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Geology Chapter 3
Transcript of Geology Chapter 3
It also gives clues about Alberta’s past. The 600 m thick section of sedimentary rock that lifts this area above the grasslands was not removed by the advancing ice sheet during the most recent Ice Age. 3.2 The Icy Epoch In northeastern Alberta, near the Athabasca River, you will find the Athabasca Dunes a field of sand piled in the middle of green forests. It migrates southward about 15 m per year, burying everything in its path. Sand came from a sheet of ice more than 1 km thick about 18 000 years ago.
1.7 mya glaciers began to advance on AB. At this time, the end of the Tertiary Period, the climate had become so cold that snow accumulated year after year.
The Pleistocene Epoch began 1.7 mya to 10,000 years ago. As the snow fell, the weight caused the snow to compact into ice and then the glaciers formed. 3.3 Explaining and Predicting
Climate Change Weather: state of the atmosphere for a particular place at a particular time (temperature, clouds, precipitations, humidity)
Climate: average daily/seasonal weather events that occur in a region over a long period of time
The Earth Simulator is the world’s most powerful supercomputer. It is used to model processes such as plate tectonics and deep ocean currents to help scientists predict future changes to Earth’s climate.
The Cenozoic Era, 65 mya to present is divided into 2 periods:
•Tertiary Period - 65 mya to 1.7 mya
•Quaternary Period – 1.7 mya to present
At the beginning of this Era, the collision between the North American Plate and the Pacific Plate was most intense and resulted in the building of the Rockies. At this time though, the mountains were round looking with V-shaped valleys.
The plate tectonics that built the mountains also caused AB and North America to slowly move northward As it did this, the climate got cooler Eventually the climate was cold enough to form glaciers that would carve out the jagged familiar features of the Rocky Mountains. This icy process began 1.7 mya.
Near the beginning of the Cenozoic Era, the Bearspaw Sea retreated southward leaving most of AB high and dry. It also dumped sediment on most of southern AB, making sedimentary rock rich in fossils. Recall, the beginning of the Cenozoic Era was marked by the Cretaceous extinction from the last period in the Mesozoic Era. Only smaller, feathered dinosaurs survived to flourish. One factor that likely led to this extinction was the drastic cooling of the global climate.
During the first 15 million years of the Tertiary Period, scientists believe there was a significant drop in the global temperatures. The cooling affected AB’s animal and plant life; tropical rain forests became temperate evergreen forests with rivers, lakes and swamps. It was cooler than the Cretaceous Period, but warmer than today.
The Cypress Hills formation contains many fossils from the Tertiary Period. There were an array of mammals and even crocodiles as well as horses and camels. After the Cretaceous Extinction, the surviving mammals were mainly small rodents. By 40 mya many new forms of mammals were appearing. The spread of these large herds of grazing species in the late Tertiary Period is closely linked to the dominance of grasses.
As the climate cooled, the deep and dense roots of grasses helped them survive. The mammals that could digest cellulose (ruminants) gave grasses a survival advantage because of their resistance to grazing and the AB landscape became grassland, home to giant herds of grazers and the predators that stalked them.
Scientists believe there was an overall cooling trend during the Tertiary Period. Absence of tree pollen suggest that the past climate was too cold for trees to survive. Foraminifera is a group of organisms that lives in oceans around the world. These tiny, single-celled animals have calcium carbonate shells and are found in a variety of marine environments. O2 present in the CaCO3 shells comes from the water. When they die, they settle to the bottom and become part of the sediment.
Formanifera are often used by scientists to deduce information about ancient environments because they have these favourable characteristics:
•hard shells preserve
•respond quickly to environmental changes During this chilly epoch, large mammals had advantages. Fossils collected near Medicine Hat reveal mammoths, horses, lamas, reindeer, camels and dogs roamed the land. As did the predators, lions, bears, sabre-tooth cats and giant birds.
In 1998, a research team drilled and extracted an ice core 2083 m long from Lake Vostok, a salty glacial lake buried under the Antarctic ice.
They analyzed this ice at each annual layer and measured concentrations of CO2, methane, and , dust to determine the average temperature at the time the layer formed. We now have a chronological record of Earth’s atmosphere for the last 420,000 years. The last glaciation was the Wisconsin Glaciation, which shows us how far south the ice had advanced before it stopped.
The largest advancing ice sheet was the Laurentide Ice Sheet, which originated west of where Hudson Bay is now. This ice entered AB and made it all the way to northern Montana before it stopped and started to melt. It fully receded around 10,000 years ago to mark the end of the Pleistocene Epoch and mark the start of the Holocene Epoch. As the ice sheet advances and retreats, it leaves characteristic landforms pg380
•drumlins: tear-drop shaped hill; one end tapered, one blunt
•kames: small round shaped hills like donuts
•sand dune: giant pile of sand, deposited by water
•hoodoos: tall, thin spire of rock; soft sediment bottom topped with harder stone
Recall that the Rockies started out with a round shape, except for the V-shaped valleys from water erosion. The jaggedness today is from the glaciers of the Pleistocene Epoch and thus inspired their name. A glacier is a large river of ice that forms on land and moves under the influence of gravity.
