Questions: (c) geology.com The best explanation for the very large crystals in this sample of pegmatite is because it had a very slow cooling rate it had a very fast cooling rate it has such a high silica content the magma that created this rock had water interact with the magma during its formation

The answer is the first one: it had a very slow cooling rate.

Explanation for each option:

1. It had a very slow cooling rate: This is the correct answer. Large crystals in igneous rocks, such as those found in pegmatite, typically form when the magma cools very slowly. Slow cooling allows the crystals more time to grow, resulting in larger crystal sizes.

2. It had a very fast cooling rate: This is incorrect. Fast cooling rates generally result in smaller crystals because the minerals do not have enough time to form large, well-defined crystals. This is often seen in volcanic rocks like basalt.

3. It has such a high silica content: While pegmatites often have high silica content, this alone does not explain the large crystal sizes. The crystal size is more directly related to the cooling rate rather than the silica content.

4. The magma that created this rock had water interact with the magma during its formation: While the presence of water can influence the formation of certain minerals and can aid in the growth of crystals, it is not the primary reason for the very large crystals in pegmatite. The slow cooling rate is the main factor.

Summary: The very large crystals in the sample of pegmatite are best explained by a very slow cooling rate, which allows the crystals ample time to grow to a large size.