When basalts erupt underwater, they commonly
form pillow lavas, which are mounds of elongate
lava "pillows" formed by repeated oozing and quenching of the 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
|Diver examines elongate pillowed flow lobe off the
coast of Kilauea volcano, Hawaii. Credit: Richard D. Grigg / U.S.
|When a lava flow enters the sea it breaks up into
a number of globular structures which may come to rest piled on top
of one another. This is known as pillow lava. The same formation can
occur if magma is expelled directly into the sea.
AND PLANETARY SCIENCE
Source: U.S. Geological