The lake lies in a volcanic depression lined with a very fine covering of volcanic ash a few feet deep which has compacted to become impervious to water.

Such eruptions usually form calderas, broad volcanic depressions created as the ground surface collapses as a result of withdrawal of partially molten rock (magma) below.

This fissure maybe a patera, or volcanic depression, in the process of forming.

Two volcanic depressions, known as paterae, lie at northern and southern ends of mountain.

Intra-patera eruptions occur within volcanic depressions known as paterae, which generally have flat floors bounded by steep walls.

A more powerful light shone through the upper crater, shedding a vague glimmer over these volcanic depressions for ever buried in the bosom of this extinguished mountain.

Caldera: a volcanic depression resulting from collapse following a volcanic eruption.

Pit crater: a volcanic depression smaller than a caldera formed by a sinking, or caving in, of the ground surface lying over a void.

Their roughly circular shapes and steep-sided rims are typical of volcanic depressions.

Io's surface is dotted with volcanic depressions known as paterae.