Chapter 3: Problem 11

Why does a dyke commonly have a fine-grained margin?

Short Answer

Expert verified

A dyke commonly has a fine-grained margin due to the more efficient heat transfer and faster cooling rate at the contact zones between the magma and the cooler surrounding rock. This faster cooling results in smaller mineral grains at the margins, giving the dyke its distinctive fine-grained appearance.

Step by step solution

Understand the formation of a dyke

A dyke is formed when magma from the Earth's mantle intrudes into the pre-existing rock layers, typically moving through fractures or cracks. As the magma cools and solidifies, it forms a sheet-like igneous rock structure known as a dyke.

Heat transfer within a dyke

The key to understanding why a dyke has a fine-grained margin lies in understanding the process of heat transfer within the dyke. When the hot magma intrudes into the colder surrounding rock, heat is transferred from the magma to the cooler rock. This process of heat transfer results in the cooling and solidification of the magma.

Cooling rates and grain size

The cooling rate of magma significantly influences the size of the mineral grains that form within it. According to the principle of nucleation and crystal growth, if the cooling rate is slow, there is more time for individual minerals to grow larger before solidifying. Conversely, if the cooling rate is fast, there is less time for minerals to grow, resulting in smaller grain sizes.

Magma cooling at the dyke margins

The cooling rate is not uniform within the entire dyke. At the contact zones between the magma and the cooler surrounding rock, heat transfer is more efficient, leading to a faster rate of cooling. As a result, the magma at the margins of the dyke cools more quickly than the magma in the center of the dyke.

Fine-grained margin formation

The faster cooling rate at the margins of the dyke leads to the formation of smaller mineral grains compared to the center of the dyke. As a result, a dyke typically has a fine-grained margin and a coarser-grained core. This fine-grained margin is characteristic of many dykes and serves as an important feature during geological interpretation. In conclusion, a dyke commonly has a fine-grained margin due to the more efficient heat transfer and faster cooling rate at the margins of the dyke compared to its core. This faster cooling results in the formation of smaller mineral grains at the margins, giving the dyke its distinctive fine-grained appearance.

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Why does a dyke commonly have a fine-grained margin?

Short Answer

Step by step solution

Understand the formation of a dyke

Heat transfer within a dyke

Cooling rates and grain size

Magma cooling at the dyke margins

Fine-grained margin formation

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