U.S. Virgin Islands Territorial Rock
Andesite is a gray volcanic rock, occasionally with tones of red, yellow, or green, composed of silica (about 52–63 percent), pyroxene, and hornblende in varying proportion. Additional minerals include zircon, apatite, and iron oxides. The texture of andesite varies from massive (of uniform grain) to vesicular (bubbly), porphyritic (containing visible crystals), or brecciated (ash matrix with fragments of rock). One reason for the variety of textures is that andesitic magmas, being relatively viscous, tend to trap bubbles of gas, resulting in violent eruptions.
A TERRITORIAL SYMBOL
While the U.S. Virgin Islands have never designated an official territorial stone, andesite, a component of the islands’ bedrock, is a good choice because it is specifically associated with the volcanic processes that shaped the islands.
The Virgin Islands lie atop a subduction zone where the Caribbean tectonic plate slides under the Atlantic plate. The accumulation of crustal mass creates pressure, and cracks along the juncture of the plates allow lava to be ejected, forming an underwater volcanic ridge. Eventually, higher mountains along this ridge may rise above sea level.
The mountain ridge arcs from Cuba to Venezuela, forming nearly the entirety of the chain of the Antilles. Andesite in island arcs appears to form when materials from the partially melted lower crust of the subducting plate combine with water and materials from the mantle.
NAME AND GEOGRAPHIC DISTRIBUTION
Andesite is named for the Andes Mountains, the volcanic chain forming the spine of South America. Geologist C.L. von Buch coined the name while studying Andean lavas in the 1830s. Andesite occurs in most young volcanic regions. Older volcanic mountains, for reasons not entirely understood, tend to be low in andesite.
In places where andesite is the prevalent stone, it has been used since ancient times for building, sculpture, and other objects. Today, andesite is quarried primarily for crushed stone used in roadbeds and asphalt; better-quality andesite, sometimes sold as "bluestone," is also made into tiles for paving or wall facing.
The vast 9th-century Borobudur Temple in Indonesia may be the single largest and most famous structure made of andesite. Two million blocks of bluish-gray andesite form a 95-foot-tall (29-meter-tall) step pyramid whose base is roughly 400 feet (122 m) square. The temple is both a Buddhist pilgrimage site and Indonesia’s largest tourist attraction.
A nine-foot (2.7-m) Aztec statue of the goddess Coatlicue, made of andesite, resides in Mexico’s National Museum of Anthropology and is one of many pre-Columbian monuments made of andesite throughout Mexico and Central and South America.
Another famous example of andesite is an event rather than an object. The famous explosion of Mt. Krakatoa in Indonesia on August 27, 1883, was an andesitic eruption of phenomenal violence. A black roiling cloud of ash rose 15 miles (25 km) into the sky. The blow, heard 3,000 miles (4,800 km) away, generated tsunamis that killed 36,000 people on Java and Sumatra. Pyroclastic flow traveled across water to incinerate villages 25 miles (40 km) away. Floating pumice carpeted the seas, preventing relief ships from reaching the area. Particulate matter thrown up into the stratosphere caused global temperatures to fall by several degrees.
Chemical Formula: (Na,Ca)(Si,Al)4O8
Color: White, gray, yellow
|Author: World Trade Press|