Download Performance Benchmark E

Earth Science Reading
Objective A1
More than 99% of the Earth’s energy comes from the Sun. This energy is transferred to Earth mostly through visible light. The Earth
also sends energy back into outer space, mostly as infrared radiation. On average, this transfer of this incoming and outgoing energy is
nearly equal. Scientist therefore state that the Earth is in radiative equilibrium with the Sun.
Of the Sun’s incoming energy, about 30% is reflected back to space. Another 19% of the incoming solar energy is absorbed by the
Earth’s atmosphere and clouds. The remaining 51% is absorbed by the Earth’s surface. Most of the energy that reaches the Earth’s
surface is in the form of visible light.
Figure 1: The global energy balance between the Earth’s surface and atmosphere (from
http://rst.gsfc.nasa.gov/Sect14/Sect14_1a.html)
To maintain equilibrium, the Earth
returns the energy it receives from the
Sun back to space as infrared light.
However, only 6% of the energy goes
directly from the Earth’s surface to
space. About 15% of the Earth’s surface
energy is absorbed by water vapor,
carbon dioxide and other gases in the
atmosphere. This is called the
greenhouse effect and is a common
condition on planets with relatively
thick atmospheres.
To learn more about the greenhouse
effect go to
http://epa.gov/globalwarming/kids/gree
nhouse.html
The remainder of Earth’s surface energy is transferred to the atmosphere in a more complex exchange involving sensible and latent
heat.
Sensible heat is the energy associated with the temperature of a body. A warm surface will be at a higher temperature. Sensible heat
flows from the surface to the atmosphere via convection (air circulations) or conduction (molecular motion).
Latent heat is the energy associated with phase changes. In the atmosphere, water vapor condenses forming clouds and precipitation.
This releases latent heat to the atmosphere. Latent heat also flows from the atmosphere to the surface during evaporation. Evaporation
cools the atmosphere.
To learn more about sensible and latent heat flux go to http://www.physicalgeography.net/fundamentals/7j.html
So, infrared radiative transfer combined with flux of sensible and latent heat provides the energy to the atmosphere. This energy,
which ultimately originated from the Sun, drives all of Earth’s weather and climate.
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Objective A2
Currently, Earth’s atmosphere consists primarily of just three gases. Argon, nitrogen and oxygen make up just over 99.9% of our
present day atmosphere (Table 1). At first blush, the atmosphere appears to be of relatively constant composition. In fact, argon,
nitrogen and oxygen, together with helium, neon and krypton, are often referred to as the “non-variable” gases of our atmosphere.
Water vapor is the most variable component, ranging from 0.01% to 3% of the atmosphere. Carbon dioxide (CO 2) has slowly, but
steadily, increased in abundance over the past several decades (Figure 1), but still constitutes less than 0.04% of the atmosphere.
Table 1. Composition of the Atmosphere
Major Permanent Gases in the Atmosphere
Nitrogen
Oxygen
Other Permanent Gases in the Atmosphere
Argon
Neon
Helium
Krypton
Hydrogen
Variable Gases in the Atmosphere
Water Vapor
Carbon Dioxide
Methane
Ozone
78.1 %
20.9%
0.9%
0.002%
0.005%
0.001%
0.00005%
0 to 4%
0.035%
0.0002%
0.000004%
Figure 1. Historic atmospheric carbon dioxide concentrations of carbon dioxide at the
Mauna Loa Observatory (from http://www.aip.org/history/climate/co2.htm)
Matter cycles through the atmosphere just as it does through other natural reservoirs. Gases might be added or removed during the
biological processes of photosynthesis and respiration. Minerals in Earth’s crust can chemically react with gases of the atmosphere
(the oxidation of iron-bearing minerals removes O2). Bodies of water dissolve certain atmospheric gases, such as CO 2, only to release
them at some later time. Further, the dissociated ions in ocean waters can react with atmospheric gases during the formation of
chemically precipitated minerals, tying up those gases in solid rock. Combustion of organic matter adds to atmospheric gases.
Volcanic eruptions and hydrothermal events can return to the atmosphere gases which have been stored for eons.
