Chapter 7: Problem 9

Why does contact metamorphism not normally take place at significant depth in the crust?

Short Answer

Expert verified

Contact metamorphism does not normally occur at significant depths in the Earth's crust due to several factors: the decreased temperature difference between the magma intrusion and surrounding rocks, the reduced likelihood of magma intrusions at depth, and the limited reaction time available due to cooling of magma and ongoing deformation of rocks. As a result, contact metamorphism is more commonly observed near the Earth's surface, where these conditions are more favorable.

Step by step solution

1. Introduction to Contact Metamorphism

Contact metamorphism is the process that occurs when rocks near a magma intrusion are subjected to heat and pressure, causing changes to their mineral composition and texture. This type of metamorphism does not require the rocks to be deeply buried, as the heat source is provided by the adjacent magma intrusion.

2. Effects of Depth on Temperature and Pressure

As we go deeper into the Earth's crust, both temperature and pressure increase. While increasing depth could potentially lead to greater metamorphism due to the increased pressure, the effect of temperature becomes less pronounced. This is because, at greater depths, the ambient temperature of the Earth's crust increases as well, reducing the temperature difference between the magma intrusion and the surrounding rocks.

3. Magma Intrusions at Depth

In general, magma intrusions are more likely to occur near the Earth's surface, where the crust is thinner and more prone to fracturing, allowing the magma to intrude and spread. At greater depths, the increased pressure makes it more difficult for magma to force its way through the denser rock layers. As a result, contact metamorphism is more likely to occur near the Earth's surface where magma intrusions are more common.

4. Limited Reaction Time at Depth

Even if a magma intrusion occurs at greater depths, the time available for contact metamorphism to take place is relatively limited. As magma cools, the heat available for metamorphism is reduced, and metamorphic reactions slow down. At significant depths, the surrounding rocks are also subject to higher strain rates due to the movement of the Earth's crust, which may cause rocks to deform and become less receptive to metamorphic reactions.

5. Conclusion

Contact metamorphism does not normally occur at significant depths in the Earth's crust due to several factors, including the decreased temperature difference between the magma intrusion and surrounding rocks, the reduced likelihood of magma intrusions at depth, and the limited reaction time available due to cooling of magma and ongoing deformation of rocks. As a result, contact metamorphism is more commonly observed near the Earth's surface, where these conditions are more favorable.

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Why does contact metamorphism not normally take place at significant depth in the crust?

Short Answer

Step by step solution

1. Introduction to Contact Metamorphism

2. Effects of Depth on Temperature and Pressure

3. Magma Intrusions at Depth

4. Limited Reaction Time at Depth

5. Conclusion

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