Interior of the Earth and Plate Tectonics
Earth's layers, seismic waves and shadow zones, continental drift, sea-floor spreading, plate tectonics, plate boundaries and the Indian plate for UPSC.
SyllabusGS I · Salient features of world physical geography
Prerequisites: Geomorphology — overview · The Universe and the Earth
The Hook: A Mountain That Will Not Stop Growing
In December 2020, Nepal and China jointly announced a new height for Mount Everest: 8,848.86 m, slightly above the long-accepted 8,848 m from the Survey of India’s measurements in the 1950s. Part of the reason a re-survey was needed is that the Himalaya is not a finished structure — it is still being pushed up.
Why would the world’s highest mountains still be rising, and why do earthquakes keep shaking the same belt from the Hindu Kush to Myanmar? The answer is that the Earth’s outer shell is broken into moving plates, and the Indian plate is still pushing into Eurasia.
The Core Concept
How we know what is inside
No one has seen the Earth’s interior. The deepest borehole (the Kola Superdeep Borehole in Russia’s Arctic) reached only about 12 km of a radius of about 6,371 km. Our knowledge comes from:
| Source | Type | What it tells us |
|---|---|---|
| Mining, deep drilling, ocean drilling projects | Direct | Composition of the upper crust |
| Volcanic eruptions | Direct | Material from the upper mantle (magma) |
| Rise of temperature and pressure with depth | Indirect | Conditions at depth |
| Meteorites | Indirect | Material similar to the Earth’s interior (iron–nickel, rocky) |
| Gravity and magnetic surveys | Indirect | Distribution of mass and magnetic minerals |
| Seismic waves | Indirect | The most important source — layer boundaries and states of matter |
Seismic waves
| Wave | Nature | Travels through | Key point |
|---|---|---|---|
| P (primary) | Longitudinal (compression) — vibrates parallel to the direction of travel | Solids, liquids and gases | Fastest; arrives first |
| S (secondary) | Transverse — vibrates at right angles to the direction of travel | Solids only | Its absence in the core reveals a liquid outer core |
| Surface waves (Love, Rayleigh) | Generated when body waves reach the surface | Along the surface | Slowest but most destructive |
Shadow zones. Seismographs within 105° of the epicentre record both P and S waves. Beyond 145° only P waves arrive. Between 105° and 145° neither arrives — the shadow zone. S waves are absent everywhere beyond 105°, which is why the S-wave shadow is much larger. The S-wave shadow tells us the outer core is liquid; the bending (refraction) of P waves at the core boundary creates the P-wave shadow.
The layers
| Layer | Extent (approx.) | Key features |
|---|---|---|
| Crust | Oceanic about 5 km thick; continental about 30 km on average, up to about 70 km under major mountain chains such as the Himalaya | Continental crust: lighter, granitic (sial). Oceanic crust: denser, basaltic (sima) |
| Mantle | From the base of the crust to about 2,900 km | Upper part includes the asthenosphere (up to about 400 km) — weak, partly molten, the main source of magma |
| Outer core | About 2,900 to 5,150 km | Liquid; iron–nickel; its movement generates the Earth’s magnetic field |
| Inner core | About 5,150 to 6,371 km | Solid (despite very high temperature) because of immense pressure; iron–nickel (NIFE) |
The lithosphere — the crust plus the rigid uppermost mantle, about 10–200 km thick — is the layer broken into plates. It “floats” on the weaker asthenosphere.
Discontinuities
| Discontinuity | Between |
|---|---|
| Conrad | Upper (granitic) and lower (basaltic) continental crust |
| Mohorovičić (Moho) | Crust and mantle |
| Repetti | Upper and lower mantle |
| Gutenberg | Mantle and outer core (about 2,900 km) |
| Lehmann | Outer core and inner core (about 5,150 km) |
From continental drift to plate tectonics
graph TD
A[Continental Drift - Alfred Wegener, 1912] --> B[Convection currents in the mantle - Arthur Holmes, 1930s]
B --> C[Ocean-floor mapping and palaeomagnetism after World War II]
C --> D[Sea-floor spreading - Harry Hess, 1961]
D --> E[Plate Tectonics - McKenzie and Parker, Morgan, 1967-68]1. Continental drift (Alfred Wegener, 1912). All continents once formed one supercontinent, Pangaea, surrounded by one ocean, Panthalassa. Pangaea split into Laurasia (north) and Gondwanaland (south), which broke into today’s continents. Evidence:
- Jigsaw fit of the coastlines of South America and Africa (a close computer fit was made at the 1,000-fathom depth line by Bullard in 1964).
- Rocks of the same age on both sides of the oceans — belts in Brazil match those in West Africa.
- Tillite (glacial deposits): the Gondwana sediments of India have counterparts in the other southern continents, pointing to a shared ice age.
- Placer gold on the Ghana coast without a local source rock — the source lies in Brazil.
- Fossils: the reptile Mesosaurus is found only in southern Africa and Brazil; lemurs occur in India, Madagascar and Africa.
Wegener proposed that pole-fleeing force and tidal force moved the continents. Most scientists found these forces far too weak — the theory’s main weakness.
