Mesozoic Era

Dramatic Rifting

After the vigorous convergent plate mountain-building of the late Paleozoic, Mesozoic tectonic deformation was comparatively mild. Nevertheless, the era featured the dramatic rifting of the supercontinent, Pangaea. Pangaea gradually split into a northern continent, Laurasia, and a southern continent, Gond-wana. Laurasia became North America and Eurasia, while Gondwana split into South America, Africa, Australia, Antarctica and the Indian subcontinent, which collided with the Asian plate during the Cenozoic, the impact giving rise to the Himalayas. The Triassic was generally dry, a trend that began in the late Carboniferous, and highly seasonal, especially in the interior of Pangaea. Low sea levels may also have exacerbated temperature extremes. Because much of the land that constituted Pangaea was distant from the oceans, temperatures fluctuated greatly, and the interior of Pangaea probably included expansive areas of desert. Abundant evidence of red beds and evaporites, such as salt, support these conclusions. Sea levels began to rise during the Jurassic, probably caused by an increase in seafloor spreading. The formation of new crust beneath the surface displaced ocean waters by as much as 656 ft. (200 m.) more than today, which flooded coastal areas. Furthermore, Pangaea began to rift into smaller divisions, bringing more land area in contact with the ocean by forming the Tethys Sea. Temperatures continued to increase and began to stabilize. Humidity also increased with the proximity of water, and deserts retreated.

Widely Disputed

The climate of the Cretaceous is less certain and more widely disputed. Average temperatures were higher than today by about 18 degrees F (10 degrees C). In fact, by the middle Cretaceous, equatorial ocean waters may have been too warm for sea life, and land areas near the equator may have been deserts, despite their proximity to water. The circulation of oxygen to the deep ocean may also have been disrupted. Large volumes of organic matter accumulated because they were unable to decompose and were eventually deposited as black shale.

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