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firefly5 March 13th, 2015 6:21am

Science Friday, the geode edition, as requested by Zod (1/3): Geodes form, with very few exceptions, almost exclusively in sedimentary rock.

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PartyFree Nowhere in Particular
03/13/15 12:35 am

The first thing I think of when I think of geodes is volcanoes, which means igneous rock. I knew this had to be false.

Laserbeam Crazy bird lady
03/12/15 11:25 pm

Wait, igneous rocks. Yeah. Right? Geology class was a long time ago lol

firefly5 the verse
03/12/15 11:33 pm

Usually igneous or sedimentary, though metamorphic can be possible.

Praetorianus Fair enough.
03/12/15 11:24 pm

I'd say no, it's volcanic rock. It needs great pressure and heat to form them.

Wert A picture of my junk
03/12/15 11:28 pm

Actually, they form closer to a process like rock candy.

firefly5 the verse
03/12/15 11:29 pm

It's true. It actually is a lot like making rock candy.

firefly5 the verse
03/12/15 11:23 pm


Geodes (in case you're wondering, they're the rocks that look like plain rocks on the outside, but have shiny crystals on the inside) form in both sedimentary (rocks formed from particles deposited by water or air) and

firefly5 the verse
03/12/15 11:29 pm

igneous (rocks formed from lava) rock. While it's possible for geodes to form in certain metamorphic rocks (rocks made from other rocks due to heat/pressure/etc.), crystals formed within metamorphic rocks tend to be quite small.

Ru12bseen Ignoring the Ignorant
03/13/15 5:20 am

I object. Do they form in sediment and lava equally? Or do they form in sediment more often? 😏

firefly5 the verse
03/13/15 6:54 am

Both are pottery conducive to it, as all they really need is porous rock with a cavity. Since sedimentary rock is more common, I imagine more geodes are sedimentary geodes.

Ru12bseen Ignoring the Ignorant
03/13/15 7:15 am

My objection stems from syntax of your question. It's essentially asking which is more common. I guessed sedimentary.

firefly5 the verse
03/13/15 7:57 am

I thought I had distinguished it well enough. But it was 3am, so I thought a lot of things.

handshown The Midwest West Coast
03/14/15 6:28 pm

I had a small pile of geodes in my back yard creek. So at one point my creek was hot and pressurized?

firefly5 the verse
03/14/15 9:57 pm

Could be. That, or there were small animals/nests/etc. encased in stone, which, if the stream has been there long enough, is totally possible.