Concern exists over the quantity of greenhouse gases (e.g., CO 2) emitted into the atmosphere as the result of burning fossil fuels. Also,
significant quantities of methane (another strong greenhouse gas) released into the atmosphere via livestock flatulence, decomposition
of animal waste, and melting of the permafrost in the artic regions, particularly in Siberia and Alaska.
To learn more about methane entering the atmosphere and atmospheric composition, go to
http://www.physicalgeography.net/fundamentals/7a.html
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Geochemists attempt to estimate how long the various gases will remain in the atmosphere as they cycle in and out of other parts of
the environment. The concept of “residence time” has been developed to address these studies. Residence time, by definition, is: the
average length of time a substance persists in a system; the capacity per rate of influx.
The residence time of nitrogen (N2) is about 44 million years; that of oxygen (O2) is 7 million years. It is easy to see why we think of
these gases as non-variable components of the atmosphere. Conversely, the residence time of carbon dioxide (CO2) is 4 years while
that of methane (CH4) is 3.6 years. It does not take long to affect the atmospheric concentration of these two gases.
The discussion on our evolving atmosphere is not complete without addressing the composition of Earth’s earliest atmosphere.
Evidence suggests our ancient atmosphere was chemically reducing (as opposed to the oxidizing atmosphere of today). Hydrogen
(H2) and helium (He) were the most abundant gases in the universe at the time of Earth’s accretion, thus it is likely they helped form
an atmosphere. Their molecular masses were too low to be easily held by Earth’s gravity and the majority of H 2 and He were lost to
space. Volcanic outgassing is believed to have contributed water vapor, carbon dioxide, carbon monoxide, sulfur dioxide, hydrogen
sulfide, chlorine and nitrogen. As gases in the primordial atmosphere reacted, ammonia and methane formed.
To learn more about Earth's early years: differentiation, water and early atmosphere, go to
http://www.globalchange.umich.edu/globalchange1/current/lectures/first_billion_years/first_billion_years.html
Over time, two processes were working to add oxygen to Earth’s atmosphere. Beginning over 4 billion years ago, during the Hadean,
ultraviolet radiation caused the photochemical dissociation of water vapor in the upper atmosphere. Hydrogen and oxygen were both
released. Some of the oxygen remained as O2, while other oxygen was converted to ozone (O3). Ultimately, photochemical
dissociation may have contributed as much as 2% of the oxygen in our current atmosphere. Atmospheric oxygen increased as
primitive photosynthetic organisms began filling the warm oceans. Oxygen was produced as CO2 was consumed. Ancient
cyanobacteria produced so much O2 during the Proterozoic that by its end, around 545 million years ago, the atmosphere became
oxidizing, as evidenced by the prevalence of deposits of red, iron-rich sediments.
Figure 2. Most of the early Earth gases were released through land volcanism (from
http://rst.gsfc.nasa.gov/Sect19/Sect19_2a.html)
Figure 3. The Earth's atmosphere for at least the first two billion years was very oxygen-poor and hence reducing (from
http://rst.gsfc.nasa.gov/Sect19/Sect19_2a.html)
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To learn more about the evolution of Earth’s atmosphere, go to
http://www.geolor.com/geoteach/How_Did_Earths_Atmosphere_Evolve-geoteach.htm
As Earth matured and cooled, its rate of geothermal outgassing decreased. While Earth’s atmosphere does still change, it does so at a
slower pace than early in our history. Natural events are still of consequence, and more recently, human activities have made
significant changes to the composition of the atmosphere. In the last 200 years, human combustion of wood and fossil fuels has greatly
increased the concentrations of CO2 and other greenhouse gases in the atmosphere.
In the past 500,000 years, atmospheric concentrations of CO 2 have been significantly greater than current levels. Evidence from ice
coring in Greenland and Antarctica shows a strong correlation between greater concentrations of atmospheric CO 2 and significantly
higher atmospheric temperatures.
To learn more about the evidence supporting Earth’s changing atmosphere, go to
http://www.niwascience.co.nz/pubs/wa/09-1/ice.htm.
Ice core data have provided a very compelling line of scientific evidence suggesting that humans maybe causing global warming and
climate change because their activities are increasing atmospheric concentrations of CO 2 and other greenhouse gases.
An excellent site discussing global climate change is found at http://www.exploratorium.edu/climate/index.html.
