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Follow an eight lesson comprehensive geologic guide
Learn about plate tectonics, the Hawaiian Hot Spot, how volcanoes form,
volcanic structure and evolution
There are three layers of the Earth: the core, the mantle, and the crust. The outermost layers of the Earth can be divided by their physical properties into lithosphere and asthenosphere.
Plate tectonics is a relatively new theory that has revolutionized the way geologists think about the earth. The theory describes the movement of the lithosphere and asthenosphere. According to this theory, the surface of the Earth is broken into large plates. The size and position of these plates change over time. The edges of these plates, where they move against each other, are sites of intense geologic activity, such as earthquakes, volcanoes and mountain building. It is based on two earlier ideas, continental drift and sea-floor spreading.
Rigid plates, called the lithosphere, are made of the crust and the uppermost mantle. The plates move on the softer, convecting mantle called the asthenosphere. Plate margins are identified by the distribution of earthquakes and volcanoes. There are 7 major plates and 20 smaller plates.
Both earthquakes and volcanoes help locate the edges of plates. Earthquakes are distributed in narrow, linear belts that circle the Earth. Volcanoes are also distributed in long belts that circle the Earth. A dramatic example is the line of volcanoes that circles most of the Pacific Ocean. This belt is known as the "Ring of Fire" because it is the site of frequent volcanic eruptions. Geologists were puzzled because not all volcanoes were at the edges of plates like the new theory proposed. In 1963, Tuzo Wilson suggested " hot spots " in the mantle supplied magma to volcanoes.
Hot Spots and Mantle Plumes
Mantle plumes are areas of hot, upwelling mantle. A hot spot develops above the plume. Magma generated by the hot spot rises through the lithospheric plate and produces an active volcano at the Earth's surface. As volcanoes move away from the hot spot, they cool and subside, producing older islands, atolls, and seamounts. The Hawaiian hot spot has been active at least 70 million years producing the Hawaiian-Emperor Volcanic Chain.
In the Pacific basin, Tuzo Wilson found three linear chains of volcanoes and seamounts. Although separated by thousands of miles, the three linear chains are parallel to each other. Wilson recorded the age of each island and found that for each chain, the islands become progressively younger to the southeast. The extreme southeast end of each chain is marked by active volcanoes.
Wilson proposed that the Hawaiian islands formed successively over a common source of magma called a hot spot. Wilson thought the hot spot was located in the relatively stagnant center of a mantle convection cell. The Island of Hawai`i is currently located above the hot spot. The older islands were once located above the stationary hot spot but were carried away as the Pacific Plate drifted to the northwest.
In 1971, W. Jason Morgan proposed that hot spots result from hot, narrow plumes of material that rise from deep within the mantle. As the hot mantle plume reaches the base of the lithosphere, it spreads laterally. Morgan proposed that radial flow away from the narrow mantle plumes caused lithospheric plates to drift. In all, Morgan proposed 20 different hot spots, some located along mid-ocean ridges and others, like the Hawaiian hot spot and Yellowstone, located within plates.
Evolution of Hawaiian Volcanoes
The Island of Hawai`i is made of five shield volcanoes. Other common types of volcanoes are stratovolcanoes, cinder cones, volcanic domes, and lava plains. The potential for an eruption of a given volcano is determined by its status. Volcanoes are classified as an extinct or dormant or active volcano. There is no precise distinction between an active and dormant volcano because some volcanoes remain inactive for thousands of years between eruptions.
The Hawaiian islands undergo a systematic pattern of submarine and subaerial growth followed by erosion. An island's stage of development reflects its distance from the hot spot. The age progression of the Hawaiian Islands was recognized by the early Hawaiians and incorporated into legends. In 1946, Harold Stearns recognized and defined the stages of development of Hawaiian volcanoes.
Structure of Kilauea Volcano
Structurally, a Hawaiian volcano can be divided into a summit, rift zones, and flank. The summit of a youthful volcano is defined by a caldera. For most eruptions, magma migrates to, and is stored in, a shallow reservoir beneath the summit. If conditions allow, magma can migrate from the reservoir to the summit to produce an eruption. In some cases, magma migrates into the rift zone to produce a dike. Magma can flow from the dike to the surface to produce a flank eruption. Geologists use ground movements, seismicity, and gas geochemistry to monitor a volcano.
Volcanic landforms vary with tectonic setting, composition of magma, conditions during eruption, and volume of eruption. Most volcanoes can be classified within a small number of features. These features are relatively easy to recognize and reflect how the feature formed and, in some cases, the tectonic setting of the volcano. Kilauea Volcano contains excellent examples of the volcanic landforms associated with shield volcanoes. The landforms reveal various stages of an eruption.
The eruptions of Kilauea begin at the caldera or a rift zone, usually as an echelon or fissure eruption with a curtain of fire which builds spatter ramparts. If the eruption continues, it usually consolidates to a single vent. Early stages of vent eruptions are typified by lava fountains, including arching fountains and dome fountains. These fountains build cinder cones and spatter cones. In 1790 and 1924 explosive eruptions occured at the summit of Kilauea. These eruptions produced large quantities of ash which accumulated into deposits called ash layers.
Lava flows from the vent down a spillway into a lava channel. Sometimes standing waves can result from the heavy flow. If the lava channel is sustained a lava tube (see lava tube formation) occurs. Skylights allow a view into the underground formations of a lava tube. Lava lakes can form at or near a vent when the cooling edges of the flow make their own levies, building a perched lava pond. Lava lakes are also formed when the flow is restricted, such as when lava flows into an existing pit crater.
An eruption ends when the pressure forcing the supply of magma to the surface is relieved. This can be caused by dwindling supply of magma from the hot spot itself, a blockage in the magma conduit or by the magma being diverted to another intrusion or eruption site. The drainback of magma after an eruption often results in a pit crater. Short lived eruptions can cause lava trees and lava tree molds to form. Fumaroles and solfatara are produced by pockets of hot magma remaining below the surface. See also: Kipuka, lava drapery and lava cascade.
Three basic types of materials are produced by volcanoes. Gases are released before, during, and after eruptions. Gases are classified by composition. Tephra and Pyroclastic materials are anything blown out of a volcano. Pyroclasts are classified by size and composition. Lava flows out of the volcano and across the adjacent land surface. Lava is classified by its physical appearance.
Adapted from A Teacher's Guide to the Geology of Hawaii Volcanoes National Park by Stephen Mattox, Ph.D.
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