Lifter: How Does Intrusive Volcanism Change the Shape of the Earth's Crust?
How does intrusive volcanism (movement beneath the earth's surface) change the shape of the earth's crust?
- knife (to be used only by an adult)
- clear plastic drinking glass
- large tube of toothpaste (remove the cap)
- 1/2 cup (125 ml) of soil <>adult helper
- Ask an adult to use a knife to cut a hole in the bottom of the glass from the inside, then use scissors to make the hole as large as the mouth of the toothpaste tube.
- Hold your finger over the hole in the bottom while you pour the soil into the glass.
- Insert the mouth of the toothpaste tube into the hole.
- Ask your helper to hold the glass while you press against the tube to force the toothpaste into the glass.
- Observe the contents of the glass as the toothpaste enters. Pay special attention to the surface of the soil.
As the toothpaste rises in the glass, the soil is pushed upward, forming a dome-shaped rise in the soil's surface.
Liquid rock beneath the earth's surface is called magma. Pressure on pools of magma deep within the earth forces the molten rock toward the surface. This movement of magma within the earth is referred to as intrusive volcanism. Intrusive volcanism is responsible for different types of intrusions (flows of magma that cool and harden before they reach the surface). Intrusions have many shapes because magma hardens in many positions as it cools. The hardened or solidified magma forms igneous rock. A dome-like intrusion is called a laccolith. A laccolith is formed when magma pushes overlying rock upward. The toothpaste simulates the formation of a laccolith. The mushroom-shaped paste pushes the overlying contents of the glass upward, producing a mound on the soil's surface.
What would happen if rock layers restricted the upward movement of the magma? Repeat the experiment, adding rocks to the soil mixture and standing a second plastic glass inside the glass of soil. Ask your helper to push down on this glass to restrict the movement of soil as you force toothpaste into the glass. Science Fair Hint: Use the description in SHOW TIME! to identify the type of intrusion formed. Label and display the glass from this and the original experiment.
- Bodies of intrusive igneous rock are classified according to their shape and relationship to surrounding rock. Use the description of each type of rock structure and the diagram to build a clay model showing the rock structures formed by intrusive activities. This model can be used as part of a project display.
- Batholiths—large intrusions below the earth's surface.
- Dikes—narrow, vertical intrusions that rise and break through horizontal rock layers.
- laccoliths—mushroom- or domeshaped intrusions that push up the overlaying rock layer.
- Sills—thin, horizontal intrusions sandwiched between other rock layers.
- Stocks—intrusions below the earth's surface; smaller than batholith bodies.
- Granite is the most common type of intrusive igneous rock. The composition of granite can vary depending on the kinds and proportions of minerals present in the magma that formed it. Different samples of granite can be purchased at a rock shop, or collect your own samples. Use these as part of a display showing the different shapes of intrusions and their composition.
CHECK IT OUT!
Domed mountains, such as the Henry Mountains of southern Utah or the Black Hills of South Dakota, are broad, circular mountains formed when layers of rock are lifted. Find out more about the surface landforms created by intrusions. What is the surface like in areas where the different intrusions are exposed when rocks around them are worn away by erosion? Examples of exposed batholiths are the Sierra Nevada mountains of California. Identify other exposed intrusion areas.
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