The asteroid that has long been believed to have wiped out the dinosaurs hit the Yucatan peninsula
formed the Chicxulub crater 65.5 mya. However, by this time, the
dinosaurs were mostly long gone (as evidenced by the severe lack of
dinosaur fossils around the world in the three meters below the
sedimentary layer containing the "iridium" left by the asteroid strike).
300,000 years prior to this dramatic event, volcanic emissions and
basalt flows on an enormous scale had already occurred on the
subcontinent of India and formed the Deccan Traps. It is believed to
have been caused by India drifting (15 million years prior to this,
India had separated from Pangea and moved northward) over a "hotspot"
(mantle plume) and/or a meteor strike hitting slightly off its western
shore (Shiva crater) that shocked the Indian continental crust and
opened up fissures. This had led to such increased CO2 and sulphur
dioxide emissions that 75% or more of all plant and animal species on
land and sea died off.
NOTE: About 250 mya, the Great Dying
extinction at the end of the Permian Era was caused by excessive
volcanic basaltic flows that formed the Siberian Traps (5 miles thick)
that also caused phenomenally extensive coal fires. The CO2 of this
global warming event raised atmospheric temperatures on land to as much
as 140 degrees F. and also acidified the oceans. This volcanic activity
is believed to have been triggered by multiple asteroid strikes on
Pangea's to be Antarctica (Wilkes Land crater).
[The concentration of
seismic waves from a meteorite impact at the antipode (opposite side of
a sphere) on the earth would be the cause of many of the massive lava
floods of the past when fissures were opened.]
Sulphur dioxide is a toxic gas that causes acid rain. Both volcanic
emissions and the burning of coal release it into the atmosphere, thus
making the Great Dying especially toxic.