Earth's Structure, Plate Tectonics, and Geological Processes
What to know
Earth has layers: crust (thin, rocky), mantle (solid but slowly flowing, source of magma), outer core (liquid iron–nickel, generates the magnetic field), and inner core (solid iron–nickel). The lithosphere (crust plus rigid upper mantle) is broken into tectonic plates that ride on the asthenosphere.
Plate boundaries: divergent (plates separate, mid-ocean ridges, new crust), convergent (plates collide - subduction zones, trenches, volcanoes, or mountain building), and transform (plates slide past, earthquakes). Convection in the mantle drives plate motion. Earthquakes occur where stress releases along faults; the focus is underground, the epicenter is on the surface, and P-waves arrive before S-waves. Volcanoes cluster at subduction zones and hot spots.
The rock cycle links igneous (cooled magma), sedimentary (compacted deposits), and metamorphic (heat and pressure) rocks. Weathering breaks rock; erosion transports sediment; deposition builds layers. Relative dating uses superposition (older below) and cross-cutting relationships; absolute dating uses radioactive isotopes. The fossil record and magnetic striping on the seafloor support plate tectonics and continental drift.
Traps
Questions confuse the mantle (solid but plastic) with the molten outer core, or mix up focus and epicenter. Watch for boundaries described backward - mid-ocean ridges are divergent, not convergent.
Rule of thumb and worked example
At a convergent boundary where an oceanic plate meets a continental plate, the denser oceanic plate subducts, forming a trench and volcanic arc. At a divergent boundary, magma rises and creates new crust, so the seafloor spreads. Use the boundary type to predict the feature.