In geologic time , the Cenozoic Era, the third era in the Phanerozoic Eon , follows the Mesozoic Era and spans the time between roughly 65 million years ago (mya) and present day. On the geologic time scale, Earth is currently in the Cenozoic Era of the Phanerozoic Eon.
The Cenozoic Era contains two geologic time periods, including the Tertiary Period (65 mya to approximately 1.8 mya) and the current Quaternary Period (1.8 mya to present day). The Tertiary Period is also sometimes referred to in terms of a Paleogene Period and a Neogene Period. When referred to in terms of a Paleogene Period and a Neogene Period, the Paleogene Period extends from approximately 65 mya to 23 mya and the Neogene Period from 23 mya to 2.6 mya. The Quaternary Period is also termed the Anthropogene Period. These periods are further subdivided into six different major epochs, including the Paleocene Epoch , Oligocene Epoch , Miocene Epoch , and Pliocene Epoch , Pleistocene Epoch and current Holocene Epoch .
The onset of the Cenozoic Era is marked by the K-T boundary or K-T event—the mass extinction of non-avian dinosaurs marking the boundary between the Cretaceous Period of the Mesozoic Era and the Tertiary Period of the Cenozoic Era.
At the start of the Cenozoic Era, North America and Europe were separated by a widening ocean basin spreading along a prominent mid-oceanic ridge. North America and South America were separated by a confluence of the future Pacific Ocean and Atlantic Ocean, and extensive flooding submerged much of what are now the eastern and middle portions of the United States. By start of the Cenozoic Era, water separated South America from Africa , and seafloor spreading continued to push the continents apart. The Australian and Antarctic continents were clearly articulated and the Antarctic continent had begun a southward migration toward its present position in the south polar region. At the outset of the Cenozoic Era, the Indian plate and subcontinent remained far south of the Eurasian plate and continent.
By 30 mya, the modern continental arrangement was easily recognizable. Although still separated by water, the land bridge between North and South America began to reemerge. Antarctica assumed a polar position and extensive ice accumulation began on the continent. The Indian plate drove rapidly northward of the equator to close with the Eurasian plate. Although still separated by a shallow straight of water, the impending collision of the plates that would eventually form the Himalayan mountain chain had begun. The gap between North America and Europe continued to widen at a site of sea-floor spreading along a prominent mid-Atlantic ridge. By mid-Tertiary Period, the mid-Atlantic ridge was apparent in a large suture-like extension into the rapidly widening South Atlantic Ocean that separated South America from Africa.
Well into the Cenozoic Era, by the start of the Quaternary Period some 2.6 million years ago, Earth's continents assumed their modern configuration. The Pacific Ocean separated Asia and Australia from North America and South America, the Atlantic Ocean separated North and South America from Europe (Euro-Asia) and Africa. Separated by the straits of Indonesia, the Indian Ocean filled the basin between Africa, India, Asia, and Australia. The Indian plate driving against and under the Eurasian plate uplifted rapid mountain building. As a result of the collision, ancient oceanic crust bearing marine fossils was uplifted into the Himalayan chain. The collision between the Indian and Eurasian plate continues with a resulting slow—but measurable—increase in the altitude of the highest Himalayan Mountains (e.g., Mt. Everest) each year. Although glacial sheets advance and recede in cyclic patterns (i.e., reestablish new terrain altering ice ages, the basic patterns of glaciation evident today were established during the Quaternary Period.
Many geologists and paleontologists assert that the K-T extinction resulted from a cataclysmic asteroid impact in an area now located underwater near the Yucatan Peninsula of Mexico. The impact caused widespread primary damage due to the blast impact and firestorms. The major damage to Earth's ecosystem occurred when the debris and smoke from the collision and subsequent fires moved into the atmosphere to block a sufficient amount of light from the Sun that photosynthesis was greatly slowed. The resulting climatic changes and food shortages led to extinction of the largest life forms (those with the greatest energy needs), including the dinosaurs.
Although mammals evolved before the Cenozoic Era, the reduction in predator species allowed land mammals to dominate and thrive—eventually setting the stage from the evolution of homo sapiens (humans).
See also Archean; Cambrian Period; Dating methods; Devonian Period; Eocene Epoch; Evolution, evidence of; Fossil record; Fossils and fossilization; Geologic time; Historical geology; Jurassic Period; Mississippian Period; Ordovician Period; Paleozoic Era; Pennsylvanian Period; Precambrian; Proterozoic Era; Quaternary Period; Silurian Period; Supercontinents; Tertiary Period; Triassic Period