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Oregon Thundereggs
How Thundereggs Form A Thunderegg is not actually a rock. It is a structure, sometimes a nodule, sometimes a geode, occurring in rhyolite, welded tuff, or perlitic rocks. However, without question, the Thunderegg is by far the most popular "rock" in Oregon. Scientists do not agree on the processes forming Thundereggs. Some insist that the characteristic and unique internal pattern of typical Thundereggs is due to expansion and rupture of rock by gases. Others claim the pattern is due to desiccation (drying) of a colloid or gel. Whatever the process, after the cavity that contains the egg is formed, further development is extremely variable in the amount of time needed to complete the egg, degree and type of infilling, and physical characteristics. Thundereggs range in size and weight from less than an inch and under one ounce to over a yard in diameter and over a ton in weight. Most eggs collected are between two and six inches in diameter. How Thundereggs Look Typically, an egg has a russet-colored outer shell that is often knobby and often has a characteristic ribbed pattern. Frequently, the inside of the outer shell has a relatively thin intermediate or transitional lining. This is sometimes composed of an iron or manganese compound, often with a thin coating of opal or chalcedony. Sometimes only opal or chalcedony is apparent. Finally, the center of an egg is usually filled with chalcedony or opal and may or may not have inclusions, pattern growth, or crystals. In some variants, the egg may be hollow or may have a thin layer of chalcedony coating the interior. This layer sometimes is topped with a coating of small quartz crystals. Growths of algalike tubes, or plumes, or "moss" of manganese or iron compounds or of clay may be free standing or partially or wholly embedded in chalcedony. Some eggs with plumes ("flowers") in chalcedony are among the most valuable specimens. Several zeolites have been observed or reported in Thundereggs; clinoptilolite is fairly common, and mordenite, natrolite, and mesolite have also been reported. Thundereggs are sometimes found with fortification banding just inside the shell, then an area of horizontal layering, with the remaining central area filled with clear chalcedony or inward-pointing quartz crystals. Banding and layering vary in color, thickness, and content. Some layers are composed of a fibrous cristobalite (lussatite). Other eggs have a partial botryoidal filling of an opal form of low cristobalite. This opal is often fluorescent because of a low content of uranium salts. One collecting site in Oregon has eggs filled with carnelian. At another, the filling may contain cinnabar, which colors it pastel to intense red. Some eggs are filled with pastel jaspers. Others may have any one of a variety of opal fillings that may be opaque blue, opaque red, translucent pastel blue, translucent yellow, translucent red, white, or colorless. Some of the opal can be faceted, and a small percentage is true precious opal. Some eggs have well-developed calcite crystals encased in chalcedony, and others contain pseudomorphs of chalcedony after calcite. Some eggs have layering that is fanned from one edge, because the egg was rotated by earth movement while the filling was being deposited. This and other features suggest that the complete development of some eggs may have taken considerable time, and the filling-in of the egg may have recorded a series of geologic events. Some eggs contain brecciated rock fragments, while others show faulting, offset, and healing. One of the most unusual Thunderegg variants is up to 3 feet long and 2 to 3 inches in diameter and looks much like a fat gray worm. In some areas, it is common to find the characteristic chalcedony core weathered out of its shell. If a complete egg is sawed in the right orientation, one or more conduits through which filling materials flowed may be found. The beauty and complexities of many of the cut and polished eggs explain why Oregon rockhounds have long been fascinated by Thundereggs. Where to Find Thundereggs in Oregon Thundereggs can be collected at many sites in Oregon. Some
localities occur in beautiful forested hill country, others in
dry, desertlike terrain. Some are "free sites," while others
are "fee sites." As Thundereggs have been collected in Oregon
for fifty years, collectors on "free sites" must expect to dig
and work for the eggs. Proper equipment, including shovel,
pick, and bar, makes the job much easier. The "fee site" will
almost always have some preparatory work (overburden removal)
done. Eggs often may be purchased and equipment rented at the
site office. Conditions change, so collectors should contact
sites for current fee status and appropriate authorities for
permission to dig. Madras Chamber of Commerce, 197 SE 5th St., Madras OR
97741, phone 541-475-2350; Maps and publications about Oregon’s Geology and its geologic treasures are available from the Nature of the Northwest Information Center, a partnership of the Oregon Department of Geology and Mineral Industries and the USDA Forest Service. Nature of the Northwest is open 9-5, Monday through Friday at: 800 NE Oregon Street #5, Suite 177, Portland Oregon 97232, phone 503-872-2750, fax 503-731-4066. Web address: http://www.naturenw.org. Information source: Nature of the Northwest
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