  We present multi-epoch photometry and spectroscopy of a light echo from eta
Carinae's 19th century Great Eruption. This echo shows a steady decline over a
decade, sampling the 1850s plateau of the eruption. Spectra show the bulk
outflow speed increasing from 150 km/s at early times, up to 600 km/s in the
plateau. Later phases also develop remarkably broad emission wings indicating
mass accelerated to more than 10,000 km/s. Together with other clues, this
provides direct evidence for an explosive ejection. This is accompanied by a
transition from narrow absorption lines to emission lines, often with broad 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 dominates the plateau. The spectral evolution of eta Car
closely resembles that of UGC2773-OT, which had clear signatures of shock
interaction. We propose a 2-stage scenario for eta Car's eruption: (1) a slow
outflow in the decades before the eruption, 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 the eccentric binary seen
today. This gives a self-consistent scenario that may explain interacting
transients across a wide range of initial mass.
