Earth's Structure and Processes
This domain covers Earth's layered interior, the plate-tectonic model that explains earthquakes, volcanoes, and mountain belts, and the surface processes that shape landforms. Items test layer properties and the seismic evidence for them, plate boundary types and their features, the rock cycle, and weathering, erosion, and deposition.
Interior layers and the evidence
- By composition: crust (oceanic basalt, ~5–10 km thick; continental granite, ~35 km), mantle (peridotite-rich silicate rock to ~2900 km), and core (iron–nickel).
- By physical behavior: rigid lithosphere (crust plus uppermost mantle, broken into plates), plastic asthenosphere (~100–350 km, flows and lubricates plate motion), solid mesosphere, liquid outer core, solid inner core.
- Seismic evidence: P waves pass through solids, liquids, and gases; S waves cannot travel through liquid, which is why they are absent beyond the 103°–143° shadow zone and why the outer core is inferred to be liquid. P-wave speed jumps at the Moho and at the core–mantle boundary.
Plate boundaries
- Divergent (mid-ocean ridges, continental rifts): new crust, shallow earthquakes, normal faulting, rift valleys; seafloor spreading recorded by magnetic stripes.
- Convergent: oceanic–continental gives a trench plus volcanic arc; oceanic–oceanic gives an island arc; continental–continental gives folded mountains with little volcanism.
- Transform: plates slide past each other, producing strike-slip faults and shallow quakes (San Andreas).
- Slab pull is the dominant driving force, assisted by ridge push and mantle convection; hotspots such as Hawaii record plate motion.
Surface processes
Weathering breaks rock in place (frost wedging, hydrolysis, dissolution); erosion transports the debris; deposition builds layers. The rock cycle links igneous, sedimentary, and metamorphic rocks through melting, cooling, burial, and uplift.
Worked example
A magnetic stripe lies 150 km from a ridge axis and is dated at 3 million years. Seafloor spreads in both directions, so that crust traveled 150 km in 3,000,000 years = 0.00005 km/yr = 5 cm/yr. This is the half-spreading rate; the full rate is 10 cm/yr.
Traps that cost marks
- Forgetting that S waves stop at the outer core, or mixing up the focus (underground) and epicenter (surface).
- Claiming continental crust is denser than oceanic crust (the reverse is true, which is why oceanic plates subduct).
- Confusing the asthenosphere with the entire mantle, or calling magma "lava" while it is underground.