Metamorphic Rocks 

Metamorphic rocks are one of the three main types of rocks found on Earth, formed through a process of transformation. These rocks begin as sedimentary or igneous rocks but undergo intense heat and pressure deep within the Earth’s crust, causing their mineral structure to change. This process, known as metamorphism, results in the formation of new rock types with unique textures and properties. Metamorphism occurs between about 10 and 50km of depth. Common examples of metamorphic rocks include marble, slate, and schist, each with distinct characteristics. In this blog, we’ll explore the formation, types, and uses of metamorphic rocks, as well as their importance in geology and construction.

The word “metamorphic” comes from Greek which means “to change form”. The transformation of pre-existing rocks into mineralogically distinct new rocks because of high temperature and pressure or both, but without melting of rock.

What Drives Metamorphism?

          ·       Heat
    ·       Pressure
    ·       Actively Fluids
    ·       Types of Parent Rocks

 

This shows how metamorphism happens 

Heat

•  Temperature increases can be caused by layers of sediments being buried deeper and deeper     under the surface of the Earth.

•  As we descend into the earth the temperature increases about 25C for every kilometer that we   descend. The deeper the layers are buried the hotter the temperatures become.

•  The great weight of these layers also causes an increase in pressure, which in turn, causes an   increase in temperature.

•  Metamorphism can take millions of years as in the slow cooling of magma buried deep under the   surface of the Earth.

Pressure

There are 3 factors that cause an increase in pressure which also causes the formation of metamorphic rocks. These factors are:

•  The huge weight of overlying layers of sediments.

•  Stresses caused by plates colliding in the process of mountain building.

•  Stresses caused by plates sliding past each other, such as the shearing stresses at the San Andreas fault zone in California, or under another, such as the Pacific & Indo-Australian plates beneath New Zealand.

Active fluids

Chemically active fluids

      •   Mainly water with other volatile components

      •   Enhances migration of ions

      •   Aids in recrystallization of existing minerals

Sources of fluids

     •   Pore spaces of sedimentary rocks

     •   Fractures in igneous rocks

     •   Hydrated minerals such as clays and micas

Type of parent Rock

• The mineral content of metamorphic rock is controlled by the chemical composition of the           parent rocks.

•  For example, marble is indicative of parent rock composed of Caco3.

•  Slate is result of metamorphism of Shale.

•  Similarly, Granite Gneiss shows metamorphism of Granite.

 

Types of Metamorphism

There are 2 main ways that metamorphism can occurs. Contact and Regional Metamorphism.

We will briefly understand each type.

Contact Metamorphism

•   Contact metamorphism occurs adjacent to pluton, when magma intrudes relatively cool.

•  The zone of contact metamorphism is called AUREOLE.

•  Dikes/Sills generally have small aureoles with minimal metamorphism whereas large           ultramafic   intrusions can have significantly thick and well-developed contact metamorphism.

•  Contact metamorphism produces non-foliated (rocks without any cleavage) rocks such as   marble,  quartzite, and hornfels.

Regional Metamorphism

•  Regional Metamorphism occurs over a much larger area than contact metamorphism.

•  This metamorphism usually produces rocks such as gneiss and schist.

•  Regional metamorphism is caused by large geologic processes such as mountain-building.

•  These rocks when exposed to the surface show the unbelievable pressure that cause the rocks  to   be bent and broken by the mountain building process.

Regions of Metamorphism

Contact & Regional Metamorphism

Progressive Metamorphism

• Change of metamorphic rocks with progressive change of PRESSURE, TEMPERATURE.

• At higher pressure and temperature, we have greater metamorphism effects, and as we get      away from the site of metamorphism progressively, the effect of metamorphism decreases.

             E.g. Shale → Slate → Phyllite → Schist → Gneiss

how one rocks change into others

Common metamorphic rocks

Foliated rocks

  Slate

     • Very fine-grained

     • Excellent rock cleavage

     • Most often generated from low-grade metamorphism of shale, mudstone, or siltstone

 Phyllite

     • Gradation in the degree of metamorphism between slate and schist

     • Platy minerals not large enough to be identified with the unaided eye

     • Glossy sheen and wavy surfaces

     • Composed mainly of fine crystals of muscovite and/or chlorite

Figure 1: Phyllite, showing the glossy sheen and fine-grain texture

Gneiss

     •    Medium to coarse-grained

     •    Banded appearance

     •    High-grade metamorphism

     •   Often composed of white or light-colored feldspar-rich layers with bands of dark ferromagnesian      minerals

Figure 2: Gneiss with its characteristic banded appearance

Non foliated rocks

Marble

    •  Coarse, crystalline

    •  Parent rock was limestone or dolostone

    •  Composed essentially of calcite or dolomite crystals

    •  Used as a decorative and monument stone

    •  Exhibits a variety of colors

Figure 3: Marble

Quartzite

   • Formed from a parent rock of quartz-rich sandstone

   • Quartz grains are fused together

Figure 4: Quartz

                                                                        Figure 5: Marble                                            

                                                                            Figure 6: Slate

 

Importance of Metamorphic Rocks in Civil Engineering

Building Materials:

Marble: This is used in the luxurious constructions, sculptures, and monuments.

Slate: Slate has found its way to the roofing and flooring industry owing to its capability to cut into thin and flat pieces.

Structural Integrity:

Granite and Gneiss: This is powerful and thick, applicable in foundations, bridges and highway construction.

Incident Resistance to adverse environmental conditions:

They are also resistant to weathering and soil erosion, which makes them the best infrastructure structures in coastal regions and those with severe weather conditions.

Quarrying and Raw Materials:

Mined to make aggregates to be used in road construction, dams, and other infrastructure projects on a large scale.