The Cenozoic Era: The Modern World Comes to Be

The Cenozoic Era

The Cenozoic Era is focused which we know today of the world, the modern world.

Paleogeography

 The two main active continental orogenic systems on the Earth today. The Alpine-Himalayan system formed when Africa, India, and Australia collided with Asia (inset). The Cordilleran and Andean systems reflect the consequences of convergent-boundary tectonism along the eastern Pacific Ocean.

During the last 65 million years, the map of the Earth has continued to change, gradually producing the configuration of continents and plate boundaries we see today. The final stages of the Pangaea breakup separated Australia from Antarctica and Greenland from North America, and formed the North Sea between Britain and continental Europe. The Atlantic Ocean continued to grow because of sea-floor spreading on the Mid- Atlantic Ridge, and thus the Americas have moved westward, away from Europe and Africa. Meanwhile, the continents that once constituted Gondwana drifted northward as the intervening Tethys Ocean was consumed by subduction. Collisions of the former Gondwana continents with the southern margins of Europe and Asia resulted in the formation of the largest orogenic belt on Earth today, the Alpine-Himalayan chain (figure above). India and a series of intervening volcanic island arcs and micro-continents  collided with Asia to form the Himalayas and the Tibetan Plateau to the north, while Africa along with some volcanic island arcs and micro-continents collided with Europe to  produce the Alps. 

As the Americas moved westward, convergent plate boundaries evolved along their western margins. In South America, convergent-boundary activity built the Andes, which remains an active orogen to the present day. In North America, convergent-boundary activity continued without interruption until about 40 Ma (the Eocene Epoch), yielding, as we have seen, the Laramide orogeny. Then, because of the rearrangement of plates off the western shore of North America, a transform boundary replaced the convergent boundary in the western part of the continent by 25 Ma. When this happened, volcanism and compression ceased in western North America, the San Andreas Fault system formed along the coast of the United States, and the Queen Charlotte Fault system developed off the coast of Canada. Along the San Andreas and Queen Charlotte faults today, the Pacific Plate moves northward with respect to North America at a rate of about 6 cm per year. In the western United States, convergent-boundary tectonics continues only in Washington, Oregon, and northern California, where subduction of the Juan de Fuca Plate generates the volcanism of the Cascade volcanic chain.

The Basin and Range Province is a rift. The inset shows a cross section along the red line.

As convergent tectonics ceased in the western United States south of the Cascades, the region began to undergo rifting (extension) in roughly an east-west direction. The result was the formation of the Basin and Range Province, a broad continental rift whose development has stretched the region to twice its original width (figure above). The Basin and Range gained its name from its topography the province contains long, narrow mountain ranges separated from each other by flat, sediment filled basins. This topography formed when the crust of the region was broken up by normal faults. Blocks of crust above these faults slipped down and tilted. Crests of the tilted blocks form the ranges, and the depressions between them, which rapidly filled with sediment eroded from the ranges, became basins. 

The Basin and Range Province terminates just north of the Snake River Plain, the track of the hot spot that now lies beneath Yellowstone National Park. As North America drifts westward, volcanic calderas formed along the Snake River Plain; Yellowstone National Park straddles the most recent caldera.

Recall that in the Cretaceous Period, the world was relatively warm and sea level rose so that extensive areas of continents were submerged. During the Cenozoic Era, however, the global climate rapidly became cooler, and by the early Oligocene Epoch (34 Ma), Antarctic glaciers reappeared for the first time since the Triassic. The climate continued to grow colder through the Late Miocene Epoch, leading to the formation of grasslands in temperate climates. About 2.5 Ma, the Isthmus of Panama formed, separating the Atlantic completely from the Pacific, changing the configuration of oceanic currents, perhaps leading the Arctic Ocean to freeze over. 

The maximum advance of the Pleistocene ice sheet in North America.

During the overall cold climate of the past 2 million years, continental glaciers have expanded and retreated across northern continents at least 20 times, resulting in the  Pleistocene Ice Age (figure above). Each time the glaciers grew, sea level fell so much that the continental shelf became exposed to air. At times, a land bridge formed across the Bering Strait, west of Alaska, providing migration routes for animals and people from Asia into North America. A partial land bridge also formed from southeast Asia to Australia, making human migration to Australia easier. Erosion and deposition by the glaciers created much of the landscape we see today in northern temperate regions. About 11,000 years ago, the climate warmed, and we entered the interglacial time interval we are still experiencing today.

The present-day Bahamas serve as an example of what the interior of the United States might have looked like during intervals of the Paleozoic. Shallow land areas were submerged and became the site of shallow-marine sedimentation.

Life evolution

When the skies finally cleared in the wake of the K-T boundary catastrophe, plant life recovered, and soon forests of both angiosperms and gymnosperms grew. The grasses, which first appeared in the Cretaceous, spread across the plains in temperate and subtropical climates by the middle of the Cenozoic Era, transforming them into vast grasslands. The dinosaurs, except for their descendants the birds, were gone for good. Mammals rapidly diversified into a variety of forms to take their place. In fact, most of the modern groups of mammals that exist today originated at the beginning of the Cenozoic Era, giving this time the nickname Age of Mammals. During the latter part of the era, huge mammals appeared (such as mammoths, giant beavers, giant bears, and giant sloths), but these became extinct during the past 10,000 years, probably because of hunting by humans.

It was during the Cenozoic that our own ancestors first appeared. Ape-like primates diversified in the Miocene Epoch (about 20 Ma), and the first human-like primate appeared at about 4 Ma, followed by the first members of the human genus, Homo, at about 2.4 Ma. Fossil evidence, primarily from Africa, indicates that Homo erectus, capable of making stone axes, appeared about 1.6 Ma, and the line leading to Homo sapiens (our species) diverged from Homo neanderthalensis (Neanderthal man) about 500,000 years ago. According to the fossil record, modern people appeared about 200,000 years ago, and initially shared the planet with two other species of the genus Homo, the Neanderthals and the Denisovans. The last Neanderthals and Denisovans died off over 25,000 years ago, leaving Homo sapiens as the only human species on Earth. 

Earth’s history reflects the complex consequences of plate  interactions, sea-level changes,  atmospheric changes, life evolution, and even meteorite impact. In the past few millennia, humans have had a huge effect on the planet, causing changes significant enough to be obvious in the geologic record of the future.
Credits: Stephen Marshak (Essentials of Geology)

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