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silica and in dissolved gases than the lavas of the Ring of Fire, mak-
ing 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 qui-
etly, it is hard to see by what process they could produce the re-
quired 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 po-
tentially lethal amounts of chemicals and cause a global mass extinc-
tion 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.
F
LOOD
B
ASALTS
The Indian Deccan traps occur over an area of at least 1 million
km
2
.
(Deccan
is Sanskrit for southern;
traps
is Swedish for staircase,
which the edge of a giant sequence of nearly horizontal lava flows
sometimes resembles.) In places they are 2 km thick. Their total
volume exceeds 1 million cm
3
, more than the outpourings of all the
Ring of Fire volcanoes put together. Scientists believe that the Dec-
can eruptions produced 30 trillion tons of carbon dioxide, 6 trillion
tons of sulfur, and 60 billion tons of halogens, gases that enhance the
greenhouse effect.
To reflect their vastness and mode of eruption, geologists call
these enormous outpourings flood basalts.
1
1
They occur in the geo-
logic record from the Precambrian to the Tertiary and on nearly
