Metamorphic rock

Topic: Minerals, Rocks and the Rock Cycle · NCERT: Beyond NCERT

Meaning

A metamorphic rock is a rock whose minerals and texture (the size, shape and arrangement of its grains) have been changed by heat and pressure while it stays solid. Its parent rock can be igneous, sedimentary or an older metamorphic rock. Common examples are marble from limestone and slate from shale. Metamorphic rocks are hard and wear away slowly, so they often stand as ridges and gorges. They also hold resources such as marble and graphite, and they make up the old crystalline base (shield) of Peninsular India.

Explanation

How it forms: cause → process → result

• Meaning of the word: meta means change and morph means form. So metamorphism is a "change of form".
• What drives it: PVT. This means changes in pressure, volume and temperature.
• What causes the PVT changes (diastrophism, the movements that bend and break the Earth's crust):
• Orogeny: mountain building, where rocks are folded and squeezed into long belts.
• Epeirogeny: large parts of the crust slowly rising or sinking.

• Earthquakes and plate tectonics (the movement of the large plates of the Earth's crust).

• Process:

• Rocks are buried, squeezed and heated.

• Their minerals change and grow again. This is called recrystallisation.

• Result:

• The grains lock tightly together, so the new rock is harder and more compact than its parent.

• Metamorphic rocks are very different from their earlier igneous, sedimentary or metamorphic form. Common examples are phyllite, schist, gneiss, quartzite and marble [2].

• Key condition: no melting.

• If the rock melts, it turns into magma.
• When that magma cools, it forms an igneous rock, not a metamorphic one.

Types of metamorphism

• By the main agent:

Type	Main agent	What happens	Chemical change
Dynamic (mechanical)	Pressure	Rock is crushed, broken and recrystallised	Little or none
Thermal	Heat	Heat makes the minerals recrystallise	New minerals may form

• Contact metamorphism: hot magma pushes into cooler rock and bakes the rock next to it. This happens over a small area.
• It takes place when hot igneous rock intrudes (forces its way) into an older rock [1].
• Contact aureole: the "baked zone" of changed rock around the intrusion. It can be a few centimetres thick around dykes (sheets of magma cutting across rock layers) and sills (sheets lying along the layers). It can be several kilometres thick around large batholiths (huge masses of magma that cooled deep underground) [3].
• Rocks in the aureole often show no foliation, because heat, not squeezing pressure, is the main agent [3].

• Hornfels is the typical rock. It is hard and fine-grained, and its crystals show little alignment. It can form at temperatures of up to 800 °C [4].

• Regional metamorphism: heat and pressure work together during mountain building and change large areas.

• Under high pressure and temperature, mica minerals grow in mudstones. Mudstones become slate, schist and gneiss; sandstone becomes quartzite; limestone becomes marble [1].

Feature	Contact	Regional
Cause	Heat from nearby magma	Heat + pressure during orogeny
Area	Small (the aureole)	Hundreds to thousands of km²
Foliation	Usually absent	Usually present
Typical rocks	Hornfels, marble near intrusions	Slate, schist, gneiss

Texture and grade

• Foliation: flat or long mineral grains line up side by side, so the rock looks striped. It forms when pressure squeezes the minerals into line [2].
• Lineation: minerals line up in one direction, like a line, not in flat layers.
• Banding: light and dark bands take turns. Light minerals (quartz, feldspar) separate from dark minerals (mica, hornblende). Banding is the mark of gneiss [1].
• Slaty cleavage: slate splits into thin, strong, long-lasting slabs. This is why it is used for roofs and writing slates [1].
• Schistosity: schist splits into thin, flaky layers [1].
• Metamorphic grade: how strongly a rock has been changed. Slate → phyllite → schist → gneiss goes from low grade to high grade.

Parent rock → metamorphic rock

Parent rock	Metamorphic product	Key feature
Shale / clay	Slate → phyllite → schist	Fine to medium grain; mica makes it shine
Limestone / dolomite	Marble	Recrystallised calcite or dolomite; takes a polish
Sandstone	Quartzite	Very hard; forms ridges
Granite	Gneiss	Coarse grain; light and dark bands
Coal	Graphite (at high grade)	Soft and black; used as a lubricant and in pencils

• Schist is medium-grained, with flaky mica that gives it a shiny surface. Gneiss is coarse-grained, with clear light and dark bands.

Where it occurs

• Peninsular shield (India): the old crystalline shield, made of Archaean and Dharwar rocks, is India's main area of regional metamorphism.
• Archaean granite-gneiss is the basement rock, the old rock that lies under all the younger layers.

