Earth’s Interior, Plate Tectonics, and Geologic History
Earth’s Interior, Plate Tectonics, and Geologic History
This domain tests how seismic evidence reveals Earth’s layers, how plate motions build and destroy crust, and how rocks record relative and absolute time.
Core rules and concepts
- P-waves (compressional) travel through solids, liquids, and gases; S-waves (shear) travel only through solids. The S-wave shadow zone from 103° to 142° proves the outer core is liquid.
- Layers: crust, mantle, outer core, inner core. Density increases with depth; the Moho separates crust from mantle; the lithosphere is crust plus rigid upper mantle above the asthenosphere.
- Plate boundaries: divergent (mid-ocean ridges, new crust), convergent (subduction, trenches, mountains), transform (faults). Hot spots record plate motion.
- Relative dating: superposition, original horizontality, cross-cutting relationships, inclusions, and unconformities. Index fossils have short ranges and wide distributions.
- Absolute dating uses half-lives; parent isotopes decay to daughter products.
Worked example
A mineral contains 100 g of parent isotope with a half-life of 50 million years. After 150 million years, 3 half-lives have passed: parent = 100 × (1/2)³ = 12.5 g; daughter = 87.5 g. The daughter-to-parent ratio is 7:1.
Traps that cost marks
- Saying S-waves pass through the outer core; they do not.
- Confusing density with pressure: density generally increases with depth, but pressure always does.
- Forgetting that a cross-cutting feature is younger than the rock it cuts.
- Assuming half-life changes with temperature or pressure; it does not for normal Earth conditions.