Problem 6 Why doesn't hornfels contain fol... [FREE SOLUTION]

Chapter 8: Problem 6

Why doesn't hornfels contain foliation? Name two other nonfoliated metamorphic rocks. (D)

Short Answer

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Hornfels does not contain foliation because it forms through contact metamorphism, which involves only heat and not the directed pressure required for foliation. During contact metamorphism, minerals recrystallize but do not develop a parallel alignment, resulting in a nonfoliated rock. Two other nonfoliated metamorphic rocks are Marble, formed from limestone or dolostone, and Quartzite, formed from quartz-based sandstone. Both rocks undergo recrystallization under heat and pressure without developing foliation due to the absence of directed pressure.

Step by step solution

1. Formation of Hornfels

Hornfels is a dense, fine-grained metamorphic rock formed through contact metamorphism. This type of metamorphism occurs when a rock is subjected to heat by being in contact with a magma body. When the magma (in the form of an intrusion) comes into contact with the country rock (rock surrounding the intrusion), it heats the rock and causes its minerals to recrystallize, forming new minerals in the process.

2. Foliation and Its Causes

Foliation is the parallel arrangement of sheet-like minerals in a metamorphic rock, such as micas, due to the directed pressure experienced during metamorphism. Foliation usually occurs in the metamorphic rocks formed through regional metamorphism, which involves intense heat and pressure over a broad area. During this process, pressure causes minerals to be aligned in a parallel manner, giving the rock a layered or banded appearance.

3. Why Hornfels Doesn't Contain Foliation

Hornfels doesn't contain foliation because it is formed through contact metamorphism, which does not involve the directed pressure that is responsible for foliation. In the formation of hornfels, rocks are only subjected to heat from the magma intrusion, which causes recrystallization but doesn't contribute to the development of parallel arrangement or alignment of minerals. As a result, hornfels is a nonfoliated metamorphic rock.

4. Two Other Nonfoliated Metamorphic Rocks

1. Marble: Marble is a nonfoliated metamorphic rock formed from the metamorphism of limestone or dolostone. During the metamorphic process, intense heat and pressure cause the calcite in the parent rock to recrystallize, forming a compact, interlocking mosaic of calcite crystals without any parallel mineral alignment. 2. Quartzite: Quartzite is another nonfoliated metamorphic rock that forms from the metamorphism of quartz-based sandstone. Under intense heat and pressure, the silica in the parent sandstone recrystallizes and forms a solid, interlocking mosaic of quartz crystals without any foliation.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Hornfels Formation

Hornfels is a fascinating type of metamorphic rock that forms under specific conditions. It is created through a process called contact metamorphism. This type of metamorphism happens when hot magma comes into contact with cooler surrounding rocks, known as country rocks. The intense heat from the magma causes the minerals in the country rock to recrystallize. However, it is important to note that during this process, pressure is not a key factor.
This is why hornfels remains dense and fine-grained without the layered look typical of rocks formed by directed pressure.

Contact Metamorphism

Contact metamorphism is a geological process that occurs when rocks are heated due to their proximity to an intruding mass of magma. This interaction primarily causes changes due to temperature, rather than pressure, unlike regional metamorphism.

The heat leads to the recrystallization of minerals in the surrounding rocks, forming new minerals in the process.
This metamorphism is usually localized near magma intrusions.

Because the pressure is not a significant factor, the rocks formed, like hornfels, don't have the banding or layering seen in foliated rocks.

Quartzite Characteristics

Quartzite is a strong, durable metamorphic rock originally formed from quartz-rich sandstone. Its transformation involves significant heat and pressure, leading to recrystallization of its constituent minerals. During this process, the silica in the sandstone fuses together, creating a rock that is both dense and hard.
Quartzite has a distinctive appearance because even though it lacks foliation, it displays a smooth, glassy surface with a granular texture.

Quartz crystals within quartzite are tightly interlocked, making it resistant to weathering and erosion.
It typically appears in a range of colors, often including shades of white, grey, or pink.

Marble Transformation

Marble is another fascinating nonfoliated metamorphic rock, formed primarily from the metamorphism of limestone or dolostone. Under conditions of high temperature, the calcite minerals in the limestone begin to recrystallize and form a dense, interlocking mosaic of calcite crystals.
The transformation makes marble a nonfoliated rock with no leaf-like mineral alignment that characterizes foliated rocks.

This rock often has a beautiful appearance with interlocking crystal patterns and a variety of colors.
Marble is highly prized for its use in architecture and sculpture.

Absence of Foliation in Metamorphic Rocks

The absence of foliation in certain metamorphic rocks is primarily due to the conditions under which they form. Foliation happens when minerals within a rock become aligned due to significant pressure, usually in regional metamorphism. However, several metamorphic rocks, like hornfels, marble, and quartzite, do not experience this kind of pressure.

Rocks like hornfels undergo contact metamorphism, involving heat from nearby magma but not the pressure needed to align minerals.
Quartzite and marble transform from their parent rocks (sandstone and limestone, respectively) through heat and minimal pressure, resulting in dense, crystalline textures without foliation.

These characteristics give nonfoliated rocks their unique and often aesthetically appealing look.

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Why doesn't hornfels contain foliation? Name two other nonfoliated metamorphic rocks. (D)

Short Answer

Step by step solution

1. Formation of Hornfels

2. Foliation and Its Causes

3. Why Hornfels Doesn't Contain Foliation

4. Two Other Nonfoliated Metamorphic Rocks

Key Concepts

Hornfels Formation

Contact Metamorphism

Quartzite Characteristics

Marble Transformation

Absence of Foliation in Metamorphic Rocks

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