• Dharwar rocks formed from sediment worn away from the Archaean rocks. Many of them have been changed into schists.

• Aravallis (Rajasthan): quartzite ridges stand up because the softer rocks around them have worn away faster.

• Marble in India (resources as on 1.4.2015) [5]:
• Total resources: 1,945 million tonnes. Only about 4.5 million tonnes (0.23%) are reserves (proved to be mineable).
• By state: Rajasthan ~63% > Jammu & Kashmir ~21% > Gujarat ~6% > Chhattisgarh ~4%.
• Rajasthan has marble in 20 of its 33 districts [5]. Its main belts are Udaipur–Rajsamand–Chittorgarh, Makrana–Kishangarh, Banswara–Dungarpur, Andhi (Jaipur)–Jhiri (Alwar) and Jaisalmer [5].
• Makrana (Nagaur district, Rajasthan): pure white crystalline marble, used for the Taj Mahal [5].
• Rikhabdeo–Kesariaji (about 60 km from Udaipur): green marble that is known around the world. Babarmal: "Indian Pink". Rajsamand: the most heavily mined area [5].

• Ambaji (Banaskantha, Gujarat): marble that is easy to carve. Mahendragarh (Haryana): "Patiala marble", with black and white bands [5].

• Bhedaghat, near Jabalpur (Madhya Pradesh): the Narmada cuts a gorge through shining marble rocks. The site is on UNESCO's World Heritage Tentative List as "Bhedaghat-Lametaghat in Narmada Valley" [6]. The marble belt continues between Jabalpur and Katni [5].

• World: regional metamorphic belts form along mountain-building zones. Contact aureoles form around granite batholiths, dykes and sills wherever magma has intruded [3].

Don't confuse with

• Igneous rock: forms when molten magma cools. Metamorphic rock forms without melting. Once a rock melts, the product is igneous.
• Sedimentary rock: forms when loose sediment is laid down and pressed into rock at surface conditions. Metamorphic rock forms when an existing rock is recrystallised by heat and pressure at depth.
• Contact vs regional metamorphism: contact is small-area, heat-driven and usually not foliated (hornfels). Regional is large-area, caused by heat plus pressure during orogeny, and usually foliated (slate, schist, gneiss).
• Geological vs commercial "marble": geologically, marble is only recrystallised limestone. In trade, any calcareous rock (rich in calcium carbonate) that takes a polish is called marble, including serpentine, onyx and travertine [5].

Prelims Hooks

• Matching pairs: shale → slate; limestone/dolomite → marble; sandstone → quartzite; granite → gneiss; coal → graphite.
• Order by grade (low → high): slate → phyllite → schist → gneiss.
• Texture clues: banding → gneiss; shiny flaky mica → schist; splits into thin sheets → slate; hard, fine-grained, unfoliated rock in a contact aureole → hornfels [4].
• Place–state pairs: Makrana, Nagaur district, Rajasthan (Taj Mahal marble); Bhedaghat marble rocks, on the Narmada near Jabalpur, Madhya Pradesh.
• Legal trap: marble is a "minor mineral" under Section 3(e) of the MMDR Act, 1957, not a major mineral [5].
• Rock-system trap: schist and gneiss are most common in Archaean rocks, not in Gondwana or Deccan Trap rocks.

Mains Points

• Metamorphism shapes landforms (GS-I):
• Mountain building raises pressure and temperature, and the rocks recrystallise.
• The new rocks are hard and wear away slowly. They stand as ridges and gorges, such as the Aravalli quartzite ridges and the Narmada's marble gorge at Bhedaghat.

• This explains much of the relief (the highs and lows of the land) of the old Peninsular shield.

• Resource base and its cost (GS-III):

• India's Archaean–Dharwar belts hold schist, gneiss and marble. Rajasthan's ~63% share of marble resources supports a large small-scale stone industry and the export of dimension stone (stone cut into blocks and slabs) [5].
• But more than 2,000 processing units in Rajasthan produce about 5–6 million tonnes of marble slurry (fine, wet cutting waste) a year. It is often dumped on open land, where it harms soil, groundwater and farmland [5].

• Possible responses: reuse the slurry in cement, bricks and road building, and regulate quarrying in the Aravallis.

• Geoheritage and tourism (GS-I): Bhedaghat–Lametaghat combines a metamorphic gorge with the Lameta beds, which hold dinosaur fossils. Its place on UNESCO's Tentative List links geology, conservation and tourism [6].

Related concepts

• Metamorphism
• Contact and regional metamorphism
• Schist and gneiss
• Marble

Read more

• Metamorphic rocks: change of form under heat and pressure