Humans have also significantly altered the atmosphere through emissions of chlorofluorocarbons (CFCs). During much of the 20 th
Century, CFCs were used to propel aerosols, and also in refrigeration. Through these uses, CFCs were released to the atmosphere.
While CFCs themselves are not harmful, they have a very long residence time in the atmosphere and will eventually migrate to the
Earth’s stratosphere. In the stratosphere, CFCs react with O3 (ozone) found naturally in this atmosphere layer. In particular, these
reactions cause stratospheric O3 concentrations to be significantly reduced over the Earth’s poles, resulting in what are commonly
called “ozone holes.” Because stratospheric O3 absorbs ultraviolet light from space that is harmful to biota, these ozone holes present a
significant danger to humans, animals, and plants in the arctic and antarctic regions. In 1993, worldwide production and use of CFCs
was virtually eliminated through international agreements. However, due to the long atmospheric residence times of CFCs, the ozone
hole phenomenon persists.
To learn more about stratospheric O3 depletion, go to http://www.epa.gov/ozone/science/index.html.
Objective A3
The greenhouse effect is an increase in the atmospheric temperature caused by water vapor, carbon dioxide and other greenhouse
gases that absorb and retain heat radiation which would otherwise escape from Earth. Jean-Baptiste Fourier, a French scientist,
speculated as to the possibility of the greenhouse effect in 1827. Seven decades later, in 1896, a Swedish chemist named Svante
Arrhenius observed, as a result of the Industrial Revolution, that more carbon dioxide was being released into the atmosphere. He
believed that carbon dioxide levels would rise as industry grew. Arrhenius predicted that this increase would eventually cause a rise in
the temperature of Earth.
Contrary to what many believe, the greenhouse effect is not a bad thing. Without the greenhouse effect, Earth’s average global
temperature would range from around 21C to 33C cooler than it is today, which would be far too cold for the flora and fauna living
on the planet. What is of concern is the “enhanced greenhouse effect,” a phenomenon in which increasing atmospheric concentrations
of greenhouse gases might lead to excessive global warming. For more on the enhanced greenhouse effect, go to
http://www.science.org.au/nova/016/016key.htm
For an overview of concepts associated with the greenhouse effect, go to http://www.ucar.edu/learn/1_3_1.htm
The temperature of Earth and its atmosphere is influenced by the atmosphere in a number of ways. Radiant energy from the sun
encounters the atmosphere. Part of the radiant energy is reflected away, part is absorbed by the atmosphere, and part passes all the
way through the atmosphere and reaches Earth’s surface (Figure 1).
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Figure 1: Interactions of Solar Radiant Energy with Earth’s Atmosphere (from National Center for Atmospheric Research,
http://www.ucar.edu/learn/1_3_1.htm)
See Benchmark E.12.A.1 for a more complete explanation of the Sun as the source of Earth’s Energy.
Clouds (and their constituent water vapor) might either reflect or absorb solar radiation. Generally speaking, clouds situated high in
the atmosphere absorb some radiation and warm the atmosphere. Clouds lower in the atmosphere, near Earth’s surface, are more
reflective and have a cooling effect. Overall, clouds seem to have a slight cooling effect on atmospheric temperature.
Another family of gases which cause cooling in the atmosphere is comprised of anthropogenic (man-made) sulfur compounds. Sulfur
dioxide and various sulfate gases released as ground-level pollution from automotive and industrial emissions block solar radiation.
The result is a cooling effect. These sulfate aerosols have a residence time of only about a week, so they do not accumulate in the
atmosphere over time the way other gases can. Thus, their effect is contingent upon their continued release in pollution.
Most people are concerned about the concentration of greenhouse gasses in the atmosphere. In too great a concentrations, the gasses
might absorb much of the infrared radiation that is radiated from Earth’s surface, and lead to a dangerous increase in global
temperatures. The following websites do a nice job of discussing greenhouse gases and how they affect atmospheric temperature.
