Large eruptions of continental flood basalts are a
probable cause of mass extinctions. Such eruptions
wreak havoc on the environment by causing the
following: acid rain (which lowers the pH of the
surface seawater), addition of volcanic ash and
carbon dioxide in the atmosphere (which alters the
climate) and depletion of the ozone. The Deccan
traps in India (an ancient continental flood basalt)
were dated and found to be active 65 million years
ago. By studying present active volcanoes we
know that they do emit a minor amount of iridium.
Thus the iridium rich layer found in the K/T clays (see Asteroid Impact
) could be linked to volcanism as well as an asteroid. However
volcanism cannot produce some of the evidence found in the K/T clays including spherules,
shocked quartz and widespread soot.