Earth’s concentric layers scale remarkably well to those of a peach. The atmosphere is proportionally as thick as the exterior fuzz. The crust corresponds to the skin, the iron core (although spherical rather than pointed) to the pit, and the rocky mantle, which constitutes eighty-four per cent of the volume of the planet, to the flesh of the fruit. The lower mantle is about eighteen hundred miles from Earth’s surface—roughly the distance from Manhattan to Denver—but is less accessible and less known to us than the face of Mars. The deepest holes ever drilled, in a bizarre late-Cold War competition between NATO and the Soviet Union, barely pricked the skin; they went about eight miles down.

We know that the mantle is solid rock, contrary to various fanciful depictions in fiction and film, because, in the wake of major earthquakes, certain seismic waves that can be transmitted only through solids are detected on the opposite side of the globe. The time it takes these waves to traverse the mantle even provides information about the minerals that they encounter along the way. We also know that the mantle, although solid, is flowing in a slow, continuous roil, like the wax in a lava lamp. This convective overturning is the principal means by which Earth cools itself, and it sets the pace for the dance of the tectonic plates at the surface. It is also crucial to the existence of the magnetic field, which arises from the motion of liquid iron in the outer core. If the mantle didn’t transport heat outward, there would be no vertical temperature contrast in the core to drive Earth’s electromagnetic dynamo, which creates an invisible aura that extends many planetary diameters away—the scent, perhaps, of the peach. This halo protects us from solar wind and space radiation.

The character and behavior of the lower mantle have been the focus of one of the most quarrelsome modern debates in the geosciences. In the early nineteen-seventies, plate-tectonic theory—then only about five years old—had already gone far in explaining how geophysical phenomena, including earthquakes, volcanism, and mountain building, result from interactions between the major plates at their boundaries. The San Andreas Fault, the Himalayas, the Andes, Krakatoa, Vesuvius, and Mt. St. Helens all lie on plate boundaries. But there remained obvious exceptions, such as the prodigious volcanic activity at Hawaii and Yellowstone, both of which are in the far interiors of their plates. Not all the unrest, in other words, was at the margins.

In a modest paper published in the March 5, 1971, issue of Nature, the geophysicist Jason Morgan proposed that Hawaii and other isolated volcanic hot spots were surface manifestations of what he called mantle plumes—columns of hot (although not molten) rock, about a hundred miles in diameter, that rose from the core-mantle boundary and generated magma as they neared the surface. Morgan’s article was rather thin on actual data, but his theory was consistent with a number of observations. First, lavas from Hawaii and similar archipelagoes, including Samoa, the Canaries, and dozens of others, differ markedly in their chemistry from those that erupt at the mid-ocean ridges, where plates diverge from each other. This suggests that they emanate from parts of the mantle that are distinct from those that produce ordinary seafloor basalt. Second, some island groups in both the Pacific and the Atlantic show a well-defined progression of ages. In the case of Hawaii, a string of successively older islands—and then an underwater mountain range, known as the Emperor chain—stretches to the northwest of the Big Island. Morgan’s greatest insight was that this pattern could be a result of the northwestward motion of the Pacific Plate. Just as the bars on a typewriter strike a fixed point on a moving sheet of paper, the stationary mantle plume was leaving its embossment on the surface of the drifting crust.

Although Morgan’s evidence was indirect, his idea quickly became reified, and most geologists forgot that mantle plumes were a conjectural concept rather than an observation. Over the next thirty years, in thousands of scientific papers, plumes and their putative effects were described, modeled, and categorized. They were invoked to explain a wide range of modern and ancient volcanic phenomena, including a billion-year-old rift beneath Lake Superior and the gigantic outpouring of lavas called the Siberian Traps, which may have triggered the Permian-Triassic extinction event, the most devastating in Earth’s history.

But, in the early two-thousands, a vocal group of skeptics began raising questions. By this time, global seismology had progressed from merely delineating Earth’s layers to measuring subtle variations in temperature or composition within the mantle using three-dimensional seismic tomography—analogous to medical CT scans. To the surprise of mainstream geoscientists, this method failed to reveal the expected columns of hot rock beneath oceanic islands. Even Hawaii, the paradigmatic plume site, did not seem to be fed by one.

Plume believers argued that seismic tomography lacked the resolution to detect relatively thin spindles of slightly warmer mantle; they remained certain that they would eventually spot the rara avis. Plume doubters, meanwhile, argued that the hypothesis was unscientific if no evidence could persuade its advocates to discard it. In their view, plumes were the phlogiston or the Sasquatch of geophysics, chimeras created to explain poorly understood phenomena that could be accounted for in other ways. A Web site, mantleplumes.org, was set up as an uncharacteristically raucous forum for exchanges between the two scientific camps.

Then, earlier this month, another paper in Nature reported what may be the first real glimpse of Morgan’s elusive plumes, some forty-four years after he first postulated their existence. Using a new, computationally intensive method of seismic tomography, a pair of geoscientists at the University of California, Berkeley, captured the mantle’s structure at unprecedented resolution. The images do at last reveal areas of warm rock emanating from the lower mantle far beneath Hawaii and other Pacific hot spots, but the columns are far larger in diameter than previously envisioned: these rare birds have stocky legs nearly five hundred miles wide. Another unexpected finding is that the plumes radiate only from certain areas along the core-mantle boundary, suggesting that they may be geochemical as much as thermal in nature. (Why the deepest mantle would be so chemically variable is a question that could become as contentious as the plume debate itself.) Finally, and perhaps most curiously, no deep plumes were detected beneath some archetypal hot spots, among them Yellowstone. The paper is so new that the scope of its influence is not yet clear. But it is a humbling reminder that this planet is one mysterious peach.