Earth's Structure and History
Earth's Structure and History
This domain tests Earth's compositional and mechanical layers, seismic evidence for internal structure, and the principles used to reconstruct geologic time.
Core concepts
- Compositional layers are crust (oceanic basalt, continental granite), mantle (silicate rock), and core (iron-nickel).
- Mechanical layers are lithosphere (rigid crust plus uppermost mantle), asthenosphere (weak, partially molten), mesosphere (stiff lower mantle), outer core (liquid), and inner core (solid).
- P waves are compressional and pass through solids and liquids. S waves are shear waves and cannot pass through liquids. The S-wave shadow zone beyond about 103° and the P-wave shadow zone from about 103° to 142° reveal the liquid outer core.
- Relative dating uses superposition, original horizontality, lateral continuity, cross-cutting relationships, inclusions, and unconformities.
- Radiometric dating uses parent-daughter ratios and half-lives; closure and contamination affect reliability.
Worked example
A zircon retains 12.5% of its original parent isotope. Each half-life halves the parent: 100% → 50% → 25% → 12.5%, so three half-lives have passed. If the half-life is 1.25 billion years, the age is 3 × 1.25 = 3.75 billion years. Reasoning: count halvings from the remaining parent fraction; 87.5% daughter corresponds to 12.5% parent, not four half-lives.
Traps
Do not confuse compositional crust/mantle/core with mechanical lithosphere/asthenosphere/mesosphere. S waves do not travel through the outer core. A parent:daughter ratio of 1:1 represents one half-life, while 1:3 represents two. An unconformity is missing time, not necessarily one erosion event.