PLATE MOVEMENTS AND CLIMATE CHANGE
Karen L. Bice
Department of Geosciences
Pennsylvania State University
University Park, PA 16802
Level: Grades 7 and above
Estimated Time Required: 50 minutes
Anticipated Learning Outcomes
Climate is simply weather "averaged" over a time period of one year or more. In general terms, the climate in most of the United States and Canada is "temperate". Moving to the south, closer to the equator, the climate is "subtropical" and then "tropical". At the Earth's poles the climate is termed "polar". Each of these climate zones is characterized by a distinctive temperature range, rainfall amount, and type of vegetation.
In a very simplified sense, climate zones are oriented
roughly parallel to lines of latitude about the Earth. However, according
to the theory of plate tectonics, the continents "ride" on dynamic
plates which make up the Earth's surface. Although the resulting movement
of the continents is very slow, over millions of years it is enough to get
a continent from one place to another, and that movement may take the landmass
through several latitudes and climate zones.
Three maps are provided showing: 1) the present-day position of the continents around the Atlantic Ocean, 2) that same area 55 My ago, and 3) 180 My ago. Maps showing the distribution of continents and oceans in the past are known to geologists as paleogeographic reconstructions. The short line segments shown in Figures 1 and 2 are portions of the Atlantic sea floor which formed at the Mid-Atlantic Ridge 55 My ago. In Figure 2, the continents have been rotated or moved to their positions at 55 My ago. The line segments representing the 55 million year-old sea floor come together and indicate the position of the mid-ocean ridge at that time (for more on the concept of sea floor spreading, see the previous exercise in this book titled "The Distance Between Us and Them"). Figure 3 shows the configuration of the super continent Pangea (Pan-GEE-uh) just as it was beginning to break apart.
In small groups or as a class, consider one or more of the following climate concepts:
TROPICAL REGIONS - discuss with students the location of and conditions (rainfall, temperatures, vegetation) in the tropics. Have them locate the tropics of Cancer and Capricorn (23.5 degrees north and south, respectively) on each of the three maps. How might the climate in Georgia, for example, have changed from 180 My to 55 My ago? From 55 My ago to the present? Because plants may be preserved as fossils in sediments laid down where the plants grew, why might it be reasonable to find tropical plant fossils in Mesozoic-aged rocks exposed in the southwestern United States today?
DESERT REGIONS - perform the same thought exercise with deserts. Because of the Earth's general atmospheric circulation pattern, the most extensive deserts form along bands around 30 degrees north and south latitude (the "horse latitudes"). Is there reason to think that deserts may have covered more of southern Africa 180 million years ago than they do today?
COASTAL MOISTURE SOURCES - the oceans are the major source of moisture for the continents. Water evaporates from the sea surface to form clouds, many of which move over continents, rise, cool, and drop rain on the land. In general, the farther clouds move inland, the more moisture they lose in the form of rain. For this reason, coastal areas commonly receive more rainfall than the interior of large continents. Compare the size of the super continent that existed 180 My ago to the sizes of the continents that formed from the breakup of Pangea. Was Maryland, for example, a coastal area about 180 My ago? How might the size of the continent of Pangea have affected the climate on land that is now part of eastern North America?
INDIA - 180 million years ago, India was connected to Africa, Antarctica and Australia. Today the collision of the Indian plate with Asia is causing the uplift of the Himalayan mountain range. What type of climate might India have experienced 180 My ago? Discuss how the climate of India may have changed dramatically as the plate moved equator-ward during the last 180 million years.
Many world atlases contain maps indicating annual rainfall, temperature, desert and rainforest distribution. Junior and senior high school geography texts may also provide information concerning present-day climate zones. Periodicals such as National Geographic often publish excellent maps and photos of polar, desert, and tropical climate regions.
Figure 1. PRESENT DAY - positions of the continents bordering the Atlantic Ocean. The line segments shown in the oceans indicate the extent of strips of sea floor rock with an age of 55 million years.
Figure 2. 55 MILLION YEARS AGO - positions of the continents during the early Tertiary period. Note that with the plates rotated to their positions 55 My ago, the strips of sea floor rock which formed 55 My ago come together and meet at the mid-ocean ridge where they formed.
Figure 3. 180 MILLION YEARS AGO - configuration of the super-continent Pangea which had just begun to split apart.
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