  We report on the shape dynamics of ice suspended in cold fresh water and
subject to the natural convective flows generated during melting. Experiments
reveal shape motifs for increasing far-field temperature: Sharp pinnacles
directed downward at low temperatures, scalloped waves for intermediate
temperatures between 5 and $7^\circ$C, and upward pointing pinnacles at higher
temperatures. Phase-field simulations reproduce these morphologies, which are
closely linked to the anomalous density-temperature profile of liquid water.
Boundary layer flows yield pinnacles that sharpen with accelerating growth of
tip curvature while scallops emerge from a Kelvin-Helmholtz-like instability
caused by counterflowing currents that roll up to form vortex arrays. By
linking the molecular-scale effects underlying water's density anomaly to the
macro-scale flows that imprint the surface, these results show that the
morphology of melted ice is a sensitive indicator of ambient temperature.
