SSC CGL Geography · Earth's Structure, Landforms and Geological Time
Earth's Interior, Rocks and Earthquakes
The Earth's layers from crust to core, the plates and their boundaries, the three kinds of rock with their examples, and the words for recording an earthquake.
Why this matters
Fifteen questions in the bank: nine from the 2023 shifts, five from the 2024 shifts and one from the 2025 shifts, 12 of them EASY. They ask an order (crust, mantle, core), a name (asthenosphere, seismogram) or a rock type, so learn the layers and the three rock types first.
Concept 1 of 2: Layers and plates
Definition
Layers, from the outside in:
- Crust: the thin outer layer. Mantle: between crust and core. Core: outer (liquid) and inner (solid).
- The lithosphere is the rigid outer part: the crust and the top of the mantle.
- The asthenosphere is the weak, partly molten upper mantle below it, on which the plates move.
- The Earth's inner heat comes from radioactive decay (radiogenic heat) and heat left from its formation (primordial heat).
Plates:
- The Pacific plate is the largest.
- Plates move apart at divergent boundaries (mid-ocean ridges), collide at convergent ones (the Mariana Trench) and slide past at transform ones (the San Andreas Fault).
| Feature | Plate boundary |
|---|---|
| Mid-ocean ridge | Divergent |
| Mariana Trench | Convergent |
| San Andreas Fault | Transform |
Practise this conceptself-check · 4 quick reps
The same idea in a real exam question:
Example 1 · Earth's Structure, Landforms and Geological Time · Earth's Interior, Rocks and Earthquakes
Stratosphere and mesosphere are in the air
Concept 2 of 2: Rocks and earthquake records
Definition
The three rock types:
- Igneous: from cooled magma or lava. Granite is igneous (quartz, feldspar, a little mica) and the commonest rock of the continental crust.
- Sedimentary: from laid-down and cemented material. Some form from salts that precipitate out of water, such as halite (rock salt), left when sea water evaporates.
- Metamorphic: older rock changed by deep heat and pressure.
- Hornblende, an amphibole mineral, is crushed for roads and railway ballast.
Earthquakes: a seismograph is the instrument; the record it makes is a seismogram; seismology is the science.
Practise this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 2 · Earth's Structure, Landforms and Geological Time · Earth's Interior, Rocks and Earthquakes
Seismograph makes, seismogram is made
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Layers and plates3 rows
| Feature | Plate boundary |
|---|---|
| Mid-ocean ridge | Divergent |
| Mariana Trench | Convergent |
| San Andreas Fault | Transform |
Watch out for (2)
- Stratosphere and mesosphere are in the air→ Layers and plates
- Seismograph makes, seismogram is made→ Rocks and earthquake records
Test yourself on Earth's Structure, Landforms and Geological Time
10 past SSC CGL questions from this chapter, timed at 6 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.