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Class 11 Interior of the Earth: cheat sheet
CBSE Social Science. How we learn about the inside of the earth, how earthquake waves reveal its layers, and the volcanoes and landforms that magma makes.
Key ideas
- Direct sources are mines, deep drilling and volcanic material; indirect sources are meteors, gravity, magnetic surveys and earthquake waves.
- The focus is where an earthquake starts inside the earth; the epicentre is the point on the surface right above it.
- P-waves are fastest and arrive first, passing through solids and liquids; S-waves travel only through solids; surface waves arrive last and do the most damage.
- S-waves cannot cross the liquid outer core, which creates the shadow zones and shows the core’s nature.
- Layers: crust (thin under oceans, thick under continents), mantle down to 2,900 km with the asthenosphere at its top, then a liquid outer core and a solid inner core of nickel and iron.
- Volcano types: shield (fluid basalt, low), composite (explosive, layers of lava and ash), caldera (collapsed), flood basalt (like the Deccan Traps) and mid-ocean ridge.
- Magma that cools inside the crust forms batholiths, laccoliths, lapoliths, phacoliths, sills and dykes.
Remember
| Within 105° of the epicentre | both P and S-waves recorded |
| 105° to 145° (shadow zone) | neither wave recorded |
| Beyond 145° | P-waves only |
| Arrival time and lag | time = distance ÷ speed; lag = time of S − time of P |
| Thickness of crust | oceanic about 5 km, continental about 30 km, up to 70 km under the Himalayas |
| Depths | core–mantle boundary 2,900 km; earth’s radius 6,378 km; deepest drill (Kola) about 12 km |
| Scales | Richter = magnitude (energy, 0–10); Mercalli = intensity (damage, 1–12) |
Watch out
- Saying S-waves pass through the outer core: they stop because it is liquid.
- Mixing up the focus (underground) and the epicentre (on the surface).
- Confusing a sill (horizontal) with a dyke (vertical wall).
- Giving the lag as the S time alone instead of S time minus P time.
Worked example
Two seismographs are (a) 34° and (b) 113° from an epicentre. Which waves does each record?
Working: within 105°: both; 105°–145°: neither; beyond 145°: P only
Answer: (a) both P and S-waves (b) neither P nor S-waves
Practise Interior of the Earth
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