Characteristics of Metamorphic Rocks (Listed and Explained)

Metamorphic rocks were initially like any other rock form, but their igneous, original, and sedimentary forms have changed significantly.

Metamorphic rocks form when rocks are exposed to high pressure, high heat, hot-mineral-rich fluids, or mostly, a combination of these aspects.

Usually, these conditions occur where tectonic plates collide or are deep in the Earth.

AnchorIn this article piece, we will be learning the characteristics of metamorphism, the process of metamorphism, rock classification, and formation of metamorphic rocks, the types and uses of metamorphic rocks.

Characteristics of Metamorphic Rocks (Let’s Get Started)

What are the Metamorphic Rocks?

The word metamorphic is derived from a Greek for ‘change of form’.

Meta means ‘change’ while morph means ‘form’, forming the name ‘metamorphic’.

Metamorphic rocks come from sedimentary or igneous rocks that have recrystallized (altered their form) through changes in their physical atmosphere.

In simple terms, metamorphic rock is a rock formed from another rock.

Characteristics of Metamorphism

Generally, metamorphic rocks have lower porosity and higher density than the rocks formed from them.

They are coarser than the original rocks.

In low-grade metamorphic circumstances, the original rocks are only condensed like in the composition of slate from shale.

We characterize the metamorphic rocks according to their texture and distinct mineral composition:

Texture

Metamorphic rocks bear a coarser texture than the protolith they are formed from.

A steady arrangement of surrounding atoms surrounds the atoms in the crystal interior.

Partially, this is missing at the crystal surface, hence forming volatile surface energy.

Recrystallization to more rough crystals lowers the surface area hence minimizing the surface energy.

Metamorphic Minerals

In metamorphic rocks, the presence of minerals is a sure bet.

Still, each mineral is steady only within some limits depending on the approximate pressures and temperatures the rock experienced a metamorphosis.

These minerals are termed ‘index minerals’.

Examples are kyanite, sillimanite, andalusite, garnet, and staurolite.

There are other minerals found in metamorphic rocks but are not essential due to the metamorphism process.

They include pyroxenes, micas, quartz, olivines, hornblende, and feldspars.

They are steady at high pressures and temperatures and can remain unaffected during the process of metamorphic. 

Classification of Metamorphic Rocks

Metamorphic rocks fall under the most extensive grouping of all rock types.

Hence, there is a wide variety of metamorphic rock forms.

We describe the rock by adding the prefix ‘meta’ to the protolith rock title.

For instance, if the protolith is basalt, the rock will be ‘metabasalt’.

If the protolith of a metamorphic rock is conglomerate, the rock will be ‘meta conglomerate.

Nevertheless, for a rock to be classified in this method, the protolith should be distinguishable from the features of the metamorphic rock itself.

According to the British Geological Society classification system, if the general type is only what can determine the protolith, such as volcanic or sedimentary, classification will be in mineral mode basis. 

How are the Metamorphic Rocks Formed?

A metamorphic rock starts as one type of rock; it eventually changes into a new rock with heat, pressure, and time.

The metamorphism process doesn’t melt rocks; it changes them into more compact and denser rocks.

The rearrangement of fluids reaction or mineral components creates new minerals.

The temperature or pressure may transform the initial metamorphosed rocks into new varieties.

Metamorphic rocks are frequently folded, smeared out, and squished.

Even with these hostile conditions, metamorphic rocks do not heat enough to melt; otherwise, they would turn to igneous rocks!

Magma intrusions and high tectonic movements form earth movements which consequently causes movement and shifting of the pre-existing rocks. 

This movement causes heat and high pressure from the rocks buried beneath the Earth’s surface, contributing to changes and accumulation of the rock’s mineralogy, chemical composition, and texture.

These changes transform the rocks’ size and crystal type and may also cause other rock essential changes.

The metamorphic heating procedure ranges between 150o and 795o  Celsius and can produce high energy to break and reform the rock’s chemical composition.

Due to the pressure coming from the overlying rocks, the transformation process is increased.

The heat that causes rock changes is a result of magma and friction heating along the fault lines. 

The rocks do not melt; what happens is that few mineral groupings rearrange the elements in line with the original minerals forming new mineral compositions.

The newly formed minerals are steady at the new atmosphere and pressures.

The chemical composition and texture changes result from extreme temperature gradients in melted magma and the country rocks.

At this stage, we find the original rocks being transformed into metamorphic rocks.

The metamorphic rocks produced due to intrusions and direct lava heating are the contact or thermal metamorphic rocks.

