Question

How did Sample I6 form? a. by magma that cooled slowly b. by fast cooling lava c. by a very fast cooling of a lava d. by a very fast cooling magma during a volcanic eruption e. by magma that was cooling slowly, then as a lava that cooled quickly

Short answer

Sample I6 likely formed by a very fast cooling magma during a volcanic eruption if it's glassy.

Step-by-step solution

Analyze Rock Properties

Examine the characteristics of Sample I6. If Sample I6 is glassy or has small, fine crystals, it likely formed from a fast cooling process. If it has large, visible crystals, it suggests slow cooling.

Interpret Cooling Process

Determine the cooling process of Sample I6. Fast cooling results in a glassy texture, while slow cooling provides time for crystals to grow larger. Mixed textures can indicate varied cooling rates.

Match Description with Options

Compare the observed characteristics and cooling interpretation with the options given: - Option (a) describes slow-cooling magma. - Option (b) describes fast-cooling lava. - Option (c) suggests very fast cooling of lava, indicating formation but no crystal development. - Option (d) describes quickly cooling magma during a volcanic eruption, resulting in a glassy texture. - Option (e) indicates initial slow cooling followed by quick cooling.

Select the Most Accurate Answer

Based on the characteristics you examined, choose the option that best fits Sample I6. If it has both large and small crystals or is glassy, consider the process it underwent.

Key concepts

Rock Cooling Processes

When it comes to understanding how igneous rocks form, examining rock cooling processes is essential. These processes dictate the texture and crystal size of the resulting rock. Let's break it down:

1. **Slow Cooling:** This occurs when magma cools gradually, typically deep underground. Because the process is lengthy, crystals have ample time to grow large. The resulting rock is coarse-grained, like granite—perfect examples of slow cooling.

2. **Fast Cooling:** In contrast, when lava erupts from a volcano and quickly flows onto the Earth's surface or ocean floors, it cools rapidly. This swift process doesn't allow much time for crystals to form, resulting in fine-grained rocks like basalt.

3. **Very Fast Cooling:** Sometimes, lava cools incredibly quickly, such as when it comes into contact with water. This instant cooling forms volcanic glass, like obsidian, with no crystal growth due to the lack of time.

Understanding these processes helps us interpret the environment and conditions under which an igneous rock formed.

Crystal Formation in Rocks

The formation of crystals in rocks depends heavily on the conditions and speed of the cooling process of magma or lava. Here's how it works:

- **Crystal Size and Cooling Rate:** Slow-cooling environments allow crystals to become large and well-developed. Think of quartz or feldspar crystals found in granite. Whereas fast-cooling environments restrict the growth of crystals, resulting in a dense network of tiny or even microscopic crystals.

- **Dual Cooling Rates:** Sometimes, cooling can start out slow, then speed up. This results in rocks with varying crystal sizes, possessing both large and small crystals—known as porphyritic texture. This texture indicates a changing environment, possibly with magma beginning cooling underground then rapidly cooling when exposed to the surface.

In igneous rocks, crystal size can tell us a lot about the history of the rock's formation and the environment it was exposed to.

Volcanic Eruption and Rocks

Volcanic eruptions play a crucial role in rock formation, influencing numerous features like texture and mineral composition of the rock itself. Here's how:

- **Lava vs. Magma:** Magma is molten rock found beneath the Earth's surface. During a volcanic eruption, this magma can reach the surface and become lava. The eruption dynamics significantly affect how lava cools and forms rocks.

- **Eruption Impact:** Explosive eruptions, which rapidly expel magma, often lead to the formation of volcanic rocks that cool quickly, such as pumice or tuff, resulting in fine-grained or glassy textures.

- **Calm Outflows:** If the eruption is less violent, lava may flow out smoothly, leading to relatively slower cooling, resulting in rocks like basalt.

Volcanic eruptions thus not only shape the landscape but also determine the type and characteristics of the rock formed.