DESCRIPTION:
North Pacific Volcanic Islands

The Pacific is the world's largest ocean -- 20,000 kilometers across from Singapore to Panama. The Pacific Islands are commonly divided into the three geographic areas of Polynesia, Micronesia, and Melanesia. These areas together contain 789 habitable islands (Douglas 1969) ranging in size from New Guinea at 800,000 square kilometers to the tiniest coral atolls. Although these geographic divisions were originally based on the appearance and culture of the peoples who lived within their boundaries (Oliver 1989), they are also somewhat meaningful from geological and biogeographical standpoints.

The Melanesian islands (Melanesia), which are close to the Asian and Australian continents, are composed of rock that originated from volcanoes or sediments similar to those found on the continents. These relatively large islands usually have many kinds of animals and plants, but few species are limited to single islands or island groups. Within Melanesia, as one travels greater distances from the Asian mainland and the distance between islands becomes greater, the total number of animal and plant species found on each island decreases, but the number of species found only on each island (endemic species) increases.

Micronesia has a large number of very small islands and a total land area of less than 3,000 square kilometers. Micronesia has two main cultural and environmental divisions, with the volcanic Palau and Mariana islands of western Micronesia differing markedly from the atolls (low-lying, ringlike coral islands) of the Caroline, Marshall, and Gilbert island groups.

Polynesia includes 287 islands and is the largest of the Pacific geographic areas, although its land area of approximately 300,000 square kilometers is much less than that of Melanesia (Bellwood 1979). Polynesia is triangular, with Hawaii, New Zealand, and Easter Island at the apexes. New Zealand (268,570 square kilometers) and the Hawaiian Islands (16,558 square kilometers) are the largest island groups, respectively accounting for 89.5% and 5.5% of the land area of Polynesia. Polynesian islands generally lack sedimentary rocks other than recent alluvial deposits, and they are much smaller and more widely separated than the islands of Melanesia. The islands of Polynesia (except Tonga and New Zealand) are formed of basaltic rocks extruded from ancient volcanoes. The only active volcanoes in Polynesia are in Hawaii and New Zealand. Although the best-known Polynesian islands are high volcanic islands with jagged profiles, deep gorgelike valleys, and cascading waterfalls (for example, Society, Hawaiian, and Samoa islands), Polynesia has many atolls. Animals and plants of Polynesian high islands originated from ancestors that came across thousands of kilometers of empty ocean; the islands generally have few animal and plant species, many of which are found nowhere else.

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Map, Major Volcanoes of the Mariana Islands, including Anatahan

The elongated, 9-km-long island of Anatahan consists of two coalescing volcanoes with a 2.5 x 5 km, E-W-trending summit depression formed by overlapping summit craters. The floor of the steep-walled crater of the younger eastern cone is only 68 m above sea level. The spareness of vegetation on the most recent lava flows on Anatahan indicate that they are of Holocene age (Meijer and Reagan 1981).

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Map, Major Volcanoes of the Mariana Islands, including Asuncion

A single large asymetrical volcano, steeper on the northeast, forms 2.6 x 3.4 kilometer wide Asuncion Island. The steep northeast flank terminates in high sea cliffs. The southern and western flanks are mantled by ash deposits that may have originated in historical time. An explosive eruption in 1906 also produced lava flows, but other historical eruption reports are of uncertain validity. Highpoint is 857 meters.

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Map, Major Volcanoes of the Mariana Islands, including Guam

Guam is in the Western Pacific and is the southern most island of the Mariana Islands and is 1,500 miles south of Tokyo and 6,100 miles west of San Francisco.

Guam: volcanic origin, surrounded by coral reefs; relatively flat coralline limestone plateau (source of most fresh water), with steep coastal cliffs and narrow coastal plains in north, low-rising hills in center, mountains in south

Elevation extremes: lowest point: Pacific Ocean, 0 meters; highest point: Mount Lamlam, 406 meters

From the air, Guam looks like it was pushed up out of the sea. Geologists say the island was formed millions of years ago when a pair of volcanoes sank beneath the ocean and left behind two separate chunks of land. The lava remains of the southern volcano, the younger of the two, eventually fused with the older northern crater whose limestone top had been formed during a long period underwater by an extensive coral polyp community.

Guam is approximately 30 miles long, stretching from Ritidian Point in the north to the village of Merizo in the south. The widest area, between Orote and Ylig Points, is approximately 12 miles; the shortest distance across the island is about four miles. A close scientific examination of Guam reveals four main physical divisions to the island: the northern limestone plateau, the dissected volcanic plateau in the south, the south-central basin area and the fringing reef areas of the coastal lowlands.

At the southern tip of Guam, protected by a barrier head, is Cocos Island and its lagoon. Cocos has a total area of approximately 2.8 square miles. It is oblong in shape, composed mostly of drift materials, and not more than 15 feet above sea level at its highest point. The lagoon is shallow, but there are several spots that are 30 feet deep. The southern volcanic half of Guam is quite irregular. A belt of mountains, running southward along the west coast, towers above the smaller plateau hills. This range includes Mount Lamlam, the island's highest peak at 1,334 feet.

