How are geodes formed? | Earth | EarthSky

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By EarthSky in
FAQs | Earth on Oct 01, 2009

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** How are geodes formed? **

Photo Credit: oldbonesPhoto Credit: oldbones
Both sedimentary and volcanic geodes need just the right combination of
water, chemicals, and pressure to produce the crystals inside.



You see geodes displayed in gift shops, sliced down the middle like a
melon. They’re dull on the outside, but hollow and glittering with
crystals on the inside.

Volcanic geodes are formed by cooling lava. Sedimentary geodes are formed
underwater, possibly when sediments collect around the lifeless body of a
sea animal. When the animal decays, it creates a cavity.

Both sedimentary and volcanic geodes need just the right combination of
water, chemicals, and pressure to produce the crystals inside.

One way to open a geode – to reveal the crystals – is with a chisel and
hammer. Score the geode completely around the outside where you want it to
crack – usually in two equal halves. Keep going until it cracks and
breaks apart. This will almost always work and won’t damage the crystals.

Trickier than opening a geode is recognizing one outdoors. Uncut, they look
like your average rock. It helps to know where to look. You want a
geodiferous outcrop of rocks. With practice, you can learn to identify a
geode. They’re usually round or egg-shaped, and the weathered ones look
like cauliflower


Source: earthsky.org/earth/how-are-geodes-formed


how are geodes formed


Geode - Wikipedia, the free encyclopedia

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** Geode **

From Wikipedia, the free encyclopedia
Jump to: navigation, search
For other uses, see Geode (disambiguation).
Inside and outside views of the same geode
Geodes in unusual rock formation
Large hollow geode at the Carefree Resort in Carefree, Arizona. Booklet
shown for scale is 8.5 by 11 inches.

*Geodes* (Greek /γεώδης - ge-ōdēs/, "earthlike")
are geological secondary structures which occur in certain sedimentary and
volcanic rocks. Geodes are essentially hollow, spherical to oblate masses
of mineral matter that may form from either the filling of vesicles (gas
bubbles) in volcanic to sub-volcanic rocks by minerals deposited from
hydrothermal fluids, or by the dissolution of sedimentary nodules or
concretions (that were deposited syngenetically within the rock formations
they are found in) and partial filling by the same or other minerals
precipitated from diagenetic or hydrothermal fluids. Geodes differ from
vugs in that the former were formed as early, rounded, structures within
the surrounding rock, whereas vugs are irregular voids or cavities within a
cross-cutting formation, usually a vein or breccia. Geodes also differ from
"nodules" in that a nodule is a mass of mineral matter that has accreted
around the nodule nucleus. Both structures had the minerals contained
within, deposited from groundwater or hydrothermal processes. Geodes
commonly have a chalcedony (cryptocrystalline quartz) shell lined
internally by various minerals, often as crystals, particularly calcite,
pyrite, kaolinite, sphalerite, millerite, barite, dolomite, limonite,
smithsonite and quartz, which is by far the most common and abundant
mineral found in geodes. Geodes are found mostly in basaltic lavas and
limestones. The Warsaw Formation in the Keokuk region near the area where
Missouri, Iowa, and Illinois join contains abundant geodes.

*Contents*

· 1 Formation
· 2 Coloration
· 3 Prevalence
· 4 Gallery
· 5 See also


Source: en.wikipedia.org/wiki/Geode

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