Guam Territorial Stone
Limestone is a hard, fine-grained to very coarse-grained sedimentary rock composed primarily of calcium carbonate and typically white to buff in color. In general, limestone is more soluble than sandstone and thus more subject to weathering, which is very much in evidence on Guam. The eroded limestone environment characteristic of Guam is known as karst topography, so the limestone of Guam is sometimes referred to as karst.
A TERRITORIAL SYMBOL
While Guam has never designated an official territorial stone, limestone has great significance for the island. It is the most common surface rock and is associated with several of Guam’s distinctive geological and biological features. Situated at the edge of the Mariana Trench, surrounded by coral reefs, and formed through a combination of tectonic/volcanic and carbonate-deposition processes, Guam exhibits a complex geology. Its many types of limestone are responsible for much of the island’s striking scenery, from limestone caverns to alutong, the native word for a type of forest that occurs only on limestone terrain.
Some of Guam’s limestone forms from fossilized coral. Some forms from volcanic ash and calcium carbonate mud. Some forms from the shells and bodies of microscopic marine organisms, while some also contains large fragments of shell. Some recrystallizes from other forms of limestone or solidifies from chemically dissolved calcium carbonate, also generally of organic origin. Regardless of the source, limestone always forms by accretion (accumulation) of particles of calcium carbonate in layers underwater. As layers are buried under layers, pressure accumulates, compacting the particles into rock. After formation, the limestone may be uplifted by tectonic action, bringing it to the surface.
Limestone covers about 65 percent of Guam. About two-thirds of this is a formation known as the Mariana Limestone, made up of material from early reefs and lagoons. Another important formation, the Barrigada limestone in the northern part of the island, is of relatively small extent (nine percent of the surface), but houses a freshwater aquifer that supplies about 80 percent of Guam’s drinking water.
Worldwide, limestone is readily available in most regions. This is because it is sedimentary in origin. While sedimentary rocks account for only eight percent of the volume of the earth’s crust, they form a thin skin that covers about 80 percent of the earth’s land and most of the ocean floor. Limestone specifically makes up about a quarter of this material.
Limestone has been used traditionally as building stone for its beauty, versatility, and prevalence. Most of Guam’s limestone, being highly eroded, is not suited to this purpose. A quarry operation at Mangilao produces crushed rock for use in roadbeds and construction on the island. Worldwide, limestone aggregate is used for many industrial applications. Because of the high pH of calcium carbonate, crushed limestone is used in agriculture to counter acidity of the soil. Finely crushed limestone is used in papermaking to increase the opacity of paper. To make cement, crushed limestone is heated with sand and clay until the components interact chemically to produce calcium silicate.
Limestone has been used for important buildings since ancient times. The Great Pyramid of Giza is made of limestone quarried near the construction site. The famous medieval cathedral at Chartres, France, is made of limestone, as are the British Houses of Parliament and New York’s Empire State Building.
Most well-known examples of karst are natural formations. The famous Three Gorges of the Yangtze River (Qutang, Wu, and Xiling Gorges) are only one spectacular example.
Chemical Formula: CaCO3
Color: White or lightly colored, usually with dark streaks.
|Author: World Trade Press|