Interior of the Earth
1. Learning Objectives
After reading these notes, you will be able to:
2. Sources of Information about Earth’s Interior
β A. Direct Sources
βοΈ Mining & Drilling
Surface rocks and rocks from mining areas give direct samples. Gold mines in South Africa go 3β4 km deep. Deepest drill at Kola (Arctic Ocean) reached 12 km. Two major projects: Deep Ocean Drilling and Integrated Ocean Drilling Project.
π Volcanic Eruptions
Molten magma thrown to the surface during eruptions is available for laboratory analysis. However, the exact depth of the magma source is difficult to confirm.
π¬ B. Indirect Sources
π‘οΈ Temperature & Pressure
Both temperature and pressure increase with depth. Density also increases. Scientists estimate values of these at different depths from known rates of change.
βοΈ Meteors
Material in meteors is similar in composition to Earth’s interior. They are solid bodies developed from same/similar materials as our planet.
π Gravity (g)
Gravity is greater near poles, less at equator. Difference between expected and recorded values = gravity anomaly β gives info about mass distribution in crust.
𧲠Magnetic Surveys
Provide information about distribution of magnetic materials in the crustal portion of the earth.
3. Earthquakes and Seismic Waves
π Types of Earthquake Waves
| Wave Type | Full Name | Speed & Order | Medium of Travel | Nature of Motion |
|---|---|---|---|---|
| P-waves | Primary Waves | Fastest β arrive first | Solid, Liquid & Gas | Parallel to wave direction; stretching & squeezing |
| S-waves | Secondary Waves | Slower β arrive later | Solid only | Perpendicular to wave direction; creates crests & troughs |
| Surface Waves | L-waves | Slowest β last to arrive | Surface only | Most destructive; cause displacement of rocks & collapse of structures |
π Shadow Zone
- Areas where earthquake waves are not recorded by seismographs are called shadow zones.
- Within 105Β° from epicentre β seismographs record both P and S-waves.
- Beyond 145Β° from epicentre β only P-waves are recorded, not S-waves.
- Zone between 105Β° and 145Β° from epicentre = shadow zone for both P and S-waves.
- The entire zone beyond 105Β° does not receive S-waves at all.
- S-wave shadow zone covers over 40% of Earth’s surface β much larger than P-wave shadow zone.
- P-wave shadow zone = a band around Earth between 105Β° and 145Β° from epicentre.
β‘ Types of Earthquakes
ποΈ Tectonic Earthquakes
Most common type. Generated due to sliding of rocks along a fault plane.
π Volcanic Earthquakes
A special class of tectonic earthquake. Confined to areas of active volcanoes.
βοΈ Collapse Earthquakes
In areas of intense mining activity β roofs of underground mines collapse causing minor tremors.
π£ Explosion Earthquakes
Caused by explosion of chemical or nuclear devices. Man-made tremors.
π§ Reservoir Induced
Occur in areas of large reservoirs. Water pressure triggers ground tremors.
π Measuring Earthquakes
β οΈ Effects of Earthquakes
4. Structure of the Earth
Outermost solid layer
Oceanic 5 km | Continental 30 km
Moho β 2,900 km
Asthenosphere = magma source
2,900β5,100 km
Liquid state
5,100β6,378 km
Solid state (NIFE)
Crust + Upper Mantle
10β200 km thick
Upper mantle, up to 400 km
Source of magma
ποΈ A. The Crust
- Outermost solid part of the earth. It is brittle in nature.
- Oceanic Crust β thinner; mean thickness 5 km.
- Continental Crust β thicker; mean thickness around 30 km.
- Continental crust is thickest under major mountain systems β up to 70 km in the Himalayan region.
π₯ B. The Mantle
- Extends from Moho’s discontinuity to 2,900 km depth.
- Upper portion = Asthenosphere β the word means ‘weak’. Extends up to 400 km.
- Asthenosphere is the main source of magma that finds its way to the surface during volcanic eruptions.
- Lithosphere = Crust + Uppermost part of Mantle. Thickness: 10β200 km.
- The lower mantle extends beyond the asthenosphere and is in solid state.
βοΈ C. The Core
- CoreβMantle boundary is at 2,900 km depth.