Earth’s largest glaciers are in the polar regions - the Antarctic and Greenland continental ice sheets. Ice is over 1 km thick at their centres, then flows outward eventually reaching the sea. Huge chunks break off and float away as icebergs (calving).
During this Epoch, snow also accumulated in the mountains and resulted in mountain glaciers.
By definition an ice age is a period during which ice sheets cover parts of the North and the South Hemisphere.
So according to this, Earth is in an ice age now.
A glaciation is a period during which polar ice sheets advance to cover large regions of North America and northern Europe.
Scientists believe there were at least 4 major glaciations during the Pleistocene Epoch. The Holocene Epoch began approx. 10,000 years ago, at the time of the great melt followed by the last glaciation. By now, people were inhabiting N. America.
The oldest archaeological evidence of humans living in N.America dates back to stone tools found in Bluefish Caves, in the Yukon, 16,000 years ago.
The rapid melting of the Laurentide Ice Sheet opened an ice-free corridor extending south through AB. Refer to Figure C3.31. It also released a large amount of water that filled enormous glacial lakes.
•cause: phenomenon that brings about an effect/result
•correlation: one phenomenon that accompanies another
•probability: a measure of how likely it is that an event will occur
The Quaternary Period was a series of repeated glaciation and warming. If this pattern continues, AB will experience its next glaciation within the next 100,000 years.
Evidence in rock strata around the world indicates that during most of history, Earth was warmer than today. This hot climate was punctuated by several long cold periods. See Fig C3.33
All the water at or near the Earth’s surface is contained in the hydrosphere.
The long periods of repeated glaciation during the Precambrian Era, Jurassic Period and Pleistocene Epoch appear to be random. This may be due to the random nature of plate tectonics.
One theory is that large land masses must be near the poles for a period of repeated glaciation to occur because the continental ice sheets must form on land to cause glaciations. Because snow is white, it reflects most of the solar energy back into space and has an overall cooling effect on the planet.
Oceans have giant convection currents that transport heat in the same way your refrigerator does. Collectively, they are called the global conveyor. Currently it has a warming effect on some parts of the world and a cooling effect on others. This system circulates warm water away from the tropics near the ocean’s surface and recirculates cold water of the polar regions using currents that flow opposite direction in the deep ocean. See page 385
Changes in the flow of ocean currents may explain the Pleistocene Epoch cold snap. 40 mya Australia separated from Antarctica creating the Indian Ocean and allowed the current to circulate around Antarctica. The cooling effect encouraged the formation of the Antarctic Ice Sheet and led to the Pleistocene Glaciation.
Old volcanic activity may have contributed to long-term climate change too. They are believed to be a major contributor to extinction during the Permian Period and to short-term fluctuations in climate.
During each major cold period, ice sheets have advanced and retreated several times. Repeated glaciations match changes in Earth’s orbit around the Sun and Earth’s rotation on its axis. This is called Milankovitch Theory.
Milankovitch found that Earth’s orbit varies in 3 ways:
•shape of the orbit (eccentricity)
•tilt of the axis of rotation
•wobble of axis of rotation
Each variation affects the amount of solar radiation reaching Earth’s polar regions. When they combine, they correlate with the timing of the Pleistocene Glaciations as shown by ocean sediment cores.
Scientists know that the Sun’s intensity changes and seems to follow a pattern determined by the frequency of sunspots Climatologists consider variations in the sunspots to have only a minor impact on climate change because the outputs are small.
There seems to be a correlation between the Maunder Minimum 1645 to 1715, a time when sunspots were few, and an unusually cold period that occurred during the Little Ice Age in Europe.
More recent theories about the cause tend to be based on changes in the North Atlantic Ocean’s circulation current. It suggests that a release of fresh water into the North Atlantic Ocean slowed the circulation current that normally warms Europe.
Complete the TABLE in your notebooks!
Top of p. 387 Earth’s atmosphere contains gases (CO2, methane) that trap heat near the Earth’s surface. Without this natural insulating effect, Earth would be much colder than even the coldest ice age.
CO2 in the atmosphere can change drastically. This can be due to natural activities like volcanic activity and weathering of carbonate rocks. These variations appear to correlate with changes in the average global temperatures over the past 160,000 years.
In more recent times, there has been a sharp increase in the CO2 concentrations in the atmosphere. This increase coincides with an exponential increase in the emissions of CO2 since the Industrial Revolution.
The largest human-caused source of atmospheric CO2 is the burning of fossil fuels. Scientists believe this increase is enhancing Earth’s natural greenhouse effect. Models predict that as CO2 emissions rise, the effect will be further increases in the average global temperature.
The prediction that global warming could end up causing a drastic cooling demonstrates the complexity of Earth’s climate. As Earth warms due to the enhanced greenhouse effect, mountain glaciers and continental ice sheets are melting. This melting releases fresh water into the North Atlantic Ocean.
If the atmospheric temperature were to get hot enough, the rate of melting would increase, resulting in less heat being pumped toward the poles from the tropics. This could have a rapid cooling effect resulting in advancing ice sheets in the north.
Chapter 3 Review
Pg 391-393 #1,3, 4, 6, 9, 17, 22, 25, 26, 28, 30