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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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

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