- Outer Core β in liquid state. (S-waves cannot pass through it.)
- Inner Core β in solid state.
- Core is made of very heavy material β mainly Nickel (Ni) and Iron (Fe). Also called NIFE layer.
| Layer | Depth | State | Key Feature |
|---|---|---|---|
| Crust | 0β100 km | Solid (Brittle) | Oceanic = 5 km; Continental = 30 km; Himalayan = 70 km |
| Asthenosphere | Up to 400 km | Semi-solid (Weak) | Main source of magma; ‘astheno’ means weak |
| Mantle | Moho β 2,900 km | Solid (lower part) | Moho’s discontinuity separates it from crust |
| Outer Core | 2,900β5,100 km | Liquid | S-waves cannot pass β proof of liquid state |
| Inner Core | 5,100β6,378 km | Solid | Nickel + Iron (NIFE); heaviest layer |
5. Volcanoes and Volcanic Landforms
π A. Types of Volcanoes
π‘οΈ Shield Volcanoes
Largest volcanoes (barring basalt flows). Made of basalt β very fluid lava. Not steep; low-explosivity. Lava comes as a fountain β forms cinder cone. Famous example: Hawaiian volcanoes.
ποΈ Composite Volcanoes
Cooler and more viscous lava. Result in explosive eruptions. Large quantities of pyroclastic material and ash accumulate in layers near vents β hence ‘composite’.
π₯ Caldera
Most explosive volcanoes. Collapse on themselves during eruption β form collapsed depressions called calderas. Magma chamber is huge and in close vicinity.
π Flood Basalt Provinces
Outpour highly fluid lava flowing long distances. Flows can be 50 m+ thick. Deccan Traps (Maharashtra Plateau) β India’s major example.
π Mid-Ocean Ridge Volcanoes
Occur in oceanic areas. Mid-ocean ridges more than 70,000 km long. The central portion experiences frequent eruptions.
πͺ¨ B. Intrusive Volcanic Landforms
π» Batholith
Large body of magma cooling at deeper depths forms large domes. Appear on surface only after denudation. Granitic bodies. Are the cooled portion of magma chambers. Cover large areas, several km deep.
ποΈ Lacolith
Large dome-shaped intrusive body with a level base, connected by a pipe-like conduit. At deeper depths. Karnataka plateau domal granite hills are examples of lacoliths.
π³ Lapolith
Lava develops into a saucer shape, concave to the sky. Forms when lava moves horizontally and rests in this saucer-like shape.
γ°οΈ Phacolith
Wavy mass of intrusive rocks found at base of synclines or top of anticlines in folded igneous country. Has a definite conduit to magma chambers below.
π Sill / Sheet
Near-horizontal bodies of intrusive igneous rock. Thin ones = sheets; thick horizontal deposits = sills.
𧱠Dyke
Lava solidifies in cracks/fissures almost perpendicular to the ground β forms a wall-like structure. Most common in western Maharashtra. Feeders for the Deccan Traps eruptions.
Summary β Quick Revision
Earth’s radius = 6,378 km. Interior studied via direct (mining, drilling, volcanoes) and indirect (seismic waves, gravity anomaly, magnetism, meteors) sources.
Deepest drill: Kola, Arctic Ocean β 12 km. Major projects: Deep Ocean Drilling and Integrated Ocean Drilling Project.
P-waves (fastest, all media) β S-waves (solid only, slower) β Surface waves (slowest, most destructive). All recorded by seismograph.
Shadow zone: 105Β°β145Β° = no direct P or S-waves. Beyond 105Β° = no S-waves. S-wave shadow zone covers 40%+ of Earth’s surface.
Earthquake measured by Richter scale (magnitude, 0β10) and Mercalli scale (intensity, 1β12). Magnitude 8+ earthquakes are very rare.
Crust: oceanic = 5 km; continental = 30 km; Himalayan = 70 km. Mantle: Moho to 2,900 km. Core: outer (liquid) + inner (solid) = NIFE.
Asthenosphere (up to 400 km) = main source of magma. Lithosphere = Crust + Upper Mantle = 10β200 km. Moho’s discontinuity = crustβmantle boundary.
