The Yap Trench, also known as the West Caroline Trench, is an

oceanic trench Oceanic trenches are prominent, long, narrow topography, topographic depression (geology), depressions of the seabed, ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers ...

near

Yap Island Yap (, sometimes written as , or ) traditionally refers to an island group located in the Caroline Islands of the western Pacific Ocean, a part of Yap State. The name "Yap" in recent years has come to also refer to the state within the Federated ...

in the western

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...

. The trench is roughly J-shaped and is 650 kilometres (400 mi) long and 8,527 metres (27,976 ft) deep at its deepest point. The trench is located on the southeastern edge of the

Philippine Sea Plate The Philippine Sea plate or the Philippine plate is a tectonic plate comprising oceanic lithosphere that lies beneath the Philippine Sea, to the east of the Philippines. Most segments of the Philippines, including northern Luzon, are part of ...

and forms the part of the

Pacific Ring of Fire The Ring of Fire (also known as the Pacific Ring of Fire, the Rim of Fire, the Girdle of Fire or the Circum-Pacific belt) is a tectonic belt of volcanoes and earthquakes. It is about long and up to about wide, and surrounds most of the Pa ...

between the

Palau Islands Palau, officially the Republic of Palau, is an island country in the Micronesia subregion of Oceania in the western Pacific Ocean. The Republic of Palau consists of approximately 340 islands and is the western part of the Caroline Islands, w ...

and the

Mariana Trench The Mariana Trench is an oceanic trench located in the western Pacific Ocean, about east of the Mariana Islands; it is the deep sea, deepest oceanic trench on Earth. It is crescent-shaped and measures about in length and in width. The maxi ...

. Researchers believe that the Yap Trench was formed during a classic intraoceanic subduction event, and that the trench is an example of early subduction zone development in the western Pacific. Studies are ongoing as to whether the trench is active. The Caroline Ridge collided with the Yap Trench at one point, effectively dividing the trench's tectonic evolution into two parts. Sato et al. observed in 1997 that microseismic activity in the trench was similar to that of active subduction zones, and Nagihara et al. noted large amounts of negative gravity along the trench, suggesting that a force was being exerted upon the crust beneath it. Recent studies agree and elaborate on these observations. Using

seismic tomography Seismic tomography or seismotomography is a technique for imaging the subsurface of the Earth using seismic waves. The properties of seismic waves are modified by the material through which they travel. By comparing the differences in seismic waves ...

, researchers showed that locations of modern earthquakes are especially concentrated in zones of high seismic velocity, which are indicative of solid rock in the mantle, and likely represent earthquakes caused by the subduction of the Caroline Plate. They further showed that subduction of the Caroline Plate is complex. It appears that subduction progressed until it was halted as the thick Southern Caroline Plateau (SCP) reached the subduction zone. The section of plate immediately east of the SCP was relatively new and still ductile, and seems to have broken and sunk into the

asthenosphere The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between c. below the surface, and extends as deep as . However, the lower boundary of the asthenosphere i ...

. The resulting rebound of the partially-subducted material may have caused a previously unexplained burst of volcanism along the Yap Arc sometime around 11-7 Mya. Researchers also noted that their ocean bottom seismometers showed a two-layer crust, like that found in Chile and Argentina, under much of the area around the Yap Trench. They believe this to be caused by the slab rebounding and reattaching to the oceanic crust above it. The upward buoyancy could explain the previously-mentioned force being exerted on the crust beneath the trench. The rest of the subducted Caroline slab, north of the SCP, can be seen as high-velocity areas in seismic tomographs. Intriguingly, it appears to have subducted and then overturned in the mantle. It is suggested this is due to a high rate of slab-pull with respect to overall plate movement, combined with the effects of asthenosphere wind pushing the plate to the west. However, the slab seems to have stopped subducting as the North Caroline Plateau (NCP) has reached the trench. This is probably because the NCP is very close to the southern end of the

, and rather than subduct, may stall until the currently subducted material breaks off, just like the southern end of the slab.

References

Oceanic trenches of the Pacific Ocean Subduction zones {{Marine-geo-stub