Geophysicist Tuzo Wilson had a creepy daydream. He imagined himself lying at the bottom of a creek, looking up at water flowing overhead. He blew bubbles. They rose, were caught by the current, and drifted away. He came back from his mini-nightmare with an idea about the way the Hawaiian islands formed. Bubbles of magma rose from deep within the Earth, broke through the crust, and drifted along because of plate tectonics.
The original Hawaiian settlers had a similar idea. They knew that the weathered northwest islands were much older than the new big island they called Hawaii. They believed that the fiery goddess Pele had successively emerged from the underworld, creating a series of volcanic islands over the ages.
Wilson was 55 at the time that his imagination discovered a way to explain the Hawaii island chain. His first attempt to publish “A Possible Origin of the Hawaiian Islands” failed when the scientists reviewing it at a leading American geophysical journal found the idea too radical. They rejected it. Tuzo scrambled to the Canadian Journal of Physics, which published his paper in 1963 – probably because they didn’t realize it was such a controversial subject, Tuzo figured.
Tuzo Wilson’s paper explained that the Hawaiian islands formed in a string because the Pacific tectonic plate slowly slid across a fixed hotspot while a plume of magma rose directly from deep below. This was an important idea because plate tectonics – new at the time – was struggling to explain anomalous volcanic activity occurring in places like Hawaii, thousands of kilometres from the rubbing and colliding spots where crustal plates met.
Some geophysicists don’t believe in plumes. They suggested that the material creating island paradises is sourced just below the lithosphere. The jury is out; however, most who have studied it for the past fifty years have cautiously agreed with Wilson’s original thesis. Similar streams of hot mantle rising in hot ribbons of rock possibly cause thermal events in Réunion, Yellowstone, Iceland, Galápagos, Tahiti and at least forty other places scattered about the globe. With his near-mystic vision, Tuzo Wilson invented a whole new way to understand what’s causing some of the Earth’s volcanic activity.
But not so fast. Ten years after Wilson, in 1972, the father and son geology team of Howard and Arthur Meyerhoff took an awkward stand against all forms of plate tectonics and against Tuzo Wilson’s plume theory. They generated vigorous pleas alerting fellow earth scientists to the pitfalls of those new ideas.
The Meyerhoffs assembled obscure facts that disputed tectonic motion. They released reasonable commentaries voicing their opposition to the nascent theory. They were usually correct in pointing out weak and contradictory aspects of plate tectonics. They asserted their opposition was based on “geological fact, which nothing can change.” Almost invariably, however, a scientist who clutches facts that “nothing can change,” is proven wrong. The ‘facts’ often change due to new data, investigation, and confirmation. The Meyerhoffs protested mobile continents, but didn’t provide an alternative that explained earth history as convincingly as plumes and plate tectonics.
In time, the younger Meyerhoff, Arthur, realized that the role of naysayer wasn’t enough. He needed to add something positive to the discussion. In 1988, he proposed a creative alternative earth-model: an interconnected near-surface world-wide plumbing system that conveyed melted igneous rocks. This relatively shallow plumbing system, he suggested, was being misinterpreted as plate tectonics. He carefully explained it in his posthumously published 1996 book, Surge Tectonics, which became popular among followers of his surge theory. And, in fact, there has been some evidence to support the notion that sub-crustal flows supply the Hawaiian island chain. In 2011, Qin Cao and her team at M.I.T. seemed to have found a hot pool of magma in the shallow mantle that could be sourcing the growth of the Hawaiian islands – possibly in a non-plume-like manner.
But plumes still has advocates. And more data has changed the idea yet again. Seismic tomography has helped by monitoring earthquake energy waves as they vibrate through the mantle. Receivers record arrivals of the earthquake signals at thousands of surface locations. Variations in those arrivals indicate waves have been influenced by variations in composition, phase, density, pressure, or temperature of mantle rock. Seismic waves travel more slowly through hotter rock. If those changes could be accurately mapped, shallow pipes and plumes might be spotted – if they exist.
Using seismic tomography, it should have been straight-forward to map pipes or channels or deep plumes surging with hot streams of mantle. But for over 50 years, we didn’t have an unequivocally clear rendering of a narrow, ribbon-like plume. The idea that hot spots exist became, to some scientists, a kind of belief system. Belief in an invisible God-like force that explained some otherwise inexplicable phenomenon of nature.
A newer technique – telescopic seismic tomography, a focused sort of tomography – recently emerged and has been enlisted in the search for plumes. Arrays of surface receivers collect billions of bits of earthquake data, collate them, and then invert the waves, mapping their probable travel paths. Yet this technique is painfully tedious and can be error-prone. The energy waves arrive steeply dipping; the Earth’s innards are far from uniform; and, a delayed arrival may be due to any combination of composition, density, pressure, phase and/or temperature differences along the seismic wave’s travel path. This underlies a fundamental problem with geophysics: almost any observation can be due to a variety of causes. Nearly every geophysics problem has multiple non-unique solutions.
Thus, for over fifty years, plumes eluded researchers. Until now, perhaps. With repeated studies and massive data-crunching, wide plumes of low velocity mantle material have finally been discerned. Last fall an important paper was released. It makes the strong and well-supported claim that plumes are real, putting plume atheists on the spot.
Barbara Romanowicz and Scott French published their findings in Nature in September. Broad Plumes Rooted At The Base Of The Earth’s Mantle Beneath Major Hotspots resulted from analysis of whole-mantle earthquake tomographic seismic data. Romanowicz’s group used full-waveform seismic computation in a process that analyzed energy waves from 273 large earthquakes and took 3 million hours of computation on a supercomputer. The data indicate that plumes are real, but are much wider than the innocent trail of creek bubbles envisioned long ago by Tuzo Wilson. From the Nature paper’s abstract:
“We describe the use of a whole-mantle seismic imaging technique— combining accurate wavefield computations with information contained in whole seismic waveforms—that reveals the presence of broad (not thin), quasi-vertical conduits beneath many prominent hotspots. These conduits extend from the core–mantle boundary to about 1,000 kilometres below Earth’s surface, where some are deflected horizontally, as though entrained into more vigorous upper-mantle circulation. At the base of the mantle, these conduits are rooted in patches of greatly reduced shear velocity that, in the case of Hawaii, Iceland and Samoa, correspond to the locations of known large ultralow-velocity zones. This correspondence clearly establishes a continuous connection between such zones and mantle plumes. We also show that the imaged conduits are robustly broader than classical thermal plume tails, suggesting that they are long-lived, and may have a thermochemical origin.”
Are we done? Probably not. This was the state of the art in September, 2015. The conclusion is that plumes are real, are much broader than hypothesized earlier, they originate at the mantle-core boundary, rise almost vertically, and (closer to surface) get distorted by the same horizontal convection currents that drive plate tectonics. It’s our best description of plumes to date, arriving at the surface 50 years after Tuzo Wilson’s first puff of air from his eerie creek bottom.