Why this matters now
The Earth’s interior underpins plate tectonics, earthquakes and volcanism. UPSC tests the layers, the seismic-wave evidence and the discontinuities.
How we know — seismic waves
Earthquakes generate seismic waves: P-waves (primary) are faster and travel through solids and liquids; S-waves (secondary) are slower and travel only through solids. Because S-waves cannot pass through the outer core, we infer the outer core is liquid. The way wave speeds change at depths reveals the boundaries (discontinuities) between layers.
The three main layers
| Layer | Features |
|---|---|
| Crust | Thin outer shell; oceanic (SiMa, basalt) is thinner/denser, continental (SiAl, granite) is thicker |
| Mantle | Largest layer; the upper mantle includes the rigid lithosphere and the plastic asthenosphere (source of magma) |
| Core | Iron-nickel; a liquid outer core (generates the magnetic field) and a solid inner core |
Discontinuities
The boundaries between layers are marked by discontinuities: the Mohorovičić (Moho) discontinuity between crust and mantle, and the Gutenberg discontinuity between mantle and core. The rigid lithosphere (crust + uppermost mantle) rides over the weaker asthenosphere — the basis of plate movement.
Significance
This layered, mobile interior drives plate tectonics, earthquakes, volcanoes and the geomagnetic field, and explains the distribution of continents, oceans and mountain belts.
UPSC angle
Remember P-waves pass through everything, S-waves only through solids (so outer core is liquid). Know the layers, the Moho/Gutenberg discontinuities and the lithosphere–asthenosphere distinction.
Frequently asked questions
What are the main layers of the Earth’s interior?
The crust, the mantle and the core (with a liquid outer core and solid inner core).
How do we know the outer core is liquid?
Because S-waves, which travel only through solids, cannot pass through it, while P-waves slow down — revealing a liquid layer.
What is the Moho discontinuity?
The Mohorovičić discontinuity — the boundary between the Earth’s crust and the mantle.
What is the asthenosphere?
A weak, partly molten layer of the upper mantle below the rigid lithosphere, over which tectonic plates move; a major source of magma.