Metamorphic rocks: change of form under heat and pressure

Detail

1. What metamorphism means

• Metamorphism (meta = change, morph = form) is a change in a rock's minerals and texture (the size, shape and arrangement of its grains). The rock does not melt. It changes while it is still solid.

• If the rock melts, it becomes magma. When that magma cools, it forms an igneous rock, not a metamorphic one. This is a common exam trap.

• PVT means the three things that drive metamorphism: changes in pressure, volume and temperature.

• What causes the PVT changes (Class 11, Geomorphic Processes, "Diastrophism"):
• Orogeny: mountain building, where rocks are folded and squeezed into long belts.
• 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).

• Result: rocks are buried, squeezed and heated, so their minerals change and grow again (recrystallise).

• Metamorphic rock is any rock that has been changed in this way. Its parent rock can be:

• igneous, for example granite → gneiss
• sedimentary, for example limestone → marble

• an older metamorphic rock, for example slate → phyllite → schist

• Metamorphic rocks are substantially changed from their earlier igneous, sedimentary or metamorphic form. Common examples are phyllite, schist, gneiss, quartzite and marble [3].

• Why they form ridges:
• Recrystallisation locks the grains tightly together, so the rock becomes harder and more compact than its parent.
• Weathering and erosion wear away the softer rocks around it more quickly.
• Result: metamorphic rock is left standing as resistant ridges. Quartzite ridges in old mountain belts such as the Aravallis are an example.

2. Types of metamorphism by the main agent

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

3. 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.
• Contact metamorphism takes place when hot igneous rock intrudes (forces its way) into an older rock [2].
• Contact aureole: the "baked zone" of changed rock around the intrusion. It can be a few centimetres thick around thin bodies such as dykes (sheets of magma cutting across rock layers) and sills (sheets lying along the layers). It can be several kilometres thick around large granite intrusions such as batholiths (huge masses of magma that cooled deep underground) [4].
• Rocks in the aureole often show no foliation, because heat, not directed pressure, is the main agent [4].

• Hornfels is the typical rock formed this way. It is hard and fine-grained, with crystals that show little alignment. It can form at temperatures of up to 800 °C [5].

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

• Under high pressure and temperature, mica minerals grow in mudstones. Mudstones become slate, schist and gneiss; sandstone becomes quartzite; limestone becomes marble [2].
• In India, the old crystalline shield of the Peninsula (Archaean and Dharwar rocks) is the main area of regional metamorphism.

Feature	Contact metamorphism	Regional metamorphism
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

4. Texture: foliation, lineation and banding

• Foliation: flat or long mineral grains line up parallel to each other, so the rock looks striped. It forms when pressure squeezes these minerals into line [3].
• Lineation: minerals line up along a line (in one direction) instead of in flat layers.
• Banding: alternating light and dark bands. Light minerals (quartz, feldspar) separate from dark minerals (mica, hornblende). It is the mark of gneiss [2].
• Slaty cleavage: slate splits easily into thin, strong and lasting slabs. This is why it is used for roofing and writing slates [2].
• Schistosity: schist tends to split into thin, flaky layers [2].

5. Parent rock → metamorphic rock

Parent rock	Metamorphic product	Key feature
Shale / clay	Slate → phyllite → schist (as heat and pressure rise)	Fine → 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, black; used as a lubricant and in pencils

• Metamorphic grade: how strongly a rock has been changed. The series slate → phyllite → schist → gneiss goes from low grade to high grade.

6. Schist and gneiss

• Schist: medium-grained. It contains flaky mica, which gives it a shiny surface. It splits into thin layers.
• Gneiss: coarse-grained, with clear light and dark bands.
• In India: both are common in the Archaean rocks, the oldest rocks of the Peninsular shield.
• Archaean granite-gneiss forms the basement rock: the old rock that lies under all the younger layers.
• The Dharwar rocks were formed from sediments eroded from these Archaean rocks. Many have been metamorphosed into schists.

