The Huronian ice ages and non-glacial periods separating them likely lasted a total of million years. Evidence suggests these glaciations reached equatorial regions at sea level.
Ice occurs in equatorial regions today, but only at high elevations. Geologic evidence of these ice ages was first discovered in , in glacial deposits near Lake Huron. Since then, geologists have discovered more evidence elsewhere in North America, as well as in South Africa, Western Australia, and northeastern Europe. Found near Whitefish Falls, Ontario, along the northern shore of Lake Huron, this dropstone landed in seafloor sediments under a floating glacier some 2.
Lindsey, USGS. The rise of oxygen did more than freeze the planet. At least twice between and million years ago, Earth fell into a deep freeze. Because the Cryogenian Period events occurred during a longer geologic era known as the Neoproterozoic Era, the deep freezes are sometimes referred to as the Neoproterozoic Snowball Earths. Scientists continue to debate the causes of Neoproterozoic freezes and the subsequent thaws.
Volcanoes may be the force that both pushed the planet into the glaciations and also pulled it out. About million years ago, most continents were clustered around the equator. Within this continental mix, geologists have identified evidence of what they call a large igneous province. Eruptions in this province could have cooled the planet in two ways. 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. The research also provides an important data point for scientists using the latest generation of global climate models.
It indicates that, for every doubling of carbon dioxide in the atmosphere, average global temperature can be expected to increase by 3. That's in the middle of the range predicted by the latest generation of climate models 1. This enabled them to directly combine fossil data about past temperatures with climate model simulations and to create maps that showed how temperatures varied in specific regions around the globe.
For more, see the University of Arizona news release. It is common in the sense that as a liquid it covers three quarters of the Earth's surface and has penetrated hundreds of feet into the ground. As ice and snow, it covers vast expanses of polar regions; and as a gas it fills the atmosphere. Water is unusual in the sense that it is one of the very few substances that exists in all three physical states liquid, solid, gas within the range of temperatures that we encounter in our everyday lives.
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