Cretaceous Period

Cretaceous Period, in geology, latest time period of the Mesozoic Era, lasting from about 145 million to about 65 million years before present. The Cretaceous was preceded by the Jurassic and followed by the Paleogene periods. The name Cretaceous alludes to the abundance of chalk (Latin creta) strata deposited during the latter part of the period in England and France and now exposed at sites such as Dover. In Europe and North America, geologists divide the period into Lower (145 to 100 million years ago) and Upper (100 to 65 million years ago) intervals.

At the beginning of the Mesozoic Era, all the continents had been joined as one landmass, Pangaea. The breakup of Pangaea in the early Mesozoic created two supercontinents: Laurasia, consisting of what are now the northern continents; and Gondwanaland, consisting of the southern (see Geology: The Geologic Time Scale). Between the two lay a vast sea, the Tethys, of which today's Mediterranean is a greatly shrunken remnant. See also Plate Tectonics.

In the Cretaceous period the African continental plate broke from Gondwanaland and drifted north, subjecting Tethys Sea sediments to powerful compressions and creating the roots of the European Alps. Later the African plate plunged beneath the Laurasian one, triggering the volcanic activity that persists today in Italy and Sicily. Meanwhile the newly formed South Atlantic Ocean widened due to seafloor spreading along the Mid-Atlantic Ridge, broadening the gap between Africa and South America. Farther east, India had separated from Gondwanaland and, drifting north, bucked eastern Tethys sediments into elongate ridges that were the forerunners of today's Himalayas. Antarctica and Australia, still joined, drifted south and westward.

The continuing westward movement of North America generated mountain-building forces that culminated in the uplift of the Rocky Mountains and California's Sierra Nevada. The rising Rocky Mountains blocked westward draining of the advancing Late Cretaceous sea, turning much of the western interior of North America into a vast swamp. In the east, sediments produced by erosion of the Appalachian Mountains formed the Atlantic coastal plain.

During the Late Cretaceous, sea levels rose worldwide, submerging about one-third of Earth's present land area. This allowed heat from the Sun to be distributed farther poleward by ocean currents, producing a warm, mild global climate with ice-free poles and Arctic water temperatures of 14°C (58°F) or higher. In such a climate, cold-blooded reptiles could exist even in northern latitudes. Fossil ferns and cycads found in Cretaceous rocks at Arctic latitudes are similar to plants growing today in subtropical rain forests. By the Late Cretaceous period the flora had taken on a modern appearance and included many of today's genera of trees, such as the oak, beech, and maple.

Despite these mild conditions, several mass faunal extinctions occurred toward the end of the period. Five great reptilian groups—dinosaurs, pterosaurs, ichthyosaurs, plesiosaurs, and mosasaurs (see Dinosaur)—that had been dominant became extinct. A recent theory is that a comet or small asteroid colliding with Earth 65 million years ago blasted enough dust into the atmosphere to reduce incoming solar radiation and temperatures worldwide, devastating the algae, vegetation, and small animals on which the large reptiles depended for food.