A8  Saturday, October 18, 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. THANKS TO VOLCANIC ERUPTIONS Volcanoes as a resource, today and yesterday

Minerals change the color of obsidian. The black obsidian (right) is from the Salton Buttes while the mahogany obsidian (left) comes from Red Hill, Ariz. PHOTO COURTESY OF IMPERIAL VALLEY DESERT MUSEUM

BY NEAL V. HITCH Special to this Newspaper/Imperial Valley f we think of volcanoes as a resource today, we might think of geother- Imal energy. But for thou- sands of years volcanoes were a resource for tool making. Volcanic rocks, or igneous rocks cre- ated when magma erupts, are the most common rocks on Earth. Rhyolite is a light-colored, volcanic rock that en- compasses many kinds of rocks, such as pumice, obsidian, and wonder stone. The volcano at Obsidian Butte near what is now the Salton Sea last erupted in 940 BCE creating valuable resources for Native Americans. Areas around Ancient Lake Cahuilla were known as important sources of rhyolite and were actively quarried. Rhyolite is formed as lava flows and cools. Different kinds of minerals in the magma create different kinds of rhyolite. Two types that were used extensively for making tools in the Imperial Valley are obsidian and wonder stone. High in silica, when fractured, they create a sharp edge that is perfect for knives and projectile points. It is only thanks to volcanic erup- ABOVE: Obsidian points come in a variety of sizes, and can be fi nely crafted de- tions, which we normally think of as a spite the brittle nature of obsidian. destructive force, that these tools could LEFT: Obsidian Butte, today called Salton Buttes, last erupted in 940 BCE. The be created. makeup of the lava meant the area became a resource for rhyolite and obsid- ian. BELOW: Wonderstone was quarried near Lake Cahuilla. Its conchoidal fractures The science of make it ideal for use as knives or projectile points. PHOTOS COURTESY OF IMPERIAL obsidian VALLEY DESERT MUSEUM Obsidian is one of the most recog- extremely important to know quarry location in Southern nizable of all the rhyolitic rocks. It is a where the artifact is from California. The geologic naturally occurring volcanic glass, or and the environment to signature of the stone lava that cools faster than minerals can which it was exposed. found at this site al- crystallize. low archaeologists to Because it is not made of mineral track the distribution crystals, obsidian is technically not a Wondering about of Rainbow Rock miles from its original source location rock at all, but a congealed liquid made Quarry stones across along trade routes. wonder stone of at least 70 percent silica. It is the ice trade and travel cube of the rock world! Wonder stone is a routes. Obsidian has no natural plane of sep- Obsidian hydration dating name for a finely Neal V. Hitch is aration. When broken, it breaks into a grained, banded director of the Im- “conchoidal fracture,” just like glass in Over time, obsidian absorbs water rhyolite. This type perial Valley Des- a window. and can be dated in a scientific process of rhyolite is hy- ert Museum. This creates an incredibly sharp edge, known as obsidian hydration dating. drothermally al- making obsidian a valued resource The water absorption occurs at the sur- tered so that min- throughout human history. face and creates a narrow rind. erals, like iron and Obsidian has been used for every- A small slice cut out of the artifact manganese, form thing from knife blades and spear and placed under a high-powered mi- bands of colors in the points to highly polished mirrors, and croscope allows a scientist to measure rock. Wonder stone jewelry. the rind and determine a date range for has 69 percent silica, The earliest known obsidian tools are the artifact. The idea is simple: the lon- less than obsidian, but still nearly two million years old from a site ger the artifact has been exposed to the enough to allow for conchoidal in eastern Africa. Even today, obsidian air, the thicker the hydration rind. fractures that make it nearly as sharp scalpels are used by doctors in special- Today, hydration dating is used ex- as obsidian. ized surgeries. tensively by archaeologists to date ob- The Imperial Valley contains a In an archaeological context, obsid- sidian artifacts at prehistoric sites both quarry of wonder stone (also called ian is also important. Every volcano in California and around the world. Rainbow Rock Quarry) near the Salton has a unique combination of trace ele- Though highly useful, the technique is Sea. It is one of the largest and most in- ments, making it possible for archaeol- not perfect. Artifacts exposed to high- tensively used quarries in the Colorado ogists to trace the origins of an obsidi- er temperatures or lower elevations Desert. Excavated by Morlin Childers, an artifact’s raw material — no matter hydrate at different rates. When the Erlinda Burton, and a field class from where it is found in the world. environment of the artifact is known, IVC in 1992, this site proved to have Obsidian was highly valued as a trade scientists can take these factors into an enormous number of cores, flakes, item and can be tracked hundreds of account during dating. Therefore, it is and tools suggesting it was a major