Unit C: The Changing Earth

Alberta’s stromatolites are evidence of cyanobacteria, which are photosynthetic organisms that produced oxygen. Scientists believe that cyanobacteria were one of the first sources of oxygen for Earth’s early atmosphere.

Deep time is the theory that Earth has gone through a very long history (4.5 billion years) of development and change.

Alberta’s oldest rock outcropping exists in the Canadian Shield area of northeastern Alberta. The one featured in this chapter is the Slave Granite, which forms a series of four famous rapids in the Slave River. The Slave Granite is two billion years old.

Plastic materials are like toothpaste—they are solid but can flow under pressure. This is how the zones of rock behave within the mantle.

Fossils of coastal organisms, such as stromatolites, have been found along the Rocky Mountains. Also present are sedimentary rocks, which are indicative of coastal river deltas. And, finally, the presence of ripple marks, cracks, and salt crystals provide evidence of an ancient coastline.

Although each cyanobacterium is very small, a single colony could consist of millions of individuals. In addition, these organisms were among the dominant life forms on Earth for two billion years—nearly half of the planet’s total existence.

Banded iron formations show that oxygen levels in Earth’s early atmosphere fluctuated. When oxygen was abundant, red seams of iron were produced. When oxygen was scarce, grey seams were produced.

At the end of the Precambrian Era, Alberta’s coastal reef was home to stromatolites, small jellyfish, worm-shaped animals, and early plants.

An animal dies and sinks to the bottom of a body of water where it is buried by sediments. Eventually, the organism is under hundreds of metres of sediment that compress the layers to solid rock. The water is drained or evaporated, or the layers of rock are lifted above the sea. Erosion exposes the rock layers that contain the fossil. After the fossil has been discovered, it may be excavated.

The law of superposition states that for a sequence of rock layers, any given layer is younger than the one beneath it.

The rock cycle is a balanced and sustainable system that could seemingly go around and around forever. It shows no sign of a beginning or an end.

An igneous intrusion must be younger than the rock layers through which it cuts because the magma invades cracks in pre-existing rock.

The process of radioactive decay occurs when the nucleus of an unstable atom disintegrates to become a different element. During the process, energy called radiation is released in the form of high-speed particles.

Radioactive decay makes a good clock for measuring the absolute age of rocks and fossils because decay occurs in uniform increments of time called half-lives. If you can determine the percentage of the parent and daughter nuclides in a sample, you can use the decay curve to determine the absolute age of the sample.

The process of radioactive decay occurring in Earth’s core releases a tremendous amount of energy that is transformed to heat. This provides the thermal energy to melt old rocks and bring this molten material to the surface as magma.

Canada’s oldest rocks are from the Acasta Gneiss in the Northwest Territories. The gneiss contains tiny crystals called zircons. Uranium radiometric dating was performed on these crystals to determine their absolute ages.

The sandstones are older than the lava flow because they are beneath the lava flow, according to the law of superposition.

The lava flow:

84% uranium-235 remaining (parent nuclide)

According to the decay curve, 0.25 half-lives have elapsed.

Age = (0.25 half-lives)(7.13 × 10⁸ a/ half-lives)
Age = 1.8 × 10⁸ a (or 180 million years)

The lava flow is 180 million years old.

17% lead-207 (daughter nuclide)

100% – 17% = 83% uranium-235

According to the decay curve, 0.33 half-lives have elapsed.

Age = (0.33 half-lives)(7.13 × 10⁸ a/ half-lives)
Age = 2.4 × 10⁸ a (or 230 million years)

The granite intrusion is 240 million years old.

The volcanic ash layer:

74% uranium-235 remaining (parent nuclide)

According to the decay curve, 0.45 half-lives have elapsed.

Age = (0.45 half-lives) (7.13 × 10⁸ a/ half-lives)
Age = 3.2 × 10⁸ a (or 320 million years)

The volcanic ash layer is 320 million years old.

Fossil A must be between 320 million years old and 180 million years old because it is located between the lava flow (180 million years old) and the volcanic ash layer (320 million years old).

Fossil B must be between 320 million years old and 240 million years old. It is younger than the volcanic ash layer (320 million years old) because it is above it. It must be older than the granite intrusion (230 million years old) because the granite cut through the layer containing the fossil.

