Mounds of elongate lava pillows
formed by repeated oozing and quenching of hot basalt
. First, a flexible glassy crust
forms around the newly extruded lava
, forming an expanded pillow. Next, pressure builds until the crust breaks and new basalt extrudes like toothpaste, forming another pillow. This sequence continues until a thick sequence may be deposited. When geologists find pillow basalts
in ancient rock sequences, they may conclude that the area was once under water.