These interior of the Earth notes for OPSC OAS Part 3 cover the evidence used to study the Earth's interior, the seismic discontinuities that separate its layers, the geological time scale, the rock cycle and mineral classification, and the geology and mineral belts of Odisha. Every section is built around the exact facts OPSC and UPSC ask, with previous year practice questions at the end.
Sources of Information about the Earth's Interior
No one can reach the Earth's centre, which lies about 6,371 km below the surface. Knowledge of the interior therefore comes from a mix of direct sampling and indirect inference.
| Direct sources | Indirect sources |
|---|---|
| Surface rocks and deep mines (South African gold mines reach 3–4 km) | Seismic waves, especially P and S wave shadow zones |
| Deep drilling, including the Kola Superdeep Borehole (about 12 km) and the Deep Ocean Drilling and Integrated Ocean Drilling Projects | Gravity anomalies and magnetic surveys |
| Magma thrown out during volcanic eruptions | Temperature, pressure and density estimates at depth |
| Meteorites, whose composition resembles the Earth's interior |
The evidence for a liquid outer core came from the S-wave shadow zone, not from direct drilling. The deepest borehole, Kola, reached only about 12 km, a tiny fraction of the 6,371 km radius.
- Deep drilling has never reached the mantle. The Moho lies 5–70 km down, far below any borehole.
- Meteorites are an indirect source, not a direct sample of the Earth's core.
Seismic Waves and the Shadow Zones
Earthquakes generate body waves that travel through the interior and surface waves that travel along the surface. Their behaviour at discontinuities is the key to mapping the Earth's layers.
| Wave | Motion | Media | Diagnostic value |
|---|---|---|---|
| P-waves (primary) | Longitudinal, push–pull | Solids, liquids, gases | Fastest; refracted at the core-mantle boundary, creating a shadow zone between about 103° and 143° |
| S-waves (secondary) | Transverse, side-to-side | Solids only | Cannot pass the liquid outer core; absent beyond about 103° from the epicentre |
The absence of S-waves beyond 103° and the P-wave shadow zone between 103° and 143° together proved that the outer core is liquid. Inge Lehmann in 1936 detected weak P-wave arrivals inside the P-wave shadow zone, which led to the discovery of the solid inner core.
P-wave shadow zone: 103°–143°. S-wave shadow zone: beyond 103°. The S-wave shadow is the proof of a liquid outer core; the inner-core reflection inside the P-wave shadow proves a solid inner core.
Layers and Seismic Discontinuities
The Earth is divided chemically into crust, mantle and core, and mechanically into lithosphere, asthenosphere, mesospheric mantle, outer core and inner core.
| Discontinuity | Boundary | Approximate depth |
|---|---|---|
| Conrad | Upper and lower crust (granitic and basaltic) | Variable; absent under oceans |
| Mohorovicic (Moho) | Crust and mantle | 5–10 km under oceans; 30–70 km under continents |
| Repetti | Upper and lower mantle | About 660–670 km |
| Gutenberg | Mantle and outer core (core-mantle boundary) | About 2,900 km |
| Lehmann (Bullen) | Outer and inner core | About 5,150 km |
OPSC and UPSC repeatedly test the pairing of discontinuity with boundary. The Gutenberg discontinuity separates mantle and outer core; the Lehmann discontinuity separates outer and inner core.
Crust, mantle and core in numbers
| Layer | Thickness / depth | State | Composition |
|---|---|---|---|
| Continental crust | About 30–40 km, up to 70 km under mountains | Solid | Granitic (Sial); density about 2.7 g/cm³ |
| Oceanic crust | About 5–10 km | Solid | Basaltic (Sima); density about 2.9–3.0 g/cm³ |
| Mantle | Up to about 2,900 km | Solid, but ductile in the asthenosphere | Silicate minerals rich in olivine, pyroxene and garnet; density about 3.3–5.7 g/cm³ |
| Outer core | 2,900–5,150 km | Liquid | Iron and nickel with lighter elements; density about 9.9–12.2 g/cm³ |
| Inner core | 5,150–6,371 km | Solid | Iron-nickel alloy (Nife); density about 12.8–13.1 g/cm³ |
The mantle makes up about 84% of the Earth's volume and 67% of its mass. The core accounts for about 16% of the volume and 33% of the mass. The lithosphere is the crust plus the rigid uppermost mantle, about 100 km thick under continents and thinner under oceans; the asthenosphere below it is a weak, low-velocity zone over which plates move.
