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 water.
|Diver examines elongate pillowed flow lobe off the
coast of Kilauea volcano, Hawai'i. Credit: Richard D. Grigg / U.S.
AND PLANETARY SCIENCE
Source: U.S. Geological Survey