Question

Each of the following statements describes one or more characteristics of a particular metamorphic rock. For each statement, identify the metamorphic rock that is being described: a. calcite-rich and often nonfoliated b. loosely coherent and composed of broken fragments that formed along a fault zone C. represents a grade of metamorphism between slate and schist d. very fine-grained and foliated; excellent rock cleavage e. foliated and composed predominately of platy materials f. composed of alternating bands of light and dark silicate minerals g. hard and nonfoliated; resulting from contact metamorphism

Short answer

a. Marble b. Fault Breccia c. Phyllite d. Slate e. Schist f. Gneiss g. Hornfels

Step-by-step solution

Identifying Calcite-Rich, Nonfoliated Rock

The statement describes a metamorphic rock that is calcite-rich and often nonfoliated. A key feature of this rock is its composition, largely of calcite, which doesn't always develop foliation. This rock is commonly known as marble, which forms through the metamorphism of limestone.

Identifying Rock with Broken Fragments in a Fault Zone

The statement refers to a rock that is loosely coherent and composed of broken fragments formed in a fault zone. This type of rock is a product of dynamic metamorphism during faulting, leading to brecciation. This rock is called fault breccia.

Identifying Rock Between Slate and Schist

This describes a metamorphic rock that represents an intermediate grade of metamorphism between slate and schist. The rock that fits this description is phyllite, which typically has a more lustrous sheen than slate but hasn't fully progressed to schist.

Identifying Fine-Grained, Foliated Rock with Excellent Cleavage

The rock in question is very fine-grained, foliated, and known for excellent rock cleavage. These characteristics are typical of slate, which forms through low-grade metamorphism of shale.

Identifying Foliated Rock with Platy Materials

The statement describes a foliated rock composed mainly of platy minerals. Commonly, these platy minerals are micas, and the rock is known as schist, which forms at intermediate to high metamorphic grades.

Identifying Rock with Alternating Light and Dark Bands

The rock described has alternating bands of light and dark silicate minerals, a hallmark feature of gneiss. Gneiss forms under high-grade metamorphism and showcases this distinctive banding.

Identifying Hard, Nonfoliated Rock from Contact Metamorphism

This describes a hard, nonfoliated rock resulting from contact metamorphism. Such rocks are typically formed by the thermal influence of magma on surrounding rocks, leading to the creation of hornfels.

Key concepts

Marble

Marble is a metamorphic rock derived from the metamorphism of limestone. It is primarily composed of calcite, a mineral form of calcium carbonate, which gives marble its distinctive white appearance. Calcite's crystalline structure can sometimes create a visual pattern of swirling, adding unique aesthetics. Due to its primary composition, marble is often nonfoliated, meaning it does not have the layered or banded appearance seen in some other metamorphic rocks.

Marble's nonfoliated nature is a result of the low pressure conditions present during its formation. This makes it different from foliated rocks, where a higher degree of pressure typically plays a role. Marble's characteristics make it popular in architecture and sculpture, as well as for use in tiles and countertops.

Contact metamorphism

Contact metamorphism occurs when rock is altered mostly by heat in the proximity of magma. This extreme heat "bakes" nearby rocks, causing the mineral structures within them to change without melting the rock completely. Unlike regional metamorphism, which usually involves both heat and pressure, contact metamorphism primarily involves intense heat with little or no pressure.

Rocks created through this process, such as hornfels, are typically nonfoliated. This lack of foliation arises because the conditions do not subject minerals to directional stress, which is necessary for the development of foliation.

Foliated rocks

Foliated rocks are characterized by a layered appearance due to the alignment of mineral grains under directed pressure during metamorphism. This process causes minerals to reorient perpendicularly to the direction of pressure, resulting in a banded texture. Foliated rocks are typically classified into different types based on the degree of metamorphism and grain size, such as slate, phyllite, schist, and gneiss.

Foliated textures are useful for identifying rocks in the field, as they can indicate the metamorphic conditions the rock has undergone. The presence of foliation is evidence of directional stress, which usually occurs over larger areas, as opposed to local heat sources like those in contact metamorphism.

Gneiss

Gneiss is a high-grade metamorphic rock recognized by its distinct banded appearance, with alternating layers of light and dark silicate minerals. This foliation is due to extreme temperature and pressure conditions, typically found deep within the Earth's crust. The light bands are usually composed of quartz and feldspar, while the dark bands consist of ferromagnesian minerals such as biotite and hornblende.

Gneiss can form from the metamorphism of many rock types, including granite and schist. Because of its strength and durability, gneiss is often used in construction and ornamental stones. Its striking banded patterns can make it aesthetically appealing for decorative stonework.

Schist

Schist is a medium- to high-grade metamorphic rock known for its well-developed foliation and platy mineral structure. Schists commonly have visible layers of minerals, such as mica, which give them a shiny and often shimmering appearance. These rocks form under conditions of more intense heat and pressure than those forming less metamorphosed rocks like slate or phyllite, but less intense than conditions forming gneiss.

Because of its platy mineral content, schist can easily break along these layers, which is why it is often sought for use in natural split stones like tiles and decorative features. However, its layered nature also means that it may not always have the structural integrity for certain applications where strength is critical.

Slate

Slate is a fine-grained, foliated metamorphic rock that originates from the low-grade metamorphism of shale. It is characterized by excellent rock cleavage, meaning it splits easily into thin sheets. This quality makes slate widely used for roofing tiles and flooring materials.

Slate's formation involves relatively low temperatures and pressures, which cause clay minerals in the original shale to transform into new minerals aligned parallel to the foliation plane. Because of its dense structure and cleavage properties, slate is also used in writing slates and as a traditional material in some musical instruments.

Phyllite

Phyllite represents a metamorphic grade between slate and schist, making it an intermediate rock type. It has a foliated texture that is slightly coarser than slate but finer than schist. The sheen visible on the surface of phyllite is due to the tiny mica flakes within the rock, which are more pronounced than in slate.

Phyllite forms under slightly higher temperatures and pressures than slate, causing the rock to have a more pronounced sheen and sometimes a wavy appearance known as crenulations. These properties make phyllite useful in decorative stone applications, similar to slate, but with a distinctive sheen that can enhance its aesthetic appeal.