Isostasy is the state of gravitational equilibrium in which the lithosphere floats on the denser asthenosphere. Higher topography has a deeper, low-density root, which is why mountain belts have thick crustal roots.
Geological Time Scale
The geological time scale divides Earth's 4.6-billion-year history into eons, eras, periods and epochs.
| Eon | Era | Key events in India |
|---|---|---|
| Hadean and Archaean (4.6–2.5 Ga) | Archaean | Formation of cratons: Aravalli, Singhbhum, Dharwar, Bastar; oldest rocks of the Indian shield; Iron Ore Group of Singhbhum-Odisha |
| Proterozoic (2.5–0.541 Ga) | Palaeoproterozoic to Neoproterozoic | Cuddapah and Vindhyan basins; Eastern Ghats granulite belt; Delhi orogeny; Purana rock systems |
| Phanerozoic (541 Ma to present) | Palaeozoic | Cambrian to Permian sequences in the Himalaya; Gondwana coal basins from the Carboniferous-Permian |
| Mesozoic | Break-up of Gondwana; Deccan Trap volcanism at the Cretaceous-Palaeogene boundary (about 66 Ma) | |
| Cenozoic | Himalayan orogeny from the Eocene; Indo-Gangetic plain; Quaternary alluvium and coastal sediments |
India's geological record is divided into four rock systems in standard textbooks: the Archaean (oldest, crystalline basement), Purana (Proterozoic, including Cuddapah and Vindhyan), Dravidian (Palaeozoic, including Himalayan fossil-bearing sequences) and Aryan (Mesozoic-Cenozoic, including Gondwana and the Deccan Traps).
The Deccan Traps erupted around 66 million years ago at the Cretaceous-Palaeogene boundary. Gondwana coal belongs to the Carboniferous-Permian, not the Cenozoic.
Rocks: Igneous, Sedimentary and Metamorphic
A rock is an aggregate of one or more minerals. Rocks are grouped into three families by mode of formation.
| Type | Formation | Sub-types | Examples |
|---|---|---|---|
| Igneous | Solidification of magma or lava | Intrusive (plutonic): slow cooling, coarse grains. Extrusive (volcanic): rapid cooling, fine grains | Granite, gabbro, pegmatite (intrusive); basalt, obsidian, pumice (extrusive) |
| Sedimentary | Deposition and lithification of fragments or chemical/organic precipitates | Mechanically formed, organically formed, chemically formed | Sandstone, shale, conglomerate (mechanical); limestone, coal (organic); chert, halite (chemical) |
| Metamorphic | Recrystallisation under heat, pressure and chemically active fluids | Foliated and non-foliated | Slate, schist, gneiss (foliated); marble, quartzite (non-foliated) |
Igneous rocks are called primary rocks because all other rocks ultimately derive from them. Sedimentary rocks are the only rocks that commonly preserve fossils. Metamorphic rocks recrystallise without melting; if melting occurs, the result is magma, not a metamorphic rock. Foliation is the layered arrangement of minerals; banding is alternating light and dark mineral layers.
Quartzite is metamorphosed sandstone; marble is metamorphosed limestone; slate is metamorphosed shale; gneiss is metamorphosed granite. UPSC 2001 asked precisely this conversion.
- Metamorphism does not involve melting. Melting produces magma and then igneous rock.
- Fossils survive best in sedimentary rocks; metamorphism usually destroys them.
- Dolomite is sedimentary, not igneous; gneiss is metamorphic, not igneous.
The rock cycle
Igneous rocks weather and erode into sediments, which lithify into sedimentary rocks. Both can be buried and metamorphosed. Metamorphic rocks can melt to form new magma. This continuous transformation is the rock cycle, driven by plate tectonics and the hydrological cycle.
IMAGE (Diagram): The rock cycle as a closed loop.
Labels: magma → igneous rock; weathering and erosion → sediments; lithification → sedimentary rock; heat and pressure → metamorphic rock; melting → magma.