Guam's northern third is a wide plateau of limestone approximately eight miles across. The highest elevation here is 600 feet at Ritidian Point on the northwest tip of the island and this height slips gradually down to the southwest until it reaches approximately 200 feet above sea level at Guam's center. The only volcanic rock in the northern sector is on Mount Santa Rosa and Mataguac Hill. Both of these were above water when the coral polyps formed the limestone roof on the sunken part of the northern volcano. Also on the northern plateau are undisturbed jungle areas that constitute one of the last major tropical limestone forests on the island. Trees such as pandanus, pugua, breadfruit and papaya are plentiful here.

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Map, Major Volcanoes of the Mariana Islands
-- includes location of a portion of the Marianas Trench

As with oceanic-continental convergence, when two oceanic plates converge, one is usually subducted under the other, and in the process a trench is formed. The Marianas Trench (paralleling the Mariana Islands), for example, marks where the fast-moving Pacific Plate converges against the slower moving Philippine Plate. The Challenger Deep, at the southern end of the Marianas Trench, plunges deeper into the Earth's interior (nearly 11,000 meters) than Mount Everest, the world's tallest mountain, rises above sea level (about 8,854 meters).

Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano. Such volcanoes are typically strung out in chains called island arcs. As the name implies, volcanic island arcs, which closely parallel the trenches, are generally curved. The trenches are the key to understanding how island arcs such as the Marianas and the Aleutian Islands have formed and why they experience numerous strong earthquakes. Magmas that form island arcs are produced by the partial melting of the descending plate and/or the overlying oceanic lithosphere. The descending plate also provides a source of stress as the two plates interact, leading to frequent moderate to strong earthquakes.

Plate Tectonics Menu

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Map, Major Volcanoes of the Mariana Islands

Human settlement of Japan can be traced for tens of thousands of years, and an unbroken line of emperors from 660 BC. Japan's first documented historical eruption was from Aso, its most prolific volcano, in 553 AD, the year after Buddhism was introduced from Korea. ...

To the south, the Mariana Islands were populated from 1500 BC and explored by Spaniards in the 15th century AD, but the islands did not come under Spanish colonial rule until 1668. The first historical eruption was documented the following year. The northern volcanic islands were sold to Germany in 1898, occupied by Japan between the two World Wars, and named a Trust Territory by the UN in 1947 administered by the U.S. The islands became a self-governing US commonwealth in 1975. ...

Most volcanoes in this region result from subduction of westward-moving oceanic crust under the Asian Plate. In the Izu-Marianas chain, however, the crust to the west is also oceanic, forming island arcs where volcanoes are largely basaltic but far more explosive than oceanic hotspot volcanoes.

(This region) ... has the largest number of submarine volcanoes, mostly extending down the Izu-Marianas arc, and the largest number of reported submarine eruptions. The many reports of water discoloration over submarine vents have also contributed to this region's record number (180) of eruptions preceded by a question mark, indicating uncertainty that the eruption actually took place.

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Map, Major Volcanoes of the Mariana Islands, including Pagan

Pagan Island, the largest and one of the most active of the Marianas Islands, consists of two stratovolcanoes connected by a narrow isthmus. Both North and South Pagan stratovolcanoes were constructed within calderas, 7- and 4-kilomters in diameter, respectively. The 570-meters-high Mount Pagan at the northeast end of the island rises above the flat floor of the caldera, which probably formed during the early Holocene. South Pagan is a 548-meter-high stratovolcano with an elongated summit containing four distinct craters. Most of the historical eruptions of Pagan have originated from North Pagan volcano.

The 1981 eruption, which sent a Plinian column to >13 kilometers elevation, was the largest eruption in Pagan's historical record. The island's 53 residents were evacuated at the start of the eruption, and have made only temporary visits since then. Despite infrequent observations by former residents, scientists, aircraft overflights, and from nearby islands, several small to moderate ash eruptions have been reported. Since the May 1981 eruption, several small to moderate ash eruptions have been observed, and plumes have occasionally been visible on satellite imagery. Seismic monitoring of Pagan ended in 1984. The pre-1981 Pagan record includes 11 eruptions dating back to the early 1800's, and a tentative eruption in 1669.

Pagan Island is in the northern Marianas arc, where the Pacific Plate is subducted beneath the Philipping Sea plate at about 10.7 centimeters per year (Circum-Pacific Map Project, Northwest Quadrant Panel, 1981).

Mount Pagan is a small stratovolcano that has grown within an unnamed 5 kilometer x 6 kilometer somma caldera (Corwin and others, 1957). Another, slightly smaller stratovolcano -- South Pagan -- has grown within a 2.5-kilometer-diameter somma caldera on the southern end of Pagan Island, 10 kilometer southwest of Mount Pagan. Several vents of South Pagan are aligned along a northeast-southwest axis, pointing toward Mount Pagan, but the subsurface relation between Mount Pagan and South Pagan is unknown.

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Map, Major Volcanoes of the Mariana Islands, including Ruby Seamount

Ruby (-230 meters), a submarine volcano that rises to within 230 meters of the sea surface northwest of Saipan, was detected in eruption in 1966 by sonar signals (Norris and Johnson, 1969). In 1995 submarine explosions were heard, accompanied by a fish kill, sulfurous odors, water bubbling, and the detection of volcanic tremor.

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