Hawaiian Islands

The Hawaiian Islands were formed almost entirely from active volcanoes on the ocean floor. Hawaiian lava has a basaltic composition, creating two distinct flow types. The lava flows are younger and lie southward in the island chain. This matches the current volcanic activity taking place on the southern portion of the island of Hawaii. Volcanoes' age progression and alignment are evidence of a hot spot beneath the islands.

Hawaii and

As a chain, the Hawaiian Islands are remarkable regarding their volcanism and geological development. Stretching from Kure Island in the northwest to Hawaii Island in the southeast, the islands, also known as the Hawaiian Archipelago, span 2,400 kilometers. In the northwestern part of the chain, the islands rise some 5,000 kilometers above their ocean floor bases; those located in the southeastern portion are higher, with Mauna Kea and Mauna Loa, Hawaii's highest mountains, reaching up to 9,000 meters. The major islands, which include Hawaii, Maui, Oahu, Kauai, Molokai, and Lanai, make up only the last 650 kilometers of the chain. The island of Hawaii is among the largest volcanic islands in the world, ranking second only to Iceland. The geological history of the islands spans many millions of years. However, the islands are young compared to the Earth's formation. Fractures were formed across a narrow northwest region of the ocean floor from which molten rocks or lava issued forth, solidified, and gradually built the volcanic mountains layer by layer. The lava that builds such massive structures consists of thousands of thin flows.

The lavas that compose the Hawaiian Islands are of a dark, iron-rich material called tholeiitic basalt and, to a lesser extent, alkalic basalt. Tholeiitic basalt has an abundance of silica. In the molecular arrangement of silica, silicon and oxygen atoms form a four-sided crystal called a tetrahedron; the silicon atoms, in the center, are surrounded by four oxygen atoms. The Tholeiitic basalt has a low percentage of alkalines, sodium, and potassium. Alkalic basalts, in contrast, have a lower percentage of silica. Both tholeiitic and alkalic basalts usually contain olivine, a dark greenish-black glassy mineral composed of iron-magnesium silicates.

Hawaiian eruptions originate at great depths in the ocean and tend to flow rather than explode. This is due to the restraining effects of the considerable weight of ocean water upon gas expansion. Lava flows tend to be dense and form a broad shield structure. As the shield structure approaches sea level, the confining water pressure decreases, and explosive eruptions become more common. As lava rises above sea level to a subaerial environment, a material called tephra is ejected along with large fragmental material called pyroclastics. Lava flows still accrete most of the magma to the volcano, as tephra comprises less than 1 percent of eruptions. Under the enormous weight of many lava flows, the summit of the shield eventually collapses along fractures or rift zones, forming a caldera.

At this point, the number of eruptions decreases, and the erupting magma shows a marked change in composition. The magma, although still of basaltic composition, has more explosive power and is richer in alkalines with a higher percentage of gases than tholeiitic basalt. As eruptions become enriched with alkalines and gases, the lavas become more viscous, forming small, steep-sided volcanic structures called spatter cones. Volcanic activity may pause for as long as 2 million years before resuming. Erosion, subsidence, and reef-building are the main processes during this stage.

Types of Lava

When volcanism resumes, the basalts are depleted in silica, forming a viscous lava type. The fluid basaltic lava tends to pour out from structures called lava fountains and is of two distinct types. Pahoehoe lava has billowy, smooth, or ropy surfaces; Aa lava has a rough, spiny, or blocky surface. Occasionally, the two types intergrade, and classification becomes difficult. Some flows change from pahoehoe to aa as they move downslope. Although the reverse does not occur—Aa lava does not change to Pahoehoe lava—the flow of Pahoehoe near its margins will sometimes burrow beneath an Aa flow, resembling such a change. Pahoehoe lava has a greater tendency to flow, and as it continues downslope—stirring, mixing, and losing volatile gases—the lava becomes more viscous and tends to change to Aa. Pahoehoe and Aa lavas are also differentiated by small gas-shaped bubbles within the flows called vesicles. Vesicles tend to be spheroidal in shape but are twisted or irregular in the aa flows. The high fluid content of the Pahoehoe lava exerts less distortion on the gas bubbles than does the Aa lava. Some flows have moved at speeds up to 55 kilometers per hour, but the entire flow as a whole advances more slowly.

The rope-like shape of pahoehoe flows results from a dragging out and wrinkling of the still plastic and not yet solidified lava by the liquid lava moving beneath the flow. As the center of the flow moves faster, the ropy texture curves outward in the flow direction, creating small, lobe-shaped protuberances that are either smooth or hummocky. The more spectacular of the pahoehoe flows are tunnels known as lava tubes. The flowing lava crusts over on the surface, forming a roof while continuing to flow beneath. Smaller tributary tubes branch off from the main tubes, allowing lava to reach the margins of the flow. The flowing lava freezes inward from the outer margins while the center remains flowing. Active movement of the liquid is confined to a cylindrical pipe-like region near the center of the flow. As the supply of lava diminishes, the liquid no longer completely fills the interior cavity. While the supply of lava continues to diminish, the liquid surface level within the cavity lowers to form a flat floor in the tube.

