Hawaiian Island Formation (World of Earth Science)
The Hawaiian archipelago is a group of 132 islands, reefs, and shoals in the North Pacific Ocean that extends about 1,525 mi (2,454 km) from Kure Atoll (29°N, 178°W) to the big island of Hawaii (19°N, 156°W). This string of geographically remote and geologically unique volcanic islands makes up the U.S. state of Hawaii, and includes the eight main Hawaiian Islands of Ni'ihau, Kauai, Oahu, Molokai, Lanai, Maui, Kahoolawe, and Hawaii. The islands are progressively younger in geologic age toward the southeast; Kauai and Ni'ihau are about 5 million years in age, and the big island of Hawaii is less than 0.5 million years old. Indeed, new volcanic rocks are being deposited at Mt. Kilauea on Hawaii today.
The Hawaiian Islands are the exposed summits of the southernmost seafloor mountains, or seamounts, in the Hawaiian-Emperor seamount chain. This 3,105 mi (5,750 km) line of 107 volcanoes has formed over the last 70 million years as the Pacific Lithospheric Plate has moved to the northwest over a stationary magmatic hot spot in the mantle. Each individual volcano in the seamount chain formed as heat from the Hawaiian hot spot melted the overlying oceanic crust, and generated buoyant molten rock, or magma, which migrated upward and erupted onto the seafloor as lava. Many sequential lava flows then amalgamated to form seamounts composed mainly of an iron and magnesium-rich, or mafic, volcanic rock called basalt. Eventually, some of these seamounts grew tall enough to emerge above sea level. The Hawaiian-Emperor seamounts are examples of basaltic volcanoes with low-angle slopes and wide bases, called shield volcanoes. Mauna Loa, the central volcanic peak on Hawaii, is the world's tallest and most massive mountain when measured from its submarine base. It has a total elevation of about 32,000 ft (10 km), and its base covers an area about the size of the U.S. state of Connecticut.
Ongoing northwestward migration of the Pacific Plate at 3.4 in/year (9 cm/yr) has carried all but the newest Hawaiian-Emperor seamounts away from the hot spot. As a seamount moves away from the hot spot, volcanic activity ceases, its rock base cools, and it begins to subside into the surrounding ocean crust. An aging oceanic island then sinks below sea level, and wave erosion levels off the volcanic peak, creating a flat-topped seamount called a guyot. Sometimes, coral reefs fringing a volcanic island continue to grow after the island has subsided below sea level, creating a ring-shaped carbonate island called an atoll, or annular island. English naturalist Charles Darwin (1809882) first suggested this explanation for the formation of atolls during the voyage of the HMS Beagle from 1831 to 1836.
By this mechanism of sequential island formation and subsidence, the Hawaiian hot spot has perforated the Pacific Plate with a line of volcanoes that are younger and higher toward the southeast. The Hawaiian-Emperor Chain propagated northward, beginning at least 70 million years ago, the age of the Meiji seamount at the Aleutian Trench. About 40 million years ago, a dogleg bend in the chain suggests a shift to northwestward plate motion, possibly due to the collision of the Indian subcontinent with Asia that created the Himalayan Mountains at that time. Since 40 million years ago, the Pacific Plate has moved northwest, bringing the hot spot to its present position beneath the southern shore of the island of Hawaii. The newest volcano in the Hawaiian-Emperor Chain, the Lo'ihi seamount, is presently forming on the seafloor about 25 mi (40 km) southeast of Hawaii.
Today, geologists at the Hawaiian Volcano Observatory at Mt. Kilauea, and visitors to Hawaii Volcanoes National Park, can observe active volcanic eruptions. Low-viscosity basaltic lava erupts from volcanic vents at about 1,830°F (1,000°C). The surface of fast-flowing lava streams cools to create a ropy-textured skin called pahoehoe. (Pahoehoe means "rope" in Hawaiian.) After the surface of a flow has cooled, lava may continue to move beneath the surface in lava tubes. Sometimes, dissolved volatile gases escape during cooling, and the lava forms a jumble of sharp blocks called aa. When lava flows into the ocean, it cools very rapidly to form pillow basalt, the most common submarine basaltic texture. The Hawaiian Islands are the world's best natural laboratory for the study of hot spot dynamics, basaltic volcanism and ocean island formation.
See also Volcanic eruptions; Volcanic vent