Lesson 7: Passive Continental Margins and Coastal Processes
Last week's lesson on continental rifting looked at active rift systems in the Great Basin and Rio Grande valley, both places where the continental lithosphere is still pretty much intact. If the rifting process continues, however, ocean floor lithosphere will develop (as it has in the Gulf of California) and the adjoining edges of the continents will become rifted or passive margins. Today, North America's passive margins lie along the Atlantic and Gulf coasts, and these areas will be the focus of this week's lesson. Next week, we'll explore an ancient passive margin that is exposed across the Colorado Plateau.
As you read through chapter 4 on modern passive margins and coastal processes it will be helpful to take notes so that you can keep track of major ideas and have ready access to what you've learned when we refer back to these concepts later in the semester. Be sure that you are prepared to meet the learning objectives outlined below before you move on to the 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:
- Distinguish the modern coastlines of the conterminous United States that are active margins from those that are passive margins and describe the characteristics that identify a coastline as one type or the other.
- Predict how the elevation of the passive margin of a rifted continent will change as it moves away from the divergent boundary where it formed and explain why this change occurs. (Note: There are two causes here: one related to the thickness of the continental crust and the other to the temperature of the lithosphere and underlying asthenosphere.)
- Identify the contiental shelf, continental slope, continental rise, and abyssal plain on a cross-section of a passive continetal margin, and indicate which of these features is underlain by continental crust, oceanic crust, or the transition zone between them.
- Describe how sand found on the beaches of national seashores along the east coast differs as you travel south from Massachusetts to Florida and relate this difference to the sands' sources.
- Predict how global climate change is likely to affect sea level and the general location of the coastline in the United States during the coming decades.
- Infer how longshore transport of sand is likely to reshape a coastline as it moves sand from exposed "upstream" areas to more sheltered "downstream" areas.
Reading and Browsing Assignment
- Read the first part of Chapter 4 (p. 72-81), focusing on the topics outlined in the learning objectives above.
- Longshore transport (drift) is the movement of sediment parallel to a coast, and it is driven by the "longshore current" mentioned in our text and this week's Hazard City exercise. To learn more about how it works, read the following article in Wikipedia and study the following animations on longshore drift (link at bottom of page) and the growth of spits and bars (scroll down to "Baymouth Bar Formation, McGraw-Hill" and then click on "Spits and Baymouth Bars").
Exercise 7 (Due by 9:00 AM on Monday, 1-Mar-2010)
After you have studied the chapter and mastered the learning objectives, please load up your Hazard City CD, click on the Shoreline Property Assessment exercise, and select version 1. Be sure to read all of the introductory sections carefully before you visit the sites you are being asked to assess. Because waves approaching this coast are coming from the east, assume that longshore transport at sites 1 and 4 is moving sediment to the northwest. Write out your "safe" or "at risk" decision for the properties at each site along with any notes you might need to help you remember why you made the decisions you did. (I suggest printing the form provided, filling it in, and then adding any annotations that might be helpful.) Finally, go to the Etudes "Assignments, Tasks, and Tests" tool and complete Exercise 7. Note that in addition to telling what your decision is for each property (safe or at risk) there are two questions about concepts (sediment erosion/deposition and sediment budget) that you learned about in the exercise.
Quiz 7 (Due by 9:00 AM on Monday, 1-Mar-2010)
After you feel you have met the learning outcomes outlined above, please complete Quiz 7 in the Etudes "Assignments, Tasks, and Tests" tool. There are ten questions, each worth one point. If you can answer all of them correctly it means that you know your way around modern passive margins and coastal processes pretty well and are ready to move on to learn about parklands that preserve ancient passive margins next week.