On the other hand, those produced from high-temperature changes and distributed pressure prompted by tectonic movements are regional metamorphic rocks.

Though metamorphic rocks mainly form deep in the Earth’s crust, they may often be exposed on the planet’s surface because of the rock and soil erosion and geologic uplift above them.

On the surface, metamorphic rocks are exposed to weathering conditions making them break down into sediment.

The sediment is compressed to produce sedimentary rocks, bringing the entire cycle into a new beginning again.   

Types of Metamorphic Rocks

We classify metamorphic rocks into two categories:

  • Foliated
  • Non-foliated

Foliated Metamorphic Rocks

These are formed from straight exposure to heat and pressure.

They have the largest metamorphic rock groupings.

They have four distinct aligned texture types with a layered or banded appearance. 

Examples include gneiss, phyllite, slate, and schist.

Non-foliated Metamorphic Rocks

Non-foliated is formed due to direct pressure or tectonic movements, which greatly depend on their pre-existing circumstances.

They lack a layered or banded appearance.

A well-known non-foliated metamorphic rock example is a marble. Other examples include hornfels, quartzite, and novaculite.

Examples of Metamorphic Rocks

There are various metamorphic rocks all over the Earth with diverse textures and compositions.

The best method of understanding their types is through proper handling and observing them in reality. Here are the most known types:  

Amphibolite

This is a non-foliated metamorphic rock made of amphibole and plagioclase (hornblende) with quartz.

In its formation, amphibolite requires directed pressure conditions and high viscosity through the recrystallization process.

Novaculite

This is a siliceous, hard, dense, and fine-grained rock.

It falls under non-foliated rocks—this type of rock forms in marine atmospheres – from sediment deposits.

Novaculite breaks with a conchoidal fracture.

Gneiss

This is a foliated metamorphic rock formed by coarse mineral grains.

It has a similar appearance to granite. It contains lots of quartz and mica bands and feldspar minerals.

Gneiss is laminated and portrays a banded appearance.

Marble

Marble is generally composed of calcium carbonate.

It falls under the non-foliated rocks and is formed from limestone or dolostone metamorphism.

It serves as a building material and for sculpture purposes.

Quartzite

This is a hard rock that consists of interconnecting quartz crystals.

It falls under the non-foliated rocks, and it’s formed during the sandstone metamorphism.

Lapis Lazuli

It’s the rarest metamorphic rock, primarily due to its blue color.

This blue color produces a blue gem material used to make beads (in the form of round-small stones) and for decoration, making Lapis Lazuli famously known. 

Soapstone

This is a metamorphic rock composed of varying mineral amounts such as micas, chlorite, carbonates, pyroxenes, and amphiboles.

It has a talk soapy feel.

Soapstone is also a soft, dense, heat-resistant rock having extraordinary heat capacity. 

Due to its assets after metamorphism, soapstone is considered highly used in architectural and artist works.

Schist

This type of metamorphic rock contains mica substantial amounts and is very well developed.

Due to high mica concentration, the schist easily splits into thin layers.

According to geologists, schist embodies the transitional metamorphic rank between phyllite and gneiss.

Slate

This is a fine-grained and low-grade rock, which can be separated into small pieces.

It falls under foliated metamorphic rocks and is produced by shale metamorphism.

Slates are mainly realigned clay crystals.

Phyllite

This type is composed of chlorite and fine-grained mica.

It falls under foliated rocks. It has a lustrous and wrinkled surface.

As per the geologists, phyllite epitomizes the transitional state between schist and slate.

Hornfels

This is a fine-grained rock formed by heat action on clay rocks through a process called contact metamorphism.

It falls under non-foliated rocks having no precise composition. Hornfels heat when on heat sources like a dike, magma chamber, or sill.

Is there any Condition Necessary for Metamorphism Formation?

The conditions are specific.

  • Expose the existing rock to high pressure, high heat, or a hot-mineral-rich fluid.
  • Beware of too much pressure or heat to prevent the existing rock from turning to magma. (It should remain solid)

The best metamorphic rocks for construction and artwork include:

  • Schist
  • Phyllite
  • Quartzite
  • Gneiss
  • Marble
  • Slate 

Metamorphic rocks exist when high pressure and temperature subject a rock to produce its chemical and physical properties.

These circumstances twist, fold and stretch the rock as it cools.

In metamorphic rock formation, few or all minerals in the original rock are substituted, atom by atom, to produce new minerals. 

characteristics of metamorphic rocks