The Burgess Shale is regarded by many geologists as the best site for Cambrian fossils on Earth. Geologists suspect that an underwater mud avalanche composed of particles of very fine silt contributed to the incredible detail of these fossils. Their preservation is so good that even soft tissue has been preserved in remarkable detail. Most sites of Cambrian fossils preserve only the hard body parts, such as shells. The large numbers and types of organisms fossilized in the Burgess Shale represent an early snapshot of an ecosystem that is more than 500 000 years old.

Seismic waves are generated by using either explosives or a large metal plate attached to the bottom of a truck that hammers the ground. In both cases, P-waves are generated that travel down into the layers of subsurface rock. The speed of the P-waves depends upon the density of the rock that they are travelling through. When a P-wave meets a boundary between rock layers with different densities, a speed change occurs, resulting in a reflection. The reflected wave is detected at the surface by devices similar to sensitive microphones, which are called geophones. Information from the geophones is sent to a seismograph and computer where the data is processed to produce a seismogram. The seismogram can provide insights about the arrangement of subsurface rock formations.

Geological phenomena often associated with boundaries between two tectonic plates are earthquakes and volcanoes.

Sea-floor spreading at ocean ridges occurs when two crustal plates pull apart due to convection currents in the mantle. Geologists refer to this as a divergent boundary.

One feature was the jigsaw fit of the South American continent into the African continent. Another feature was the great similarity in fossils found on both continents. An example is the mesosaurus.

The first scientific reason why you should not purchase this fossil is that by 48 million years ago, the dinosaurs had already fallen victim to the Permian Extinction and were extinct. The second reason is that carbon-14 dating is only useful for dating things that are less than about 40 000 years old. Radiometric dating can be used to date fossils that are millions of years old, but the isotope used has to be one with a longer half-life. Another reason is that a footprint is a trace fossil and would not contain carbon for analysis.

If 93% of the parent nuclide has decayed, then only 7% of the parent nuclide remains. Using the “Radioactive Decay Curve” on page 557, this means that four half-lives have occurred. By referring to “Elements for Radioactive Dating” on page 557, you can determine that the half-life of uranium-235 is 7.04 × 10⁸ years.

The age of the basement rock is approximately 2.8 × 10⁹ a.

The “Table of Index Fossils” was used to answer this question.

Elrathia is a middle-Cambrian fossil, so the shale containing it is about 550 million years old.

The shale with the didymograptus fossil is from the Ordovician Period, so this shale is between 440 and 500 million years old.

Mucrospirifer is a fossil from the middle Devonian Period, so the limestone is about 385 million years old.

The layer of sandstone is between the two limestone layers. As a result, it must be between about 250 million and 380 million years old.

Bactrites is a Permian fossil, so this layer of limestone is between 250 and 290 million years old.

The geological phenomena likely observed on the continental crust beside each trench would be volcanic activity and earthquakes.

The rate of sea-floor spreading could be measured by using radiometric dating techniques to date the oceanic crust on the sea floor in several places on either side of the ocean ridge. If you knew the age of the crust and its distance from the ridge, you could calculate the rate of sea-floor spreading on each side of the ridge.

The rate of sea-floor spreading, which adds oceanic crust to Earth’s surface, must be balanced by the rate of subduction, which removes oceanic crust from Earth’s surface. Since Earth has a fixed surface area, any process that adds crust must be balanced by a process that removes crust.

The North American Plate is the over-riding plate because the arc of volcanoes always occurs on the over-riding plate. Therefore, the Juan de Fuca Plate is the subducting plate.

Since both arrows show the plates moving away from the ridge, the Juan de Fuca Ridge is a location of sea-floor spreading. Therefore, the mantle material below the ridge is moving up to the surface to spill out as lava on the sea floor.

The labels are as follows—the North American Plate is F., the Juan de Fuca Plate is C., and the Pacific Plate is A.

The Juan de Fuca Plate moved 12 m between 1700 and 2000.

When the elastic energy stored in the crust is suddenly released, the crust springs back. This motion involves the movement of a huge mass of crust, so an earthquake occurs. The seismic waves generated by this event carry the energy to other parts of Earth’s crust.

