Resources

What are thunder eggs?  According to Indian lore, the spirits of the various volcanoes  would become angry with each other and throw hot volcanic stones back and forth - hence, thunder eggs.  Thunder eggs are volcanic in nature.  Their usual make-up includes a volcanic matrix, usually a form of rhyolite (an igneous rock), and an inner core of agate, quartz, zeolite, jasper, opal, or a combination of these.  The colors of the different eggs are due to the various elements present at the time of their formation.  For instance, a high concentration of iron would produce a reddish color, and so on.

How are they formed?  No one really knows for sure.  This may sound surprising, but no one has really ever seen a thunder egg in the process of forming.  If you search the web, you will find many and varied explanations and theories as to their origin, but no one really knows for sure.  My opinion?  If you look closely at the USGS map of Northwest volcanoes below, you will notice that there are a lot of volcanoes!  And it is clear from research that they erupted many different times in the past.  I am guessing that the various eruptions produced molten globules filled with liquid silica and water.  The silica content by the way, is what allows these stones to be polished.  That is why agate polishes so beautifully.  As these "globs" cooled, the various center cores hardened into agate and other material.  These "globs" were either projectiles propelled with the various eruptions, or were carried here and there by way of volcanic flows.  The Columbia Plateau, for example, appears to be made up of many such flows in some cases hundreds of feet thick.  One thing is certain, however, things were a whole lot different in the past than they are today!  The kind of eruptions on the scale in the past that is apparent are certainly not going on today.  It would have taken catastrophes on a very grand scale to produce the features present today in the Northwest.  Look at the thunder egg photos below and see if you can spot the various minerals.  In order from left to right - rhyolite with an agate core of deep blue, rhyolite with a mixed agate/quartz core, rhyolite with a quartz hollow center, and the last photo is of rhyolite matrix and blue opal cores.  Notice the difference in rhyolite colors, too.

The Northwest is home to an active volcanic landscape.  And although no one has ever seen thunder eggs form, it is clear from their composition that they are volcanic in nature.  The differences from thunder egg site to thunder egg site suggests that at several times in the past various eruptions of the volcanoes listed on the USGS map at the left produced different kinds of flows which carried slightly different chemical compositions.  The many and varied thunder eggs found in this area really are distinct from one site to the next. 

Are thunder eggs unique to the Northwest United States?  No.  Thunder eggs are found all over the world - but only where there has been evidence of some type of volcanic activity such as an eruption or volcanic flow.  Below are examples of thunder eggs from Australia, Mexico, Poland and Germany.

Maps courtesy of the Unites States Geological Survey