Marine Resources Assessment for the Marianas Operating ... - SPREP

AUGUST 2005 FINAL REPORT
and the Aleutian Islands of Alaska. Unlike passive margins, active continental margins do mark the
boundary between two crustal plates. Due to the collision of the crustal plates, active margins are
associated with deep oceanic trenches, the formation of seamounts, seismic activity, and volcanism.
The bathymetry of the study area can be divided into three main areas: the Mariana Trough, the Mariana
Ridge, and the Mariana Trench.
Mariana Trough—The Mariana Trough (or Basin) spans the region to the west of the Mariana Ridge in
the region of the study area (Figure 2-2). The basin formed as the crustal plate spread between the West
Mariana Ridge and the Mariana Ridge. The Mariana Trough attains its widest spread (approximately 250
km) at about 18°N (Yamazaki et al. 1993). The spreading center is located on the eastern side of the
basin. The spreading of the seafloor between the two ridges is believed to have begun approximately 6
million years ago. The area between the two ridges is a flat plain averaging approximately 3,500 m in
depth and is spreading at a rate of 0.3 to 1.0 cm/yr in the northern region (Taylor and Martinez 2003;
Yamazaki et al. 1993).
Mariana Ridge—The Mariana Ridge consists of both active and extinct volcanoes. The latter are the
islands of Guam, Rota, Tinian, Saipan, and FDM (Figure 2-2). In general these islands are surrounded by
shallow fringing reefs with the occasional boulder breaking the water surface. There are barrier reefs on
the leeside of the islands of Guam and Saipan and a large shoal area 2 km north of FDM at a water depth
of 36.5 m (Randall 1979; Eldredge 1983). The Mariana Ridge formed as active volcanoes emerged from
the ocean floor over the subducting Pacific Plate. As the subduction zone moves to the east, the Mariana
Ridge will eventually subside and become submerged beneath the surface of the Pacific Ocean
(Thurman 1997).
Mariana Trench—The major physiographic feature of the study area is the Mariana Trench. The trench
runs from approximately 11°N, 141°E to 25°N, 143°E in an arc-like pattern extending over 2,270 km in
length (Figures 2-1 and 2-2). The trench is the result of the collision and subduction of two crustal plates,
the faster moving Pacific Plate and the slower moving Philippine Plate. Water depths in the trench range
from 5,000 to 11,000 m with the deepest locations being southwest of Guam and becoming shallower
northward (north of 14°N, the Mariana trench shallows to a depth less than 9 km; Fryer et al. 2003;
Figures 2-1 and 2-2). Located within the trench is Challenger Deep (11,034 m; 11°22’N, 142°25’E) and
HMRG Deep (10,732 m; 11°50’N, 144°30’E) (Fryer et al. 2003; Figures 2-1 and 2-2). Water mass
characteristics at varying depths within the trench suggest that the waters of the Mariana Trench are not
significantly different from those found on the abyssal plain north of the Marshall Islands (2,000 km to the
east) (Mantyla and Reid 1978).
2.3.1.2 Bottom Substrate
The bottom substrate covering the seafloor in the study area is primarily volcanic or marine in nature
(Eldredge 1983). Large flats of the seafloor are covered with a pavement-like covering of volcanic mud.
Patches of Globigerina ooze, the calcareous shells of foraminiferan cells, also form large patches on the
seafloor. Closer to island land masses are regions of coral debris, formed from the skeletons of corals
comprising the fringing and barrier reefs found throughout the Mariana archipelago (Eldredge 1983). The
Mariana Trench seafloor is comprised mostly of reddish-brown, pumiceous sand and silty clays (Ogawa
et al. 1997). Sediment cores of the Mariana Trench seafloor also contain radiolarians, pollen, sponge
spicules, diatoms, and benthic foraminiferans (Ogawa et al. 1997).
2.4 PHYSICAL OCEANOGRAPHY
2.4.1 Circulation
The water column can be divided into three separate water masses: a surface layer, an intermediate layer
of rapidly changing temperature referred to as the thermocline, and a deepwater layer (Pickard and
Emery 1982). Wind and water density differences drive the circulation of water masses in the ocean.
Surface currents are primarily driven by the wind (wind-driven circulation), which affects the upper 100 m
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