Inside the Earth — Layers, Discontinuities, the Three Rock Families and the Rock Cycle

You Cannot Dig to the Centre — So How Do We Know?

The deepest hole ever drilled reaches about 12 km. The centre of the Earth is 6,371 km down. Everything we claim about the inside of the planet is therefore indirect evidence, and almost all of it comes from one source: the way earthquake waves bend, slow down, or refuse to pass through.

That single idea carries the chapter. P-waves travel through solids and liquids; S-waves travel only through solids. When S-waves vanish on the far side of the planet, whatever they hit must be liquid. That is how the outer core is known to be molten without anyone ever seeing it.

The rocks under your feet are the same story at a smaller scale: three families, one cycle, and a short list of "which turns into which" pairs that papers ask almost word for word. Learn the layers and the pairs, and the marks become routine.

1. Which discontinuity separates the crust from the mantle?

2. Fossils are found in which type of rock?

3. Marble is the metamorphic form of which rock?

4. The outer core of the Earth is known to be liquid mainly because:

5. The Deccan Trap of peninsular India is composed mainly of which rock?

6. Which of the following parent-to-metamorphic pairs is correctly matched?

7. SIAL, SIMA and NIFE refer respectively to:

8. Igneous rocks are called primary or parent rocks because: