Thunderegg

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A thunderegg from the Black Rock Desert in Nevada Black rock desert thunderegg.JPG
A thunderegg from the Black Rock Desert in Nevada

A thunderegg (or thunder egg) is a nodule-like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers. [1] Thundereggs are rough spheres, most about the size of a baseball—though they can range from a little more than a centimeter to over a meter across. They usually contain centres of chalcedony which may have been fractured followed by deposition of agate, jasper or opal, [1] either uniquely or in combination. Also frequently encountered are quartz and gypsum crystals, as well as various other mineral growths and inclusions. Thundereggs usually look like ordinary rocks on the outside, but slicing them in half and polishing them may reveal intricate patterns and colours. A characteristic feature of thundereggs is that (like other agates) the individual beds they come from can vary in appearance, though they can maintain a certain specific identity within them.

Contents

Thunderegg is not synonymous with either geode or agate. A geode is a simple term for a rock with a hollow in it, often with crystal formation/growth. A thunderegg on the other hand is a specific geological structure. A thunderegg may be referred to as a geode if it has a hollow in it, but not all geodes are thundereggs because there are many different ways for a hollow to form. Similarly, a thunderegg is just one of the forms that agate can assume.

Occurrence

Thundereggs are found globally where conditions are optimal. In the US, Oregon is one of the most famous thunderegg locations. Germany is also an important center for thunderegg agates (especially sites like St Egidien and Gehlberg). Other places known for thundereggs include Ethiopia, [2] Poland, [3] Romania, Turkey, Mexico, Argentina, [4] Canada, Mount Hay [5] and Tamborine Mountain, [6] (Australia) and the Esterel massif, [7] (France).[ citation needed ]

Formation

Thundereggs are found in flows of rhyolite lava. They form in the lava from the action of water percolating through the porous rock carrying silica in solution. The deposits lined and filled the cavity, first with a darker matrix material, then an inner core of agate or chalcedony. The various colors come from differences in the minerals found in the soil and rock that the water has moved through. [8]

State rock designation

On March 30, 1965, the thunderegg was designated as the Oregon state rock by a joint resolution of the Oregon Legislative Assembly. [9] [10] [11] While thundereggs can be collected all over Oregon, the largest deposits are found in Crook, Jefferson, Malheur, Wasco and Wheeler counties. [12] The world's largest thunderegg, a 1.75 ton specimen, is housed by the Rice Northwest Museum of Rocks and Minerals in Hillsboro, Oregon. [13]

Legend

Native American legend reportedly considers the rocks to be the eggs of the thunderbirds which occupied Mount Hood and Mount Jefferson. Thunder Spirits on the mountains hurled the "eggs" at each other. [11]

Images

See also

Related Research Articles

Agate Rock consisting of cryptocrystalline silica alternating with microgranular quartz

Agate is a common rock formation, consisting of chalcedony and quartz as its primary components, consisting of a wide variety of colors. Its IMA symbol is Aga. Agates are primarily formed within volcanic and metamorphic rocks. The ornamental use of agate dates back to Ancient Greece in assorted jewelry and in the seal stones of Greek warriors. Use of beads necklaces with pierced and polished agate goes further back to the 3rd millennium BC in the Indus Valley Civilisation.

Chalcedony Microcrystalline varieties of silica, may contain moganite as well

Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. Chalcedony's standard chemical structure (based on the chemical structure of quartz) is SiO2 (silicon dioxide).

Jasper Chalcedony variety colored by iron oxide

Jasper, an aggregate of microgranular quartz and/or chalcedony and other mineral phases, is an opaque, impure variety of silica, usually red, yellow, brown or green in color; and rarely blue. The common red color is due to iron(III) inclusions. Jasper breaks with a smooth surface and is used for ornamentation or as a gemstone. It can be highly polished and is used for items such as vases, seals, and snuff boxes. The specific gravity of jasper is typically 2.5 to 2.9. A green variety with red spots, known as heliotrope (bloodstone), is one of the traditional birthstones for March. Jaspillite is a banded-iron-formation rock that often has distinctive bands of jasper.