PhysicalGeography.Net: Introduction to the Atmosphere
http://www.physicalgeography.net/fundamentals/7h.html
Australian Greenhouse Office
http://www.greenhouse.gov.au/education/factsheets/what.html
National Oceanic and Atmospheric Administration
http://lwf.ncdc.noaa.gov/oa/climate/gases.html#cd
The greenhouse gases which are of greatest importance are: water vapor, carbon dioxide, methane, nitrous oxide, chlorofluorocarbons
and halocarbons. Water vapor is the most abundant greenhouse gas, and is very efficient in absorbing infrared radiation, but its
atmospheric concentration is highly variable. The rapid turnover of water vapor in the lower atmosphere, via the water cycle, prevents
it from accumulating over time. Water vapor has a limited effect over land, but the high concentration of water vapor over warm
oceans, such as the Pacific Ocean, can lead to what has been termed a “supergreenhouse effect” and a very significant increase in the
temperature of the ocean surface as well as the air above it. This effect can contribute to greater storm intensity.
Carbon dioxide is the gas most often thought of during discussions on the greenhouse effect. Levels of carbon dioxide, in modern
times, began increasing with the Industrial Revolution. Figure 2 “ Trends in Atmospheric Concentrations…” depicts the trend in
atmospheric carbon dioxide concentration over the past 250 years. Carbon dioxide concentrations have been obtained both from
measurements taken from ice cores as well as directly from the atmosphere. Clearly, carbon dioxide levels have significantly
increased.
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Figure 2. Trends in Atmospheric Concentrations and Anthropogenic Emissions of Carbon Dioxide
(http://www.eia.doe.gov/oiaf/1605/ggccebro/chapter1.html)
There is debate over what this increase means. Some scientists are convinced further increases will inevitably lead to a disastrous
increase in global temperature, with ice caps melting and sea levels rising. Other scientists point out that modern levels of
atmospheric carbon dioxide are far lower than levels during our geologic past (Figure 3). The highest concentrations of CO2 during
the previous 600 million years of history occurred during the Cambrian Period (nearly 7000 ppm -- almost 19 times higher than the
360 ppm of today). Scientists who argue there are many more factors to consider than just carbon dioxide point out there was an ice
age during Late Ordovician while CO2 concentrations (at 4400 ppm) were around 12 times higher than today. If high levels of carbon
dioxide must lead to global warming, Earth should have been exceedingly hot. Instead, global temperatures were cooler than today.
Figure 3. Global Temperature and Atmospheric CO2 over Geologic Time (from Climate and the Carboniferous Period,
http://www.clearlight.com/~mhieb/WVFossils/Carboniferous_climate.html)
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Methane (CH4) is added to the atmosphere via microbial fermentation in wetlands, emissions from coal mines, gas pipelines and oil
wells, additionally Methane is produced in the stomachs of ruminant animals, such as cattle, and is produced in large quantities by
termite colonies. While methane exists at far lower concentrations (1.65 ppm) than does CO 2, it is far more effective at absorbing
infrared radiation than is carbon dioxide, and its concentration must be taken seriously (Figure 4).
Figure 4. Concentrations of Greenhouse Gases (from Hyperphysics, http://hyperphysics.phyastr.gsu.edu/hbase/thermo/grnhse.html#c3)
Nitrous oxide (N2O) is added to the atmosphere through the burning of biomass, the use of chemical fertilizers, and to a small extent
from the combustion of fossil fuels. Nitrous oxide has a lengthy residence time, almost 120 years. Because of this, it tends to
accumulate over time, causing problems in both the troposphere, where it contributes to warming, and in the stratosphere, where it
contributes to the destruction of ozone.
Chlorofluorocarbons and halocarbons are found at levels from 10 ppt to 20 ppt, but, as shown in Figure 4, are around 10,000 times
more effective at absorbing infrared radiation than carbon dioxide. These are introduced to the atmosphere strictly by human activity.
They are used as refrigerants, solvents and fire retardants. We have begun to use a less damaging form of chlorofluorocarbon in
domestic and automotive air conditioners in an attempt to ameliorate the damaging effects to our atmosphere.
For greater detail on the various greenhouse gases, visit “Greenhouse Gases and Society” at,
http://www.umich.edu/~gs265/society/greenhouse.htm.
The greenhouse gases act in concert to warm Earth. As the incoming solar radiation reaches Earth, the shorter wavelength radiation
(such as visible light and ultraviolet light) is able
to pass through the atmosphere more easily than can the longer wavelength radiation, such as infrared radiation, microwaves and
radiowaves (Figure 5). Greenhouse gases in the upper atmosphere absorb some of the infrared radiation, but much of the infrared
coming from the sun is reflected back out to space.
