Earth Materials, Structure, and Plate Tectonics
Minerals and the rock cycle
A mineral is naturally occurring, inorganic, solid, with a definite composition and an ordered internal structure - glass fails that last test. Know Mohs hardness (talc 1, quartz 7, diamond 10; quartz scratches glass), cleavage versus fracture, streak, luster, and the silica tetrahedron as the building block of most crustal minerals. Rock textures record origin: igneous textures record cooling rate (phaneritic = slow, intrusive; aphanitic = fast, extrusive; porphyritic = two stages; vesicular or glassy = rapid cooling or gas-rich). Sedimentary rocks record transport and deposition (sorting, rounding, cross-bedding, ripple marks, graded beds). Metamorphic rocks record pressure and temperature (foliation, index minerals, grade).
Interior layers and seismic evidence
Separate compositional layers (crust, mantle, core) from mechanical layers (lithosphere, asthenosphere, mesosphere, outer and inner core). The lithosphere is crust plus uppermost mantle, broken into plates; the asthenosphere below is solid but ductile and flows. P waves (compressional, fastest, travel through solids and fluids) arrive before S waves (shear, solids only) - the S-wave shadow is the evidence that the outer core is liquid. Surface waves are slowest and most destructive. At subduction zones, earthquake foci deepen inland along the Wadati–Benioff zone.
Boundaries and traps
Divergent boundaries create crust (ridges, normal faults, shallow quakes); convergent boundaries destroy or thicken crust (subduction with composite volcanoes and deep quakes, or continental collision producing fold mountains); transform boundaries slide past with strike-slip faulting and no crust created. Hot-spot chains record plate speed and direction.
Traps: equating lithosphere with crust; calling the mantle liquid; confusing focus (where slip occurs) with epicenter (surface point above it); confusing magnitude with intensity. Rule of thumb: oceanic crust is denser than continental crust, so ocean–continent convergence always subducts the oceanic plate; continent–continent collision subducts neither.