General Geology and Earth Materials
What to know
The Earth is described two ways: compositionally (crust, mantle, core) and mechanically (lithosphere, asthenosphere, mesosphere, outer core, inner core). The Mohorovičić discontinuity is the crust–mantle boundary, but the lithosphere–asthenosphere boundary is a strength/creep transition that generally lies within the mantle. Plate tectonics unifies seismicity, volcanism, and topography: divergent boundaries give normal faulting and new crust, convergent boundaries give thrust/reverse faulting and subduction, transform boundaries give strike-slip faulting. Isostasy (Airy and Pratt models) explains why thick, low-density crust stands high.
Relative dating rests on superposition, original horizontality, lateral continuity, cross-cutting relationships, inclusions, and faunal succession. Absolute dating rests on radioactive decay, where the fraction of parent remaining is f = (1/2)^n with n = number of half-lives elapsed.
Traps
Equating lithosphere with crust; assuming the asthenosphere is molten (it is solid but weak); mixing half-lives among isotopic systems (14C ≈ 5,730 yr; 40K ≈ 1.25 Ga; 238U ≈ 4.47 Ga; 87Rb ≈ 48.8 Ga); confusing rank of era/period/epoch; treating a mineral date as a rock-formation age without considering closure temperature.
Rule of thumb
Count half-lives from the surviving parent fraction, not from the daughter. If 12.5% parent remains, that is 1/2, 1/4, 1/8 - three half-lives. A granite with 1/8 of its 40K remaining is about 3.75 Ga old only if the system stayed closed.