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We present multi-epoch photometry and spectroscopy of a light echo from η 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 ∼150 km s⁻¹ at early times, up 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 …