Figure 5. A Comparison of Wavelengths of Energy from Solar Radiation (from National Center for Atmospheric Research,
http://www.ucar.edu/learn/1_3_1.htm)
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Some of the shorter wavelength radiation which reaches Earth’s surface is absorbed, a process which converts it into infrared
radiation. This warms the surface, which in turn radiates the infrared energy outward and into the atmosphere. Greenhouse gases in
the atmosphere also absorb this radiation, further increasing the temperature of the atmosphere, and re-radiating the infrared back to
the Earth. Some scientists liken this to having “trapped” the infrared radiation by an atmosphere which acts as a thermal blanket.
The greenhouse gases in Earth’s atmosphere currently block and absorb only a portion of infrared (heat) radiation (Figure 6). The
question is what concentration of greenhouse gases will cause excessive absorption of infrared radiation. It is a question, currently,
without a definitive answer, as there is not consensus across the scientific community.
Figure 6. The Greenhouse Effect (from
Saskatchewan Interactive,
http://interactive.usask.ca/ski/media/drawings/agriculture/greenhouse.jpg)
Objective C1
Imagine that your favorite author has just released a book and you were able to attain an advanced copy. You set aside some time to
begin this much anticipated adventure only to find out that the book is incomplete. Upon turning to the first chapter, all that is written
is the title followed by 30 pages of blank paper; worse still this pattern repeats for the next several chapters. Eventually a single word
appears, followed by mainly blank pages in which only a few single words are scattered. An appreciable way into the book you
discover short phrases, eventually followed by longer phrases, and finally complete sentences. From this point the remaining portion
of the story is captivating, and you experience a robust story of life, death, and change with fully developed characters and an
intriguing narrative. At the end of the book you are left with many questions as to how this story began and to the characters’ origins
and early experiences. Just like the above scenario, the story of Earth and the life it supports has changed throughout time. The
charge of the geoscientist is to piece together the parts of the story that are incomplete.
One of the challenges facing geoscientists is to determine the exact age and history of Earth that has brought it to its current situation.
Earth is a dynamic planet that has evolved over time. As a result, continents have been moved, mountains created and leveled
(eroded), meteors impacted, volcanoes erupted, earthquakes jolted, and rocks recycled and destroyed by the process plate tectonics.
Earth’s biologic diversity and evolution have also been critical players in our changing planet. To help piece together the story of
Earth an understanding of the rock cycle, importance of fossils, geologic timescale, and age dating techniques – both relative and
absolute - need to be explored.
Rock Cycle
Rocks are naturally occurring aggregates of one or more minerals. All rocks are composed of material that has been present on Earth
since it first formed – excluding that material which has been delivered by meteorites. Although the constituent material used to
create rocks has not changed appreciably over time, their arrangements have. The rock cycle (Figure 1) is a model that illustrates the
changes to rocks that have taken place through time. Rocks are recycled into other rocks through processes which occur in mainly
two locations; at or near Earth’s surface such as weathering, erosion, and deposition; and deep below the surface such as melting and
increased heat and pressure. Most rocks are formed from other rocks and a “rock” may take more than one path through the rock
cycle. The arrows on the rock cycle diagram shows various processes one rock type (i.e. metamorphic) could advance through in
order to become a different rock type (i.e. sedimentary).
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Using the rock cycle diagram, let’s examine the processes a metamorphic rock passes through to become a different rock type.
Starting at metamorphic rock and following the outside arrows clockwise our metamorphic rock would need to experience an increase
in temperature to the point of melting it, creating magma. Eventually this magma body would enter an environment where the heat
contained would transfer from it (cooling) and the process of solidification (crystallization) occurs. This rock is now classified as an
igneous rock. Continuing clockwise from igneous, several more changes must occur in order to turn this igneous rock into a
sedimentary rock. The igneous rock needs to be subjected to the agents of weathering and erosion, which over geologic time creates
pieces or fragments of rock called sediment. As this sediment piles up, compaction and cementation turn the loose sediment into a
solid rock through the process of lithification. This rock is now classified as a sedimentary rock. Continuing clockwise this
sedimentary rock will become a metamorphic rock with the addition of heat and pressure causing a partial melting of some of the
minerals in the sediment. This process is referred to as metamorphism and results in creation of a metamorphic rock. The straight
arrows within the rock cycle diagram indicate that any one rock type can turn into any other rock type by passing through several
common processes.