2. Convection currents (Arthur Holmes, 1930s). Heat from radioactive decay sets up convection currents in the mantle that could drag the crust — a driving force Wegener lacked.
3. Sea-floor spreading (Harry Hess, 1961). Ocean-floor mapping showed mid-ocean ridges, deep trenches, young ocean-floor rocks and thin sediments. Hess proposed that magma rises at the ridges, forms new crust and pushes the older floor aside; old crust sinks back into the mantle at trenches. Proof: symmetrical bands of magnetic polarity on either side of ridges, and the fact that ocean-floor rocks are nowhere older than about 200 million years, while some continental rocks are over 3,000 million years old.
4. Plate tectonics (1967–68). The lithosphere is broken into rigid plates moving over the asthenosphere. It is the continents and the ocean floor that move together, as parts of plates.
The plates
Seven major plates: Pacific; North American; South American; Eurasian; African; Indo-Australian; Antarctic.
Important minor plates: Cocos (off Central America), Nazca (off South America), Arabian, Philippine, Caroline, Fiji, Juan de Fuca, Scotia.
Plate boundaries
| Boundary | Motion | Crust | Landforms and examples |
|---|---|---|---|
| Divergent (constructive) | Plates move apart; new crust forms | Created | Mid-ocean ridges (Mid-Atlantic Ridge); continental rifts (East African Rift); Iceland sits on the ridge |
| Convergent (destructive) — ocean–ocean | One oceanic plate subducts | Destroyed | Deep trench + volcanic island arc (Mariana Trench; Japan) |
| Convergent — ocean–continent | Oceanic plate subducts under continental plate | Destroyed | Trench + fold mountains with volcanoes (Andes; Peru–Chile Trench) |
| Convergent — continent–continent | Neither subducts easily; crust crumples | Neither created nor destroyed much | Fold mountains, few volcanoes (Himalaya; Alps) |
| Transform (conservative) | Plates slide past each other horizontally | Neither | Transform faults; shallow but damaging earthquakes (San Andreas Fault, California) |
Rates of spreading differ widely: the Arctic Ridge spreads slowest (under 2.5 cm a year) and the East Pacific Rise fastest (over 15 cm a year), according to NCERT.
The Indian plate
- India was part of Gondwanaland. After the break-up of Pangaea, the Indian plate moved north across what was then the Tethys Sea.
- During its journey, a huge outpouring of basaltic lava formed the Deccan Traps (beginning about 60 million years ago).
- India collided with Eurasia about 40–50 million years ago; the Tethys sediments were squeezed up to form the Himalaya, which are still rising.
- Boundaries: north — the Himalaya (continent–continent convergence); east — the Arakan Yoma of Myanmar, continuing along the Java (Sunda) Trench (subduction); west — the Kirthar range of Pakistan and the Makran coast; south — oceanic ridges separating it from the African and Antarctic plates.
- This is why the Himalayan arc and the Andaman–Sumatra arc are among the most earthquake-prone regions in the world.
Memory Anchors
Discontinuities, top to bottom: “Can My Rat Get Lost?” → Conrad · Moho · Repetti · Gutenberg · Lehmann.
Seismic waves: P = Push–pull, Passes everything, arrives at Pole position. S = Shake sideways, Solids only, Second.
Shadow zone numbers: “105 to 145” — the gap where neither wave arrives; “beyond 105, no S”.
Boundary types: Divergent = Divorce (move apart, ridge). Convergent = Crash (trench or mountains). Transform = Traffic passing side by side (fault).
Three Cs of convergence: Ocean–Ocean = island arcs; Ocean–Continent = Andes; Continent–Continent = Himalaya.
For Prelims
- S waves cannot pass through liquids — the S-wave shadow zone (beyond 105°) shows the outer core is liquid.
- The shadow zone for both P and S waves lies between 105° and 145° from the epicentre.
- Moho separates the crust from the mantle; Gutenberg separates the mantle from the core; Lehmann separates the outer and inner core.
- The asthenosphere (upper mantle) is the main source of magma; the lithosphere (crust + upper mantle) forms the plates.
- Continental drift — Wegener (1912); sea-floor spreading — Hess (1961); plate tectonics — 1967–68.
- Ocean-floor rocks are much younger (not older than about 200 million years) than the oldest continental rocks.
- Transform boundaries neither create nor destroy crust — San Andreas Fault is the classic example.
- The Himalaya formed by continent–continent convergence; the Andes by ocean–continent convergence.
For Mains
- Dimensions: evolution of the theory (drift → spreading → plate tectonics); evidence; plate boundaries and their landforms; distribution of earthquakes, volcanoes and fold mountains; the Indian plate and Himalayan seismicity; implications for disaster management.
- Keywords: lithosphere, asthenosphere, convection currents, palaeomagnetism, subduction zone, Benioff zone, island arc, transform fault, Wilson cycle, isostasy.
- Value-addition: the distribution of earthquakes and volcanoes along plate margins (Pacific Ring of Fire, Alpine–Himalayan belt, mid-ocean ridges) is itself the strongest evidence for plate tectonics; the Indian plate’s continuing push explains Himalayan uplift and seismic hazard.
