Lesson 3: Geologic Dating and Crustal Deformation
Before we dive into the details of the geologic processes that have shaped the North American continent and America's national parklands we're going to spend a little time learning about some key geologic principles. This week's lesson and the two that follow are drawn from Chapter 2 and introduce geologic concepts that we'll come back to repeatedly throughout the semester.
As you read about relative and absolute dating and about the folds and faults produced by crustal deformation this week it will be useful to take careful notes so that you'll be able to keep track of major ideas and apply them to the specific examples we'll encounter during the coming weeks. Be sure that you are prepared to meet the learning objectives outlined below before you move on to the exercise and quiz at the bottom of the page.
Weekly Learning Objectives
Upon successful completion of this week's lesson, a student is expected to be able to:
- Reconstruct the sequence of geologic events that shaped an area using the principles of relative dating and an understanding of the different types of unconformities.
- Calculate the radiometric age of a geologic sample knowing the relative amounts of parent and daughter isotopes it contains and the half-life of the decay system.
- Identify common types of faults and folds depicted on geologic maps or cross-sections and relate them to the type of stress (extensional, compressional, or shear) and the crustal depth (shallow or deep) under which they were formed.
Reading and Browsing Assignment
- Read pages 22-30 of Chapter 2, and focus on how (1) relative dating principles such as superposition and fossil succession enable us to infer the sequence of geologic events that shaped a landscape; (2) the decreasing abundance of a radioactive parent element in a sample over time–and corresponding increase in the abundance of its stable daughter–enable us to determine the absolute ages (in years) of rock units; and
(3) different types of faults develop in the brittle upper crust due to different types of stress whereas folds develop in the ductile lower crust due to compression. With regard to relative dating principles, here is a brief summary that expands a little on what the author of our text wrote:
- Superposition: in sequences of strata (layered sedimentary or volcanic rocks) the oldest layer is at the bottom and successively younger layers are towards the top.
- Original horizontality: sequences of strata are commonly deposited as flat-lying layers, and if they are found tilted or folded it is because of later deformation.
- Lateral continuity: many strata are deposited as continuous two-dimensional sheets that can be correlated across wide areas in spite of later disruption by faulting or erosion.
- Cross-cutting relationships: if one rock body or geologic structure (e.g., a fault or unconformity) cuts across another, the cross-cutting body or structure is younger.
- Inclusion relationships: if a piece of one type of rock is found as an inclusion (isolated piece) in another, the included rock is older than the one that encloses it.
- Fossil succession: life on Earth changed in a regular way through time so that the assemblages of fossil plants and animals found in successively younger rocks are more and more similar to modern communities of living things.
- For a little more insight into both relative and radiometric dating, check out this PDF of a PowerPoint presentation on Geologic Time written by Ronald Parker at Earlham College.
- Review how geologists use fossils to date geologic events by browsing through the US Geological Survey's Fossils, Rocks, and Time site. Note, especially, how different groups of plants and animals appeared at different times in Earth's history on the page entitled "Fossil Succession".
- Browse through the joint US Geological Survey/National Park Service website on geologic maps . It gives a clear overview of the meanings of the symbols commonly found on these maps which we'll be using, along with geologic cross-sections, to understand park geology throughout the semester. Just two notes about this presentation. First, the base maps on which most geologic maps are drawn depict the shape of the land surface using topographic contours. These are imaginary lines of equal elevation drawn on Earth's surface and should not be confused with geologic contacts or faults. Second, the map symbols for fold axes are different for anticlines and synclines. Paired arrows on either side of an axis line point inward for a syncline because beds dip inward towards the center of such a fold (see location 6 on the website). Similarly, the paired arrows point outward for an anticline because the beds dip away from its center.
- Apply the relative dating principles you've learned by interpreting the geologic histories of one or two of the hypothetical cross-sections on Athro's Interpreting Geologic Sections site. (These are fun; just choose the events in the correct sequence from the drop down menus and then run the animation!)
Exercise 3 (Due by 9:00 AM on Monday, 1-Feb-2010)
This exercise will give you practice applying the relative dating principles you've learned to work out the geologic history of a famous national park region: the Grand Canyon. Your task is to study the accompanying cross-section of the canyon and, based on what you observe, answer the questions below. Be sure to write your answers down and then go to Exercise 3 in the Etudes "Assignments, Tasks, and Tests" tool to record them.
Quiz 3 (Due by 9:00 AM on Monday, 1-Feb-2010)
After you feel you have met the learning outcomes outlined above, please complete Quiz 3a in the Etudes "Assignments, Tasks, and Tests" tool. Remember, this quiz covers all of the learning outcomes above, not just relative dating. There are ten questions, each worth one point. If you can answer all of them correctly it means that you know your way around geologic dating and crustal deformation pretty well and are ready to move on to a more detailed look at earth materials (rocks and minerals) next week.