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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 irid-
ium, 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 by-
products 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 mag-
mas 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 bot-
tle of carbonated beverage, shooting plumes of dust and ash into the
stratosphere and showering debris for thousands of kilometers. In
1980, 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
