In 1991 we took a horse trip into Mt. Edziza Park, an inactive
volcano in western-central British Columbia, Canada. While we were there
we saw some egg-shaped rocks, 10-15 cm in diameter. Several had split
open to reveal an onion-layer type structure. Each layer was between 0.75
- 1 cm thick. The centre of the "egg" seemed to be made of the same rock
as the layers. There was a "clutch" of around 12 of these. We've asked a
geologist and he had no idea what they would be. Do you have any ideas? I
have a picture if you'd be interested (I'll have to get it scanned first).
Thanks for your curiosity
Richard Gaskell
Hi Richard,
What you are describing sound like geodes, sometimes called "thunder
eggs". They are a type of secondary formation that form in volcanic
places, usually long after a volcano has gone extinct. They form when
hot fluids migrate through pore spaces in rocks underground. These hot
fluids are carrying high concentrations of dissolved minerals and if
conditions are right they can start to precipitate these minerals in
pre-existing cavities. If there is a somewhat spherical cavity a portion
of this mineral-laden fluid might get trapped there and slowly start to
cool. As it cools it will first crystallize out the minerals that are the
least soluable, then the next least soluable, and finally the most
soluable minerals will crystallize. That is why the geodes have
layers--the outermost minerals crystallized first then the next then
finally the innermost ones. They usually have very nicely-formed crystal
forms that point inward. You can imagine the geode while it was still
forming. There were mineral crystals growing into the remaining fluid in
the center of the geode.
Geodes are often found in rock shops. People cut them in half and polish
the cut surface.
I guess the other possibility is that they are real dinosaur egges.
Dinosaur eggs have actually been found in very few places so if that is
the case then you have quite a find. Of course dinosaurs pretty much
went extinct about 65 million years ago so that would imply that your
volcano was active about the same time.
"Volcanoes of North America" by Chuck Wood and Jurgen Kienle says that
Edziza Peak was active from around 7.5 million years ago until about 1300
years ago so that kind of puts an end to the dinosaur egg idea.
We'd still like to see the photos. 10-15 cm is pretty good size for
geodes.
Sincerely,
Scott Rowland
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