7. Marble: India's main metamorphic building stone

• Geological meaning: marble is limestone that has been recrystallised by thermal and regional metamorphism [6].
• Commercial meaning: traders call any calcareous rock (a rock rich in calcium carbonate) that takes a polish "marble". This includes serpentine, onyx and travertine [6].
• Legal status: marble is a "minor mineral" under Section 3(e) of the MMDR Act, 1957 (Mines and Minerals (Development and Regulation) Act). The name comes from the Latin and Greek words for "shining stone" [6].
• Resources (as on 1.4.2015) [6]:
• Total resources of all grades: 1,945 million tonnes. Only about 4.5 million tonnes (0.23%) are "reserves" (proved to be mineable); the remaining 99.77% are "resources".
• By grade: about 17% white, 55% off-colour and 27% unclassified.

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

• Rajasthan: marble is found in 20 of its 33 districts (IMYB 2016) [6]. The main belts are:

• Udaipur–Rajsamand–Chittorgarh
• Makrana–Kishangarh
• Banswara–Dungarpur
• Andhi (Jaipur)–Jhiri (Alwar)

• Jaisalmer

• Makrana (Nagaur district, Rajasthan) is famous for pure white crystalline marble, which was used for the Taj Mahal [6].

• Other notable varieties [6]:
• Green marble from Rikhabdeo–Kesariaji, about 60 km from Udaipur. It is known around the world.
• "Indian Pink" from Babarmal.
• Off-white to greyish-white marble from Rajsamand, the most heavily mined area.
• Marble from Ambaji (Banaskantha, Gujarat), known for being easy to carve.

• "Patiala marble", with black and white bands, from Mahendragarh (Haryana).

• 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" [7].

• The marble belt continues between Jabalpur and Katni, where off-white, fine-grained banded marble is quarried [6].

• Waste problem: 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 [6].

Prelims Hooks

• Metamorphism happens without melting. If the rock melts and the magma then cools, the result is an igneous rock.
• Matching pairs: shale → slate; limestone/dolomite → marble; sandstone → quartzite; granite → gneiss; coal → graphite.
• Order by grade (low → high): slate → phyllite → schist → gneiss.
• Contact metamorphism: small area around an intrusion (the aureole); typical rock hornfels; usually not foliated. Regional metamorphism: large area, mountain building, foliated.
• Banding (alternating light and dark bands) → gneiss. Shiny flaky mica → schist. Splits into thin sheets → slate.
• Makrana is in Nagaur district, Rajasthan, and supplied the white marble of the Taj Mahal. Bhedaghat marble rocks are on the Narmada, near Jabalpur, Madhya Pradesh.
• Marble resources by state (1.4.2015): Rajasthan (~63%) > J&K (~21%) > Gujarat (~6%) > Chhattisgarh (~4%).
• Marble is a minor mineral under the MMDR Act, 1957. Trap: it is not a "major mineral".
• Schist and gneiss are most common in Archaean rocks, not in Gondwana or Deccan Trap rocks.

Mains Points

• Metamorphism and landforms (GS-I):
• Mountain building raises pressure and temperature, and rocks recrystallise.
• The new rocks are hard and resist erosion. They stand as ridges and gorges, such as the quartzite ridges of the Aravallis and the marble gorge of the Narmada at Bhedaghat.

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

• Where metamorphic resources are found (GS-I/III):

• India's Archaean–Dharwar metamorphic belts hold schists, gneisses and marble.

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

• Environmental cost (GS-III):

• Marble quarrying in the Aravalli belt produces 5–6 million tonnes of slurry a year in Rajasthan. The slurry harms soil, groundwater and farm land.

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

• Heritage and geotourism (GS-I):

• Bhedaghat–Lametaghat combines a metamorphic gorge with the Lameta beds, which hold dinosaur fossils.
• Its place on UNESCO's Tentative List shows how geology, conservation and tourism are linked.