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| TIPS: Targeted Interventions for Proficiency in Science | |||
Earth Science
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Students know how successive rock strata and fossils can be used to confirm the age, history, and changing life forms of the Earth, including how this evidence is affected by the folding, breaking, and uplifting of layers. E/S 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 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.
For an interactive rock cycle animation, go to For additional information related to the rock cycle and three classifications
of rocks, go to
Fossils For an animation showing how fossils form, go to http://www.classzone.com/books/earth_science/terc/content/visualizations For detail on the Cambrian explosion, go to 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 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
Geologic Time
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 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 The Smithsonian interactive geologic timeline is a really great resource
for background information related to geologic time, along with detailed
surface and atmospheric conditions,
Dating Methods
Relative Dating
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 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).
For an animation showing the steps to form an unconformity, go to 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.
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.
Answer from oldest to youngest; For more background information and examples of relative dating of
rock layers, go to
Absolute Age Dating of Rocks
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.
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
Radiocarbon dating Extensive background information on radiocarbon dating can be found
at
Performance Benchmark E.12.C.1 Students know how successive rock strata and fossils can be used to confirm the age, history, and changing life forms of the Earth, including how this evidence is affected by the folding, breaking, and uplifting of layers. E/S Common misconceptions associated with this benchmark: 1. Students have difficulty with the numerical literacy required to identify with geological and biological concepts. We have all been there: pose a question to students such as “How tall is that building?” and their typical responses range from 100 feet to two miles! Now think about the human life span of experience. We can expect to live 60 or 80, maybe even 100 years. To a high school student of 16 years old…100 appears incredibly far away. Now consider the age of the Earth and the geologic time scale. When talking about dates in Earth’s history such as; 10 thousand years ago the last ice age ended, or 65 million years ago to dinosaurs went extinct, or further still the formation of the Earth 4.6 billion years ago students will have difficulty relating to these large numbers. In the intervention strategies and resources section of this benchmark, several activities are presented to assist students in gaining a deeper understanding of scale and time. For further information on this misconception and for strategies
to address it, visit the Action Bioscience website from the American
Institute of Biological Sciences at Overcoming geological misconceptions article from Planet
No.17, December 2006 How big is 1 billion – scale perspective activity
2. Students incorrectly believe that fossil evidence does NOT support evolution because there are too many “missing links” or missing transitional fossils. A transitional fossil is one that links a more modern organism with a more primitive organism. A transitional fossil would have characteristics in common with both the primitive organism and the more modern organism. Transitional fossils are often called “missing links.” According to evolutionary theory, however, all organisms are in transition, and therefore, a specific “missing links” may not actually exist as organisms evolve. In addition, there are many organisms that have existed in the past for which no fossils will ever be found, so there will always be gaps in the fossil record. This is because conditions required for fossilization to occur are not always present when on organism dies. Many examples of transitional fossils do exist, providing evidence that species do transition. Several examples are listed below, with links for additional information. For greater detail on this misconception visit
3. Students incorrectly believe that fossils are pieces of dead animals and plants. While in some unique circumstances actual remains of organisms become preserved (such as saber-toothed cats in tar, mosquitoes in amber, and a mammoth in ice), the majority of fossils are not actually pieces of dead animals and plants. They are only the impression or cast of the original organism or plant. The actual living portion of the organism decay away but their shape is permanently recorded in the rock as it formed. To learn more about this misconception go to A straight forward overview of what fossils are can be found at
4. Students incorrectly think that fossils of tropical plants cannot be found in deserts. Fossils and the rocks in which they are found record ancient environments present during the time the rocks were deposited. The climate and topography in a particular region could have been very different in geologic past from current day conditions. For example, during the Paleozoic Era, Southern Nevada was a shallow sea teeming with marine life, in sharp contrast from the desert conditions of today. Plate tectonics resulted in shifting land masses, carrying continents from tropical regions of the planet to temperate and polar regions. As a result of this movement, fossils can be found in areas that today could not support those types of organisms. For more information on this and other misconceptions go to, To see the distribution of key fossils, access the following link.
Once there, “click” the reptile image to see where fossils
were found. For a review of the geologic evolution of Southern Nevada (part of
the virtual field trip of Frenchman Mountain), go to
5. Students may incorrectly believe that radiometric dating is unreliable. When presenting the facts about radiometric dating to students, it is important that they understand how isotopic age dating works and that it is very accurate – typically less than 1% error of measurement. Methods used for radiometric dating are based on sound physics, the same physics that is at work in the students’ everyday lives. Often times students only know about one or two isotopes (typically U-238 and/or C-14 ) and are not aware of other common radioactive isotopes (see figure 9 for a partial list) that can be tested within the same material yielding similar ages – supporting the reliability of radiometric dating. For more information on this and other misconceptions, go to For an activity that models radioactive decay titled Determining
Age of Rocks and Fossils, Performance Benchmark E.12.C.1 Students know how successive rock strata and fossils can be used to confirm the age, history, and changing life forms of the Earth, including how this evidence is affected by the folding, breaking, and uplifting of layers. E/S Sample Test Questions
Use the rock cycle diagram above to answer question 1 and 2 1. Sandstone is a sedimentary rock which forms
as a result of 2. What must occur in order for an igneous rock
to form? 3. Which statement is most accurate regarding
the current fossil record? Base your answers to questions
4 and 5 on the diagram below,
4. Which layer is the oldest? 5. Which geologic principle of relative age dating
provides the reasoning for why layer C is the youngest? 6. The study of the rock record suggests that 7. The half-life
of carbon-14 is approximately 5700 years Use the diagram below to answer question 8
8. How many additional boxes would need to be
shaded in order to accurately represent the additional decayed material
formed during the second half-life? 9.
Which of the graphs above most accurately represents
changes in life on Earth over time? Use the following graph to answer questions 10-12 Radiometric dating graph showing
the decay of the
10. What fraction of radioactive parent atoms
remain after 2 half-lives? 11. What would happen to the half-life of this
radioactive isotope if it were taken to the below-freezing temperatures
of the North Pole? 12. The amount of initial radioactive material
is doubled from 1000 to 2000 atoms. What affect does doubling the
radioactive material have on its half-life?
Students know how successive rock strata and fossils can be used to confirm the age, history, and changing life forms of the Earth, including how this evidence is affected by the folding, breaking, and uplifting of layers. E/S Answers to Sample Test Questions Performance Benchmark E.12.C.1 Students know how successive rock strata and fossils can be used to confirm the age, history, and changing life forms of the Earth, including how this evidence is affected by the folding, breaking, and uplifting of layers. E/S Intervention Strategies and Resources The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark. 1. Who’s On First? A Relative Age
Dating Activity To access this activity go to http://www.ucmp.berkeley.edu/fosrec/BarBar.html.
2. Relative Dating – Telling Time
Using Fossils The unit summary can be found at To download this PDF Lesson visit http://www.pbs.org/americanfieldguide/teachers/fossils/fossils.pdf
3. Teaching Geoscience with Visualizations:
Using Images, Animations, and Models Effectively The link to visualizations can be accessed at On the Cutting Edge search page for activities, assessments, and
more is found at
4. Virtual Fossil Museum – Fossil
Image Directory Link to Virtual Fossil Museum homepage, The fossil image directory can be directly accessed at
5. Construct Seven Paper Models that Describe
Faulting of the Earth To reach the teachers guide for this activity, go to To access the print friendly models, go to
6. Smithsonian National Museum of Natural
History – Geologic Time: The Story of a Changing Earth To access this site, go to
7. Geologic Time Activity To access this activity, go to
8. Understanding Geologic Time To access this educational module, go to |
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