Page 65 - The Restless Earth Fossils

64 Fossils

when geologists found magnetic anomalies deep beneath north-
ern Mexico. Further studies revealed a buried impact crater of the
right size for such a collision. Walter Alvarez describes many of
the details in his book T. rex and the Crater of Doom.
When it strikes, an object that size vaporizes itself and its
target with enough force to put tons of debris into Earth’s orbit.
The “dirt ring” blocks sunlight and causes green plants and the
animals that depend on them to die. Flaming rocks falling back to
Earth cause massive, worldwide fires. The blast itself kills directly
and creates huge ocean waves, or tsunamis.
But the evidence for mass extinction at extinction boundar-
ies sometimes comes from fields of study other than geology.
For example, paleontologist Peter Ward has spent much of his
career studying the worst mass extinction of all time—the one
at the boundary between the Paleozoic and Mesozoic. A 60-foot-
thick layer of mudstone separating hundreds of feet of limestone
marks this layer, which is located not far from Luning, Nevada.
“The limestones above and the limestones below are packed with
life,” he says in his book Under a Green Sky. “What a supreme
difference those two worlds show with clearly almost no survivors
of some catastrophe grabbing the river of life and giving it a 90-
degree kink into a whole new assemblage. . . .”
The culprit for this end-Permian extinction and many others,
Ward believes, is rapid and severe climate change. Earth has many
mechanisms for maintaining climate balance, but sometimes the
scales tip wildly. Long and extensive volcanic eruptions much
greater than anything seen in human history may sometimes
shift the balance. Such events happened both at this boundary
and during the KT event. The volcanoes poured vast amounts of
carbon dioxide and sulfur dioxide into the air. Carbon dioxide
acts like a thermal blanket, warming the Earth while sulfur diox-
ide forms sulfuric acid when dissolved in rainwater. The Earth’s
climate also changes more slowly as continental landmasses shift
positions and ocean currents change. Long-term variations in the
Earth’s orbit and tilt relative to the sun also contribute to alter-
nating periods of heat and cold.

RE_Fossils2print.indd 64 3/17/09 9:00:08 AM