How do Geodes form?
Where do geodes actually come from? It turns out that what looks like a solid entity on the outside begins with an absence of material. A geode typically begins when a cavity forms in a rock, which can happen several ways.
Cavities are most common in igneous rock created by cooling lava or magma. Usually this happens when a bubble of carbon dioxide and water vapor forms in flows of lava. As the molten rock cools and the gas dissolves, empty space is left behind.
A cavity can also form when lava solidifies underwater. Occasionally, the outside of the melt solidifies before the inside does. It gets brittle,and the weight of liquid inside causes it to break and leak. Once the liquid has leaked out, an empty “pillow” of lava is left.
Geodes can also form in sedimentary rocks such as limestone or sandstone. The cavity in these rocks is usually formed from a solid core. A mass of minerals, or nodule, in the sediment may begin to dissolve and leave space behind. In other cases, organic matter such as coral, a fossil, or a piece of wood buried in the sediment weathers out over time.
The result of this hollowing is a shell waiting to be filled. When the geode forms, its outside crust will be older than the crystals inside. This is possible because geodes, which appear solid, are actually porous—its just that the pores are so small that you need a microscope to see them.
The next step, for both igneous and sedimentary geodes, is for mineral-rich groundwater or rainwater to seep through the porous rock and fill the cavity.
The water lines a thin crust of minerals inside of the cavity. As water flows through the geode, additional mineral layers are deposited in its hollow interior. Over thousands of years, these layers of minerals build crystals that eventually fill the cavity. How long this takes depends on the size of the geode — the largest crystals can take a million years to grow.
Which minerals end up as crystals in a geode varies by location and conditions such as temperature, acidity, and the type of rock the geode forms from. For instance, quartz crystals are most common in igneous geodes. Silica is more common in geodes that form in sediment.
Many other minerals can be found inside geodes, too. Geodes found in Brazilian quarries host soapstone and several kinds of quartz, while geodes from central Spain are filled with gold-laced hematite (the stone they use to make that magnetic jewelry). Many of these minerals, in addition to others including dolomite, calcite, and “fool’s gold,” have been found among the geodes in America’s Midwest.
Such mineral crystallization forges a rock fit to cut in half, polish and use as bookends. Not all geodes, however, are the perfect size to corral books. Some are small enough to fit in the palm of your hand or to catch the light while perched on your windowsill. At the other end of the scale, the largest gypsum crystals, from a cave in Naica, Mexico, have grown up to 36 feet long.
Where are Geodes found?
Geodes are found throughout the world, but the most concentrated areas are located in the deserts. Volcanic ash beds, or regions containing limestone, are common geode locations.
There are many easily accessible geode collecting sites in the western United States, including in California, Arizona, Utah and Nevada. The state of Iowa also has geodes, in fact, the geode is their state rock.
A geode looks like a regular spherical rock from the outside, perhaps a bit lumpy, but inside it contains mineral deposits or crystals. The outer layer of rock, usually limestone, is called the “rind.” Hollow geodes may have quartz crystals inside them. Sometimes the mineral deposits completely fill the inside; this kind of formation is referred to as a nodule.
Geodes are most easily collected when their host rocks have weathered away. This can occur because basalt, limestones, dolomites, and shales weather much more readily and rapidly than the quartz and chalcedony that typically form the outer layer of a geode. The host rock weathers away and the geodes are left on the surface, washed into a stream, or stranded in a residual soil. In these situations the geodes are easily found and collected. Some geodes are produced by mining the host rock, but that method is difficult, costly, and often damages the geode.
This Earthcache
Note that the given coordinates are a starting point. You must walk down either road to find geodes!
If you visit this earthcache location, you will find more geodes than you can shake a stick at. They are mostly about baseball size and literally cover the ground in some places. Many are cracked open, revealing their inner crystals of quartz; however, most are not. You may want to bring a hammer with you to break one open if you want to see the interior of one without the dirt and weathering. Most here are not gem show quality, but all are unique and interesting.
The host rock for these geodes is limestone, which formed at the bottom of a shallow sea. The voids created in the limestone were probably created by coral and other materials that slowly dissolved after they were deposited and buried. If you have a sharp eye, you will find fossilized coral in many places along this road.
The crystals within these geodes are quartz.
To reach this earthcache location, start in Whispering Pines at the junction of Houston Mesa Rd, and the Control Road (FR 64). Head east on the Control Road for 3.0 miles to a non-descript dirt road on your right, labelled FR 1600. You can park near this junction if you have a very low clearance vehicle, or are very averse to Arizona pinstriping. If you already have pinstriping, most cars can easily make it 0.7 miles to the first rock that may threaten your oil pan (N34° 22.176' W111° 15.183'), and where you will find parking for one or two cars. If you have a jeep or ATV, you can drive all the way to the earthcache coordinates.
You will have to go through a barbed wire gate at N34° 22.010' W111° 15.294'. From there, follow the road down the hill and to the west until you reach a fork at the earthcache coordinates. Up till now, you have not seen a single geode. At the junction, you can take either road to find geodes, but the right fork is shorter, and an easier hike. Follow this road until you see baseball-sized rocks that look like tiny human brains. These are the geodes. The farther you go down this road, the more abundant they become. At N34° 22.164' W111° 15.612', I was able to find many specimens of fossilized coral on and off the road.
If you bring a hammer to break open a geode, please bring protective eye gear, as either rock chips, or steel chips from the hammer can cause nasty damage to your eyes.
Note that the given coordinates are a starting point. You must walk down either road to find geodes!
Geodes and Petrified Coral from this Earthcache
To claim credit for this Earthcache, you must visit the site and answer the following questions:
- What is the host rock for these geodes?
- Look at the crystals inside several broken geodes. Are they all the same size? Same color? Do you think they are all the same mineral?
- Can you find any "nodules", i.e. geodes with no hollow space inside?
- About what size is the largest crystal you found inside a geode from this excursion?
- What is the range in sizes of the geodes you found compared to balls used for games and sports, e.g. "marbles to basketballs", or "golf balls to softballs".
Please go ahead and log your geocache as soon as you send your answers.