The Alaska Earthquake of 1964 was the second-largest earthquake ever recorded, and it involved Earth’s crust suddenly moving 9 m. If the North American Plate around Vancouver suddenly moved 12 m, it could be larger than the Alaska earthquake.

As the time since the last major earthquake grows, the Juan de Fuca Plate continues to move toward the North American Plate. This causes the continental crust of the North American Plate to continue to bend. The effect is similar to a spring that is compressed farther and farther with each passing year. The more time elapses, the greater the elastic energy that can be stored in the system. Therefore, there is more energy available to be released as an earthquake.

The Cascadia Megathrust Earthquake of 1700 is thought to have been one of the largest earthquakes to ever occur in North America. Geologists estimate that the Richter magnitude of this earthquake was more than 9.0.

The difference in arrival times between the S-wave and the P-wave leads to the following distances between each of the seismic stations and the epicentre of the earthquake: #1—540 km; #2—400 km; and #3—680 km.

The order of the stations from closest to furthest is #2, #1, and #3. This order is confirmed by the maximum amplitude of the S-wave from each station. Since #2 is the closest, the S-wave can transfer more energy. As a result, the maximum amplitude of the S-wave is largest for #2. As the stations get farther away, the maximum amplitude of the S-wave becomes smaller.

The first step in locating the earthquake epicentre is to locate each of the map’s seismic stations. Then use the map’s scale to draw a circle around each seismic station by using the distance to the epicentre as the radius of each circle. Since the circle around each station shows what places are the given distance from the station, the earthquake epicentre must lie somewhere on this circle. The one place that satisfies this condition for all three circles is the point where the three circles intersect—this is the epicentre of the earthquake.

This table shows how the Richter magnitude was determined from the data on the three seismograms.

Using the table, the Richter magnitude is approximately 6.7 for this earthquake.

The research and essay components that answer each of these questions will vary depending upon the most recent findings available on each topic and on the information you choose.

The Big Rock at Okotoks was carried from the Rocky Mountains by an advancing ice sheet during the last glaciation.

Ice flows like water under the influence of gravity’s pull, but at a much slower pace. Just like water, ice causes erosion by carrying material along as it flows.

The Cypress Hills were Nunatak during the last ice age. This means that the height of the plateau protected them from the grinding of the ice sheets that scoured lower areas. The ice that eroded away many other rock formations flowed around the Cypress Hills.

The drainage of the Bearspaw Sea was caused by the uplift of the Rocky Mountains. The increased height of that part of Alberta caused the Bearspaw Sea to drain to the southeast.

Glaciation occurs when continental ice sheets form on land masses near Earth’s polar regions and then advance outward to cover other parts of the globe.

Before the Rocky Mountains had experienced repeated glaciations, they were much more rounded with V-shaped valleys due to water erosion.

There was a cooling of Alberta’s climate during the Tertiary Period. A possible cause of this cooling trend could be that North America was moving farther north due to plate tectonics.

Mammals and feathered dinosaurs became Alberta’s dominant organisms after the extinction of the large dinosaurs.

Alberta was cooler than it had been in millions of years. It was a grassland home to large grazing animals including rhinoceroses and elephant-like woolly mammoths.

Scientists can analyze layers of calcium carbonate left in ocean sediments from the last 65 million years. The ratios of oxygen-18 to oxygen-16 incorporated into the shells of Foraminifera fossils in the layers are indicators of average temperature.

The climate in the Tertiary Period was cooling but was still warmer than it is today. There were repeated glaciations during the Quaternary Period, at which time ice sheets advanced to cover Alberta. At present, Earth is between glaciations.

A continental ice sheet forms when there is an annual accumulation of snow on a land mass. When the snow becomes thick enough, it is compacted into ice and begins to flow outward in all directions. Modern examples of continental ice sheets exist in Greenland and Antarctica.

A mountain glacier forms when snow accumulates year after year on a mountain. Eventually, the snow compacts into ice and begins to flow down the mountain. Probably the most famous glacier in Alberta is the Athabasca Glacier, but there are many other examples.

The glacier-fed creek on the left has exposed an outcropping of rounded boulders, gravel, and sand known as glacial till. Outwash from the glacier cuts the stream bed. This shows that a glacier must have covered this area at one time. This photo on the right shows the characteristics of mountain glacier landforms—U-shaped valleys with steep sides that end in sharp ridges. If the optional extension activity titled “Naming Glacial Landforms” is done, then you can also name some specific features. The round bowls are cirques, the peaks are horns, the ridges are arêtes, and the green lakes are tarns. Moraines can also be seen.