Figure 1. The Rock Cycle (from http://piru.alexandria.ucsb.edu/collections/geosystems/geosystems11-06.jpg).
For an interactive rock cycle animation, go to
http://www.classzone.com/books/earth_science/terc/content/investigations/es0602/es0602page02.cfm
For additional information related to the rock cycle and three classifications of rocks, go to
http://www.windows.ucar.edu/tour/link=/earth/geology/rocks_intro.html
Fossils
Fossils are the remains, molds, or traces of living organisms preserved in media such as sedimentary rock (i.e. sandstone, siltstone,
shale or limestone), amber, ice, or tar. Fossils provide evidence of past forms of life and are used by paleontologists, geologists,
biologists and others to learn about the past history of Earth. The oldest fossils found indicate life on our planet began at an age of
well over 3 billion years ago. These organisms were simple, single-celled organisms. Increasingly complex multi-cellular organisms
began to evolve, as preserved in rock, dating from about one billion years old. A significant number of fossils (and diversity) begin
around 550 million years ago, as organisms with hard parts burst onto the scene – this is referred to as the Cambrian explosion.
For an animation showing how fossils form, go to
http://www.classzone.com/books/earth_science/terc/content/visualizations/es2901/es2901page01.cfm?chapter_no=visualization
For detail on the Cambrian explosion, go to
http://www.ucmp.berkeley.edu/cambrian/camb.html
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The Principle of Superposition states that, for an undisturbed rock sequence the oldest rock layer is on the bottom, and the higher up
one travels, the younger the rock layers become. When comparing fossils in undisturbed strata, fossils can be found in upper strata
which, although different from fossils in lower strata, resemble those fossils. This suggests links between modern forms and older
forms, as well as divergent pathways from common ancestors. If we examine fossils found in various layers of rock, and look at
progressively older layers, we can see that there is a layer below where no human fossils are naturally found. As we progress
backward in time, we will eventually see a layer below where no fossils of birds, no mammals, no reptiles, no fish, and eventually, no
animal of any kind are found. This is evidence that the kinds of plants and animals, and other organisms have changed over time and
is called the Law of Fossil Succession.
Further detail about fossils can be found in the TIPS L12D3 performance benchmark.
To learn more about the importance of the fossil record and the law of fossil succession, go to
http://pubs.usgs.gov/gip/fossils/succession.html and http://www.fossilmuseum.net/fossilrecord.htm
There are gaps in the branches of the fossil records of life. Gaps exist in the fossil record, partly because plants, microorganisms, and
soft shelled organisms (majority of marine animals), are not likely to fossilize. Even hard bodied organisms do not frequently
fossilize. In addition, changes in the land resulting from forces on our dynamic planet (i.e., erosion, metamorphosis, and geological
events) can destroy fossils if they were present. However, the fossil record does provide significant evidence of evolution and of the
history of life on earth.
For more information on fossils, visit
http://www.museum.vic.gov.au/prehistoric/what/index.html and http://www.museum.vic.gov.au/dinosaurs/sciprocess.html
Geologic Time
Earth's history has been divided into a series of time intervals (Figure 2). These time intervals are not equal in duration like hours in a
day. Instead they are variable in duration because geologic time is divided using significant events in the history of the Earth. For
example, the boundary between the Permian and Triassic is marked by a global extinction in which a large percentage (nearly 90%!)
of Earth's plant and animal species went extinct – the end-Permian mass extinction is perhaps the most severe extinction the planet has
seen. Another example is the boundary between the Precambrian and the Paleozoic which is marked by the first appearance of
animals with hard parts, called the Cambrian explosion occurring some 550 million years ago.
Figure 2. These time scales are drawn to scale so you can compare the relative lengths of geologic time divisions.
