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Fire vs Water: Erosional/Depositional Geology, Hawaiian Islands
browse graphic Even a casual, untrained observer will see evidence that opposing forces have formed the Hawaiian Islands. The massive and lofty volcanoes have been scoured, abraded, and lacerated by streams and the sea. On the oldestislands (in the northwest) the volcanic mountains have been eroded and deeply cut by canyons. Even farther north are coral reefs and atolls--all that remain of former islands. This slide set examines volcanic features and their erosion by waves, glaciers, streams, and storms. The images include dramatic examples of Hawaii's unique geology. The Erosional and Depositional Geology of the Hawaiian Islands. Even a casual, untrained observer will see evidence that opposing forces have formed the Hawaiian Islands. The massive and lofty volcanoes have been scoured, abraded, and lacerated by streams and the sea. The early Hawaiians themselves recognized this conflict between the volcanoes that built the islands and the forces that eroded them. The people created stories about two goddess sisters: Pele and Na Maka O Kaha'i. Pele, the goddess of fire was depicted as always building a new home for herself ( volcano). Na Kaka O Kaha'i, the evil goddess of the sea, continually hunted out Pele and attempted to destroy her home. This is an apt description of the geology of the islands. On the oldest islands (in the northwest), the volcanic mountains have been eroded and deeply cut by canyons. Even further north are coral reefs and atolls all that remain of former islands. The older islands show evidence of a resurgence of volcanism. The myths attribute this to Pele's attempt to fight back against the force of water that sought to destroy her homes. Scientists believe that continents and ocean basins drift, or move slowly, relative to eachother and to Earth's hot interior. The Pacific Ocean floor is drifting to the northwest over a hot spot the source of the molten material which is building the volcanic islands. While olderislands erode, new islands form to the southeast as new sections of the ocean floor move over thehot spot. On these islands, the geological features are on a grand scale. On the Big IslandHawaii), Mauna Loa and Mauna Kea rise from the ocean depths of 5,486 m (18,000 ft) to a maximum elevation above sea level of nearly 4,267 m (14,000 ft). They are among the tallest mountain masses on Earth. The islands are characterized by cliffs, or palis, some of which may have originated as the subaerial scars above giant submarine landslides. These cliffs are striking and steep, and deeply grooved as though Na MaKa O Kaha'i herself had attempted to draw more land into the sea with her long fingernails. As a new Hawaiian volcano was born on the ocean floor, pillow lava and a rubble of fragmented lava formed the submarine portion of the future island. When the volcano finally emerged above the sea, the submarine rubble and debris were covered by solid layers of lava flows. The weight of these layers pressed on the underlying weak debris. As the huge shield volcano settled under its own weight, cracks opened that permitted magma to intrude laterally and reach the surface on the flanks of the shield. Some of the cracks became planes of weakness in the underlying rubble, allowing the volcanic slopes to slide massively down and out onto the deep floor. On the emergent part of the volcano, lava flows continued to build the island, while the forces of erosion, waves, wind, rain, freshwater streams, landslides, and even glaciers carved the land and delivered sediment to the sea. Some of the larger valleys and canyons have eroded where streams took advantage of weak zones in faulted bedrock. Hawaii's beaches have many colors of sand. White sand, the most abundant, is of marine origin, derived from skeletal fragments of shallow-water marine organisms. Dark-gray detrital sand is eroded basalt, deposited mainly by streams, but also by waves. Tan sand is a mixture of the white and gray. Green sand is created as waves attack olivine-bearing cinder and tuff. Black sand commonly forms when hot lava enters the cooler ocean, fragments, and is deposited on the beach. All these features combine to give the islands of Hawaii a unique and incomparable beauty.