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Chapter 6: Problem 8

How does grain size reflect the cooling time of a magma?

Short Answer

Expert verified
In summary, the grain size in igneous rocks reflects the cooling time of the magma from which they form. Fine-grained rocks indicate rapid cooling, usually as a result of volcanic eruptions, while coarse-grained rocks indicate slow cooling that occurs deep within the Earth's crust. This relationship is important for understanding geological processes and the history of a particular region.

Step by step solution

01

Overview of Magma Cooling and Grain Size Formation

Magma is a molten mixture of minerals, gases, and sometimes dissolved crystals that forms deep within the Earth's crust. When magma cools and solidifies, it forms igneous rocks. The rate at which the magma cools has a direct impact on the size of the mineral grains, or crystals, that develop within the solidified rock. As magma cools, the minerals within the magma come together to form solid crystals. The longer the cooling process, the longer the minerals have to grow, which can result in larger, more visible crystals in the final rock.
02

Fast Cooling: Fine-Grained Rocks

When magma cools rapidly, there is less time for the minerals to come together and grow. As a result, the mineral grains in the resulting igneous rock will be smaller and less visible to the naked eye. Fine-grained rocks, also known as aphanitic rocks, are typically formed during volcanic eruptions when lava is expelled onto the Earth's surface and cools quickly in contact with the atmosphere or water.
03

Slow Cooling: Coarse-Grained Rocks

In contrast, when magma cools slowly, the minerals within it have more time to grow and develop into larger crystals. This can result in coarse-grained rocks, also known as phaneritic rocks, with crystals visibly noticeable. Slow cooling typically occurs when magma solidifies deep within the Earth's crust, where it is insulated from the surface and can take a longer time to cool down.
04

Relationship between Grain Size and Cooling Time

In summary, the grain size of an igneous rock is a reflection of the cooling time of the magma from which it was formed. Fine-grained rocks are indicative of fast cooling times, such as those resulting from volcanic eruptions, while coarse-grained rocks are indicative of slow cooling times, such as those that occur deep within the Earth's crust. Understanding this relationship can be crucial to interpreting the geological history and processes that have shaped a particular region or environment.

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Most popular questions from this chapter

Describe the way magmas are produced in subduction zones.

What factors control the viscosity of a melt?

If you look at the Moon, even without a telescope, you see broad areas where its surface appears relatively darker and smoother. These areas are individually called mare (plural: maria), from the Latin word for sea. The term is misleading, for they are not bodies of water but rather plains of igneous rock formed after huge meteors struck the Moon and formed very deep craters. These impacts occurred early in the history of the Moon. Propose a cause for the igneous activity, and suggest the type of igneous rock that fills the mare. (Hint: Think about how the presence of a deep crater affects pressure in the region below the crater, and think about the viscosity of a magma that could spread over such a broad area.)

Why does melting take place beneath the axis of a midocean ridge?

What does the mixture of grain sizes in a porphyritic igneous rock indicate about its cooling history?
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