Geologic Setting and Geomorphology of Coral Reefs in the Mariana Islands (Guam and Commonwealth of the Northern Mariana Islands)

Abstract

These westernmost territories of the United States are where “America’s day begins”, and contain the most diverse coral reefs under the US flag (Randall 1995; Paulay 2003). The Mariana Islands have a long history of dedicated coral reef investigations by international and US researchers begun by Agassiz (1903). Despite having been discovered by the Spanish in 1521, these islands were rarely visited by Westerners due to their isolation. The Spanish used the Marianas as a trade-route stop-over between the Philippines and South America, a history that later attracted others interested in finding treasure. In 1742, Tinian was visited by the British Commodore Anson who became rich and famous after pirating Spanish treasure, and in 1765 and 1767, Byron and Wallis revisited the island in continued search of treasure and territories. Accounts of Anson’s and Byron’s voyages were some of the earliest documentations of the Marianas. In Guam, research received a boost when the USA acquired it from Spain following the Spanish American War, and in the CNMI, when Germany bought the islands in 1899. Coral reef investigations began in 1903 when Agassiz reported about the Marianas in the course of his decade of coral reef expeditions around the world from 1893 to 1902. The German explorer Prowazek, reported on the “raised coral limestones” of the CNMI in 1913. Under Japanese mandate, Sugawara (1934) investigated the coral reefs and Holocene limestones on Rota just prior to the Second World War. At a similar time, Stearns (1937, 1940a, b) reported on Guam’s geology, hydrogeology and drew attention to the erosional notches, which have remained informative features for many subsequent investigation (Shepard et al. 1967; Easton et al. 1978; Kayanne et al. 1993; Dickinson 2000). From these beginnings, perspectives regarding eustacy in the entire Pacific Ocean were gained. After the war, intensive US-funded mapping and geologic research led to a series of United States Geological Survey publications (Cloud et al. 1956; Doan et al. 1960; Tracey et al. 1964). The seventh International Coral Reef Symposium took place in Guam and highlighted carbonate and reef studies in the Marianas, and Siegrist and Randall (1992) provide an informative overview and guide. Another excellent review of geology and hydrogeology is Mink and Vacher (1997). In addition to the raised limestone islands of the southernmost Mariana islands, the archipelago also includes nine emergent, purely volcanic islands.