The size and temperature of the keyhole and melt pool on the top surface of a metal plate being welded with a “keyhole” has been calculated, based on the power loss resulting from evaporation of an element in the chemistry of the plate and a minimum power density required to produce a keyhole. The evaporative power loss is set equal to the Gaussian power density, which is used to represent the laser beam. The result is a temperature distribution across the keyhole. The results indicate that peak powers below the minimum peak power density yield temperature that are indicative of melting only: peak powers equal to a minimum power density yield temperatures that reach the evaporation temperature of the element; and that peak powers greater that the minimum power density yield temperatures greater than the evaporation temperature of the element. The results will be compared to some visual observations of the laser melt pool.