Chert Hard, fine-grained sedimentary rock composed of cryptocrystalline silica

Chert is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood.

Trachyte Extrusive igneous rock

Trachyte is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and alkali metals. It is the volcanic equivalent of syenite.

Onyx Banded variety of the mineral chalcedony

Onyx primarily refers to the parallel banded variety of the silicate mineral chalcedony. Agate and onyx are both varieties of layered chalcedony that differ only in the form of the bands: agate has curved bands and onyx has parallel bands. The colors of its bands range from black to almost every color. Commonly, specimens of onyx contain bands of black and/or white. Onyx, as a descriptive term, has also been applied to parallel banded varieties of alabaster, marble, obsidian and opal, and misleadingly to materials with contorted banding, such as "Cave Onyx" and "Mexican Onyx".

Geode Hollow formation inside a rock

A geode is a geological secondary formation within sedimentary and volcanic rocks. Geodes are hollow, vaguely spherical rocks, in which masses of mineral matter are secluded. The crystals are formed by the filling of vesicles in volcanic and sub-volcanic rocks by minerals deposited from hydrothermal fluids; or by the dissolution of syn-genetic concretions and partial filling by the same, or other, minerals precipitated from water, groundwater or hydrothermal fluids.

Vug Small to medium-sized cavity inside rock

A vug, vugh, or vugg is a small to medium-sized cavity inside rock. It may be formed through a variety of processes. Most commonly, cracks and fissures opened by tectonic activity are partially filled by quartz, calcite, and other secondary minerals. Open spaces within ancient collapse breccias are another important source of vugs. Vugs may also form when mineral crystals or fossils inside a rock matrix are later removed through erosion or dissolution processes, leaving behind irregular voids. The inner surfaces of such vugs are often coated with a crystal druse. Fine crystals are often found in vugs where the open space allows the free development of external crystal form. The term vug is not applied to veins and fissures that have become completely filled, but may be applied to any small cavities within such veins. Geodes are a common vug-formed rock, although that term is usually reserved for more rounded crystal-lined cavities in sedimentary rocks and ancient lavas.

Lithophysa felsic volcanic rock

A lithophysa is a felsic volcanic rock with a spherulitic structure and interior cavity with concentric chambers. Its outer shape is spherical or lenticular. They vary in size from very small up to twelve feet in diameter depending on the age of the magma chamber. These rocks are usually found within obsidian or rhyolite lava flows. Lavas low in feldspar minerals may produce a version known as snowflake obsidian.

Moss agate

Moss agate is a semi-precious gemstone formed from silicon dioxide. It is a form of chalcedony which includes minerals of a green color embedded in the stone, forming filaments and other patterns suggestive of moss. The field is a clear or milky-white quartz, and the included minerals are mainly oxides of manganese or iron. It is not a true form of agate, because it does not have concentric banding.

Quartz-porphyry Type of volcanic rock containing large porphyritic crystals of quartz

Quartz-porphyry, in layman's terms, is a type of volcanic (igneous) rock containing large porphyritic crystals of quartz. These rocks are classified as hemi-crystalline acid rocks.

Lake Superior agate

The Lake Superior agate is a type of agate stained by iron and found on the shores of Lake Superior. Its wide distribution and iron-rich bands of color reflect the gemstone's geologic history in Minnesota, Wisconsin, Nebraska, Iowa, Kansas and Michigan. In 1969 the Lake Superior agate was designated by the Minnesota Legislature as the official state gemstone.

Diamond Peak (Oregon) Mountain in the United States

Diamond Peak is a volcano in Klamath and Lane counties of central Oregon in the United States. It is a shield volcano, though it might also be considered a modest stratocone. Diamond Peak forms part of the Cascade Volcanic Arc, a segment of the Cascade Range in western North America extending from southern British Columbia through Oregon to Northern California. Reaching an elevation of 8,748 feet (2,666 m), the mountain is located near Willamette Pass in the Diamond Peak Wilderness within the Deschutes and Willamette national forests. Surrounded by coniferous forest and visible in the skyline from foothills near Eugene, Diamond Peak offers a few climbing routes and can be scrambled. Diamond Peak is one of Oregon's Matterhorns.