The ice core was drilled at Lake Vostok in Antarctica. It contained annual snowfall layers for the past 420 000 years. Oxygen-18: oxygen-16 ratios were analyzed in each layer to determine the temperature changes over that time period.

The Wisconsin Glaciation reached its maximum extent 18 000 years ago when the Pleistocene Ice Sheet advanced to cover nearly all of Alberta. It melted approximately 10 000 years ago, ushering in the Holocene Epoch.

Scientists can determine the direction of ice-sheet movement by analyzing the shapes of glacial landforms— such as drumlins—which literally point in the direction of ice-sheet flow. They can also observe the direction of scratch marks, formed as fill was dragged over bedrock.

The many continental glacial landforms, such as moraines, drumlins, eskers, kettle lakes, kames, and outwash plains, do not extend south of Wisconsin in the east and northern Montana in the west.

Mountain glaciers have given the Rockies a much more jagged appearance due to the formation of mountain glacial landforms, such as horn peaks, cirques, and arêtes.

Glaciers aid basic needs by providing fresh, clean drinking water. Thirsty Edmontonians obtain water from the North Saskatchewan River, which is fed by the Saskatchewan Glacier. Thirsty Calgarians obtain water from the Bow River, which originates in the Bow Glacier.

The current global warming trend is resulting in more annual melting than annual snowfall. Due to this, many mountain glaciers and ice sheets are shrinking.

Weather includes variables, such as temperature and precipitation, on a daily or even an hourly basis. Climate is an average of these variables for a much longer period of time—at least 30 years.

To increase the certainty in predictions about future climate changes, scientists are working to understand how the global climate system operates. Scientists are also trying to improve ways to record present conditions.

During Earth’s history, there have been about five major cold periods. Each period has lasted millions of years and featured repeated glaciations. Earth is currently experiencing one of these cold snaps. Luckily, people are enjoying a temporary warm period between glaciations.

Continents can drift to the polar regions where snow accumulates. Once ice sheets form, they have a global cooling effect. When oceans change shape due to plate tectonics, ocean-current patterns are changed, and these are significant determiners of climate.

Ice sheets are white. This means that they reflect all the colours of sunlight back out into space, rather than absorbing them. This helps ice sheets stay frozen and keeps Earth cool.

When the three types of variations—shape of orbit, axial tilt, and axial wobble—are combined, they closely correlate with the timing of the repeated glaciations during the Pleistocene Period.

The Vostok ice core shows a strong correlation between the atmospheric carbon dioxide concentration and the average temperature. Average temperature is determined by analyzing oxygen-18: oxygen-16 ratios, while concentrations of carbon dioxide can be measured from little bubbles of atmospheric gases trapped in annual ice layers.

People are mainly adding to the atmospheric carbon dioxide concentration by burning fossil fuels. This increase in atmospheric carbon dioxide is believed to be enhancing the natural greenhouse effect, causing an artificial increase in the average global temperature.

The enhanced greenhouse effect could cause a continued melting of the Greenland Ice Sheet to the point that it slows down the North Atlantic Conveyor. This would cause a cooling of northern places like Canada and, especially, northern Europe.

An inference that could be made from this evidence is that at some time it must have been much warmer in the Arctic.

Glacial ice is produced over many years as snow accumulates and is packed down. Gravity then acts on the ice to pull it down the slope.

Recall from Chapter 1 that a plastic is a material with the properties of a solid, but it can also flow under pressure. Glacial ice is said to have the properties of a plastic because a glacier flows very slowly as gravity acts to pull the ice down the slope.

The fresh water that flows from glaciers is used for drinking water, hydroelectric power, irrigation, and recreation.

The brewing controversy is the validity in predicting and understanding climate change as developed by computer modelling.

Although there are uncertainties in the ultimate cause of climate change, all indications point to human influence as a contributing factor.

The identification of human influence as a contributing factor implies that action should be taken to identify how people are causing climate change. Steps should then be taken to reduce or eliminate the human impact.

