vesicles & amygdules

The Dictionary of Geologic Terms (R. Bates & J. Jackson, eds) defines vesicle as "a small cavity in an aphanitic or glassy igneous rock, formed by expansion of a bubble of gas or steam during solidification of the rock." Such a rock is said to be vesicular. Only igneous rocks - rocks that cooled from a molten magma - can have vesicles. Very few (less than 1 in 1000) meteorites have interior vesicles because the interor of most meteorites was never molten. Many terrestrial rocks have vesicles, however. Also, almost every sample of industrial slag that we've seen has vesicles. Vesicles and metal together in the same "rock" are a good field mark for slag.

Pictured here is a vesicular basalt from Hawaii. The field of view is about 6 cm.

The surface at the top of the photo is where the molten lava was exposed to air. The exposed portion cooled quickly, leaving a glassy, shiny surface. The surface somewhat resembles a meteorite fusion crust. Meteorite fusion crusts are usually smoother than this, however. Also, you can see the circular shapes of broken gas bubbles in the crust of this rock; such features are very rare in meteorite fusion crusts. Click on image for enlargement.
This is also a vesicular basalt. As on the photo above, the crust on top is where the molten lava was exposed to air. Basalts come in a variety of colors, mostly gray or black to rust.
A highly vesicular basalt from Hawaii. Highly vesicular volcanic rock is also known as pumice.
Vesicular rocks occur all over the world. This one is from Australia.
(Photo by Max McCosker)


An amygdule is "a gas cavity or vesicle in an igneous rock which is filled with such secondary minerals as zeolites, calcite, quartz, or chalcedony." Such a rock is said to be amygdaloidal or amygdular. Amygdules form when fluids containing dissolved minerals flow through the rocks and deposit the minerals as solids in the vesicles. Lunar basalts are not amygdaloidal because the Moon is so dry that there are no fluids (and, apparently, there never were).

Basalts are the rocks that form when volcanic lava or magma cools. Not all basalts are vesicular, but vesicular basalts are very common on Earth.

Bottom line: If you have a vesicular rock it's probably not a meteorite. Such rocks are very common on Earth but are exceedingly rare among meteorites.

vesicles in meteorites

vesicles in meteorite fusion crusts

Prepared by:

Randy L. Korotev

Department of Earth and Planetary Sciences
Washington University in St. Louis

Please don't contact me about the meteorite you think you’ve found until you read this and this.