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We present multi-epoch photometry and spectroscopy of a light echo from etaCarinae's 19th century Great Eruption. This echo shows a steady decline over adecade, sampling the 1850s plateau of the eruption. Spectra show the bulkoutflow speed increasing from 150 km/s at early times, up to 600 km/s in theplateau. Later phases also develop remarkably broad emission wings indicatingmass accelerated to more than 10,000 km/s. Together with other clues, thisprovides direct evidence for an explosive ejection. This is accompanied by atransition from narrow absorption lines to emission lines, often with broad PCygni profiles. These changes imply that the pre-1845 luminosity spikes aredistinct from the 1850s plateau. The key reason for this change may be thatshock interaction dominates the plateau. The spectral evolution of eta Carclosely resembles that of UGC2773-OT, which had clear signatures of shockinteraction. We propose a 2-stage scenario for eta Car's eruption: (1) a slowoutflow in the decades before the eruption, driven by binary interaction thatproduced a dense equatorial outflow, followed by (2) explosive energy injectionthat drove CSM interaction, powering the plateau and sweeping slower CSM into afast shell that became the Homunculus. We discuss how this sequence could arisefrom a stellar merger in a triple system, leaving the eccentric binary seentoday. This gives a self-consistent scenario that may explain interactingtransients across a wide range of initial mass.

Light-echoes from the plateau in Eta Carinae's Great Eruption reveal a two-stage shock-powered event