction that occurred around 47 million years ago, the rapid southward migration of the Emperor hotspot prior to this, and the lack of the heatflow anomaly expected for a plume."16
Notice that the first two eruptions, Meiji and Detroit seamounts, are the largest plateaus in the entire chain. The first is also oriented at a different angle from the rest of the Emperor chain.
Assuming the tensional stress-crack alternative to the plume model described above is correct, it becomes a matter of determining what pulled on the crust. Clearly it must be the motion of Alaska (red arrows, below). White arrows indicate the orientation of fissures along the chain. The sudden bend may mark the initial collision of Alaska with North America. Drag of the merged landmasses north would then have increased tension on the Pacific crust, leading to increased volcanism towards the end of the Hawaiian chain.
A long rift or fault, often overlooked, branches off of the Emperor Seamount chain. It also indicates pull-apart stretching towards Alaska.
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John Michael Fischer, 2006