Marble

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

Meaning

Marble is a metamorphic rock that forms when limestone or dolomite is heated and squeezed until its grains of calcite or dolomite grow again into new crystals (recrystallise), while the rock stays solid. It is India's main metamorphic building stone. It is also a common exam topic because of famous places like Makrana (white marble of the Taj Mahal) and the marble gorge of the Narmada at Bhedaghat.

Explanation

How marble forms (cause → process → result)

• Cause: PVT changes. PVT means changes in pressure, volume and temperature. They come from:
• Orogeny: mountain building, where rocks are folded and squeezed.
• Epeirogeny: the slow rise or sinking of large parts of the Earth's crust.

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

• Process: recrystallisation without melting.

• Limestone or dolomite gets buried, heated and squeezed.
• Its calcite or dolomite grains recrystallise and lock tightly together.

• The rock does not melt. If it melted, the magma would cool into an igneous rock, not marble.

• Result: a harder, more compact rock.

• Marble is limestone that has been recrystallised by thermal and regional metamorphism [4].
• It takes a polish. That is why it is valued as a building and carving stone.
• Its name comes from the Latin and Greek words for "shining stone" [4].

Two routes: contact and regional metamorphism

• Contact metamorphism: hot magma pushes into cooler rock and bakes the rock next to it.
• This happens over a small area called the contact aureole (the "baked zone" around the intrusion).
• Marble can form where limestone lies next to such an intrusion.

• Rocks here are usually not foliated (their grains are not lined up in stripes), because heat, not directed pressure, does the main work [2].

• Regional metamorphism: large areas are changed by heat and pressure together during mountain building.

• In this setting, limestone becomes marble, sandstone becomes quartzite, and mudstones become slate, schist and gneiss [1].
• In India, the old crystalline shield of the Peninsula (Archaean and Dharwar rocks) is the main area of regional metamorphism.

Geological meaning vs trade meaning vs legal status

• Geological meaning: only recrystallised limestone (or dolomite) counts as marble [4].
• Commercial meaning: traders call any calcareous rock (a rock rich in calcium carbonate) that takes a polish "marble". This includes serpentine, onyx and travertine [4].
• Legal status: marble is a "minor mineral" under Section 3(e) of the MMDR Act, 1957 (Mines and Minerals (Development and Regulation) Act) [4].

How much India has (as on 1.4.2015) [4]

• Total resources: 1,945 million tonnes of all grades.
• Only about 4.5 million tonnes (0.23%) are "reserves", which means proved to be mineable.

• The remaining 99.77% are "resources".

• By grade: about 17% white, 55% off-colour, 27% unclassified.

• By state: Rajasthan ~63% > Jammu & Kashmir ~21% > Gujarat ~6% > Chhattisgarh ~4%.

Where it occurs

• Rajasthan: marble is found in 20 of its 33 districts (IMYB 2016) [4]. The main belts are:
• Udaipur–Rajsamand–Chittorgarh
• Makrana–Kishangarh
• Banswara–Dungarpur
• Andhi (Jaipur)–Jhiri (Alwar)

• Jaisalmer

• Makrana (Nagaur district, Rajasthan): famous for pure white crystalline marble, which was used for the Taj Mahal [4].

• Rajsamand (Rajasthan): off-white to greyish-white marble. It is the most heavily mined area [4].
• Rikhabdeo–Kesariaji (about 60 km from Udaipur): green marble, known around the world [4].
• Babarmal (Rajasthan): "Indian Pink" marble [4].
• Ambaji (Banaskantha, Gujarat): marble known for being easy to carve [4].
• Mahendragarh (Haryana): "Patiala marble", with black and white bands [4].
• Bhedaghat, near Jabalpur (Madhya Pradesh): the Narmada cuts a gorge through shining marble rocks.
• It is on UNESCO's World Heritage Tentative List as "Bhedaghat-Lametaghat in Narmada Valley" [5].
• The marble belt continues between Jabalpur and Katni, where off-white, fine-grained banded marble is quarried [4].

Don't confuse with

• Limestone: this is the sedimentary parent rock. Marble is the metamorphic product, recrystallised and harder, and it takes a polish.
• Quartzite: this is metamorphosed sandstone, not limestone. It is very hard and forms ridges, such as those in the Aravallis.
• Hornfels: this is the typical rock of contact metamorphism. It is hard, fine-grained and has little crystal alignment [3]. Marble can also form near intrusions, but its parent is always limestone or dolomite.
• Trade "marble" (serpentine, onyx, travertine): these are sold as marble because they take a polish, but geologically they are not true marble [4].

Prelims Hooks

• Matching pairs: limestone/dolomite → marble; sandstone → quartzite; shale → slate; granite → gneiss; coal → graphite.
• Makrana → Nagaur district, Rajasthan, which supplied the Taj Mahal's white marble. Bhedaghat → Narmada, near Jabalpur, Madhya Pradesh.
• Marble resources by state (1.4.2015): Rajasthan (~63%) > J&K (~21%) > Gujarat (~6%) > Chhattisgarh (~4%) [4].
• Trap: marble is a minor mineral under Section 3(e) of the MMDR Act, 1957, not a "major mineral" [4].
• Trap: marble forms without melting. Melting followed by cooling gives an igneous rock.
• Variety–place pairs: green marble → Rikhabdeo–Kesariaji (near Udaipur); "Indian Pink" → Babarmal; "Patiala marble" → Mahendragarh (Haryana); easy-to-carve marble → Ambaji (Gujarat) [4].

Mains Points

• Metamorphism shapes landforms (GS-I):
• Mountain building raises pressure and temperature, so limestone recrystallises into hard marble.
• Hard rock resists erosion, so rivers cut steep gorges through it. The Narmada's marble gorge at Bhedaghat is an example.

• Bhedaghat–Lametaghat also holds the Lameta beds with dinosaur fossils. Its place on UNESCO's Tentative List [5] links geology, conservation and geotourism.

• Resource base and industry (GS-III):

• Marble is concentrated in Rajasthan (~63% of resources, 2015) [4].
• It supports a large small-scale stone industry and the export of dimension stone (stone cut into blocks and slabs).

• But only 0.23% of resources are proved reserves [4], so more exploration is needed.

• Environmental cost (GS-III):

• More than 2,000 processing units in Rajasthan produce about 5–6 million tonnes of marble slurry a year. Slurry is the fine, wet waste from cutting marble. It is often dumped on open land [4].
• The slurry harms soil, groundwater and farm land.
• Possible responses: reuse slurry in cement, bricks and road building, and regulate quarrying in the Aravalli belt.

Related concepts

• Metamorphic rock
• Metamorphism
• Contact and regional metamorphism
• Schist and gneiss

Read more

• Metamorphic rocks: change of form under heat and pressure