If you look at lava flows — such as in the Lava Beds National Monument or along highways 140 or 66 east of the Bear Creek Valley — you might notice little round holes in some flows, especially near their tops.
The holes, varying in number, size and shape, are called vesicles and may be totally or partially filled by secondary minerals.
Though created by gas, vesicles aren't caused by eating too many bean burritos. Magma (liquid rock) rising from depth, where the pressure is much greater than on the Earth's surface, holds gas as a dissolved phase within the magma, much like caps on pop bottles hold dissolved carbon dioxide under pressure in soda. As magma rises, the confining pressure progressively decreases and gas comes out of solution, expanding like the belly of a middle-aged desk jockey.
If the gas pressure is immense, an explosion results, scattering magma fragments as volcanic ash or as larger lava "bombs." It's analogous to shaking a pop bottle then giving it to an unsuspecting victim to open.
If the lava freezes before the gas can escape, it will be trapped much like the "vesicles" on the top of gassy cow pies (come on, we've all seen if not stepped in them).
Trapped gases produce spherical or irregular shapes depending on the type of gas and its ability to push on the hot, sticky magma. Numerous irregular holes, common in High Cascade lavas, form a closely packed, sieve-like texture called diktytaxitic (the term rolls easily off the tongue, doesn't it?).
Minerals may precipitate from the gas into vesicles as the lava cools. Through time, liquids may seep through the rock, partially or totally filling vesicles to form amygdules that look like tiny white eggs. Seeping liquids also may fill open cracks in rocks to form thin veins. Common fillings are chalcedony (agate), quartz, calcite and zeolites (soft white minerals). Variable colors are due to different impurities carried by water percolating through the rock.
The Oregon state rock, the "Thunder Egg," forms in large vesicles and, being harder than the rock that surrounds it, often remains once the lava weathers and crumbles away, littering the landscape with treasures. Though much smaller than Thunder Eggs, agates in the Agate Desert near White City or Agate Flat in the Cascade-Siskiyou National Monument formed in such a manner. Thunder eggs were named by Central Oregon Native Americans who believed they were thrown during thunderstorms by quarrelsome Thunder Spirits of Mount Hood and Mount Jefferson, a situation rather analogous to our present political environment.
Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at firstname.lastname@example.org.