A8  Saturday, October 4, 2014  Imperial Valley Press Land of Extremes

QUESTIONS? Contact Local Content Editor Richard Montenegro Brown at [email protected] or 760-337-3453.

EDITOR’S NOTE A series of stories on the history of man in our desert and the e orts of the Imperial Valley Desert Museum to tell that story will run through October, replacing the Teen page until a new crop of interns return in the fall connected to the IVHigh journalism program. CRYSTALLIZED MINERALS nature’s GEODES: treasure box A geode cut in half at the Gem and Min- eral Society building re- veals a hidden treasure. IVP FILE PHOTO

Geode A hollow concretionary or nodular stone, often lined with crystals.

BY NEAL V. HITCH Special to this Newspaper/Imperial Valley here is a reason that the most fun thing to do at the California Mid-Win- ter Fair & Fiesta is cutting open a ge- Tode at the Gem and Mineral Society building. Opening a geode is like finding a trea- sure box; you never really know what will be inside. Most people know that roundings. geodes are filled with crystal- While most geodes are lized minerals. small, they have been found Fewer people know that up to several feet in diameter. they are made by volcanoes. Geodes are found around the Geodes are some of the world, with the most concen- most recognizable of all volca- trated geode “beds” found in nic rocks. deserts. They start as a simple cavity Riverside and Imperial ABOVE: Geodes usually look like lumpy rocks until they are cut inside volcanic rock. Igneous County have what is often open. rocks form the cavities when considered the best con- LEFT: Map of Salton Butte volcano. bubbles of carbon dioxide and centration of geode sites in BELOW: A lava rock from Imperial County with hundreds of water vapor get trapped in Southern California. holes fi lled by quartz crystals. PHOTOS COURTESY OF IMPERIAL VALLEY cooling lava, like the bubbles DESERT MUSEUM in a soft drink. When the lava cools and the The volcanoes of Obsidian Butte. This outcrop of obsidian gas dissolves, an empty cavity Imperial County Lava domes are a type of sits just southeast of the is left behind. volcano that builds up by slow Salton Sea. As mineral-rich water When we think of volcanoes eruptions of highly viscous Today, it looks almost like moves through the hole in the in California, we generally lava. an island extending into the rock, tiny crystals form along think of Mount Shasta or one The viscosity of the lava, water. the inner walls of the cavity. of the other active volcanoes high in silica and rhyolite, Obsidian Butte last erupted Over thousands of years in Northern California. prevents it from flowing far in about 940 BCE (Before these crystals build up In 2012, however, the U.S. rhyolitic lava dome volcanoes: from its original vent, creat- Common Era), which makes forming large crystals, or Geological Survey elevated Mullet Island, Red Island ing a dome-like shape of lava it one of California’s youngest mini-crystals packed so tight- the Salton Buttes to active (two domes), Rock Hill and that slowly cools. volcanoes. ly together you can’t tell one status. The best known of the It became the primary from another. The Salton Buttes Imperial County source of obsidian for the The result is a rock inside of volcano actual- lava domes is southernmost part of Califor- a rock. ly consists Obsidian nia. As the softer volcanic rock of five Butte. Artifacts made from Obsid- erodes and weathers, the small ian Butte rock begin to show harder crystalline rock re- up in archaeological sites with mains. This is the geode. dates around 510 BCE. Before this it is possible that the buttes were covered The science of by water from Lake Cahuil- la. geodes Obsidian Butte would Most geode interiors are have been underwater made of quartz, chalced- when ancient Lake Ca- ony or calcite, although huilla was at its highest. agate, jasper and other The water would have minerals can also occur. needed to recede to 180 Different colors form in feet before the volcano the crystals based on the was visible as an island. different minerals in the But even so, it would water that formed them. have been about seven There is no real way of and a half miles from the knowing what is inside a lake’s shoreline. geode until it is actually The water would have cut open, although geodes needed to drop to 230 in one location will tend feet below sea level be- to have the same inte- fore Obsidian Butte was riors since the minerals connected to dry land and that formed them are the its readily resources avail- same. able. The exterior of a geode tends to be an unremark- Neal V. Hitch is director able, lumpy chalcedony of the Imperial Valley that blends into its sur- Desert Museum.