• life begins in sea
• algae, bacteria, primitive worms

• explosion of life
• increase in atmospheric oxygen
• jellyfish
• crustaceans
• snails

• ammonoides (ammonites)
• crinoids
• eurypterids
• water scorpions
• corals

• predatory fish
• large carbonate reefs

• much of Alberta’s oil and gas deposits

• carbonate shells

• first oil discovered from this time period

huge swamps, but largely marine

• giant dragonflies
• fin-backed reptiles

• large reptiles
• early dinosaurs

• oil and gas

• dinosaurs ruled

• oil and gas

• dinosaurs continue to roam
• T-Rex
• triceratops
• duck-bill dinosaurs
• flowering plants
• snakes
• Rocky Mountain building starts
• major extinction caused by asteroid impact and cooling climate

• oil sands
• oil and gas
• coal

• rise of mammals
• dominance of grasses and grazing mammals

• mammoths
• 80 000 years ago last ice age begins

• most rocks and fossils from early in Earth’s history destroyed by rock cycle

• new technological advances—ground-penetrating radar, GPS satellites, radiometric-dating techniques help scientists overcome challenges

• uniformitarianism versus catastrophism
• development of plate tectonic theory
• development of radiometric dating and dating Earth’s age
• various theories concerning climate change

• computer modelling
• mass spectrometer
• seismometers
• GPS
• ground-penetrating radar
• satellite imaging
• computer modelling
• uncertainty concerning theories about climate changes affects public policy

• P-waves
• S-waves
• surface waves

• technology used to track earthquakes
• lives saved by earthquake predictions

• inner core
• outer core
• mantle
• crust

• seismic waves used to probe internal structure of Earth

• sea-floor spreading
• subduction

• predictions of volcanic eruptions save lives
• GPS used to track, predict plate movement

• Juan de Fuca Plate being subducted under North American Plate

• lava lamp also works using the concept of convection

• radiometric dating
• uranium
• carbon

• moulds
• imprints
• tracks
• trails
• burrows
• actual remains

• fossil fuels Alberta’s main energy source
• presence of fossils in drill-core samples—a useful indicator of petroleum

• principle of superposition

• used to correlate drill cores for oil exploration

• mud cracks
• salt deposits
• limestone
• marine fossils
• oxygen-18: oxygen-16 ratios from ocean sediments

• understanding climate changes of past helps scientists predict changes in future and determine extent and nature of human impacts on climate

• cyanobacteria help provide oxygen to early atmosphere
• exponential increase in atmospheric CO2 from burning fossil fuels

• human-induced enhanced greenhouse effect

• drumlins
• eskers
• kames
• kettles
• till deposits

• sharp peaks of Alberta’s mountains largely due to glaciation
• much of Alberta’s drinking water from glaciers

• Vostok ice core from Antarctica record of 420 000 years climate and atmospheric composition

• Vostok ice core used to support theories about causes of climate change—used to make predictions concerning climate change

• plate tectonics affect continent position, ocean currents
• Milankovitch theory
• variations in solar radiation (sunspots)
• changes in atmospheric composition (carbon dioxide)
• enhanced greenhouse effect

• theories about causes of climate change form foundation for models used to predict future climate change—public policy based on scientific data concerning degree of human influence on global climate

The theory of uniformitarianism argues that Earth’s history of change can be explained in terms of geological processes still in operation.

Parent material: 93%

According to the decay curve, 0.10 half-lives have elapsed.

Age = (0.10 half-lives)(7.04 × 10⁸ a/half-lives)
Age = 7.0 × 10⁷ a or 70 million years old

Layer C is 70 million years old.

Answers will vary but could include one of the following:

• The organism is buried rapidly by fine material before the remains are subjected to the effects of scavengers and decomposition.
  
  
• The organism must be in an environment that is favourable to preservation when it dies (low oxygen, smothered in tree sap, freezing, etc.)
  
  

• Various rock units correlate with each other (must give examples such as sandstone layers or lava flows).
  
  
• The ages of the volcanic lava match.
  
  
• The pattern of magnetic reversals (paleomagnetism) matches.
  
  
• The overall pattern of rock units correlate well, from lava flow downward.

Answers will vary but could include the following:

• The rate of sea-floor spreading was faster during the reversed period.
• The period of magnetic reversal lasted longer than the normally magnetized period.