1. Understanding Earth: Chapter 8 - Clocks in Rocks: Timing the Geologic Record
2. Earth Revealed. Courtesy of Anneberg Media, URL <http://www.learner.org/resources/series78.html>.  Requires Windows media Player.  Sign in and view #10 Geologic Time,  and #11 Evolution through Time
3. LABS: Relative Dating Geologic Events (preliminary exercise) / Investigating the geology of Forest River Park (pdf)

Recommended:

• Explore Geologic Time with Enchanted Learning Software. This interactive site lets you explore the events of each period.
• Tour geologic time and learn about the history of the time scale from the University of California's Museum of Paleontology
• Online Labs from sciencecourseware.com > Virtual Dating

Terms: relative age, absolute age, isotope, laws of relative age dating (superposition, cross-cutting relationships, faunal succession, inclusion), index fossil, correlation

So just how are geologic events recorded in rocks?

Rock and the features they contain are the product of past events and can therefore be used to unravel what has happened in the past. Here are some examples.

Features	Event and relevant information
Sedimentary rocks	Deposition: land subsidence or rise in sea level, prevailing climate and geologic processes
Igneous rocks	magmatism: rifting, hotspot activity, or subduction
Regional Metamorphism	Deformation: tectonic activity
Folds	Deformation: tectonic activity
Faults	Deformation: tectonic activity
Unconformities (buried erosion surfaces)	Erosion, often accompanying and following uplift, tectonic activity

Relative Age vs. Absolute Age

Relative age dating entails placing events in sequential order, from oldest to youngest. In doing so there are a number of common sense principals or laws that are applied.

• Laws:
  • Law of original horizontality: Sediments are typically deposited in horizontal layers parallel to the Earth's surface. Therefore unless they were disturbed by folding and faulting sedimentary rock formations are horizontal. This law sets the stage for the following Principal of Superposition.
  • Principal of superposition: Younger sedimentary rocks are deposited on older rocks. Therefore the rocks on top are younger than those on the bottom.
  • Law of cross cutting relations: A dike, fault, or feature that cuts across a body of rock has to be younger than the rock it cuts.
  • Law of inclusion: The lithology included in another rock body such as a clast in a conglomerate or a xenolith in a pluton has to be older than the layer or pluton that includes it.
  • Law of faunal succession: Groups of fossil animals and plants have succeeded one another in a definite and non-repeating order. Fossils used to date rocks are called index fossils, and must have had a wide geographic distribution, short geologic life span as a species, and readily preservable hard parts.

Exercises in relative age dating

Problem 1:

List the events (A-D) in order of their relative age. Remember the oldest (first) event is always on the bottom. 4. __________________ 3.___________________ 2. ___________________ 1. ___________________

Feature 12.______ 11.______ 10.______ 9._______ 8._______ 7._______ 6._______ 5._______ 4._______ 3._______ 2. _______ 1._______	Event _____________________ _____________________ _____________________ _____________________ _____________________ _____________________ _____________________ _____________________ _____________________ _____________________ _____________________ _____________________

Examples of possible events that may or may not be found: folding and uplift, faulting (related to extention or compression), sedimentation, igneous activity (related to convergence or extension), erosion, etc.

Problem 4: Print out the following pages and practice your skills at relative age dating and critical thinking. You may have to look up some terms.

• Relative Dating Geologic Events (Hanson)
• Grand Canyon stratigraphy. Univ. of Akron Geologic Instruction Resource site

Absolute Age Dating

Absolute Age dating is determining the actual age of an event or feature in years. There are numerous methods of absolute age dating, such as counting tree rings, coring glacial ice, and counting sediment varves. However, these methods are only useful in dating recent events, that is those occurring within the last few thousand or million years. Radiometric dating, which uses the rate of disintegration of radioactive isotopes to daughter products, enables geologist to date events as recent as a few thousand years and as old as a several billions years. An half life is the time it takes for 50% of the parent isotope to decay to its daughter product. Once the half life of an isotope is known, the age of a rock containing it can be determined by measuring the relative proportion of the radioactive parent and its daughter product.

• To learn more about absolute dating go to Geology Labs On-Line's Virtual Dating Page and complete the Virtual isochron dating exercise for rocks.
• For a good introduction to absolute dating visit the Absolute Dating page of the Department of Geological Sciences, University of Saskatchewan, (Geol. 118.3).

Geologic Time Scale

  The Earth is 4.6 billion years old.  Most people cannot image the vastness of geologic time, often referred also as deep time.  Repetatively ocean basins are created and destroyed, mountains are built and erased by erosion, and lifeforms are evolved to later dissapear. Most geologic processes operate at rates that are barely perceptible to humans, but over a vast amount of the time are capable of large scale changes.  Complete the following exercise to help you understand the scale of geologic time.

An exercise in understanding deep time:

Go to Metaphors for Geologic time (www.athro.com) and answer the following in units of time.  If all of geologic time were compressed into one year, recorded human history started_______________ago, Homo sapiens have existed for ______________ago, the Phanerozoic began ___________ago, and the extinction of the dinosaurs that marked the end of the Mesozoic and beginning of the Cenozoic occurred approximately____________ hours ago.

Geologic time on Earth is divided into four large divisions called Eons that extend over 500 million years. Eons are divided into eras, which are subdivided divided into Periods. Because our knowledge of recent events is more detailed, these Periods are subdivided further into Epochs.  The boundary between each era is defined by a major extinction event.  The greatest extinction event, which killed 95% of all existing species, occurred at the end of the Paleozoic (Permian Period).  The cause of the event is unkown.  The best understood extinct occurred 65 millionyears ago (end of the Mesozoic) when a meteor impact killed the dinosaurs and 75% the lifeforms existing during the Cretaceous Period.  With each major extinction new life forms flourish.  So each boundary is marked by the termination of one fossil assemblage and beginning of another.

Historic Events - Travel Through Time

 Visit the Field Museum's  Tour Through Time and answer the following questions:

1. By what time did the first single-celled organisms (prokaryotes) develop? __________
2. The first complex organisms with soft bodies? ____________
3. Organisms with external skeletons?