Lúdent or Lúdentarskál is an ash ring formed in phreatic explosions as water came into contact with magma in the feeder conduit causing steam explosions. The change from water into steam at 1 bar pressure increases the volume more than a thousand times. Basaltic magma is of low viscosity and as a rule dissolved gases (including steam) are released rather gently during decompression of the upflowing magma as lava fountains. Lúdent is among the oldest eruptive units of Heiðarsporðar. There must have been a lake at the site when the eruption occurred or rock of high permeability and water table. Craters such as Lúdent occur primarily in shallow water. Surtsey is an example. Eruptions of this type are called surtseyan.