Geology Lesson 3. Where did the oceans go?
OverviewIn this lesson, students will examine two common external factors that control geologic landscapes: climate and tectonics. When complete, students should understand the role of each in the geology of the Guadalupe Mountains and in modern geologic events.
The learner will:
• Understand the external forces that aid landscape evolution.
• Apply this understanding to the Guadalupe Mountains, Carlsbad Caverns National Park, the Delaware basin area, or your local geography.
While landscape evolution is dependent upon internal mechanisms to control the ways in which it changes (mineralogy, lithology, fractures, etc.), external driving mechanisms are responsible for causing landscape changes. While there are many geologic responses to external forces and feedbacks associated with each, the primary external mechanisms for landscape change are climate change and tectonics.
Climate change is driven by changes in incoming solar energy, which affects the global hydrologic, oceanic and gas cycles. Levels of solar energy fluctuate regularly over various time spans. The most recognizable fluctuations in solar energy, or solar cycles, are the Milankovitch cycles that occur approximately every 21,000 years, 41,000 years, and 100,000 years due to alterations in the Earth’s precession (the wobbly path traced out by Earth’s spinning axis which varies), tilt, and eccentricity around the sun, respectively. These cycles alter the Earth’s climate by increasing or reducing the energy that drives global cycles such as some atmospheric cycles (by increasing or decreasing greenhouse gases such as carbon dioxide, water vapor, methane, etc.), the hydrologic cycle (through increasing temperature and changing evaporation/ condensation rates which in turn change the global cloud and ice coverage and thus lead to a rise or fall in sea level), and the ocean cycle, or ocean conveyor belt (a global circulation of the oceans that mixes the warm water with the cold water). An example of a climate change due to a change in the oceanic cycle is El Niňo, which occurs every 3 to 7 years due to shifting of the warm surface water in the Pacific Ocean.
Each of these cycles affects the global climate and in turn alters regional ecology and geology. Thus, geological and ecological records are excellent measures of past climate changes. Records that are reliable measures of past climate changes are rock records (geology), ice cores, ocean cores, tree rings, pollen records, paleontological records, speleothems, and archaeological records.
Tectonics also plays a major role in shaping the global landscape. While plate tectonics is a major force in forming and shaping the global landmasses, small scale structural changes in regional geology also play a large role in the evolution of landscapes. Plate tectonics affects landscapes through either compressional or extensional stresses. These stresses create folds, bends, or wrinkles in rock layers, and/or faults, fractures in rocks where one body of rock slides past another, in the rocks they are acting upon. Folds may be simply divided into two major types: those where rocks are buckled up into a hump, called anticlines, and those where layers of rock are folded into a U-shape, called synclines. There are also several different types of faults, however all faults are described by the relative motion of the upper side of the break, called the headwall or hanging wall, in respect to the lower side or the footwall. A normal fault is a fault in which the hanging-wall block moves down the fault slope in relation to the footwall whereas a reverse, or thrust fault, is a steeply dipping fault on which the hanging wall slides up relative to the footwall.
Both climate change and tectonics have affected the geology and ecology of the Guadalupe Mountains and Carlsbad Caverns National Park area since the Permian Period. At the end of the Permian Period, the sea level in the area dropped leaving behind thick evaporite deposits. At this time, a massive global extinction also occurred and nearly 90% of all life on the Earth, such as most of those fossils seen at Guadalupe Mountains and Carlsbad Caverns National Parks, became extinct. Millions of years later, in the Cretaceous, a shallow sea returned to North America leaving marine deposits in mid-continent America once again. During the last million years, in the Pleistocene Period, North America has been subjected to glacial advances—the most recent of which occurred a mere 18,000 years ago. While the glaciers did not advance as far south as New Mexico and Texas, the climate in the region is believed to have been considerably cooler and wetter. It was at this time that many of the cave decorations, or speleothems at Carlsbad Caverns National Park were probably formed.
Tectonics also played a large role in the formation of the Guadalupe Mountains and Carlsbad Cavern. After the Capitan Reef was formed in the Permian Period, the basin filled with evaporites and other sediments. However, during the Tertiary Period, regional uplift began to raise the sturdy Permian deposits and the overlying material was eroded off. This increasing uplift allowed the groundwater, rich in sulfuric acid from the mixing of H2S gas with the oxygen in the water to lower in level and carve out the lower caves. Therefore, caves deepest in the mountains are those most recently formed. These caves are later believed to have been decorated during the more moist Pleistocene Epoch when the abundant water flowed through the rocks, dissolved some of the limestone, and redeposited the calcite as cave decorations or speleothems.
• Global Geography slides (slides #11-18)
• Clay (3 sticks for each pair of students)
• Structure slides (slides #112 – 115)
• Science notebooks
The teacher will:
• Define climate change.
• Discuss different mechanisms for climate change (solar flux and Milankovitch cycles, ocean conveyor belt, greenhouse gases, etc.) and show the paleogeography changes through time.
• Divide class into four groups with the following headings and have each person in the group research a topic under the heading (these topics may be modified depending on the size of the class and the resources available):
• Causes for past climate changes
• Solar fluctuations• Ocean fluctuations
• Tectonics (mountain building and volcanics)• Greenhouse gases
• Records of climate change
• Ice cores
• Marine cores
• Environments in the Guadalupe Mountains and Carlsbad Caverns National Park through time
• Permian Period• Cretaceous Period• Pleistocene Epoch
• Future Climate Change
• Potential causes• Possible effects• Potential cures
• Have each group present research and then individual students should write a summary/thought paper on what past climate change may tell us about the future climate.
• Review plate tectonics. Discuss how this might have affected the southwest and the Guadalupe Mountains National Park and Carlsbad Caverns National Park.
• Discuss the smaller scale effects this has on a local region (define types of folds and faults).
• Split students up into pairs and have them model each of the folds and faults in three layers of rock made out of clay. Have them draw the cross-sections of these features in their science notebooks.
• Look at the present configuration of the Guadalupe Mountains and Carlsbad Cavern—discuss the role of tectonics in the formation of these features.
• Structure diagrams
The facts that nowadays the Guadalupes are mountains formed from the elevation of an ancient reef and it is located next to the Chihuahuan desert where water is scarce, lead us to the idea that some kind of forces have influenced the actual landscape making the ancient sea to disappear.
• Have students construct paper fault models from the USGS (http://geomaps.wr.usgs.gov/parks/deform/7modelsa.html)
• Have students research some of the famous faults and folds (San Andreas Fault in California, the Black Hills dome, etc.).
• Research the tectonic activity in your area—are there faults? Are there folds?
Bebout, D.G. and C. Kerans. 1993. Guide to the Permian Reef Geology Trail, McKittrick Canyon, Guadalupe Mountains National Park, West Texas. Guidebook 26. Austin, TX: Bureau of Economic Geology.
Caves Reveal Climates Project Proposal: http://www.ncdc.noaa.gov/ogp/papers/gonzale.html
Hill, C.A. 1996. “Geology of the Delaware Basin Guadalupe, Apache, and Glass Mountains New Mexico and West Texas.” Permian Basin Section – SEPM. Pub. No. 96-39.
Marshak, S. 2001. Earth Portrait of a Planet. New York: W.W. Norton & Co.
Paleogeography and geologic evolution of North America http://www2.nau.edu/rcb7/nam.html
Rubrics for Teachers
NOAA’s “Paleo-perspective” webpage: http://www.ngdc.noaa.gov/paleo/globalwarming/home.html
Additional reading and other resources
Climate Internet links: http://www.istl.org/01-fall/internet.html