Common Traps
In the News
- December 2020 — Nepal and China jointly announce Mount Everest’s height as 8,848.86 m.
- February 2023 — Major earthquakes in Türkiye and Syria occurred along the East Anatolian Fault, a transform-type boundary between the Anatolian and Arabian plates. See Geophysical Phenomena.
Practice
Prelims
Consider the following statements about seismic waves:
- P waves can travel through solid, liquid and gaseous materials.
- S waves travel faster than P waves.
- The absence of S waves beyond 105° from the epicentre indicates that the outer core is liquid.
Which of the statements given above is/are correct?
Solution
- Statement 1 — correct. P waves are compressional and pass through all states of matter.
- Statement 2 — wrong. P (primary) waves are the fastest and arrive first; S (secondary) waves arrive later.
- Statement 3 — correct. S waves cannot travel through liquids; their absence beyond 105° shows the outer core is liquid.
Answer: B
Which of the following were cited by Alfred Wegener or later geologists as evidence of continental drift?
- Glacial deposits (tillite) of the same age in India and other southern continents
- The occurrence of Mesosaurus fossils in southern Africa and Brazil
- The fit of the coastlines of South America and Africa
Select the correct answer using the code given below.
Solution
All three are standard evidence. Tillite shows a shared glaciation of the Gondwana landmasses; Mesosaurus, a small freshwater reptile, could not have swum across an ocean, so its distribution suggests the continents were joined; the jigsaw fit was confirmed by Bullard’s computer fit in 1964.
Answer: D
Match the following plate boundaries with their examples:
| Plate boundary | Example |
|---|---|
| A. Divergent | 1. San Andreas Fault |
| B. Ocean–ocean convergence | 2. Mid-Atlantic Ridge |
| C. Transform | 3. Mariana Trench |
Select the correct answer using the code given below.
Solution
- Divergent → Mid-Atlantic Ridge (2): plates move apart and new crust forms.
- Ocean–ocean convergence → Mariana Trench (3): the Pacific plate subducts beneath the Philippine plate, forming the deepest trench.
- Transform → San Andreas Fault (1): the Pacific and North American plates slide past each other.
Answer: A
Mains
How does the theory of plate tectonics explain the global distribution of earthquakes, volcanoes and fold mountains? Illustrate with examples.
Solution
Introduction: Plate tectonics holds that the lithosphere is divided into rigid plates that move over the asthenosphere, driven largely by mantle convection. Most earthquakes, volcanoes and young fold mountains lie along plate margins — a pattern the theory explains.
Body:
- Divergent margins: rising magma creates new crust; shallow earthquakes and fissure volcanoes along mid-ocean ridges (Mid-Atlantic Ridge, Iceland); continental rifting (East African Rift).
- Ocean–ocean convergence: subduction creates trenches and volcanic island arcs with deep-focus earthquakes (Mariana Trench, Japan).
- Ocean–continent convergence: subduction melts the descending plate; explosive volcanoes and fold mountains (Andes over the Nazca plate).
- Continent–continent convergence: crust crumples into fold mountains with frequent earthquakes but few volcanoes (Himalaya, Alps).
- Transform margins: plates slide past each other; shallow, damaging earthquakes without volcanoes (San Andreas Fault).
- Global belts: the Circum-Pacific “Ring of Fire” (subduction around the Pacific) and the Alpine–Himalayan belt (Africa, Arabia and India colliding with Eurasia).
- Exceptions: intraplate hotspots (Hawaii) and intraplate earthquakes show that not all activity is at margins.
Conclusion: By linking earthquakes, volcanoes and mountain-building to plate interactions, plate tectonics turned scattered observations into a single explanation — and it guides hazard zoning, such as for India’s Himalayan and Andaman regions.
Quick Revision
| Point | One-liner |
|---|---|
| Most important source on the interior | Seismic waves |
| P vs S | P: all media, fastest; S: solids only |
| Shadow zone | 105°–145° for both; beyond 105° no S waves |
| Discontinuities | Conrad, Moho, Repetti, Gutenberg, Lehmann (top to bottom) |
| Liquid layer | Outer core |
| Asthenosphere | Upper mantle, up to ~400 km; source of magma |
| Continental drift | Wegener, 1912; Pangaea, Panthalassa |
| Sea-floor spreading | Hess, 1961; magnetic striping |
| Plate tectonics | 1967–68; seven major plates |
| Himalaya | India–Eurasia collision, ~40–50 million years ago; still rising |
Key Takeaways
- Seismic waves are the X-ray of the Earth: the S-wave shadow proves a liquid outer core.
- Continental drift supplied the evidence; sea-floor spreading supplied the mechanism; plate tectonics united them.
- The type of plate boundary predicts the landform and the hazard — ridges, trenches, island arcs, fold mountains or faults.
Related Topics
- Landforms and Agents of Erosion
- Geomorphology — quick revision
- Geomorphology — practice questions
- Earthquakes, Tsunamis and Volcanoes
- Physiography of India