Dacite

Dacite ( /ˈdeɪsaɪt/) is an igneous, volcanic rock. It has an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. The relative proportions of feldspars and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram. Dacite is also defined by silica and alkali contents in the TAS classification.

The word dacite comes from Dacia, a province of the Roman Empire which lay between the Danube River and Carpathian Mountains (now modern Romania) where the rock was first described.

Composition[link]

Dacite consists mostly of plagioclase feldspar with biotite, hornblende, and pyroxene (augite and/or enstatite). It has quartz as rounded, corroded phenocrysts, or as an element of the ground-mass.

The plagioclase ranges from oligoclase to andesine and labradorite. Sanidine occurs, although in small proportions, in some dacites, and when abundant gives rise to rocks that form transitions to the rhyolites.

The groundmass of these rocks is composed of plagioclase and quartz.

Texture[link]

Grey, red, black, altered white/tan, flow-banded pumice dacite

In hand specimen many of the hornblende and biotite dacites are grey or pale brown and yellow rocks with white feldspars, and black crystals of biotite and hornblende. Other dacites, especially pyroxene bearing dacites, are darker colored.

In thin section, dacites may have an aphanitic to porphyritic texture. Porphyritic dacites contain blocky highly zoned plagioclase phenocrysts and/or rounded corroded quartz phenocrysts. Subhedral hornblende and elongated biotite grains are present. Sanidine phenocrysts and augite (or enstatite) are found in some samples. The groundmass of these rocks is often aphanitic microcrystalline, with a web of minute feldspars mixed with interstitial grains of quartz or tridymite; but in many dacites it is largely vitreous, while in others it is felsitic or cryptocrystalline.

Geological context and formation[link]

Dacite usually forms as an intrusive rock such as a dike or sill. Examples of this type of dacite outcrop are found in northwestern Montana and northeastern Bulgaria. Nevertheless, because of the moderate silica content, dacitic magma is quite viscous^[1] and therefore prone to explosive eruption. A notorious example of this is Mount St. Helens in which dacite domes formed from previous eruptions.

Dacitic magma is formed by the subduction of young oceanic crust under a thick felsic continental plate. Oceanic crust is hydrothermally altered causing addition of quartz and sodium.^[2] As the young, hot oceanic plate is subducted under continental crust, the subducted slab partially melts and interacts with the upper mantle through convection and dehydration reactions.^[3] The process of subduction creates metamorphism in the subducting slab. When this slab reaches the mantle and initiates the dehydration reactions, minerals such as talc, serpentine, mica and amphiboles break down generating a more sodic melt.^[4] The magma then continues to migrate upwards causing differentiation and becomes even more sodic and silicic as it rises. Once at the cold surface, the sodium rich magma crystallizes plagioclase, quartz and hornblende.^[5] Accessory elements like pyroxenes provide insight to the history of the magma.

The formation of dacite provides a great deal of information about the connection between oceanic crust and continental crust. It provides a model for the generation of felsic, buoyant, perennial rock from a mafic, dense, short-lived one.

Dacite's role in the creation of Archean continental crust[link]

The process by which dacite forms has been used to explain the generation of continental crust during the Archean eon. At that time, the fabrication of dacitic magma was more ubiquitous due to the availability of young hot oceanic crust. Today, the colder oceanic crust that subducts under most plates is not capable of melting before the dehydration reactions therefore inhibiting the process.^[6]

Molten dacite magma at Kīlauea[link]

Dacite magma was encountered in a drillhole during geothermal exploration on Kīlauea in 2005. The magma was encountered at a depth of 2488 m and was repeatedly drilled as it surged up the drillhole and quenched over a range of 8 m. The dacite magma is a residual melt of the typical basalt magma of Kīlauea.^[7]

References[link]

Wikimedia Commons has media related to: Dacite

Superstition Wilderness
IUCN category Ib (wilderness area)
The Flat Iron peak
Location	Maricopa / Pinal / Gila counties, Arizona, USA
Nearest city	Apache Junction, Arizona
Coordinates	33°28′41″N 111°14′20″W / 33.47806°N 111.23889°W / 33.47806; -111.23889Coordinates: 33°28′41″N 111°14′20″W / 33.47806°N 111.23889°W / 33.47806; -111.23889
Area	159,757 acres (64,651 ha)
Established	1939
Governing body	U.S. Forest Service

Miners Needle from Bluff Spring Trail. Note prominent bedding planes in the volcanics.

The Superstition Mountains (Yavapai: Wi:kchsawa), popularly called "The Superstitions", are a range of mountains in Arizona located to the east of the Phoenix metropolitan area. They are anchored by Superstition Mountain, a large mountain that is a popular recreation destination for residents of the Phoenix, Arizona area.

The mountain range is in the federally designated Superstition Wilderness Area, and includes a variety of natural features in addition its namesake mountain. Weaver's Needle, a prominent landmark and rock climbing destination set behind and to the east of Superstition Mountain, is a tall erosional remnant ^[1] that plays a significant role in the legend of the Lost Dutchman's Gold Mine. Peralta Canyon, on the northeast side of Superstition Mountain, contains a popular trail that leads up to Freemont Saddle, which provides a very picturesque view of Weaver's Needle. Miner's Needle is another prominent formation in the wilderness and a popular hiking destination.

As with most of the terrain surrounding the Phoenix metropolitan area, the Superstition Mountains have a desert climate, with high summer temperatures and a handful of perennial sources of water. The altitude in the more remote, eastern portion of the wilderness is higher than the western portion, which lowers temperatures slightly. Numerous hiking trails cross the mountains from multiple access points, including the Peralta Trailhead, the most popular.^[2] The Lost Dutchman State Park, located on the west side of Superstition Mountain, includes several short walking trails.

The Superstition Mountains are bounded roughly by U.S. Route 60 on the south, State Route 88 on the northwest, and State Route 188 on the northeast.

The mountains were once known in Spanish as Sierra de la Espuma.

Superstition legends[link]

View from the Flat Iron hiking trail, February, 2008

The legend of the Lost Dutchman's Gold Mine centers around the Superstition Mountains. According to the legend, a German immigrant named Jacob Walzer discovered a mother lode in the Superstition Wilderness and revealed its location on his deathbed in Phoenix in 1891 to Julia Thomas, a boarding-house owner who had taken care of him for many years. Several mines have been claimed to be the actual mine that Walzer discovered, but none of those claims have been verified.^[1]

Some Apaches believe that the hole leading down into the lower world is located in the Superstition Mountains. Winds blowing from the hole are supposed to be the cause of severe dust storms.^[3]

Nearby towns and cities[link]

Petroglyphs in Superstition Wilderness

• Apache Junction, Arizona
• Tortilla Flat, Arizona
• Superior, Arizona
• Phoenix, Arizona
• Gold Canyon, Arizona

See also[link]

• List of U.S. Wilderness Areas
• List of Arizona Wilderness Areas
• List of mountain ranges of Arizona
• Wilderness Act

References[link]

External links[link]

Wikimedia Commons has media related to: Superstition Mountains

• Apache Trail | Superstition Wilderness - Sunset Route 88
• Superstition Wilderness Area - Tonto National Forest
• Superstition Wilderness - Wilderness.net
• Superstition Wilderness - GORP
• Superstition Mountain Hikes
• History of the Superstitions