This activity aligns with the following National Geography Standard: Standard 8: The Characteristics and Spatial Distribution of Ecosystems on Earth's Surface
Many people think of deserts as vast expanses of blowing and drifting sand. As the Mojave illustrates, deserts are anything but one-dimensional. All deserts feature very low precipitation levels, but they can be hot or cold, sparsely vegetated or full of life. Most deserts occur between the latitudes of 15º and 40º on either side of the equator. They cover approximately 20 percent of the Earth's land area. There are several types of deserts, categorized on the basis of the conditions that created them.
Subtropical deserts lie along the Tropics of Cancer and Capricorn. Near the equator, hot, moist air rises and then cools, dropping heavy rains on tropical areas. The resulting cooler, drier air then descends, creating zones of high atmospheric pressure as the air moves away from the equator. The descending air hinders cloud formation and precipitation. The air also warms up as it sinks, absorbing any available moisture. Africa's Sahara Desert is the best-known example of a subtropical desert.
Coastal deserts also develop because of high-pressure weather systems. Damp, chilly fog forms when air cooled by water contact as it blows toward shore meets warm air over land. Although humidity is high, atmospheric disturbances that cause rainfall are not present. The Atacama in Chile and the Namib in southern Africa are both coastal deserts.
Interior deserts, such as Asia's Gobi Desert, exist because they are too far from the ocean. By the time ocean winds reach the center of a large landmass, their air is very dry.
Rain shadow deserts are created when mountain ranges lie parallel to moist, coastal areas. Prevailing winds moving inland cool as air is forced to rise over the mountains. Moisture falls on slopes facing the winds. When the winds move over the mountain crests and down the far side, they are very dry. Descending air also makes it hard for additional clouds and precipitation to form. Without another source of moisture, rain shadow deserts are formed on the far side of these mountains. As mentioned in the article, the Mojave is a rain shadow desert.
Assign small groups of students to research and report on some of the world's deserts. In their reports, groups should locate their desert on a world map, describe its physical features and the way in which it was created, and if possible, include illustrations of typical plants and animals. Students should be encouraged to use the Internet as a resource in their research; some good websites to use as starting points are: The World's Biomes: Deserts at www.ucmp.berkeley.edu/glossary/gloss5/ biome/deserts.html and Desert Biome at http://mbgnet.mobot.org/pfg/diverse/ biomes/desert/index.htm.
This activity aligns with the following National Science Education Standard: Content Standard D: Earth and Space Science--Structure of the Earth System
A fault is a fracture in the Earth's crust along which rocks on one side have been displaced in relation to rocks on the other side. Earthquakes are caused by the release of large amounts of energy as one or more rock masses under stress shift or slide past each other suddenly along a fault. Because the Mojave Desert region is being crushed between the Pacific and North American tectonic plates, it experiences relatively frequent earthquakes along many different faults.
Scientists use seismographs to record earthquake vibrations. These instruments help scientists determine the location of an earthquake and its strength, or magnitude. Students can make their own version of a seismograph to see how it works.
1. Place a chair approximately 50 cm in height on top of a table.
2. Using a string and masking tape, suspend an uncapped, fine-tip marker from the seat of the chair. Adjust the length of the string so that the tip of the marker just barely reaches the tabletop.
3. Next, take a strip of adding-machine tape about 75 cm long and center one end of it beneath the marker.
4. Now, let the shake, rattle, and roll begin. Have one or two students gently shake the table from side to side, while another slowly pulls one end of the adding machine tape lengthwise under the marker. The students shaking the table should gradually increase the force of the quaking.
What happens to the marks on the tape as the shaking increases? (The marker tracings extend further and further on both sides of the paper's center line.)
This activity aligns with the following National Science Education Standard: Content Standard C: Life Science - Populations and Ecosystems; Diversity and Adaptations of Organisms
To teach how competition with weeds can threaten the survival of native plants, have students set up some plant competitions in the classroom. You'll need three medium-sized pots filled with potting soil.
1. In the first pot, plant a handful of quick-germinating grass seed, such as wheat, barley, oats, or rye. In the second, plant a half dozen or so flower seeds, such as marigolds or zinnias. In the third pot, plant a mixture of the grass and flower seeds.
2. Cover the seeds in each pot with a thin layer of potting soil.
3. Place the three pots next to each other in a sunny location and water each pot regularly, using equal amounts of water.
4. Once a week over the course of four to six weeks, have students monitor the growth in each of the pots, measuring the height of the grass and the flower stems.
Which flower plants appear healthier after several weeks? (Among characteristics students can look for are: height, overall color, and number and size of leaves. The flowers planted separately should appear healthiest.) Why? (These flowers had full access to water and to the nutrients in the soil.) Describe the grass in the two pots. (The grass planted alone should appear healthiest, but the grass planted with the flowers should also appear healthy.) Why is the grass in the pot with the flowers likely to be healthier than the flowers in the same pot? (Its root system is more extensive and often grows faster than the plant's root system, making it better able to take in water and nutrients.) Explain that this is similar to what happens in the desert, where native annual flowers are reduced when nonnative grasses such as cheat-grass are introduced.