Northern Mariana Islands Territorial Rock
(Proposed) Territorial Rock of the Northern Mariana Islands
The Tagpochau limestone is a complex that includes a variety of textures of sedimentary calcareous stone formed in the Miocene epoch (5–23 million years ago). It is most commonly massive (of consistent texture), pinkish, and compact, with grains of varying sizes. In places, it incorporates volcanic ash and rubble. In others, it grades into marl limestone, which contains a large component of mud or clay. Fossils of various gastropods such as snails, whelks, and conches are also preserved in the rock.
REPRESENTATIVE OF THE TERRITORY
While the Northern Mariana Islands has never designated an official territorial stone, the Tagpochau limestone deserves recognition as characteristic of the islands. Named for Mt. Tagpochau, Saipan’s highest elevation (1555 ft/474 m), it is the most common surface rock, accounting for half the area of Saipan and most of the highlands.
The Tagpochau, in combination with other limestones, covers 90 percent of the Northern Marianas, for although these islands have a volcanic core, they are covered with a skin of sedimentary deposits. Limestone is not an inert presence on the Marianas, but is intricately involved in the native ecosystems of the islands. The limestone forest that once covered large parts of the islands is best preserved on Rota, where extensive tall-canopy forests remain as habitat for numerous endemic plants and animals.
Among the endangered birds living here are the Mariana crow, the Rota bridled white-eye, and the Mariana common moorhen. The rare sheath-tailed bat lives in limestone caverns, while the Marianas flying-fox nests on limestone cliffs on Asuncion. Without limestone, the native flora and fauna of the Marianas would not exist. In addition, the limestones harbor aquifers that supply 90 percent of Saipan’s municipal water.
FORMATION AND OCCURRENCE
Limestone 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 90 percent of the Marianas, which are sometimes described as carbonate islands because of their surface terrain. (They are also classed as volcanic islands because of the basaltic bedrock, which surfaces over 10 percent of the islands’ area.) 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 a building stone for its beauty, versatility, and prevalence. While some of the Marianas’ limestone is too highly eroded to be used for building, there are exceptions. Latte stones, ancient pillars found in groupings in the Marianas, are made of limestone as well as of basalt.
Worldwide, aggregate for industrial applications is the primary product of limestone quarries. Because of the high pH of calcium carbonate, crushed limestone is also 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.
Chemical Formula: CaCO3
Hardness (Mohs): 3-4
Specific Gravity: 2.7
Color: White or lightly colored, usually with dark streaks.
Streak: Not Found