The block-like surface of an Aa flow covers a massive, dense interior. The central portion of the flow moves, while the top is merely carried along. As the flow advances, irregular fragments called clinker fall from the top and are buried by the advancing lava, resulting in a layer of clinker on the bottom of the flow. The clinker fragments covering the flows tend to be very sharp and spiny and have been known to cause painful cuts and even to slash leather boots.

Pillow lava is created when the water is of sufficient depth to allow the confining pressure to prevent an explosive eruption. This also occurs when the surface of the lava is cooled by contact with ocean water to form an outer crust over a molten core. This lava comprises ellipsoidal masses the size and shape of pillows. Close examination reveals a radial internal structure.

Hawaiian Hot Spot

Radiometric methods have dated the volcanic series of the major volcanoes of the Hawaiian Islands. Results show that the lava flows of volcanoes are progressively older toward the northwest part of the chain: On the island of Hawaii, for example, the age of flows is 500,000 years, while on the island of Midway, the age of flows is 11 million years. This age progression and the chain's linear trend indicate that the newest volcanic activity should occur toward the southeast. The Hawaiian Island chain is only one of several linear chains in the Pacific, including the Marshall-Gilbert and Tuamotu Island chains, which display the northwest alignment.

It has been suggested that the Hawaiian Islands developed as the Pacific plate moved northwestward over a deep-seated melting region, or hot spot, within the mantle. A hot spot, or mantle plume, would supply magma to the overriding lithospheric plate, thus creating volcanoes. Volcanoes that had already developed would gradually move away from the hot source and eventually become dormant. If the location of the hot spot within the mantle were fixed for a relatively long time, the lithosphere would transport the volcanoes beyond the source. As this process continued, a succession of volcanic mountains would develop in the direction of plate motion. Although they are responsible for less than 1 percent of total volcanic activity, hot spots have several distinctive features consistent within the Hawaiian chain. They occur in isolated regions distant from the major lithospheric plate boundaries. They are located in regions of crustal uplift and produce linear island chains that show an age progression, and they produce volcanoes that are basaltic in composition.

The exact origin and nature of the Hawaiian hot spot have yet to be discovered. Still, it is possible to speculate on its size and location when compared with the positions of the Hawaiian Islands. Hawaiian volcanoes show alignment in two different loci. A locus of volcanic activity includes the islands of Kauai, West Molokai, Lanai, East Hawaii, and the new seamount, Loihi. Another locus of points falls across East Molokai and West Maui and includes Mauna Kea and Kilauea. Volcanoes falling on the two loci that are within the given region may experience simultaneous eruptions; it may then be said that the size of the Hawaiian melting spot approximates the distance between volcanic pairs or loci.

The hot spot hypothesis is, nonetheless, undermined by so-called rejuvenated volcanism. This is volcanic activity that resumes after a period of dormancy. The Honolulu volcanic series on the island of Oahu generated new magma after a quiet period of more than a million years, with the probable location of the hot spot some 500 kilometers away. In addition, hot spots may not be entirely stationary; for example, evidence indicates that the Hawaiian hot spot has drifted at rates of 1 to 2 centimeters per year. Plotting the locations of hot spots concerning the Hawaiian volcanoes gives significant positional errors. If hot spots are not stationary features, determining the nature of plate motion beyond 30 million years ago becomes difficult.

Geologists tend to agree, nonetheless, that whatever the nature and size of the Hawaiian hot spot, its present position is southeast of Kilauea and Mauna Loa, near the Loihi seamount. Located 40 kilometers east of Hawaii's South Point and 5,000 meters under the ocean, Loihi's long base and oval summit seem appropriate for its name, which is the Hawaiian word for “long.” Although its growth rate is unknown, its summit is estimated to break through the surface in the next 100 to 10,000 years. Loihi's caldera generally lacks large marine life, perhaps because of the high carbon dioxide emitted and the instability of the terrain resulting from earthquakes, eruptions, or rock slides on an active volcano. Temperatures of as much as 48 degrees Celsius have been measured on the summit compared to 22 degrees Celsius for the surrounding seas at that depth.

Principal Terms

aa flow: a basaltic lava flow with a surface characterized by large angular blocks

basalt: a dark-colored, fine-textured igneous rock rich in iron and magnesium with a low percentage of silicon

caldera: a large crater resulting from subsidence or collapse, with a diameter many times greater than its depth

harmonic tremor: a movement or shaking of the ground accompanying volcanic eruptions

lava tube: a cavern structure formed by the draining out of liquid lava in a Pahoehoe flow

pahoehoe flow: a lava flow having a ropy or billowy surface of basaltic composition

pillow basalt: spheroidal masses of igneous rock formed by extrusion of lava under water

pyroclastic rocks: rocks formed in the process of volcanic ejection and composed of fragments of ash, rock, and glass

shield volcano: a large, gently sloping flat lava cone with the shape of a shield and composed of numerous flows of basaltic lava

tephra: a general term that describes all volcanic ejecta

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