Interior of Earth - UPSC NOTES GS1 2026

GEOMORPHOLOGY 

1. Interior of earth 

2. Continental drift theory 

3. Plate tectonic theory 

4. Earthquakes 

5. Volcanism 

6. Rocks 

7. Weathering and erosion 

8. Mountain & Buildings

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1. Interior of earth

Sources of Information about Earth’s Interior

Earth’s radius: 6,370 km. Direct study is limited, so knowledge comes from both direct and indirect sources. 1. Direct Sources Surface rocks & laboratory experiments: Provide clues about internal conditions. Mining & drilling: Deepest mine < 5 km; Kola superdeep borehole (Arctic) reached 12 km. Deep ocean drilling projects: Collected crustal samples. Volcanic eruption: Magma gives information about material from deeper layers.

2. Indirect Sources Meteors: Composition often similar to Earth’s interior. Gravitation (gravity anomalies): Indicate distribution of mass in the crust. Magnetic field: Helps locate magnetic materials in the crust. Seismic activity: Most reliable source; S-waves travel only in solids, giving information about internal layers. Temperature: Increases by about 1°C per 32 m, reaching ~4000°C at the core. Density: Increases with depth (2.7 g/cm³ at crust → up to 11 g/cm³ at core). Pressure: Rises greatly with depth due to overlying rocks. The Earth is made up of concentric layers: Crust, Mantle, Outer Core, and Inner Core. Each has unique physical and chemical properties

1. Crust Thickness: 8–40 km (thinnest under oceans, thickest under continents). Nature: Brittle solid. Volume & Mass: ~1% of Earth’s mass, ~15% of volume. Composition: Silica (Si) + Aluminium (Al) → called SIAL. Discontinuity: Conrad Discontinuity (between crustal layers). 2. Mantle Thickness: ~2,900 km. Nature: Denser, tightly packed rocks; partly plastic at depth. Volume & Mass: ~67% of mass, ~84% of volume. Composition: Silica (Si) + Magnesium (Mg) → called SIMA. Discontinuity: Mohorovičić Discontinuity (Moho) between crust and mantle.

3. Core Thickness/Radius: ~3,500 km. Nature: Divided into Outer Core → liquid/semi-liquid. Inner Core → solid, dense. Volume & Mass: ~32% of mass, ~15% of volume. Composition: Nickel (Ni) + Iron (Fe) → called NIFE. Discontinuity: Gutenberg Discontinuity (mantle–core boundary). 4. Additional Zones Lithosphere: Crust + rigid upper mantle (~100 km thick). Asthenosphere: Ductile, partially molten zone below lithosphere; allows plate movement. Source of Heat: Radioactive decay generates heat in mantle. Paleomagnetism Meaning: Study of Earth’s past magnetic field preserved in rocks. Material: Magnetic traces in igneous rocks and sediments (with magnetite). Findings: Ocean floor has parallel magnetic bands of alternating polarity. These bands are symmetrical on either side of mid-oceanic ridges. Evidence for sea-floor spreading and plate tectonics.

The interior of earth can also be studied by various methods and techniques, one of them is the study of seismic waves to know the interior of earth.

1.  Seismic waves :- 

1. Hypo center:- The point inside earth , where seismic waves got generated or earthquakes began.

2. Epicenter:- The point on the surface of the earth , nearest to the focus or the point where seismic waves reach first. 

3. All natural Earthquakes Occur in the lithosphere. 

2. The study of S waves :- 

1. The study of s waves revealed that there is a shadow zone for s wave beyond 105⁰ from epicenter  

2. In this shadow zone s waves aren't obtained at any location 

3. Since, it is known that s waves cannot travel through Liquid medium, it was developed that there must be a liquid layer in the interior of the earth , and Must be at 2900km from the surface. 

3. The study of p waves : 

1. The study of p waves was necessary to analyze the depth up to which the liquid layer existed

2. When the variation in velocity of P waves with the depth was studied it was seen that velocity gradually increased upto the depth of 2900 km which was obvious because of increased.

3. At this there was a sudden drop in velocity that indicates a liquid zone as was clear by the study Of s waves also. 

4. Beyond this depth velocity again increased gradually because of density , but around 5150 km there was sudden increase in velocity , which is only possible if there is a change in medium. 

5. Thus beyond the 5150 km there is solid Again. 

6. The study of p waves also revealed that there is a shadow zone of p-waves which extends from 105⁰ to 145⁰ angular distance from the epicenter of the earthquake.The p waves undergo drastic refraction at 2900 km and again at 5150 km of depth.

7. The shadow zone helped in analysing  the depth of inner solid Core .

4. Discontinuities 

1. The region in the interior of earth where the velocity of seismic waves changes abruptly. 

2. The change can be because of change in composition of medium or change in state of medium. 

3. C M R G L 

Why Inner Core is Solid and Outer Core is Liquid

1. Composition

1. Both are mainly made of iron (Fe) and nickel (Ni).

2. Temperature vs. Pressure Balance

1. The outer core has very high temperatures (similar to the inner core), enough to keep iron–nickel in a molten (liquid) state because the pressure there is not extreme enough to force atoms into a solid arrangement.

2. The inner core, though hotter, is under tremendously higher pressure due to the weight of all the overlying layers.

3. This extreme pressure prevents atoms from moving freely, forcing them into a solid crystalline structure, despite the high temperature.

3. At last

1. Outer core → liquid because high temperature overcomes pressure.

2. Inner core → solid because pressure dominates over temperature, keeping it tightly packed.

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