Evidence of the once-equatorial large igneous province that may have kickstarted the Cryogenian is preserved in Nunavut, Canada. Sills—intrusions of volcanic material into older rock layers—cut across older, sand-colored rock. The bands in the lighter rock result from the coastline rising after the glaciers that had weighed down the coast retreated. Image from Mike Beauregard, Wikimedia Commons. When volcanoes release sulfur dioxide, the gas undergoes chemical reactions in the atmosphere to form highly reflective sulfates—particles that block out sunlight, like billions of tiny mirrors.
Likewise, when volcanoes extrude large volumes of basalt, the rock weathering that follows can cool the planet. Over time, rain, wind, and chemical changes all eat away at volcanic rocks. Rainwater and groundwater percolating through rock can dissolve carbon dioxide, stripping it from the atmosphere and ultimately trapping it as carbonate minerals such as limestone.
Geologists have identified two glaciations during the Neoproterozoic: the Sturtian about to million years ago and the Marinoan about to million years ago.
Rock layers from these times show the most extensive evidence of extreme glaciations so far found in the geologic record. In between these deep freezes, Earth appears to have endured an equally remarkable hothouse.
This climate extreme, too, might be down to volcanic activity. Over the long term, volcanic emissions of carbon dioxide and the depletion of carbon dioxide by weathering of rocks can keep each other in check. But as ice enrobed most of the planet hundreds of millions of years ago, weathering probably slowed as conditions turned too cold for heavy precipitation. If the most extreme ice ages in Earth history were true Snowball Earth events—with no open ocean—our planet may have looked like a supersized version of Enceladus.
Volcanoes, however, kept cranking out carbon dioxide. With little rock-weathering or photosynthetic activity left to draw from the atmosphere, the greenhouse gas would have accumulated, leading to a gradual increase in global temperatures.
Once conditions warmed enough to melt tropical ice, the temperature increase would have accelerated. The subsequent big melt might have caused such dramatic, rapid weathering that it led to the second glaciation. As in the Huronian, glaciations of the Cryogenian Period reached sea level at the equator. But just how complete the Neoproterozoic ice coverage was—whether it was a Snowball Earth or a Slushball Earth —remains an area of active research.
The rock record indicates that nothing as extensive as the Huronian and Cryogenian glaciations has happened in the last million years, even though geologists have found evidence of several more ice ages. Although it has some competition from cold conditions occurring between and million years ago, the most significant ice age in the last half a billion years may be the most recent.
Striking during the time period known as the Pleistocene Epoch, this ice age started about 2. Like all the others, the most recent ice age brought a series of glacial advances and retreats. In fact, we are technically still in an ice age. All of human civilization—everything from the earliest scripts such as cuneiform to smartphones and tweets—has occurred within an interglacial.
About 50 million years ago, the planet was too warm for polar ice caps, but Earth has mostly been cooling ever since. Starting about 34 million years ago, the Antarctic Ice Sheet began to form.
Besides nauseating generations of ocean travelers, the Drake Passage opening created the Antarctic Circumpolar Current. Circling the now-frozen continent, the current may have reduced the amount of ocean heat reaching Antarctica, enabling Antarctic ice to form and grow.
Wind and waves make trips through the Drake Passage memorable. Its appearance due to plate tectonics maybe have contributed to the development of the Antarctic Ice Sheet. CC license by Flickr user Christopher Michel. Another land movement likely plunged the planet into its most recent ice age.
Prior to its formation, the Atlantic and Pacific Oceans freely exchanged tropical waters. By cutting off that exchange and sending warm, salty ocean water northward, the isthmus increased precipitation at high latitudes in the Northern Hemisphere. Snow accumulated into glaciers and eventually into ice sheets. Once Earth was cold enough for ice sheets to form, they waxed and waned over timescales of about 20, to , years, due partly to Milankovitch Cycles. Since most of the water on Earth's surface was ice, there was little precipitation and rainfall was about half of what it is today.
There were winters and summers during that period. While Homo sapiens evolved, many vertebrates, especially large mammals, succumbed to the harsh climate conditions of this period. One of the richest sources of information about life in the Pleistocene Epoch can be found in the La Brea Tar Pits in Los Angeles, where remains of everything from insects to plant life to animals were preserved, including a partial skeleton of a female human and a nearly complete woolly mammoth.
In addition to the woolly mammoth, mammals such as saber-toothed cats Smilodon , giant ground sloths Megatherium and mastodons roamed the Earth during this period. Other mammals that thrived during this period include moonrats, tenrecs hedgehog-like creatures and macrauchenia similar to a llamas and camels.
Although many vertebrates became extinct during this period, mammals that are familiar to us today — including apes, cattle, deer, rabbits, kangaroos, wallabies, bears, and members of the canine and feline families — could be found during this time. Other than a few birds that were classified as dinosaurs, most notably the Titanis , there were no dinosaurs during the Pleistocene Epoch.
The cold periods are called glacials ice covering and the warm periods are called interglacials. There were at least 17 cycles between glacial and interglacial periods. The glacial periods lasted longer than the interglacial periods.
The last glacial period began about , years ago and lasted until 25, years ago. At that point our Homo sapien ancestors had migrated from the warm African heartland into northern European and Eurasian latitudes severely impacted by the sinking temperatures.
Armed with big, creative brains and sophisticated tools, though, these early modern humans—nearly identical to ourselves physically—not only survived, but thrived in their harsh surroundings. For our Homo sapien forebears living during the last ice age, there were several critical advantages to having a large brain, explains Brian Fagan, an emeritus professor of anthropology at the University of California, Santa Barbara, and author of many books, including Cro Magnon: How the Ice Age Gave Birth to the First Modern Humans and Climate Chaos: Lessons on Survival from our Ancestors.
With the advent of language, knowledge about the natural world and new technologies could be shared between neighboring bands of humans, and also passed down from generation to generation via storytellers. Also through music, dance and art, our ancestors collected and transmitted vast amounts of information about the seasons, edible plants, animal migrations, weather patterns and more. The elaborate cave paintings at sites like Lascaux and Chauvet in France display the intimate understanding that late ice age humans possessed about the natural world, especially the prey animals they depended on for survival.
This painting in the Chauvet Cave in southern France dates to around 32,, B. The last ice age corresponds with the Upper Paleolithic period 40, to 10, years ago , in which humans made great leaps forward in toolmaking and weaponry, including the first tools used exclusively for making other tools.
One of the most important of these was called a burin, a humble-looking rock chisel that was used to cut grooves and notches into bone and antler, lightweight material that was also hard and durable. The intricate spearheads and harpoon tips made from that bone and antler were small and light enough to be carried on foot by hunters over long distances, and were also detachable and interchangeable, creating the first compound tools.
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