Hogg Rock Mountain in the Cascade Range of northern Oregon

Hogg Rock is a tuya volcano and lava dome in the Cascade Range of northern Oregon, located close to Santiam Pass. Produced by magma with an intermediate andesite composition, it has steep slopes and thick glassy margins. Hogg Rock exhibits normal magnetic polarity and is probably about 80,000 years old.

Chrome chalcedony

Chrome chalcedony is a green variety of the mineral chalcedony, colored by small quantities of chromium. It is most commonly found in Zimbabwe, where it is known as Mtorolite, Mtorodite, or Matorolite.

Rice Northwest Museum of Rocks and Minerals United States historic place

The Rice Northwest Museum of Rocks and Minerals is a non-profit museum in Hillsboro, Oregon, United States. Located just north of the Sunset Highway on the northern edge of Hillsboro, the earth science museum is in the Portland metropolitan area. Opened in 1997, the museum's collections date to the 1930s with the museum housed in a home built to display the rock and mineral collections of the museum founders. The ranch-style home is listed on the National Register of Historic Places, the first of its kind listed in Oregon. In 2015 the museum became a Smithsonian Affiliate museum.

Ochoco Mountains

The Ochoco Mountains are a mountain range in central Oregon in the United States, located at the western end of the Blue Mountains. They were formed when Permian, Triassic, and Jurassic rocks were slowly uplifted by volcanic eruptions to form the Clarno Formation. Today, the highest point in the range is Lookout Mountain. The dominant vegetation on the west side of the range is old-growth ponderosa pine; on the east side, western juniper is common. The western area of the mountains is administered by the Ochoco National Forest, while the southeastern section is part of the Malheur National Forest. The Ochoco Mountains are used for hiking, camping, bird watching, rockhounding, and hunting, as well as cross-country skiing in the winter.

Enhydro agate

Enhydro agates are nodules, agates, or geodes with water trapped inside its cavity. Enhydros are closely related to fluid inclusions, but are composed of chalcedony. The formation of enhydros is still an ongoing process, with specimens dated back to the Eocene Epoch. They are commonly found in areas with volcanic rock.

References

  1. 1 2 "America's Volcanic Past - Oregon". USGS Cascades Volcano Observatory. Retrieved 2007-06-08.
  2. Rix, David. "Thundereggs from Ethiopia". Eibonvale Thunderegg Gallery. Retrieved 19 May 2017.
  3. Rix, David. "Thundereggs from Poland". Eibonvale Thunderegg Gallery. Retrieved 19 May 2017.
  4. Rix, David. "Thundereggs from Argentina". Eibonvale Thunderegg Gallery. Retrieved 19 May 2017.
  5. "Mount Hay". Dig the Tropic: Outback to the Reef. Capricorn Enterprise. Retrieved 19 May 2017.
  6. "Tamborine Mountain". Sydney Morning Herald. The Sydney Morning Herald. 15 August 2008. Retrieved 19 May 2017.
  7. Richard, Jean-Jacques (1 February 2009). "Les Oeufs de Tonnerre". Bijoux et pierres precieuses (in French). Retrieved 19 May 2017.
  8. Thunderegg Oregon State Rock, StateSymbols USA
  9. "Oregon Symbols". SHG Resources. Retrieved 2007-09-17.
  10. Chapter 186 — State Emblems; State Boundary 2017 Oregon Revised Statutes
  11. 1 2 "Rock Hounding". Nature of the Northwest. Archived from the original on 2007-09-28. Retrieved 2007-09-17.
  12. "Oregon Almanac". Oregon Blue Book. Retrieved 2007-09-19.
  13. Christie, Tim (March 26, 2007). "Rock hounds check out goods at 18th annual Gem Faire". The Register-Guard. Eugene, Oregon.