Free Republic
Browse · Search
General/Chat
Topics · Post Article

Skip to comments.

Geology Picture of the Week: Thunder Egg
Stone Museum of Geology ^

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.


TOPICS: Arts/Photography; Education; Outdoors; Science; Travel
KEYWORDS: collecting; concretion; crystals; gases; rhyolite
I wanted to show a picture of the interior and exterior of a thunder egg; most pictures on the Web show only the polished interior. The picture below is an example:


1 posted on 06/13/2006 11:01:23 AM PDT by cogitator
[ Post Reply | Private Reply | View Replies]

To: 2Trievers; headsonpikes; Pokey78; Lil'freeper; epsjr; sauropod; kayak; Miss Marple; CPT Clay; ...

** ping **


2 posted on 06/13/2006 11:02:20 AM PDT by cogitator
[ Post Reply | Private Reply | To 1 | View Replies]

To: cogitator

What is the difference between a "thunder egg" and a "geode"?


3 posted on 06/13/2006 11:08:12 AM PDT by Jemian (PAM of JT)
[ Post Reply | Private Reply | To 1 | View Replies]

To: Jemian

You beat me to it, I think they're the same thing.


4 posted on 06/13/2006 11:09:19 AM PDT by conservativewasp (Liberals lie for sport and hate our country.)
[ Post Reply | Private Reply | To 3 | View Replies]

To: cogitator

Fascinating! I am familiar with geodes but had never heard of thunder eggs.


5 posted on 06/13/2006 11:13:14 AM PDT by kayak (Praying for MozartLover's son, Jemian's son, all our military, and our President every day!)
[ Post Reply | Private Reply | To 2 | View Replies]

To: kayak

I always thought they were the same too! Neat...


6 posted on 06/13/2006 11:38:26 AM PDT by geezerwheezer (get up boys, we're burnin' daylight!!!)
[ Post Reply | Private Reply | To 5 | View Replies]

To: Jemian; conservativewasp; geezerwheezer
What is the difference between a "thunder egg" and a "geode"?

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:

Concretions, thunder eggs, and geodes

7 posted on 06/13/2006 11:47:15 AM PDT by cogitator
[ Post Reply | Private Reply | To 3 | View Replies]

To: cogitator; Xenalyte; Miss Behave; theDentist; Gabz
And we were just disgusting whether or not we'd ever want to see Natlie and the other Dixie Chicks lay "thunder eggs" on stage in their latest tour!
8 posted on 06/13/2006 11:53:17 AM PDT by Robert A. Cook, PE (I can only donate monthly, but Hillary's ABBCNNBCBS continue to lie every day!)
[ Post Reply | Private Reply | To 1 | View Replies]

To: cogitator
I knew an old geologist that used the term Vugh when referring to geodes.
9 posted on 06/13/2006 11:58:06 AM PDT by DocRock
[ Post Reply | Private Reply | To 7 | View Replies]

To: cogitator; Lil'freeper

I have several of them.


10 posted on 06/13/2006 1:46:43 PM PDT by sauropod ("Heaven on my left, Hell on my right and the Angel of Death behind me" - Dune)
[ Post Reply | Private Reply | To 1 | View Replies]

To: sauropod
No comment.

:)

11 posted on 06/13/2006 1:52:31 PM PDT by Lil'freeper (You do not have the plug-in required to view this tagline.)
[ Post Reply | Private Reply | To 10 | View Replies]

To: cogitator

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?


12 posted on 06/13/2006 1:58:30 PM PDT by HeadOn (Life is hard. It's harder if you're stupid.)
[ Post Reply | Private Reply | To 1 | View Replies]

To: cogitator

Thanks. I have learned something today. Good on ya'!


13 posted on 06/13/2006 2:19:58 PM PDT by Jemian (PAM of JT)
[ Post Reply | Private Reply | To 7 | View Replies]

To: HeadOn

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.


14 posted on 06/13/2006 3:23:20 PM PDT by Fire_on_High (I am so proud of what we were...)
[ Post Reply | Private Reply | To 12 | View Replies]

To: Fire_on_High

Cool. Never heard that one. Thanks.


15 posted on 06/14/2006 7:48:36 AM PDT by HeadOn (Life is hard. It's harder if you're stupid.)
[ Post Reply | Private Reply | To 14 | View Replies]

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.

Free Republic
Browse · Search
General/Chat
Topics · Post Article

FreeRepublic, LLC, PO BOX 9771, FRESNO, CA 93794
FreeRepublic.com is powered by software copyright 2000-2008 John Robinson