Plate Tectonics, Earth's Interior, and Structural Geology
Domain 4 connects plate motion to earthquakes, volcanoes, Earth’s layered interior, and rock deformation.
Core rules and concepts
- Plate boundaries: divergent (extension, ridges, normal faults), convergent (compression, subduction or collision, reverse faults), transform (shear, strike-slip).
- Evidence includes fit of continents, matching fossils and rocks, magnetic stripes, seafloor age, and GPS motion.
- Earth’s interior: crust, mantle, outer core (liquid), inner core (solid). Lithosphere rides on the weaker asthenosphere.
- P waves are fastest, compressional, and travel through solids and liquids; S waves are slower, shear, and cannot pass liquids. The S-wave shadow zone helps locate the liquid outer core.
- Stress produces folds and faults; anticlines have oldest rocks in the core, synclines youngest.
- Earthquakes occur at faults; the focus is underground, the epicenter is at the surface.
Worked example
A seismic station records P arrival 5.0 s before S. If Vp = 6.0 km/s and Vs = 3.5 km/s, distance d satisfies 5.0 = d(1/3.5 − 1/6.0). The bracket is 0.2857 − 0.1667 = 0.1190 s/km, so d ≈ 42 km. The lag grows with distance; the calculation converts arrival-time difference into epicenter distance, and three stations locate the epicenter.
Traps that cost marks
Do not reverse P and S speeds. S waves do not travel through the outer core. Normal faults indicate extension, not compression. An epicenter is not the focus. Anticline cores are oldest, not youngest. Magnetic stripes record reversals plus seafloor spreading, not a changing magnetic field strength alone.