Earth as a Planet: Origin, Interior and Geological Time
What the domain covers
This domain, Earth as a Planet: Origin, Interior and Geological Time, tests how Earth formed, how its internal layers are inferred, and how geological events are placed in time. Expect questions on the nebular hypothesis, accretion and differentiation, seismic discontinuities, and relative and absolute dating.
Core rules
- The nebular hypothesis explains the Solar System from a rotating cloud of gas and dust. Planetesimals accreted, and early Earth heated, melted and differentiated into core, mantle and crust.
- P-waves are compressional and pass through solids, liquids and gases. S-waves are shear waves and cannot pass liquids. The S-wave shadow zone beyond about 103 degrees proves a liquid outer core.
- The Moho separates crust from mantle. The Gutenberg discontinuity separates mantle from core. The Lehmann discontinuity marks the inner-core boundary.
- Relative dating uses superposition, original horizontality, cross-cutting relationships and faunal succession. Absolute dating uses radioactive decay, such as U-Pb, K-Ar and Rb-Sr.
- The geological time scale is divided into eons, eras, periods and epochs, for example Phanerozoic, Mesozoic and Cretaceous.
Worked example
A mineral grain contains one-eighth of its original parent isotope. Each half-life is 1.25 Ga. After one half-life one-half remains; after two, one-quarter; after three, one-eighth. Therefore the age is 3 times 1.25 Ga, or 3.75 Ga. The reasoning uses the fraction of parent remaining, not the daughter amount alone.
Traps
Do not claim that S-waves pass through the outer core; they do not. Do not confuse the Moho, which is crust-mantle, with the Gutenberg discontinuity, which is mantle-core. In dating problems, count half-lives from the parent fraction remaining and check whether the question gives parent, daughter or parent-to-daughter ratio.