DARWIN RISE

submarine topographic rise underlying a vast area of the western and central Pacific, corresponding in location to a large topographic rise that existed during the Mesozoic Era (65,000,000 to 225,000,000 years ago), and named in honour of Charles Darwin. The rise stretches more than 6,000 miles (10,000 km) roughly from the area just east of the Mariana Trench southeast to the Tumotu Archipelago and is about 2,500 miles (4,000 km) wide. It is characterized by many flat-topped seamounts and coral atolls; maximum depth to the seamount tops is about 5,200 feet (1,600 m). Several ridges and troughs are oriented parallel to the rise, with transverse fracture zones cutting across the rise topography. A vast archipelagic apron characterized by floods of fluid lava covers most of the Darwin Rise. Presently, the area is seismically inactive and exhibits normal heat flow through the floor of the ocean. That the area was formerly a rise is suggested by the ridge and trough topography, the former volcanic activity, and its position in the middle of an ocean basin. There is ample evidence for subsidence: shallow water organisms have been found on the seamount tops, and oxidized iron has been found in dredged samples, suggesting that erosion and weathering have taken place. Many volcanic cones on the Darwin Rise must have been exposed above the sea in the past. Charles Darwin recognized the area as one of subsidence from the appearance of upward-growing coral atolls, in accordance with his theory of atoll formation by ocean floor subsidence. The subsidence is believed to have occurred continuously since the Cretaceous Period. The existence of such an area of subsidence is significant in marine geology because it confirms that large vertical movements of the Earth's crust have occurred beneath the sea.