Plate 121

Deformation by impacting objects: bomb sags

Pieces of lava ejected by volcanoes (cinder) may spin and assume a spindlelike shape during their flight; a solid crust starts to envelop them. These volcanic bombs,  upon landing on fine-grained, compressible materials like ash or mud, sink and leave more or less deep imprints, called bomb sags (A).

If the lava is still plastic or semifluid when it impacts, the sag is shallow and the bomb squeezes and deforms like a liquid, with a curious affinity to dung (B).

Quaternary pyroclastic deposits of Lipari, Eolian Islands (A) and Procida (B), Tyrrhenian Sea.