Caption: The rock cycle links the three rock families through melting, weathering, deposition and metamorphism.
Find it: NCERT Class 7 Our Environment chapter on Inside Our Earth, or USGS 'Rock cycle' diagram.
Minerals: Classification and Rock-Forming Minerals
A mineral is a naturally occurring, inorganic solid with a definite chemical composition and a crystalline structure. The crust is dominated by silicate minerals, which contain silicon and oxygen tetrahedra.
| Mineral group | Examples | Where they dominate |
|---|---|---|
| Ferromagnesian silicates (mafic) | Olivine, pyroxene, amphibole, biotite | Mantle and oceanic crust; dark, dense |
| Non-ferromagnesian silicates (felsic) | Quartz, feldspar, muscovite mica | Continental crust; light coloured, less dense |
| Oxides | Haematite, magnetite | Iron ores |
| Carbonates | Calcite, dolomite | Limestone and marble |
| Sulphides | Galena, sphalerite, pyrite | Ore deposits |
Feldspar and quartz are the two most common minerals in the crust. Silicates make up the overwhelming bulk of crustal and mantle minerals; the mantle is dominated by olivine and pyroxene, which is why ultramafic rocks such as peridotite are greenish and dense.
Oxygen is the most abundant element in the crust (about 46.6%), followed by silicon (27.7%). Aluminium is the most abundant metal (about 8.1%), followed by iron (about 5%).
Odisha Angle: Cratons, Granulites and Mineral Belts
Odisha lies on the eastern fringe of the Indian Peninsular shield and has one of the richest geological and mineral endowments in the country. About 72.5% of the state's area is occupied by Precambrian metamorphic rocks of Archaean and Proterozoic age, which host most of its minerals.
Major geological units
| Unit | Location in Odisha | Rocks and minerals |
|---|---|---|
| Singhbhum-Odisha Craton | Northern Odisha, Mayurbhanj, Keonjhar, Sundargarh | Archaean gneisses, granites and migmatites; Iron Ore Group (BIF) of the Badampahar-Gorumahisani and Bonal-Keonjhar belts; chromite and PGM in ultramafic intrusives |
| Bastar Craton | Western Odisha | Archaean gneisses, granites and migmatites; pegmatites with strontium, tantalum and niobium |
| Eastern Ghats granulite belt | Central and southern Odisha | Khondalite, charnockite, migmatite, anorthosite and alkaline rocks; bauxite, manganese, graphite and gemstones |
| Gondwana basins | About 8% of the state, chiefly in the west and north-west | Coal-bearing Gondwana sediments of the Mesozoic |
| Cenozoic coastal plains | Eastern coastal belt | Alluvium, laterite, beach sand minerals and deltaic fans offshore |
Odisha's share of India's reserves is striking: about 83% of chromite, 92% of nickel, 55% of bauxite, 38% of iron ore and 26% of coal. The Sukinda valley in Jajpur district is one of Asia's largest chromite deposits, hosted in the ultramafic rocks of the Singhbhum-Odisha Craton.
The Eastern Ghats granulite belt is Proterozoic in age and is distinct from the Archaean Singhbhum craton. Bauxite in Odisha is associated with lateritic weathering of khondalite and charnockite, not with the Gondwana coal measures.
Singhbhum-Odisha Craton (Archaean) → iron ore, chromite, nickel. Eastern Ghats granulite belt (Proterozoic) → bauxite, graphite, gemstones. Gondwana basins (Mesozoic) → coal. Coastal Cenozoic → beach sand minerals.
IMAGE (Map): Geological and mineral map of Odisha.
Labels: Singhbhum-Odisha Craton (north), Bastar Craton (west), Eastern Ghats granulite belt (centre and south), Gondwana basins, coastal alluvium; mark Sukinda (chromite), Keonjhar-Barbil (iron ore), Panchpatmali (bauxite), Talcher and Ib valley (coal).
Caption: Major geological units and mineral belts of Odisha.
Find it: Odisha Directorate of Geology and Mining mineral map, or the Geology and Mineral Resources of Orissa report.
Practice Questions
Which one of the following is the correct pairing of a seismic discontinuity with the layers it separates?
