Contact & Compression (milliseconds): the impactor contacts the target, launching a hemispherical shock wave with pressures up to 10³ GPa. At >50 GPa, rocks melt; at >100 GPa, vaporize. Excavation (seconds to minutes): the shock wave opens a transient crater; an excavation flow carries ejecta outward in a conical curtain. Simple-to-complex transition at ~2–4 km on Earth. Modification (minutes to hours): gravity collapses the transient cavity; complex craters develop central peaks (peak-ring rebound), slump terraces, and melt sheets. Scaling law: D_final ≈ 1.17 × (E/ρ_t)^(1/3.22) × g^(-0.22) (Pi-group scaling). The Chicxulub impactor (~12 km, ~20 km/s) released ~10⁸ megatons.