Plate Tectonics and Earth's Interior
~11 min read
- Interior: Three layers: crust (5-70 km), mantle (2900 km thick), core (outer liquid + inner solid; ~3500 km radius).
- Plates: Lithosphere broken into 7 major + several minor plates, drifting on the plastic asthenosphere.
- Boundaries: Convergent (collision/subduction), divergent (rifting), transform (sliding) — each makes different landforms.
Earth's outer shell (lithosphere) is broken into ~15 rigid plates that float on the semi-fluid asthenosphere. Their slow movement causes earthquakes, volcanoes, mountains and ocean ridges. NDA tests both the interior structure and the plate-boundary types.
Layers of Earth's Interior
| Layer | Thickness | Composition |
|---|---|---|
| Crust | 5–70 km (oceanic 5-10, continental 30-70) | Silicate rocks — granite (continental), basalt (oceanic) |
| Mantle | ~2,900 km | Silicate minerals, semi-fluid in asthenosphere (upper mantle, plastic layer) |
| Outer core | ~2,260 km | Liquid iron + nickel; source of Earth's magnetic field |
| Inner core | ~1,220 km | Solid iron-nickel; T ~5,400 °C |
The boundary between crust and mantle is called the Moho (Mohorovičić discontinuity). Crust + uppermost mantle together = lithosphere (the rigid plates that move).
Tectonic Plates
Seven major plates: Pacific, North American, South American, Eurasian, African, Indo-Australian, Antarctic. Minor plates include Arabian, Caribbean, Cocos, Nazca, Philippine, Scotia, Juan de Fuca.
The Indian plate is moving north-northeast at about 5 cm per year. Its collision with the Eurasian plate ~50 million years ago raised the Himalayas — which are still rising by ~5 mm per year.
Plate Boundary Types
| Boundary | Motion | Result | Example |
|---|---|---|---|
| Convergent | Plates collide | Mountains, trenches, volcanoes, earthquakes | Himalayas (India + Eurasia), Andes (Nazca + S. America) |
| Divergent | Plates pull apart | Mid-ocean ridges, rift valleys | Mid-Atlantic Ridge, East African Rift |
| Transform | Plates slide past | Earthquakes, no new crust | San Andreas Fault (California) |
Earthquakes and Volcanoes
Earthquakes occur where plates grind against each other; magnitude is measured on the Richter Scale. The Pacific "Ring of Fire" accounts for ~75% of the world's earthquakes and active volcanoes.
Volcanoes: shield (Hawaii, gentle slopes), composite/stratovolcano (Fuji, steep), cinder cone (small). India has only one active volcano — Barren Island in the Andaman Sea.
NDA PYQ Examples
Q: The Indian Plate is moving towards the:
(a) South-West (b) North-East (c) East (d) North-West
Answer: (b) North-East — colliding with the Eurasian plate, raising the Himalayas.
Q: The 'Ring of Fire' is associated with:
(a) Atlantic Ocean (b) Indian Ocean (c) Pacific Ocean (d) Arctic Ocean
Answer: (c) Pacific Ocean — encircles the Pacific basin.
Q: The outer core of the Earth is in which state?
(a) Solid (b) Liquid (c) Gas (d) Plasma
Answer: (b) Liquid — molten iron and nickel; source of Earth's magnetic field.
Drill Plate Tectonics and Earth's Interior for NDA
NDA-pattern items on Plate Tectonics and Earth's Interior with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
What causes earthquakes?
Sudden release of energy along plate boundaries or faults, mostly at convergent or transform boundaries. The point inside the Earth where rupture occurs is the focus; the point directly above on the surface is the epicentre.
Which is the only active volcano in India?
Barren Island in the Andaman Sea. Narcondam Island nearby has a dormant volcano.
How is the Himalayan range still growing?
The Indian Plate continues to push northward into the Eurasian Plate at ~5 cm/year. The collision uplifts the Himalayas by about 5 mm per year (though weathering takes some of this back).
What is the Moho discontinuity?
The boundary between Earth's crust and mantle, named after seismologist Andrija Mohorovičić. Seismic waves change speed sharply at this boundary.
How are mountains formed?
By convergent plate boundaries — folding (Himalayas, Alps), faulting (Sierra Nevada), or volcanic activity (Andes' volcanic arcs). All three are direct results of plate tectonics.