- Mohorovicic – mantle and outer core
- Gutenberg – crust and mantle
- Lehmann – outer core and inner core
- Repetti – upper crust and lower crust
Show answer
(c) – Lehmann separates outer and inner core; Moho separates crust and mantle; Gutenberg separates mantle and outer core; Conrad separates upper and lower crust.
Consider the following statements:
- The Mohorovicic discontinuity lies at a greater depth under continents than under oceans.
- The Gutenberg discontinuity lies at about 2,900 km depth.
- The Conrad discontinuity is present everywhere, including under the oceans.
How many of the statements given above are correct?
- Only one
- Only two
- All three
- None
Show answer
(b) – The Moho is deeper under continents (30–70 km) than under oceans (5–10 km), and Gutenberg is at about 2,900 km. The Conrad discontinuity is not ubiquitous and is generally absent under oceans.
The evidence that the outer core is liquid comes primarily from:
- Deep drilling to the core-mantle boundary
- The S-wave shadow zone
- The P-wave shadow zone alone
- Magnetic surveys of the ocean floor
Show answer
(b) – S-waves cannot travel through liquids and are absent beyond about 103° from the epicentre, proving a liquid outer core. No drilling has reached the core.
Consider the following statements about the Earth's layers:
- The mantle accounts for about 84% of the Earth's volume.
- The inner core is solid despite temperatures above the melting point of iron because of extreme pressure.
- The asthenosphere is a rigid layer over which the lithospheric plates move.
How many of the statements given above are correct?
- Only one
- Only two
- All three
- None
Show answer
(b) – The mantle is about 84% of the volume and the inner core is solid due to pressure. The asthenosphere is weak and ductile, not rigid; the lithosphere above it is rigid.
The Deccan Trap volcanism occurred at the boundary between which two geological periods?
- Permian and Triassic
- Triassic and Jurassic
- Cretaceous and Palaeogene
- Palaeogene and Neogene
Show answer
(c) – The Deccan Traps erupted around 66 million years ago at the Cretaceous-Palaeogene boundary.
Which one of the following is the correct metamorphic product of the original rock given against it?
- Limestone → quartzite
- Sandstone → marble
- Shale → slate
- Granite → basalt
Show answer
(c) – Shale metamorphoses to slate; limestone to marble; sandstone to quartzite. Granite to basalt is not a metamorphic conversion.
Consider the following statements about sedimentary rocks:
- They are the only rock type that commonly preserves fossils.
- Coal and limestone can be organically formed sedimentary rocks.
- They are called primary rocks because they form first in the rock cycle.
How many of the statements given above are correct?
- Only one
- Only two
- All three
- None
Show answer
(b) – Fossils survive mainly in sedimentary rocks, and coal and limestone can be organically formed. Igneous rocks, not sedimentary, are called primary rocks.
Which one of the following is a ferromagnesian silicate mineral?
- Quartz
- Feldspar
- Olivine
- Muscovite mica
Show answer
(c) – Olivine, pyroxene, amphibole and biotite are ferromagnesian (mafic) silicates, rich in iron and magnesium. Quartz, feldspar and muscovite are non-ferromagnesian.
Consider the following statements about Odisha's geology:
- The Sukinda valley chromite deposits are hosted in ultramafic rocks of the Singhbhum-Odisha Craton.
- The Eastern Ghats granulite belt is Archaean in age and hosts the Gondwana coal deposits.
- Odisha accounts for more than 80% of India's chromite reserves.
How many of the statements given above are correct?
- Only one
- Only two
- All three
- None
Show answer
(b) – Sukinda's chromite is in ultramafic rocks and Odisha holds about 83% of India's chromite. The Eastern Ghats granulite belt is Proterozoic, and the Gondwana coal lies in separate Gondwana basins, not in the granulite belt.
Which of the following is the correct order of the Earth's layers from the surface to the centre?
- Crust → asthenosphere → lithosphere → outer core → inner core
- Lithosphere → asthenosphere → mesospheric mantle → outer core → inner core
- Crust → outer core → mantle → inner core
- Lithosphere → mesospheric mantle → asthenosphere → inner core → outer core
Show answer
(b) – Mechanically, the sequence is lithosphere, asthenosphere, mesospheric (lower) mantle, outer core and inner core.