Question

Two things that a geologist first considers when looking at a metamorphic rock are what the parent rock might have been, and what type of metamorphism has taken place. This can be difficult to do, even if you have the actual rock in your hand, but give it a try for the following metamorphic rocks: 1\. Chlorite schist 2\. Slate 3\. Mica-garnet schist 4\. Amphibolite 5\. Marble

Short answer

1. Chlorite schist: Parent rock: Shale or Mudstone Type of metamorphism: Regional 2. Slate: Parent rock: Shale Type of metamorphism: Low-grade regional 3. Mica-garnet schist: Parent rock: Shale or Felsic Igneous rock Type of metamorphism: Regional (Medium to High grade) 4. Amphibolite: Parent rock: Basalt or Gabbro Type of metamorphism: High-grade regional 5. Marble: Parent rock: Limestone or Dolostone Type of metamorphism: Contact or Regional

Step-by-step solution

1. Chlorite schist

: Chlorite schist is formed by regional metamorphism from sedimentary rocks like shale or mudstone. Chlorite is a greenish mineral found in this rock. Parent rock: Shale or Mudstone Type of metamorphism: Regional

2. Slate

: Slate is a foliated metamorphic rock that forms from the metamorphism of shale, a sedimentary rock. It is characterized by the parallel arrangement of clay minerals that confer a characteristic splitting ("slaty cleavage"). Parent rock: Shale Type of metamorphism: Low-grade regional

3. Mica-garnet schist

: Mica-garnet schist is a medium- to high-grade metamorphic rock characterized by the presence of micas (typically muscovite) and garnet crystals, usually formed by regional metamorphism of a protolith like shale or a felsic igneous rock. Parent rock: Shale or Felsic Igneous rock Type of metamorphism: Regional (Medium to High grade)

4. Amphibolite

: Amphibolite is a non-foliated metamorphic rock formed from basalt or gabbro, with amphibole minerals (e.g., hornblende) as its main constituent. The formation of amphibolite involves high-grade metamorphism, typically occurring in regional metamorphic settings. Parent rock: Basalt or Gabbro Type of metamorphism: High-grade regional

5. Marble

: Marble is a non-foliated metamorphic rock that forms from limestone or dolostone, sedimentary rocks composed of calcium carbonate (limestone) or calcium-magnesium carbonate (dolostone). The metamorphism often occurs under contact or regional metamorphic conditions, resulting in the recrystallization of the carbonate minerals. Parent rock: Limestone or Dolostone Type of metamorphism: Contact or Regional

Key concepts

Type of Metamorphism

Understanding the type of metamorphism is crucial for geologists trying to decipher the history of metamorphic rocks. Metamorphism, the alteration of rocks by heat, pressure, and chemically active fluids, can occur in various environments and intensities. The primary types include regional and contact metamorphism.

Regional metamorphism transpires over large areas during tectonic processes like mountain building. It's associated with large-scale structural deformations and can give rise to different grades of metamorphism—ranging from low to high—depending on the conditions. For example, slate represents low-grade regional metamorphism, whereas amphibolite signifies a high-grade version of this transformation.

Contact metamorphism, on the other hand, occurs when rocks are heated by nearby magma intrusion. The temperature gradient this causes means the process is localized rather than regional. Marble can form through contact metamorphism when limestone or dolostone is exposed to high heat and recrystallizes. This focused heating can alter the rock significantly without high pressure, distinguishing it from regional metamorphism.

Parent Rock Identification

Identifying the parent rock from which a metamorphic rock originated is akin to unraveling the life story of the rock. The original rock, also known as the protolith, undergoes profound physical and sometimes chemical changes to become a metamorphic rock. For example, the common parent rock for slate and schist is shale, a sedimentary rock composed of clay minerals.

For geologists, clues like mineral composition and textural features are vital. If you see a metamorphic rock with a greenish mineral known as chlorite, it likely evolved from a shale or mudstone. The formation of marble from limestone or dolostone is another classic example, as the calcite or dolomite crystals reorganize themselves into a denser, crystalline structure.

Each metamorphic rock may have a unique mineral assemblage or texture that points back to its parent rock, helping geologists reconstruct geological events and understand the conditions under which metamorphism occurred.

Regional Metamorphism

Regional metamorphism dominates over extensive geographic areas, often associated with the massive forces exerted by plate tectonics. This type of metamorphism can lead to the creation of magnificent mountain ranges and alters vast quantities of rock under differential stress, resulting in foliation, or the layering within rock structures.

Examples include the transition from shale to slate, then to schist, and even to gneiss, each reflecting progressively higher grades of metamorphism. Chlorite schist and mica-garnet schist from our examples can form through regional metamorphism. As these rocks are buried deeper under the Earth's surface during tectonic activity, they undergo dynamic changes. The pressure aligns platey minerals and may introduce new minerals, as seen with the garnet crystals in mica-garnet schist, indicating a medium to high-grade regional metamorphism.

Geologists map out these transformations through index minerals that point to the pressures and temperatures endured by the rocks. This practice allows them to outline the metamorphic grades present over regions, deciphering a part of Earth's geological history.

Contact Metamorphism

Contact metamorphism occurs when rocks are significantly heated by the intrusion of hot magma from the Earth's mantle. Unlike regional metamorphism, this process does not typically involve high pressures but is limited to changes in temperature. Thus, contact metamorphic rocks often do not exhibit the foliation typical of their regionally metamorphosed counterparts.

Marble, which can also be formed through regional metamorphism, is a prime example of a contact metamorphic rock when subjected to extreme heat from nearby magmatic activities. These conditions trigger the recrystallization of carbonate minerals within limestone or dolostone, resulting in a dense, crystalline rock. This heat-driven recrystallization enhances the textural and mineralogical uniformity of the original rock.

Contact metamorphism's effects are generally restrained to zones surrounding the magmatic intrusion, known as metamorphic aureoles. The extent of these aureoles depends on factors like the size and temperature of the intrusion and the existing temperature gradient of the surrounding rocks.