Many volcanic islands, such as those of the Hawaiian chain in the Pacific, Reunion in the Indian Ocean, and the Canary Islands and Tristan da Cunha in the Atlantic, are built through a combination of volcanic flows adding material to a small area in the center of the island. Frequent submarine landslides cause the islands to collapse, spreading the rocks from these flows across a wide area. Undersea mapping of the Hawaiian chain using sonar systems that can produce detailed views of the seafloor have shown that the island chain is completely surrounded by a series of debris fans and aprons from undersea landslides, covering a much larger area than the islands themselves. Submarine mapping efforts have discovered more than 70 giant landslide deposits along the 1,360-mile- (2,200-km-) long segment of the island chain from Hawaii to Midway Island. The age of the islands and flows increases from Hawaii to Midway, and studies have suggested that the average recurrence time between giant submarine slides along the Hawaiian chain is 350,000 years. The youngest giant slides on Hawaii, the Alika slides, are estimated to be several hundred thousand years old, suggesting that parts of the Hawaiian chain could be close to being ready to produce another giant slide.
Many of the landslides on the Hawaiian Islands are 100-200 miles (150-300 km) long, but seem to be somewhat shorter for the older volcanoes to the west. The largest ones have displaced volumes of material up to 1,200 cubic miles (5,000 km3). Many started on the top of the volcano near where different rift zones meet at the flank of the volcano. Giant slides that start near the topographic highs of the islands carve out a semicircular amphitheater on the island, and repeated slides from different directions can carve out the island into the shape of a star. The starlike shape of many volcanic islands is therefore the result of repeated volcanism-landslide cycles, acting together to build a high and wide volcanic edifice.
Giant landslides on volcanic islands may be initiated by many causes. Most seem to be triggered by earthquakes or by the collapse of slopes of volcanoes that are inflated by magma and ready to erupt. However, in other cases the volcanoes have built up such steep and unstable slopes that relatively minor events have triggered the release of giant landslides. These events have included the stresses from storm surges, internal waves in the ocean at depth, or pronounced rainfall events.
When the slopes of the volcanoes collapse, they typically produce several different types of submarine slides simultaneously. The submarine slides may begin at the surface or underwater as slumps, where large volumes of material move outward and downward on curved fault surfaces, some of which extend to about six miles (10 km) depth. These slumps can carve out huge sections of the island, but usually move too slowly to produce tsunamis. However, this is not always the case, and the Hawaiian Islands are famous for having earthquakes generated by fast-moving slumps that in turn do produce tsunamis. For instance, in 1868 a magnitude 7.5 earthquake was generated by a slump that produced a 66-foot- (20-m-) high tsunami, killing 81 people. In 1975 a 3.5 magnitude quake occurred on Kilauea when a 37-mile- (60-km-) long section of the flank of the volcano slumped 25 feet (8 m) laterally and 12 feet downward (3.5 m), forming a 47-foot- (14.3-m-) high tsunami that killed 16 people on the shoreline. Similar slumps and earthquakes are frequent occurrences on the Hawaiian Islands, but only some produce tsunamis.
The most dangerous submarine slides are the giant and fast moving debris avalanches. These chaotic flows can start as slumps, then break into incoherent masses of moving debris that flow downslope at hundreds of miles (km) per hour. The large volumes and high speeds of these flows make them very potent tsunami generators. The largest known submarine debris flow around the Hawaiian Islands is the 200,000-year-old Nuuanu debris avalanche on the northern side of Oahu. This flow deposit is 150 miles (230 km) long, covers an area of 14,300 square miles (23,000 km2) and is more than a mile (2 km) thick at its source, making it one of the largest debris avalanches known on Earth. The sheer volume of the material in this flow, which probably moved at hundreds of miles (km) per hour, would have sent huge tsunamis moving around the Pacific. Models suggest that this debris avalanche would have caused tsunami run-ups of more than 65 feet (20 m) along the west coast of the United States.
some of the younger submarine slide deposits around Hawaii that are much smaller than the Nuuanu slide have produced wave run-ups of up to 1,000 feet (305 m) on nearby islands, showing how locally devastating these slide-generated tsunamis may become. on the southwest side of the main island of Hawaii, two moderate-size slides, the 900-square-mile Alika 1 slide and the 650-square-mile Alika 2 slide (2,300 and 1,700 km2, respectively) released about 150 cubic miles (600 km3) of rock from Mauna Loa, excavating steep-sided amphitheaters on the island and sending the debris shooting downslope to the Pacific seafloor. Nearby islands have uncharacteristically high beach deposits that are a couple of hundred thousand years old and are probably tsunami deposits related to these slides. For instance, on the islands of oahu, Molokai, and Maui, tsunami-related beach deposits are found at elevations of 213-260 feet (65-80 m) above sea level. On Lanai, boulder ridges form dunelike features that were deposited at more than 1,000 feet (326 m) above sea level by a catastrophic tsunami from this event. A wave with a run-up height of 1,000 feet (305 m) would need to be at least 100 feet (30 m) tall when it crashed on the coast. Areas closer to the coast on Lanai and Kahoolawe were stripped of their cover and soil to heights of 300 feet (100 m), dumping this material near the shore where it was redeposited in tsunami beds from later waves in this series of crests. This wave was so powerful when it struck the little island of Lanai that it not only removed the soil cover, but cracked the bedrock to 30 feet (10 m) depth, removing huge pieces of fractured bedrock, and filled the fractures with tsunami debris.
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