TOEFL Reading練習 #032 — The Hotspot That May Not Have Stayed Put(日本語解説)

The Hotspot That May Not Have Stayed Put

¶1 A sharp bend in a chain of underwater volcanoes running across the Pacific floor has long been read as a kind of frozen compass reading. It is said to record the moment a tectonic plate changed the direction it was sliding. That reading rests on one hidden assumption: that the volcanic source feeding the chain never moves itself. Testing the assumption directly took decades, since the source lies far too deep to observe. When geologists finally found an indirect way to check it, the bend turned out to preserve a more complicated story than a single, sudden turn.

¶2 The chain in question is the Hawaiian-Emperor seamount trail, stretching more than five thousand kilometers across the Pacific floor. The islands nearest Hawaii are youngest; age increases toward the older, more eroded Emperor Seamounts to the northwest. Geologists explain the pattern with the hotspot model, first outlined by Tuzo Wilson in 1963 and developed further by Jason Morgan in 1971. A plume of unusually hot rock stays fixed deep in the mantle while the plate slides past above it, marking the seafloor with volcanoes the way a stationary blowtorch would mark a moving sheet of metal. Under this model, the sharp bend where the younger Hawaiian chain meets the older Emperor chain, dated to around forty-seven million years ago, records nothing about the plume itself; it records only the moment the plate changed course. No one can watch a plume rise through thousands of kilometers of solid rock over tens of millions of years. So in 2001, a research team drilled into two of the oldest seamounts, Detroit and Suiko, to recover cylinders of solidified lava. They wanted to know whether the rock's own magnetism could reveal where the plume had stood when it erupted. Cooling lava locks tiny magnetic minerals into place at the angle of the planet's magnetic field at that instant, called inclination, which converts directly into paleolatitude: the latitude at which the rock formed.

¶3 Two outcomes were possible once the cores came up. If the plume had stayed exactly where it was while only the plate turned, every seamount along the chain, regardless of age, should show close to the same paleolatitude as Hawaii's present location, near nineteen degrees north. If instead the plume itself had drifted, paleolatitude should shift steadily with age, oldest to youngest. The results favored drift. Basalt from Detroit Seamount, eighty-one million years old, had formed at roughly thirty-six degrees north; basalt from Suiko Seamount, twenty million years younger, had formed at roughly twenty-seven degrees north. Rather than holding steady near nineteen degrees, the source had moved south by close to nine degrees of latitude over that span. John Tarduno, the geophysicist who led the analysis, calculated a drift rate of more than forty millimeters a year. A plume moving that fast could no longer be treated as a fixed anchor for reconstructing anything.

¶4 The drift data did not settle the question of what produced the bend. A southward shift of nine degrees of latitude is real, but it unfolds gradually over twenty million years, while the seamount chain itself bends by roughly sixty degrees in compass direction, concentrated in a much shorter stretch. [A] Later studies using computer models of deep mantle flow estimated that the plume drifted no more than about four to nine degrees of latitude over that same span. [B] That gap left room for the older explanation to survive alongside the newer one. [C] A change in the direction the Pacific Plate itself was moving, close to the same forty-seven-million-year mark, still appears necessary to account for the sharpness of the bend. [D] The two processes, a drifting plume and a turning plate, are not mutually exclusive, and most geologists now suspect the bend records some combination of both.

¶5 The stakes reach well beyond one bend in one chain. Geologists have long used hotspots as a fixed grid for reconstructing how the planet's plates have moved relative to one another. If a hotspot can drift, part of that grid has to be redrawn, and no one yet agrees by how much. Comparisons with other hotspot chains crossing the Pacific Plate, which should show a matching bend if a plate-direction change alone explained the pattern, have so far produced only partly consistent results. More than two decades after the first cores came up from Detroit Seamount, geologists still cannot say how much of the Hawaiian-Emperor bend belongs to a plume that wandered and how much belongs to a plate that turned, or why the deep mantle beneath the Pacific pushed the plume south exactly when it did.