understand how supervolcanoes work, imagine a blazing abscess moving and growing under your skin, suffusing the flesh below with fiery pus. In geological terms, this abscess is known as a hot spot, and the Earth’s skin, or crust, is moving over it. In Windows into the Earth, Robert Smith and his cowriter, Lee. J. Siegel, former science editor of the Salt Lake Tribune, explain that most hot spots are “columns or plumes of hot and molten rock that begin 1,800 miles underground at the boundary between Earth’s core and lower mantle, then flow slowly upward [because heat rises] through the entire mantle and crust.”
Hot spots are typically located at the boundaries of tectonic plates, which essentially float on seas of molten rock. Hot spots tend to be located along the seafloor, since most of the Earth is covered by water. The molten churn of these hot spots is composed largely of basalt, which tends to seep and flow rather than explode.
“Of the roughly thirty active hotspots on Earth, almost all except Yellowstone are beneath oceans or near coastlines or other boundaries between tectonic plates. The best known of the other hotspots are those that produced Iceland [including Surtsey], the Hawaiian Islands, and the Galapagos Islands,” write Smith and Siegel.
The Yellowstone hot spot, by contrast, is smack dab in the middle of our continent. It’s quite far from any ocean or plate boundary, the closest one being roughly at the Pacific coast. And the Yellowstone hot spot does not extend down into the Earth nearly as far as the others. According to current estimates, it reaches a depth of only about 125 miles, less than a tenth the normal. Thus its impetus does not come from the Earth’s molten core. Rather it appears to have been formed largely from the heat produced by decay of vast amounts of uranium and other radioactive elements in the region, heat that then melts iron-rich basalt rock, great blobs of which periodically plume to the top.
“The molten blobs of basalt heat overlying crustal rock, creating a ‘magma chamber’ in which silica-rich crustal granite partially melts, formally a molten rock known as rhyolite when it erupts… . Because molten rhyolite is thick and viscous, major eruptions from the Yellowstone hot spot have been explosive, unlike the basalt that erupts more gently from oceanic hot spots [such as Surtsey],” explain Smith and Siegel.
Think of the way a pot of thick stew left on the flame too long might all of a sudden splatter the kitchen, where a pot of watery soup would bubble and boil over less explosively.
The Yellowstone hot spot appears to have formed some 16.5 million years ago, beneath the areas where Oregon, Nevada, and Idaho meet. Since then it has had several dozen eruptions, each of which would have devastated any civilization that might have existed at the time.
A macabre illustration of Yellowstone’s handiwork was discovered by Professor Michael Voorhies, of the University of Nebraska. After heavy rains, Voorhies went to the little town of Orchard, Nebraska, to do some fossil hunting. What he found was an archaeologist’s dream and everyone else’s nightmare: hundreds of skeletons of rhinos, camels, horses, lizards, and turtles, most in their prime, all killed abruptly 10 million years ago, almost certainly coinciding with a Yellowstone blast. The skeletons in this mass catastrophe were covered with a white film, forensic evidence that the animals died of something akin to Marie’s disease, a lung disorder most likely contracted by the inhalation of volcanic ash.
Slowly and steadily, the murderous hot spot has shifted some 500 miles in a northeasterly direction to its present location in northwestern Wyoming, where its caldera bulges menacingly under Yellowstone National Park. Like any other abscess continuously scratched and abraded, the hot spot will pop and spew. Once emptied, it will settle back down and then slowly refill over the next 600,000 years or so, until it explodes again. Also like any other abscess, there is not necessarily any preset, optimal explosion moment, just a range of “ripeness.”
The swirl of data and innuendo surrounding Yellowstone’s current seismic activity is almost as thick as the molten rhyolite that one day will pop its cork. Anecdotal reports of impromptu police action, unannounced trail closures, discoveries of heat and seismic sensors, and other appurtenances of heightened watchfulness abound on the Internet, contrasting starkly with official, what-me-worry? attitudes.
An uptick in Yellowstone’s seismic activity would be a tipoff that an eruption was coming; dozens of seismographs have been installed in and around the park, to relay as early as possible the slightest bad news. Swarms of tiny earthquakes, a chemical change in the composition of lava, gassing from the ground, cracking of the land—all these are potential signs that eruption is imminent. A rapid and substantial rise in the elevation of the caldera, swelling, it would be assumed, with magma and volcanic gas, would be an obvious tipoff.
The problem, as the producers of the BBC specials on Yellowstone quickly found out, is that for reasons unspecified much of this data is unavailable to the public. For example, numerous reports of a 100-foot bulge in the bottom of Lake Yellowstone, a normally cold, high mountain lake whose water temperatures have somehow hit the mid-80s, have gone unconfirmed, and also unchallenged, by park authorities. For the most part, however, one is forced to rely on unofficial information sources. According to Bennie LeBeau, from the Eastern Shoshone Nation in Wyoming, a number of new steam vents have formed along the Norris Geyser Basin, where soil temperatures reached nearly 200 degrees in 2003, at which point the entire 200-square-mile basin was closed.
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