Sedimentary rock

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

Meaning

A sedimentary rock forms when loose material settles in layers, mostly in water, and is then pressed and glued into solid rock. That material can be bits of older rock, shells and the remains of plants and animals, or minerals that come out of water as solids. Sedimentary rocks cover about three-quarters of the Earth's land surface. They hold most fossils, and they also hold coal, oil, natural gas, cement limestone and most of the world's iron ore.

Explanation

How it forms: from loose sediment to rock

• The name comes from the Latin sedere, "to settle".
• Sediment is loose material such as gravel, sand, silt, clay or shell bits that wind, water or ice has carried and dropped.
• The main processes are weathering, erosion, dissolution, precipitation and lithification [3].
• Lithification (turning loose sediment into solid rock) happens on the floor of a sea or lake. The weight of the sediment above slowly presses it into rock [3]. It has two steps:
• Step 1, compaction: new layers pile up on older ones. Their weight squeezes out water and pushes the grains closer together.

• Step 2, cementation: groundwater flows through the gaps between grains. Minerals dissolved in it, mainly silica (SiO₂) or calcite (CaCO₃), come out as solids. They act like glue and bind the grains together.

• Result: a layered (stratified) rock that keeps a record of where and how it formed.

Key features: layers, fossils and fuels

• Area against volume:
• These rocks cover about 75% of the land surface.

• By volume, though, they are only a thin skin on a crust that is mostly igneous and metamorphic rock.

• Stratification (layering):

• Each layer is a stratum (plural strata). Each one stands for one period of deposition.

• Older layers usually lie at the bottom, so geologists can read geological time from the rocks.

• Fossils:

• Plants and animals buried in soft sediment are kept as fossils.

• Fossils survive mostly in sedimentary rock. In igneous rock, the heat of magma destroys them. In metamorphic rock, heat and pressure destroy them.

• Minerals in beds or layers:

• Coal forms when buried plant matter is pressed and heated over a long time.

• Evaporites (salts left behind when water evaporates), such as gypsum, potash salt and sodium salt, form mostly in dry regions.

• Petroleum traps:

• Oil and gas form from buried marine organic matter.
• They move up through porous rock (rock with open spaces), such as limestone or sandstone.
• They collect under a layer that fluids cannot pass through, such as shale. The trap can be an anticline (an upward fold) or a fault trap.

Three types by origin

Type	How it forms	Examples
Clastic (mechanically formed)	Pieces of older rock are carried, dropped, compacted and cemented	Sandstone, conglomerate, shale, loess (wind-blown silt), tillite (glacial)
Chemically formed	Dissolved minerals come out of water as solids, often when the water evaporates	Rock salt, gypsum, potash, some limestone, chert
Organically formed	Remains of living things pile up	Coal (plants), chalk and much limestone (shells), geyserite

• Clastic rocks are graded by grain size. From coarsest to finest: conglomerate and breccia → sandstone → siltstone → mudrock (shale, claystone, mudstone) [3].
• Conglomerate has rounded pebbles. Breccia has sharp, angular pieces.

• Tillite is glacial till (the mixed rubble a glacier drops) that has turned into rock.

• Chemical rocks other than carbonates are rare in the rock record but important for the economy. They include cherts (silica rocks), phosphorites (phosphate rocks), evaporites, iron formations and ironstones, and carbon-rich deposits [3].

• Coal stages: peat → lignite → bituminous → anthracite. Carbon content and heating value rise at each stage. Coal forms from plants pressed together over millions of years [4].

Important sedimentary rocks

• Sandstone:
• It is made of sand-sized grains (about 0.06–2 mm), mostly quartz. The grains are cemented by silica, calcite or iron oxide. Iron oxide makes it red.

• It is porous, so it can be an aquifer (rock that stores and passes groundwater) and can also hold oil.

• Shale:

• It is clay and mud pressed into very thin layers. It is the most common sedimentary rock.

• Almost no fluid passes through it, so it often seals oil traps.

• Limestone:

• It is mostly calcium carbonate (CaCO₃) and can be chemical or organic.
• It is the raw material for cement.
• It is also a flux in blast furnaces (a substance that removes impurities from molten iron as slag).

• Rainwater is a weak carbonic acid. When it dissolves limestone or dolomite, it creates karst topography: sinkholes, caves, stalactites, stalagmites and underground streams.

• Banded iron formation (BIF):

• It is a very old chemical rock. Thin bands of iron oxide (haematite, magnetite) take turns with bands of silica (chert or jasper).
• How it formed: early oceans had no oxygen, so iron stayed dissolved in seawater. Photosynthesis by early microbes turned this iron into iron oxide that does not dissolve. The iron oxide sank to the sea floor in layers.
• Isotope evidence suggests that some early BIF iron was oxidised by anoxygenic photosynthesis (an older kind of photosynthesis that releases no oxygen) [4].
• BIFs formed between 3.8 and 1.8 billion years ago and are no longer forming today [4]. NCERT says they formed mostly 3.5–1.8 billion years ago.