Volcanoes: Shield (basalt, low-explosive; Hawaiian), Composite (viscous, explosive), Caldera (most explosive), Flood Basalt (Deccan Traps), Mid-Ocean Ridge.
Intrusive forms (cooling inside crust): Batholith, Lacolith, Lapolith, Phacolith, Sill/Sheet, Dyke β all plutonic rocks.
Dykes = most common in western Maharashtra; feeders for Deccan Traps. Karnataka plateau domal hills = examples of Lacoliths/Batholiths.
Important Terms to Remember
- Seismograph: An instrument that records earthquake waves reaching the surface of the earth.
- Focus / Hypocentre: The point inside the earth where energy is released during an earthquake.
- Epicentre: The point on the surface directly above the focus β the first point to experience earthquake waves.
- Fault: A sharp break in crustal rocks along which rocks tend to move in opposite directions, eventually releasing energy as an earthquake.
- P-waves (Primary Waves): Fastest body waves that can travel through solids, liquids, and gases. They vibrate parallel to the wave direction, creating stretching and squeezing.
- S-waves (Secondary Waves): Body waves that can travel only through solid materials. They vibrate perpendicular to wave direction, creating crests and troughs.
- Surface Waves (L-waves): Slowest but most destructive earthquake waves that travel along the surface. Last to be recorded on seismograph. Cause structural collapse.
- Shadow Zone: Areas on Earth’s surface where earthquake waves are not recorded. Zone between 105Β° and 145Β° from epicentre = shadow zone for both P and S-waves.
- Gravity Anomaly: The difference between the expected and the recorded value of gravity (g) at a place. Gives information about mass distribution in Earth’s crust.
- Richter Scale: Scale for measuring the magnitude (energy released) of an earthquake. Range: 0β10.
- Mercalli Scale: Scale for measuring the intensity (visible damage) of an earthquake. Range: 1β12. Named after Italian seismologist Mercalli.
- Moho’s Discontinuity: The boundary between the crust and the mantle of the earth.
- Asthenosphere: The upper portion of the mantle (up to 400 km). The word means ‘weak’. It is the main source of magma that finds its way to the surface during volcanic eruptions.
- Lithosphere: The crust and the uppermost part of the mantle together. Thickness ranges from 10β200 km.
- NIFE Layer: The core of the earth, composed mainly of Nickel (Ni) and Iron (Fe).
- Magma: Molten rock material present inside the earth (upper mantle/asthenosphere).
- Lava: Magma that has started moving towards the crust or has reached the surface of the earth during a volcanic eruption.
- Active Volcano: A volcano from which materials are currently being released or have been released in the recent past.
- Caldera: Collapsed depressions formed when extremely explosive volcanoes collapse on themselves rather than building a tall structure.
- Pyroclastic Material: Fragmented rock material (ash, dust, cinders, volcanic bombs) ejected explosively during volcanic eruptions.
- Deccan Traps: A flood basalt province in India, covering most of Maharashtra plateau. Formed by flood basalt type volcanic eruptions.
- Batholith: Large dome-shaped granitic intrusive body that cools deep inside the crust. The cooled portion of magma chambers; covers large areas.
- Lacolith: Large dome-shaped intrusive body with a level base connected by a pipe-like conduit. Located at deeper depths. Karnataka plateau hills are examples.
- Lapolith: A saucer-shaped (concave to the sky) intrusive igneous body formed when horizontally moving lava rests in a saucer-like form.
- Phacolith: Wavy mass of intrusive rocks found at the base of synclines or top of anticlines in folded igneous country.
- Sill / Sheet: Near-horizontal bodies of intrusive igneous rock. Thin deposits = sheets; thick horizontal deposits = sills.
- Dyke: Wall-like intrusive structures formed when lava solidifies in vertical cracks/fissures perpendicular to the ground. Most common in western Maharashtra; feeders for Deccan Traps.
- Tsunami: Giant ocean waves generated by earthquake tremors when the epicentre is below oceanic waters and the magnitude is sufficiently high.
- Tectonic Earthquake: The most common type of earthquake, caused by sliding of rocks along a fault plane.