The Creeping Hot Spots
Students learn about the Mantle Plume Theory, plate tectonics, and Idaho geography by experimenting with a map of Idaho.
1 to 2 hours
The theory of plate tectonics explains much of Earth's geology. For example, California's famous San Andreas fault is formed where two tectonic plates come together and slide by each other. The tension that is created along the fault is periodically released, like a rubberband stretched to its breaking point, resulting in earthquakes.
Other geologic phenomena require a combination of theories to be explained. For example, events creating the Snake River Plain that stretches across southern Idaho from Oregon to Wyoming cannot be described as easily as the San Andreas fault. A combination of "Rifting," "Basin and Range Faulting," "the Mantle Plume Theory," and Plate Tectonics are required to explain that big crescent. For a concise description of these theories and the likely sequence of events that made the Snake River Plain see pages 2.1 to 2.5.
The student activity that follows will require that you understand the basics of these theories.
Show the students the enclosed map, a copy of which they will soon receive. Emphasize that if Idaho were really this big, the crust would be about the thickness of the paper. Idaho is part of a massive piece of crust called the North American Plate and it slides as a unit over the Earth's mantle.
With the Idaho map you will demonstrate the Mantle Plume Theory, Rifting, and Plate Tectonics. The students will then repeat the demonstration on their own maps individually and in teams, adding information to their maps as they work.
Part 1 - Demonstrate Rifting
The Western Snake River Plain was created through the formation of a rift valley. The crust was pulled apart in that area, resulting in lava welling up to the surface. Imagine that the inner part of a Snickers bar is molten and under pressure and kept in place by the chocolate coating. If you were to stretch the bar, "faults" and "fractures" would appear in the chocolate "crust" and its "molten" candy center would well up to the surface. This is what happened in the Western Snake River Plain during the last 17 million years.
Locate the Western Snake River Plain on your map and weaken the area by punching several pencil holes through paper "crust." Then use your fingers to tug at the paper and create small tears and -voila- you've created a rift valley.
As another way of demonstrating the process you may give each student a Snickers bar and let them pull and twist it to create faults and fractures before they eat it. You may wish to do this at the end of the class. Perhaps the Snickers bar could be used as a reward for demonstrating their knowledge to you about the formation of the Snake River Plain and geography of Idaho.
Part 2 - Demonstrate the Great Rift
In regards to the Western Snake River Plain, we spoke of rifting on a grand scale. A smaller example of rifting is the Great Rift, which runs 60 miles north-south and ranges from one to five miles wide. Here the crust was pulled apart, creating many fissures which allowed magma to escape. There are 25 cinder cones and 60 lava flows along the Great Rift. Locate the line through Craters of the Moon on your map that represents the Great Rift. Punch pencil holes along the line and tear slightly to demonstrate the rifting here.
Part 3 - Demonstrate Mantle Plume Theory
Let the candle represent a hot spot in the mantle. Geologists think this heat source may have nuclear origins, like the heat generated in a nuclear reactor (it represents no threat to life because it is buried deep in the Earth). If you did "The Earth, from Core to Crust" activity you might want to get one of the models out to demonstrate the Mantle Plume Theory. Imagine some the of the deep mantle material squeezing upward toward the surface in a 'Plume." That hotter, deep mantle rock creates the heat necessary for the volcanic activity expressed at the surface.
Light the candle to symbolize the hot spot. Hold the crust (your map of Idaho) over the stationary candle high enough so that it won't scorch. From your reading of 2.1 to 2.5 you know that the Mantle Plume Theory applies only to the Eastern Snake River Plain and that 10 million years ago the Plume was located under what is now Twin Falls. You also know that the North American Plate has crept southwest over the eons and that 600,000 years ago the Plume or hot spot was active under Yellowstone.
As you describe this to the kids, lower the Twin Falls area to just the point of scorching and slowly move the crust (the map) southwest to form the Eastern Snake River Plain between Twin Falls and Yellowstone. You or a student can say, "10 million years ago, 9 million years ago," and so on as you slowly move the paper over the candle toward Yellowstone so that by the time you get there you're at "one million years ago." Let the scorching paper represent the creation of the Eastern Snake River Plain.
Part 4 - Student Maps
Issue your students copies of the provided maps. Have them label their maps with the following to improve their knowledge of Idaho geography. Then they should create the Western and Eastern Snake River Plain and the Great Rift (have them use a marking pen or highlighter instead of a candle for demonstrating the Mantle Plume Theory. Make sure they hold the pen stationary beneath the moving crust and count backward slowly from 10 million years ago until they arrive at Yellowstone).
Note: Make sure the students understand that the above geologic processes happened concurrently over millions of years and that this explanation is a simplified version of reality.
The Creeping Hot Spots
Did You Know?
Watch out for bombs! Before they cooled, volcanic bombs were hot globs of lava that were hurled from volcanoes along the Great Rift. They form a variety of interesting shapes described as "breadcrust", "spindle" and "ribbons" by geologists.