Where it occurs

India: rock systems

Rock system	Sedimentary link
Dharwar (Archaean–Palaeoproterozoic)	First metamorphosed sedimentary rocks; BIF iron ore, manganese, gold
Cuddapah (Proterozoic)	Unmetamorphosed sandstone, shale, limestone, quartzite
Vindhyan (Proterozoic)	Red sandstone, limestone used for cement, Panna diamond-bearing conglomerate
Gondwana (Late Carboniferous–Jurassic)	Most of India's coal, in the Damodar, Mahanadi and Godavari valleys; tillite
Tertiary (Cenozoic)	Petroleum in Assam, Gujarat and Mumbai High; lignite at Neyveli (Tamil Nadu) and in Rajasthan and Gujarat

India: rocks, landforms and ore

• Red Vindhyan sandstone:
• It comes from rocks that stretch across Madhya Pradesh, Rajasthan and Uttar Pradesh.

• It was used to build the Red Fort (Delhi), Fatehpur Sikri (Uttar Pradesh) and the Sanchi stupas (Madhya Pradesh).

• Gondwana tillites are evidence that ancient glaciers once covered India.

• Karst caves: Guptadham (Bihar), Borra Caves (Andhra Pradesh) and the caves of the Meghalaya plateau.
• Banded iron formations:
• Precambrian BIF lies in the Archaean schist belts (old belts of metamorphosed rock) of Jharkhand, Odisha, Karnataka, Chhattisgarh and Goa. It also occurs in the granulite terrain of Tamil Nadu and Kerala [2].
• India's haematite belongs to the Precambrian Iron Ore Series and lies inside banded iron formations [1].
• In the Dharwar Craton (Karnataka), the lower Bababudan Group has shelf sediments rich in iron and manganese [2].
• Main ore belts: Bailadila (Dantewada, Chhattisgarh), Kudremukh and Bababudan (Karnataka), Keonjhar–Sundargarh–Mayurbhanj (Odisha), Noamundi–Gua (Singhbhum, Jharkhand) and Goa.

World

• Loess: Loess Plateau, China.
• Karst: the Karst region (Slovenia) and Guilin (China).
• BIF iron ore: Hamersley Range (Australia), Carajás (Brazil) and Lake Superior (USA and Canada).

Don't confuse with

• Igneous rock: it forms when magma or lava cools. It is not laid down in layers, and it has almost no fossils because magma heat destroys them.
• Metamorphic rock: it is an existing rock changed by heat and pressure. Limestone → marble, shale → slate, sandstone → quartzite. Marble, slate and quartzite are metamorphic, not sedimentary.
• Area against volume of the crust: sedimentary rocks cover about 75% of the land area but are only a thin skin by volume. A statement that they make up most of the crust is false.
• Clastic against chemical types: loess and tillite are clastic, even though wind and ice carry them. Chert and gypsum are chemical. Coal and chalk are organic.

Prelims Hooks

• Lithification order: compaction comes first, then cementation. The cement is mainly silica or calcite carried by groundwater [3].
• Grain size, coarsest to finest: conglomerate/breccia → sandstone → siltstone → shale [3].
• Red Vindhyan sandstone: Red Fort (Delhi), Fatehpur Sikri (Uttar Pradesh), Sanchi (Madhya Pradesh).
• Fuel and ore by rock system: Gondwana → coal; Tertiary → petroleum and lignite; Dharwar → BIF iron ore.
• BIFs: formed about 3.8–1.8 billion years ago, no longer forming today [4], and hold most of the world's iron ore.
• Iron ore figures:
• Haematite resources (1.4.2015): Odisha 34% > Jharkhand 23% > Chhattisgarh 22% > Karnataka 11% > Goa 5%.
• Magnetite: Karnataka alone has 72%.
• Production 2017-18: Odisha > Chhattisgarh > Karnataka > Jharkhand [1].

Mains Points

• Basis of energy and steel security (GS-III):
• Sedimentary rocks hold almost all fossil fuels (Gondwana coal, Tertiary oil and gas). They also hold evaporites, cement limestone and BIF iron ore.

• So India's energy and steel security depends on the geology of its sedimentary basins.

• Uneven iron ore and weak reserves (GS-III):

• Haematite is in the east and magnetite is in the south (Karnataka). This shapes where steel plants are built (Jamshedpur, Rourkela, Bhilai) and the export ports (Paradip, Visakhapatnam, Mormugao) [1].
• Mining in Adivasi forest areas (Bailadila, Keonjhar) causes displacement and conflicts under the Forest Rights Act and PESA.
• Only 24% of haematite resources are proven "Reserves", and fines were about 67.5% of output in 2017-18 [1].

• The answer is more exploration, plus beneficiation (upgrading low-grade ore) and pelletisation (rolling fines into small balls).

• Record of Earth's history and heritage (GS-I):

• Strata and fossils record geological time. BIFs record how Earth's oceans and air gained oxygen.
• Fossil sites and Vindhyan sandstone monuments need geoheritage protection.

Related concepts

• Lithification
• Clastic, chemical and organic sedimentary rocks
• Sandstone
• Banded iron formation

Read more

• Sedimentary rocks: layers, fossils and fuels