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Chapter 6: Problem 6

What factors control the viscosity of a melt?

Short Answer

Expert verified
The viscosity of a melt is controlled by three primary factors: temperature, composition, and pressure. As temperature increases, the viscosity decreases due to increased molecular movement. Magma with higher silica content has higher viscosity, while magma with lower silica content and higher iron and magnesium content is less viscous. Additionally, volatiles like water vapor, carbon dioxide, and sulfur dioxide can influence viscosity. Lastly, viscosity increases with increasing pressure, as it forces molecules closer together and increases friction.

Step by step solution

01

Define Viscosity

Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a moving fluid. In the context of geology, a melt refers to molten rock, also known as magma. Understanding the viscosity of a melt is important as it affects the movement of magma and its ability to generate volcanic eruptions.
02

Effect of Temperature on Viscosity

Temperature plays a significant role in controlling the viscosity of a melt. As the temperature of a melt increases, its viscosity decreases. This is because the heat provides more energy to the molecules within the melt, allowing them to move more freely, and thus reducing the internal friction. In magma, which is composed of various minerals, an increase in temperature allows more elements to become fluid and mobile, thereby reducing the overall viscosity.
03

Effect of Composition on Viscosity

The composition of a melt directly affects its viscosity. A melt's composition refers to the elements and compounds present within it, such as silica, iron, magnesium, and other elements. Magma with a higher silica content has a higher viscosity, as silica forms a polymer-like network that increases internal friction. Conversely, magma with a lower silica content and a higher proportion of elements like iron and magnesium is less viscous. Additionally, the presence of volatiles (gases) such as water vapor, carbon dioxide, and sulfur dioxide can also influence the viscosity of the melt.
04

Effect of Pressure on Viscosity

Pressure also affects the viscosity of a melt. As pressure increases, viscosity tends to increase as well, because increased pressure forces the molecules closer together. This results in a higher friction between the molecules and a higher viscosity. In the context of magma, higher pressure is often found at greater depths within the Earth's crust, and this pressure can cause an increase in the viscosity of the magma. In conclusion, the viscosity of a melt is controlled by three primary factors: temperature, composition, and pressure. By understanding and examining these factors, one can better comprehend the behavior of magma and its implications for volcanic eruptions and related geological phenomena.

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Most popular questions from this chapter

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

What factors control the cooling time of a magma within the crust?

What is a large igneous province (LIP)? How might the formation of LIPs have affected the Earth System?

What is the source of heat in the Earth? How did the first igneous rocks on the planet form?

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.)
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