(from http://wrgis.wr.usgs.gov/docs/parks/gtime/gtime2.html)
The geologic time scale consists of (from longest to shortest duration) eons, eras, periods, and epochs. The eon is the largest division
of geologic time, which is made up of several eras and continues for hundreds of millions or billions of years. Essentially there are
two eons, the Precambrian (which covers ~90% of all geologic time) and the Phanerozoic. An era is a geologic division including
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several periods, but of shorter duration than an eon. In general, eras last for many tens or hundreds of millions of years, and are often
characterized by distinct life-forms. The Paleozoic era “age of ancient life” was a time dominated by marine invertebrates. The
Mesozoic era “age of middle life” was a time dominated by reptiles, of which the most famous were the dinosaurs. The Cenozoic era
“age of recent life” is a time dominated by mammals, of which the most famous are reading this TIPS benchmark. The period is the
most commonly used unit of geologic time, representing one subdivision of an era. Each period generally lasts for some thirty to
eighty million years. Lastly, epochs, only found in the Cenozoic, are smaller divisions of a period. To describe the current geologic
time (from smallest to largest division), we are in the Holocene Epoch of the Quarternary Period within the Cenozoic Era of the
Phanerozoic Eon.
For detailed information related to the geologic time scale and an interactive model of major geologic events within the various
divisions of time, go to
http://www.palaeos.com/Timescale/default.htm
The Smithsonian interactive geologic timeline is a really great resource for background information related to geologic time, along
with detailed surface and atmospheric conditions,
http://paleobiology.si.edu/geotime/main/
Dating Methods
Dating the age of rocks is critical to reconstructing Earth's history. Geologists rely on two basic types of dating: relative dating and
absolute dating. Relative dating places historical events in their correct order, but does not yield numerical estimates of how many
years ago the events happened. Absolute dating establishes how many years ago a given event took place. The most important
methods of absolute dating are based on the decay of naturally occurring radioactive elements. It may seem odd that the two
procedures are kept distinct - if scientists can determine the numerical ages of rocks and fossils, they should be able to put the
evidence of ancient life in the correct historical order. The problem is that only some types of rocks and fossils can be numerically
dated, so all other evidence of ancient life must be related to age-dated material by the techniques of relative dating.
Relative Dating
James Hutton (known as the Father of Modern Geology) advanced the concept of geologic time and strengthened the belief in an
ancient world. Hutton first proposed formally the fundamental principle used to classify rocks according to their relative ages. He
concluded, after studying rocks at many outcrops, that each layer represented a specific interval of geologic time. Further, he
proposed that wherever undisturbed layers were exposed, the bottom layer was deposited first and was, therefore, the oldest layer
exposed; each succeeding layer, up to the topmost one, was progressively younger. This came to be known as the Principle of
Superposition.
Figure 3. Superposition (from http://cse.cosm.sc.edu/hses/RelatDat/pages/superpos.htm).
Hutton also proposed the Principle of Uniformitarianism, which states “The present is the key to the past”. This manner of thinking
assumes that geologic forces and processes (gradual as well as catastrophic) acting on the Earth today are the same as those that have
acted in the geologic past. An example of uniformitarianism would be to observe today how rocks weather into sediment and pile up
(typically in a body of water) and, as more and more weight is added, the lower layers become compacted and cemented together
forming a sedimentary rock – with layers building horizontally (known as the Principle of Original Horizontality). Upon seeing a
sequence of faulted, folded, or tilted rock layers, one can assume that these layers were originally deposited horizontal. Once formed,
these layers were then subjected to geologic forces that altered their original state.
To see an animation of how originally horizontal layers become tilted, go to
http://www.classzone.com/books/earth_science/terc/content/investigations/es2903/es2903page04.cfm
An unconformity is a buried erosion surface separating two rock layers of different ages. An unconformity represents time during
which no sediments were deposited and the local record for that time interval is missing. The steps required to form an unconformity
are; deposition of sediments creating horizontal rock layers, uplift and tilting, erosion (removal of material), followed by further
deposition (Figure 4).
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Figure 4. Steps to create an unconformity
(from http://www.winona.edu/geology/MRW/mrwimages/Earth%20History/angular_uncof.JPG).
For an animation showing the steps to form an unconformity, go to
http://www.classzone.com/books/earth_science/terc/content/visualizations/es2902/es2902page01.cfm?chapter_no=visualization
The Principle of Cross-Cutting Relations states that any feature that cuts across a layer must be younger than the layer it cuts through.
