Chapter 8, Problem 1GST

(a)

To determine

To identify: The calcite rich and non-foliated metamorphic rock.

Answer to Problem 1GST

Marble is a calcite-rich, non-foliated rock.

Explanation of Solution

The metamorphic rocks that are formed around igneous intrusions where the temperature is high and pressure is low and are distributed equally in all directions are termed as non-foliated metamorphic rocks. During the formation of non-foliated metamorphic rocks, the minerals within the rock recrystallize into large grains and the atoms will become tightly packed; thereby making it denser than the pre-existing rock.

A calcite-rich, non-foliated metamorphic rock is marble, which is formed by the metamorphism of limestone and dolostone. It is a medium- to coarse-grained rock with interlocking crystals of calcite.

(b)

To determine

To identify: The metamorphic rock that is loosely coherent and composed of broken fragments that formed along a fault zone.

Answer to Problem 1GST

The loosely coherent rock that is formed along a fault zone and composed of broken fragments is termed as fault breccia.

Explanation of Solution

When two blocks of rocks that have a brittle solid nature moves along a fault zone, fractures are created in the rocks, thereby grinding the rocks. As a result, the loosely coherent rock that is composed of broken and crushed fragments is formed and it is known as fault breccia.

(c)

To determine

To identify: The metamorphic rock that represents a grade of metamorphism between slate and schist.

Answer to Problem 1GST

The rock that represents a grade of metamorphism between slate and schist is phyllite.

Explanation of Solution

Metamorphism of shale, mudstone, or siltstone results in the formation of slate, phyllite, and schist.

When the metamorphic intensity increases, that is with progressively higher temperature and pressure conditions, shale is converted to slate, then to phyllite and to schist.  Grain size increases with increasing levels of metamorphism.

A fine-grained rock that is composed of very fine-grained platy minerals, such as muscovite, chlorite or both, that breaks along the wavy surfaces is called phyllite. The grain size are larger than that of slate but not recognized by unaided eye. It has a glossy sheen and it represents the degree of metamorphism between the slate and schist.

(d)

To determine

To identify: The metamorphic rock that is composed of tiny chlorite and mica grains and display excellent rock cleavage.

Answer to Problem 1GST

The metamorphic rock that is composed of tiny chlorite and mica grains and exhibits excellent rock cleavage is slate.

Explanation of Solution

The tendency of a rock to split into thin pieces when hammered is called rock cleavage. It develops well in slate and hence termed as slaty cleavage. Slaty cleavage develops due to low-grade metamorphism that involves the folding of shale beds and metamorphism to produce slate. Folding occurs by the deformation of rock due to compressional stress. Further deformation recrystallizes the clay minerals in shale into small pieces of chlorite and mica, and they orient perpendicular to the maximum stress direction.

(e)

To determine

To identify: The metamorphic rock that is foliated and composed predominantly of platy minerals.

Answer to Problem 1GST

The metamorphic rock that is foliated and composed predominantly of platy minerals is schist.

Explanation of Solution

A planar or layered structure exhibited by platy minerals is termed as schistosity and it is usually exhibited by schist. A higher pressure and temperature conditions converts the small flakes of mica and chlorite present in slate to larger muscovite and biotite crystals by recrystallization. Apart from platy minerals, flattened or lens-shaped deformed quartz and feldspar are embedded with mica grains.

(f)

To determine

To identify: The metamorphic rock that is composed of alternating bands of light and dark silicate minerals.

Answer to Problem 1GST

The metamorphic rock that is composed of alternating light and dark bands is gneiss.

Explanation of Solution

Segregation of minerals due to ion migration results in the formation of alternating bands of white and dark silicate minerals.

A metamorphic rock that is characterized by alternating bands of light and dark silicate minerals is termed as gneiss. A high-grade metamorphism of shale generates metamorphic gneiss. At the time of high-grade metamorphism, the minerals are segregated as a result of ion migration and it leads to the formation of alternate white and dark bands of silicate minerals. White or reddish feldspar constitutes the white bands and dark ferromagnesian minerals constitute the dark layer. In some cases, the evidence of deformation due to folds and faults can also be traced in banded gneiss.

(g)

To determine

To identify: The hard and non-foliated metamorphic rock produced by contact metamorphism.

Answer to Problem 1GST

The hard and non-foliated metamorphic rock produced by contact metamorphism is quartzite.

Explanation of Solution

A type of metamorphism in which the role of pressure is minimal and occurs at the Earth’s surface or sub-surface by the intrusion of a hot molten material into a country rock is contact metamorphism. The intrusion of a hot molten magma results in a large contrast in temperature between the intruded magma and surrounding country rock. As a result, the area surrounding the igneous intrusion will be baked due to the heat released by the magma and this zone is termed as contact aureole.

Quartzite is a very hard, massive rock that formed by the metamorphism of quartz sandstone. When sandstone is subjected to medium to high-grade metamorphism, the quartz mineral grains present in sandstone are compacted together. Pure quartzite is white, but iron oxide may produce reddish or pinkish stains, while dark mineral grains may impart shades of green or gray.