Q: How are geodes formed? (Golly, someplace, Michigan)

My three geodes, each lined with sparkling quartz crystals. Photo by author.

A:  A trio of geodes gleams on my desk, white crystals glittering in the light.  Geode — the name comes from the Greek (geoides), meaning earthlike, a reference to their rounded shape.  Mine are all cut in half but their outsides are tough, rounded rocks that have endured much, reminiscent of planet Earth. 

One outside part of the geode is clearly lava, another perhaps limestone.  At least, it looks all-over sandy but parts are rough and baked like those parts had been molten once:  another igneous rock. The third is the most interesting of all:  an igneous rock with white, black and bronze marbled veins going from outside to inside.  I can see the vein pattern even through the inside dark purple (amethyst) and white (quartz) crystals. 

A close up of one geode. Photo by author.

My geodes range in size from 2 to 3.5 inches (5 to 9 cm) in diameter, and that's a common size.  On May 28, 2000, however, a Spanish geologist, Javier Garcia-Guinea, tracked down a giant geode in an old silver mine in northeastern Spain, near the coast.  This geode — big enough for ten people to crawl into — is 25 feet (8 m) long and 6 feet (1.7 m) across with transparent crystals of up to 6 feet (2 m) in length:  huge protruding pyramids.  Garcia-Guinea thinks that the geode might have formed about six million years ago when the Mediterranean Ocean evaporated, leaving thick salt layers and briny fluids, which could have percolated into the geode cavity.

Now, your question:  how are geodes formed?  We don't know but we have a theory for two types.  Partly hollow balls filled mostly with quartz crystals or agate — commonly called geodes — have at least two entirely different origins, says geologist professor Gary A. Smith of the University of New Mexico.  They form in:

• lava (rhyolite) and
• dolomite, a sedimentary rock resembling limestone.

No matter what kind of rock, however, embeds the geodes originally, the rock must contain cavities for the geode's crystals to grow into, says geologist Jonathan H. Goodwin of the Illinois State Geological Survey.

So, how do the cavities form, and how do the crystals grow inside?

It depends on the original rock.  Let's consider lava and other igneous rocks first.  Long ago, as molten rock cooled, dissolved gasses formed bubbles (like dissolved air forms little bubbles in ice cubes as the water freezes).  When the lava hardened, the bubbles became cavities — the barest beginning of thundereggs.  The minerals forming the crystals could have come from hot water moving through cracks in the cooling lava rock, or later when mineralized groundwater oozed through, says Goodwin.  Either way, quartz and other minerals precipitated out of the water into the thunderegg cavity.  The crystals began to grow.

How geodes formed from dolomite is a more complicated matter.  Geologist Robert Maliva, formerly of Harvard University and now a consulting hydrogeologist, came up with a theory in 1987, which explains most geode features. 

It goes like this:  The geode cavity formed in a roundabout way  — from a hard little nodule that later dissolved away, leaving a void. The process started about 350 million years ago when limey sediments built up in warm, shallow seas that covered what is now central United States.  The salty warm waters seeped through calcite-rich limey sediment; the salt reacted with the calcite (which is an ingredient in limestone, chalk and marble), and transformed the calcite into the minerals: dolomite and anhydrite.  That's how dolomite enters the picture. 

The anhydrite (related to the gypsum used to make wallboard and plaster) is important, too:  it formed hard spherical nodules within the dolomite as the dolomite solidified from sediment into rock.  Now, the interesting part.  Water that is slightly acidic will dissolve anhydrite.  As the water percolating through the dolomite became acidic over time, the acidic, mineral-laden water dissolved the outer edges of the anhydrite nodules and replaced them with small fibrous crystals of quartz.  The interiors of the anhydrite nodules also dissolved away, "leaving a spherical void with a rind of the tiny quartz crystals," says Smith.  The pre-geode then had its cavity.

Mineral-rich water moving through the rock provided quartz (and other minerals in some cases); the crystals inside the cavity grew inward.

Later, the younger rocks eroded away, and exposed the dolomite, which then largely weathered away.  The hard quartz resisted weathering, separated from the dolomite and accumulated on the ground as geodes.

Further Reading:

Geodes — small treasure vaults in Illinois,  Illinois State Geological Survey

Geodes: a look at Iowa's state rock, Iowa State Geological Survey

Geode images, Texas Rock Shop & Lapidary

Cave of crystals, BBC News Online and FreeUK.net

(Answered June 20, 2006)