Earth's Structure and Materials
This domain examines Earth's internal layers, seismic evidence, minerals and rocks, plate tectonics, and deformation. Questions often ask you to infer composition or process from a rock's texture, read a boundary map, or use seismic arrival times.
Core rules and concepts
- Layers. Compositionally: crust, mantle, core. Rheologically: lithosphere (crust plus uppermost mantle), asthenosphere (weak, ductile mantle), mesosphere, outer core (liquid), inner core (solid). The Moho marks the crust–mantle boundary.
- Seismic evidence. P-waves are compressional and travel through solids and liquids; S-waves are shear waves and cannot pass through liquids. The S-wave shadow zone (beyond ~103°) and the P-wave shadow zone reveal a liquid outer core and a solid inner core.
- Minerals. Naturally occurring, inorganic, crystalline solids with a definite composition (solid solution allowed). Key properties: hardness (Mohs), cleavage versus fracture, luster, streak, specific gravity. Silicates dominate the crust; Bowen's reaction series predicts crystallization order.
- Rocks. Igneous textures record cooling rate (phaneritic versus aphanitic; porphyritic). Sedimentary rocks are clastic, chemical, or biochemical and form by weathering, transport, deposition, and lithification. Metamorphic rocks are foliated or nonfoliated; contact metamorphism is local, regional is pressure-temperature driven.
- Tectonics. Divergent boundaries create new crust and rift valleys; convergent boundaries produce subduction, trenches, volcanic arcs, and mountain belts; transform boundaries produce strike-slip faults. Earthquakes and volcanoes trace plate margins. Isostasy explains crustal rebound.
Worked example
A seismic station 42 km from an earthquake records S-waves 5 s after P-waves. If V_P = 6 km/s and V_S = 3.5 km/s, verify the delay. t_P = 42/6 = 7 s; t_S = 42/3.5 = 12 s; t_S − t_P = 5 s. The reasoning: use each wave's speed over the same distance, then subtract. This S–P interval is the basis for locating an epicenter with three stations.
Traps that cost marks
- Saying S-waves travel through the outer core; they do not, which is why the shadow zone exists.
- Assuming continental crust is denser than oceanic crust; it is thicker but less dense (granitic versus basaltic).
- Thinking metamorphism requires melting; metamorphism occurs in the solid state.
- Calling glass a mineral; lack of a crystalline lattice disqualifies it.