Earth's Interior, Rocks and Landforms
This domain tests how geographers infer an unseen interior from earthquake waves, then connect rock types to the landforms they build or resist.
Probing the interior
Drilling reaches only a few kilometres, so the interior is mapped by seismology. P waves are longitudinal and pass through solids, liquids and gases; S waves are transverse and cannot travel through liquids. The Mohorovičič discontinuity separates crust from mantle, the Gutenberg discontinuity separates mantle from core, and the Lehmann discontinuity lies within the core. Because the outer core is liquid, S waves are absent beyond about 103° from the epicentre, creating the shadow zone. Oceanic crust is basaltic and thin (about 5 km); continental crust is granitic and thick (30–70 km), thickest under young fold mountains. The rigid lithosphere floats on the plastic asthenosphere.
The three rock families
Igneous rocks cool from magma: intrusive types (granite, dolerite) have large crystals, extrusive types (basalt) are fine-grained. Sedimentary rocks are layered and may carry fossils - sandstone, limestone, shale. Metamorphic rocks are altered by heat and pressure: limestone becomes marble, sandstone quartzite, shale slate, granite gneiss.
Worked example
A seismograph records the P wave at 10:05:00 and the S wave at 10:10:00, so the S–P lag is 300 s. Taking VP = 6 km/s and VS = 3.5 km/s, distance d satisfies d(1/3.5 − 1/6) = 300, i.e. d × 0.119 = 300, giving d ≈ 2,520 km to the epicentre.
Traps that cost marks
- Claiming S waves cross the outer core; they do not, which is the whole basis of the shadow zone.
- Mixing up anticline (upfold) and syncline (downfold), or confining folding to igneous terrain.
- Writing that continental crust is thinner than oceanic crust.
- Ignoring that denudation is weathering plus erosion plus transport, not weathering alone.