We present multi-epoch photometry and spectroscopy of a light echo from eta Carinae's 19th century Great Eruption. This echo's light curve shows a steady decline over a decade, sampling the 1850s plateau of the eruption. Spectra show the bulk outflow speed increasing from similar to 150 km s(-1) at early times, up to similar to 600 km s(-1) in the plateau. Later phases also develop remarkably broad emission wings indicating mass accelerated to more than 10 000 km s(-1). Together with other clues, this provides direct evidence for an explosive ejection. This is accompanied by a transition from a narrow absorption line spectrum to emission lines, often with broad or asymmetric P Cygni profiles. These changes imply that the pre-1845 luminosity spikes are distinct from the 1850s plateau. The key reason for this change may be that shock interaction with circumstellar material (CSM) dominates the plateau. The spectral evolution of eta Car closely resembles that of the decade-long eruption of UGC 2773-OT, which had clear signatures of shock interaction. We propose a two-stage scenario for eta Car's eruption: (1) a slow outflow in the decades before the eruption, probably driven by binary interaction that produced a dense equatorial outflow, followed by (2) explosive energy injection that drove CSM interaction, powering the plateau and sweeping slower CSM into a fast shell that became the Homunculus. We discuss how this sequence could arise from a stellar merger in a triple system, leaving behind the eccentric binary seen today. This gives a self-consistent scenario that may explain interacting transients across a wide range of initial mass.