Chapter 11: Problem 3

What factors determine whether a rock will behave brittlely or plastically?

Short Answer

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The behavior of a rock as brittle or plastic depends on several factors, including temperature, pressure, strain rate, rock composition, and the presence of fluids. Higher temperatures and pressures generally promote plastic deformation, while low temperatures, low pressures, and high strain rates lead to brittle behavior. Additionally, the rock composition and presence of fluids can also influence its response to stress, making it more susceptible to plastic deformation or brittle failure.

Step by step solution

Factor 1: Temperature

The behavior of a rock is greatly influenced by the temperature. When the temperature increases, rocks generally become more ductile or plastically deformed. This is because the higher temperature allows the minerals in the rock to become more mobile, making it easier for them to change shape or flow. Conversely, at lower temperatures, the minerals are less mobile, and the rock becomes more prone to brittle failure.

Factor 2: Pressure

Pressure also plays a crucial role in determining whether a rock behaves brittlely or plastically. Rocks subjected to high pressure are generally more prone to plastic deformation. The elevated pressure tends to close the micro-cracks present in the rock and thus promotes ductile deformation mechanisms, such as creep or dislocation motion. On the other hand, under low pressure, rocks are more likely to behave brittlely and experience fracturing or faulting.

Factor 3: Strain Rate

The strain rate, which is the rate at which deformation is applied to the rock, also affects its behavior. High strain rates can lead to brittle behavior, as the rock doesn't have enough time to adjust and deform plastically. In contrast, when the strain rate is low, the rock has more time to adjust to the applied stress, allowing it to deform in a more plastic manner.

Factor 4: Rock Composition

The composition of the rock also plays a role in determining its behavior. For example, some rocks, such as quartz-rich rocks, are generally more brittle than rocks containing a higher proportion of clay minerals or other ductile materials. The physical and chemical properties of the minerals within a rock influence how it responds to stress and, therefore, whether it will behave plastically or brittlely.

Factor 5: Fluid Presence

Fluids present within a rock can affect its behavior under stress. When water or other fluids are present in the pore spaces of a rock, they can reduce the rock's strength and make it more susceptible to plastic deformation. This can occur through processes such as pressure solution and mineral recrystallization facilitated by the presence of fluids. In summary, the behavior of a rock as brittle or plastic depends on several factors, including temperature, pressure, strain rate, rock composition, and the presence of fluids. Understanding the interplay of these factors helps in predicting and interpreting the deformation behavior of rocks in various geological settings.

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What factors determine whether a rock will behave brittlely or plastically?

Short Answer

Step by step solution

Factor 1: Temperature

Factor 2: Pressure

Factor 3: Strain Rate

Factor 4: Rock Composition

Factor 5: Fluid Presence

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