Olcanic Iridium

The picture of iridium anomalies as uniquely diagnostic of meteorite impact began to cloud in the mid-1980s, lending additional credence to the volcanism theory. The chemistry of aerosols (suspensions of fine solid or liquid particles in gases) emitted from Kilauea Volcano in Hawaii had been under investigation by scientists from the University of Maryland.5 Although for five years they detected no iridium, aerosols from the 1983 eruption unexpectedly contained up to 10,000 times as much iridium as the Hawaiian basalts. Officer and Drake pointed out that the iridium in the airborne particles was "comparable to concentrations associated with meteorites."6

The picture quickly clouded further: High iridium levels were discovered in particles emitted by a volcano on the remote island of Reunion in the western Indian Ocean' and in the ejecta of silicic volcanoes on Kamchatka.8 Iridium levels as high as ',500 ppt, comparable to the K-T levels, were found in layers of volcanic dust buried in the Antarctic ice sheet.9 Thus, contrary to the view that prevailed when the Alvarez theory was first introduced, as the 1980s progressed it began to appear that certain volcanic processes can concentrate iridium and in amounts approaching K-T boundary levels.

In 1996, Frank Asaro, an original member of the Alvarez team, and Birger Schmitz of the University of Gothenburg in Sweden, reported iridium measurements in a number of ash deposits, including some near the K-T boundary.10 They confirmed the discovery that some types of explosive volcanism produce ash with up to ',500 ppt iridium, but said that by comparing the levels and ratios of various chemical elements in volcanic rocks and meteorites, it is easy to tell that these were terrestrial iridium anomalies rather than impact-related ones. They found no iridium in the types of ashes studied by the Russian geologists, whose claim, they therefore said, needed further confirmation. They thought that the Antarctic iridium, rather than stemming from volcanism, might be derived from the meteoritic dust that has settled there for millennia. The other clay layers they studied contained no iridium.

For the volcanic alternative to be viable, volcanoes must have emitted large enough volumes of lava to allow their by-products, such as carbon dioxide or dust, to cause a mass extinction. Those byproducts would have to include the iridium, shocked quartz, and the spherules found at the K-T boundary, all of which the volcanoes would have to distribute around the globe. The difficulty is that although all types of volcanoes taken collectively might explain these observational facts, none of the individual types do. We know that volcanoes such as Krakatoa, and those of the Ring of Fire—the group of active volcanoes that encircle the Pacific Ocean basin from Tierra del Fuego around to the Philippines—explode suddenly and unexpectedly. They do so because the chambers beneath them hold a volatile mixture: magmas (subterranean lavas) rich both in silica and in gases kept in solution under high pressure. These silicic magmas are thick and viscous, like molasses, which causes them to clog their volcanic conduits, trapping the dissolved gases. The gases can then burst free in a gigantic explosion, like a too-rapidly opened bottle of carbonated beverage, shooting plumes of dust and ash into the stratosphere and showering debris for thousands of kilometers. In '980, Mount St. Helens blasted itself to pieces in an explosion that sent fine ash wafting over most of the United States. By the time it reached the eastern states, however, the heavier fraction of the ash had already settled out, leaving suspended a portion so fine as to be almost invisible.

K-T quartz grains and spherules are much larger and heavier than fine volcanic ash. Had they erupted into the stratosphere, they would quickly have fallen back to earth. No one has been able to show how explosive volcanism can send large particles winging around the globe; in any case, volcanic explosions produce angular glass shards, not rounded spherules. And as noted in Chapter 5, the multiple, crisscrossing planar deformation features common in boundary clay quartz have never been found in volcanic products (including the quartz from Mount St. Helens).

The high iridium levels measured in volcanic aerosols from Hawaii came from a different type of volcano than those of the Ring of Fire. The Hawaiian variety erupts basalt, which is lower both in silica and in dissolved gases than the lavas of the Ring of Fire, making it less viscous and more able to flow. For this reason, basaltic eruptions are quiescent rather than explosive and their lavas are restricted to the nearby area. Although gases emitted from these basaltic volcanoes might convey iridium around the globe, the lavas themselves contain almost none. Because basaltic lavas erupt quietly, it is hard to see by what process they could produce the required worldwide distributions of iridium, shocked minerals, and spherules, especially since basalt contains negligible iridium and no quartz. Thus basaltic volcanism also fails to explain all the evidence.

Although silicic volcanoes contain quartz and explode, they emit smaller volumes of material, and for more limited periods of time, than the basaltic variety. The famous eruptions of Toba, Krakatoa, and Mount St. Helens did not come close to causing a mass extinction. The only volcanoes known to erupt large enough volumes of lava over a long enough period of time to produce potentially lethal amounts of chemicals and cause a global mass extinction are basaltic, yet basaltic volcanoes emit their products so quietly that they do not receive worldwide distribution. It is hard to put all this together into a satisfactory substitute for meteorite impact. Nevertheless, one of the most massive outpourings of basalt in earth history did erupt in India close to the time of the K-T boundary, a worrisome coincidence for the pro-impactors. Another is that the greatest mass extinction of them all—the one between the Permian and Triassic periods—occurred at nearly if not exactly the same time as a huge outpouring of basaltic lava in Siberia.

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