Geologically, the islets are a coral
atoll, built on the Kyushu–Palau Ridge, the westernmost part of the
Izu–Bonin–Mariana Arc system. The name
Parece Vela Basin has been given to the extinct
back-arc basin that lies immediately to the east (the northern half of this back-arc basin is known as the
Shikoku Basin). This back-arc basin was formed by
seafloor spreading between the late
Oligocene and
Miocene. The
Parece Vela Basin contains the longest
megamullion in the world. The original Spanish name of the islets is normally used for the
geological formations, hence,
Parece Vela megamullion, Parece Vela
ridge, or Parece Vela
basin. The waters around the reefs are potentially rich in
oil and other mineral and fisheries resources and it lies in an area of potential military significance. At
high tide, one area of the reef is , roughly the size of a twin bed, and pokes just out of the ocean. The other is , the size of a small bedroom, and rises , about twice as high. The entire reef consists of approximately , most of which is submerged even at low tide. The area has three tiny individual
islets: • Higashi-Kojima (東小島, "Eastern Islet") • Kita-Kojima (北小島, "Northern Islet"), nevertheless rather in the "West" • Minami-Kojima (南小島, "Southern Islet") Minami-Kojima is a completely artificial islet created in shallow water. But also the two original islets appear completely artificial today, with little if any trace of the two natural rocks that still appear on photographs of 1987. In 1925, there were still five above-water rocks, which have eroded since. A report from 1947 mentions five above-water rocks. Three smaller ones were on the west side, nearly impossible to see from seaward because of the breaking waves. The larger rocks on the southwest side and on the northeast side, possibly Kita-Kojima and Higashi-Kojima, were reported to be high, respectively. The original rocks appeared barren, without any
terrestrial vegetation. The current artificial dry land areas with their concrete surfaces appear unfit to support terrestrial vegetation either. After
concrete encasing, each of the islets appears as a circle with a
diameter of 60 meters (196 ft) on detailed
satellite images, which would correspond to a land area—albeit mostly artificial—of per islet, or in total. In addition, there is a platform on
stilts in the shallow part of the lagoon east-northeast of the southern islet, built by the Japan Marine Science and Technology Center in 1988, which appears as a rectangle of . The platform has a
helicopter landing pad and a large three-story building with a marine investigation facility and a meteorological station. The rocks are in the western part of a
lagoon surrounded by a submerged coral reef, over which the waves break, and that extends east-west and north-south, with an area of roughly 5 km2 within the rim of the reef. The lagoon is 3 to 4.6 meters deep, but there are numerous coral heads of lesser depths throughout the area. The fringing reef of the atoll is pear-shaped in an east–west direction with its greatest width at the eastern end. There is a small boat channel into the lagoon in the southwest, about wide and deep, southeast of the artificial islet. ==Administration==