Igneous intrusions, faults, and erosion surfaces can cut across any features, including other igneous rocks, other faults, and erosion
surfaces. As a consequence, the principle of cross-cutting relations is extremely important in narrowing the relative age of a geologic
event.
Figure 5.
Cross-cutting by
erosion.
Figure 6.
Cross-cutting by
fault.
Figure 7. Cross-cutting by igneous rock.
(from http://cse.cosm.sc.edu/hses/RelatDat/pages/crosscut.htm)
The sequence of geologic events in an area can be solved by applying the fundamental principles of geologic relative age dating (e.g.,
Principle of Superposition). In this diagram, which shows a hypothetical exposure of rock, it is possible to determine the order in
which the various geologic events occurred. See if you can tell the story of how this sequence can to be the way it is – from oldest to
youngest.
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Figure 8. Hypothetical exposure of rock (from http://www.nysedregents.org/testing/scire/es606.pdf).
Answer from oldest to youngest;
1. Conglomerate 2. Shale 3.Sandstone 4. Siltstone 5. Limestone 6. Breccia (using the Principle of Superposition), followed by; 7.
Basalt Intrusion (and contact metamorphism)
8. Fault (using the Principle of Cross Cutting Relations) 9. Erosion.
For more background information and examples of relative dating of rock layers, go to
http://gpc.edu/~pgore/geology/historical_lab/relativedating.htm and
http://gpc.edu/~pgore/geology/historical_lab/reldat_exercises.html
Absolute Age Dating of Rocks
Radioactive elements are unstable; they breakdown spontaneously into more stable atoms over time, a process known as radioactive
decay. Radioactive parent elements decay to stable daughter elements. This decay occurs at a constant rate, specific to each
radioactive isotope (Figure 9), and is not affected by changes in temperature and pressure.
Figure 9. Parents and daughters for some isotopes commonly used to establish numeric ages of rocks
(from http://pubs.usgs.gov/gip/fossils/numeric.html).
Each radioactive isotope has its own unique half-life. A half-life is the time it takes for half of the radioactive isotope (parent
material) to decay to a stable element (daughter product). The proportion of parent to daughter reveals the number of half-lives, which
can be used to find the age in years.
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Figure 10. Radioactive decay graph showing parent isotope decay (red line) and time for each half-life (blue line). From
http://www.palaeos.com/Geochronology/radiometric_dating.htm
For example, if there is an equal amount of parent and daughter (such as; 500 atoms of both carbon-14 and nitrogen-14 in the graph
above), then one half-life has passed. If two complete half-lives have passed then 25% (or 250 atoms) of radioactive parent and 75%
(or 750 atoms) of daughter product are present.
For more information about radioactive half-life, visit TIPS P12C4 performance benchmark.
Radiometric dating has been used to determine the ages of the Earth, Moon, meteorites, ages of fossils, including early man, timing of
glaciations, ages of mineral deposits, recurrence rates of earthquakes and volcanic eruptions, the history of reversals of Earth's
magnetic field, and many of other geological events and processes.
For additional information related to radiometric dating visit
http://www3.interscience.wiley.com:8100/legacy/college/levin/0470000201/chap_tutorial/ch01/chapter01-3.html and
http://wrgis.wr.usgs.gov/docs/parks/gtime/ageofearth.html#date
Radiocarbon dating
All living plants and animals have a constant ratio of carbon-14 (radioactive carbon) to carbon-12 (nonradioactive carbon). After the
death of an organism, the amount of radiocarbon gradually decreases as it radioactively decays to nitrogen-14. Radiocarbon dating
works by measuring the amount of radioactivity remaining in organic materials (amount of carbon-14). From this the age of the
organic material can be determined.
For example, if carbon from a sample of wood is found to contain only half as much carbon-14 as that from a living plant, the
estimated age of the old wood would be 5730 years. If only ¼ as much carbon-14 was present, then the estimated age would be
11,460 years [2 half-lives x 5730 years/half-life].
The radiocarbon clock has become an extremely useful and efficient tool in dating the important episodes in the recent prehistory and
history of man, but because of the relatively short half-life of carbon-14, this method can be used for dating events that have taken
place only within the past 50,000 years, and therefore is not useful for dating older geological events.
Extensive background information on radiocarbon dating can be found at
http://www.c14dating.com/k12.html
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