Skip to comments.Geology Picture of the Week: Thunder Egg
Posted on 06/13/2006 11:01:16 AM PDT by cogitator
Thunder eggs are spherical objects which form in some types of silica-rich volcanic rocks (e.g. rhyolites). As the volcanic lava cooled, trapped steam and other gases formed an expanding bubble. Silica and feldspar minerals often crystallise around the bubble or grow crystal fibres which radiate outwards from the its centre. These mineral-filled bubbles with a radiating structure are called spherulites.
Internal gas pressure forces the spherulite apart to form a central hollow, later filled with more minerals. Adjacent wedge-shaped segments of the cracked and expanding spherule move outwards and away from each other, helping form the typical star-shaped interior. Silica gels and clays filling the cavity can later dry out, shrink and crack, producing more internal structures such as interesting patterns of mineral-filled cracks.
Later, silica-rich solutions may enter the cavity and fill it with banded agate, chalcedony, clear quartz crystals or amethyst. Solutions of different composition seep in at various times, leaving behind several layers of different minerals.
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What is the difference between a "thunder egg" and a "geode"?
You beat me to it, I think they're the same thing.
Fascinating! I am familiar with geodes but had never heard of thunder eggs.
I always thought they were the same too! Neat...
Two things: geodes are partially hollow inside, allowing the formation of crystals, rather than the more common contents of thunder eggs, various forms of amorphous silica (agate, jasper), including some with valuable opal.
Second, and perhaps to geologists more importantly, geodes form in different environments; thunder eggs always occur in volcanic rhyolites. Geodes can occur in igneous or sedimentary formations -- in sedimentary formations the outer shell is limestone or dolomite.
I knew some of this (especially the distinction between partially hollow and solid-filled), but I learned more here:
I have several of them.
Very interesting. Reminds me of "pop rocks" (NOT the candy). Perhaps someone on this thread can tell me about them.
In the red clay and sand here in West Tennessee, there are often erosions we call "gullies". Growing up, my cousins and I had all kinds of fun playing in the gullies, and we'd often find what my grandfather called "pop rocks". He said if you put them in a fire, they would burst and make a loud popping noise, because they were hollow. We never tried that, but we almost always broke them open. In the cavity, there was usually some really pretty purple sand. Sometimes it was more red than purple.
Do these have a name? How are they formed?
Thanks. I have learned something today. Good on ya'!
I always heard those referred to as "Indian Paint" as a kid...the story they told us was that they'd crack them open and use the sand for war paint. I'm in northern GA and it's red clay here also.
Cool. Never heard that one. Thanks.