120 million years ago, an extraordinary burst of submarine volcanism struck
the Pacific Basin. It released vast amounts of gas-laden lava onto the ocean
floor. The volcanic spasm is evidenced by a collection of massive undersea
lava plateaus that formed almost simultaneously, the largest of which, the
Ontong Java, is about two-thirds the size of Australia. It contains at least 9
million cubic miles of basalt, enough to bury the entire United States under
3 miles of lava.
The surge of volcanism increased the production of oceanic crust as
much as 50 percent.This rise in volcanic activity provided perhaps the great-
est contribution to the warming of Earth by producing 4 to 8 times the pre-
sent amount of atmospheric carbon dioxide. As a result, worldwide
temperatures averaged 7.5 to 12.5 degrees higher than today.
For the next 40 million years, Earth’s geomagnetic field, which normally
reverses polarity quite often on a geologic time scale (Table 10), stabilized. It
assumed a constant orientation due to several mantle plumes that produced
tremendous basaltic eruptions.This greater volcanic activity increased the car-
bon dioxide content of the atmosphere, producing the warmest global climate
in 500 million years. Carbon dioxide also provided an abundant source of car-
bon for green vegetation and contributed to its prodigious growth, supplying
a substantial diet for herbivorous dinosaurs.
Polar forests extended into latitudes 85 degrees north and south of the
equator, as indicated by fossilized remains of an ancient forest that thrived on the
now frozen continent of Antarctica. Evidence of a warm climate that supported
lush vegetation is provided by coal seams running through the TransantarcticHistorical Geology
TABLE 10 COMPARISON OF MAGNETIC REVERSALS
WITH OTHER PHENOMENA (IN MILLION YEARS AGO)Magnetic Unusual Meteorite Sea Level Mass
Reversal Cold Activity Drops Extinctions0.7 0.7 0.7
1.9 1.9 1.9
2.0 2.0
10 11
40 37–20 37
70 70–60 65
130 132–125 137
160 165–140 173