Geology – Overview of Metamorphic Rocks

In the conclusion of our three-part series looking at various types of rock, we delve into the aftermath of the application of heat and pressure to find out about metamorphic rocks. As you may gather from the name and your knowledge of the term “metamorphosis” as it relates to the life cycle of a butterfly or frog, for example, metamorphic rocks are all about change.

Metamorphic rocks form from the alteration of pre-existing rocks of any type through heat, pressure, and chemical processes. As with igneous and sedimentary rocks, metamorphic rocks provide valuable clues to geologists about different processes that occur deep within the Earth’s crust and mantle, and about the history of different regions.

Formation of Metamorphic Rocks

As mentioned, metamorphic rocks form from the alteration of pre-existing rocks through heat, pressure, and chemical processes. This metamorphism can occur through several different mechanisms, including contact metamorphism, regional metamorphism, and dynamic metamorphism. Contact metamorphism occurs when rocks are altered by heat and pressure from nearby magma bodies, while regional metamorphism occurs when rocks are altered by heat and pressure from tectonic forces over a large area. Dynamic metamorphism occurs when rocks are altered by pressure and shear forces in fault zones – which can happen relatively quickly if a fault is active.

Varieties of Metamorphic Rocks

Metamorphic rocks can be classified into two main types based on their texture: foliated and non-foliated. Foliated metamorphic rocks have a layered or banded texture due to the alignment of mineral grains, while non-foliated metamorphic rocks have a uniform texture due to the absence of mineral alignment.

• Foliated Metamorphic Rocks: Foliated metamorphic rocks include rocks like gneiss, slate, and schist. Gneiss is a high-grade metamorphic rock that is composed of alternating layers of feldspar and quartz. Slate is a low-grade metamorphic rock that is composed of clay minerals that have been compacted and recrystallized. Schist is a medium-grade metamorphic rock that is composed of mica minerals that have been aligned in a parallel orientation.
• Non-Foliated Metamorphic Rocks: Non-foliated metamorphic rocks include rocks like marble and quartzite. Marble is a metamorphic rock that is composed of recrystallized calcite or dolomite. Quartzite is a metamorphic rock that is composed of recrystallized quartz.

Identification and Properties of Metamorphic Rocks

Metamorphic rocks can be identified based on their texture, mineral composition, and degree of metamorphism. Foliated metamorphic rocks are characterized by their layered or banded texture, while non-foliated metamorphic rocks have a uniform texture. The mineral composition of metamorphic rocks can vary depending on the parent rock and the degree of metamorphism. Some common minerals found in metamorphic rocks include mica, quartz, and calcite.

The properties of metamorphic rocks vary depending on their texture, mineral composition, and degree of metamorphism. Foliated metamorphic rocks are typically harder and more durable than non-foliated metamorphic rocks due to the alignment of mineral grains. Metamorphic rocks are often denser and more resistant to weathering and erosion than sedimentary rocks due to the recrystallization and compaction of mineral grains.

Types of Metamorphic Rock

Some common types of metamorphic rocks include gneiss, slate, schist, marble, and quartzite. Gneiss is a high-grade metamorphic rock that is composed of alternating layers of feldspar and quartz. Slate is a low-grade metamorphic rock that is composed of clay minerals that have been compacted and recrystallized. Schist is a medium-grade metamorphic rock that is composed of mica minerals that have been aligned in a parallel orientation. Marble is a metamorphic rock that is composed of recrystallized calcite or dolomite. Quartzite is a metamorphic rock that is composed of recrystallized quartz.

Uses for Metamorphic Rocks

Metamorphic rocks have a wide range of uses in human society. Some common uses of metamorphic rocks include:

• Building Stone: Metamorphic rocks like marble and slate are often used as building stone due to their durability, strength, and aesthetic qualities.
• Dimension Stone: Some types of metamorphic rocks, such as marble and gneiss, are highly valued as dimension stone for use in sculpture, monuments, and high-end architecture.
• Crushed Stone: Some types of metamorphic rocks, such as quartzite, are used as crushed stone for road construction, railroad ballast, and concrete aggregate.
• Industrial Minerals: Some types of metamorphic rocks, such as talc and graphite, are used in a variety of industrial applications, including the production of ceramics, lubricants, and pencils.

To wrap things up in a TL;DR fashion, metamorphic rocks provide valuable information about the conditions and processes that occur within the Earth’s lithosphere. They are formed through the alteration of pre-existing rocks through heat, pressure, and chemical processes, and can be classified as foliated or non-foliated based on their texture. Metamorphic rocks can be identified based on their texture, mineral composition, and degree of metamorphism, and have a wide range of uses in human society, including as building stone, dimension stone, crushed stone, and industrial minerals.