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Geologic Provinces of the United States: Hawai'i

Hawai'i - A Chain of Shield Volcanoes

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Kilauea caldera
A lava pond formed over the new vent of Pu'u O'o, and its frequent overflows built a broad, low shield that reached its maximum height of 55 m in less than a year. From HVO photo gallery.

The Islands of Hawa'ii are a series of shield volcanoes that have built up from the ocean floor. Some of the islands are composites of extinct volcanoes, while Maui's Haleakala and four of the big island of Hawaii's volcanoes - Mauna Loa, Loihi, Hualalai and Kilauea - have erupted in the past 2,000 years.

List of National Parks exhibiting Hawaiian Geology

Shield volcanoes are the result of multiple eruptions, most of which have produced fluid basaltic lavas, which flow and harden. Due to the low viscosity of these lavas, the flows are relatively thin and travel far, building up slowly to produce a volcanic profile like that of a shield lying on the ground.

Most of the volcanoes' lavas flows from a central vent, or group of vents, although side vents are common. Basaltic lava flows also tend to form lava tubes, which allow the flow of lava to extend much further than would otherwise be possible. The tube protects the lava from cool air temperatures. Heat loss is the enemy of flowing lava; it causes crystals to form in the melt, inhibiting the movement of the lava, which becomes thicker and slower until it stops and totally crystallizes. With its sides and top insulated, lava flowing through tubes can remain melted for longer and travel much further. On Hawai'i some lava tubes feed lava directly into the sea. Click here to learn more about the Kilauea lava tubes at the HVO web site.

Kilauea caldera
Scientist takes advantage of a skylight to measure height of lava flowing in a lava tube, Kilauea Volcano, Hawai`i. From HVO photo glossary.

Underwater basaltic lava flows form things called pillow lavas. These are forms that have been quenched quickly, leaving them in globular, tooth-paste-like forms. Close to the sea level, lava that comes into contact with water shatters, forming piles of glassy shards. As these build up lava flows and hardens on top of these shards, kept out of the water by the thickness of the deposits from previous eruptions. However the layers of glassy shards are not stable, and can create hazards after many eruptions. The ground which is formed above the shards appears stable; smooth thick cliffs of unbroken lava benches look over the sea all around the coast of Hawai'i. People don't realize that the ground is prone to landsliding. Bench collapse, or the sliding of an entire section of the coast into the ocean, is a major hazard at Hawai'i Volcanoes National Park. Also hazardous are the plumes of volcanic gasses. Volcanic fume consists primarily of water vapor, carbon dioxide, and sulfur dioxide. If you visit, don't walk through the plumes, which can be poisonous!

To learn more about the hazards of Hawaii's volcanoes from the HVO website, click here.

Some of the largest volcanoes in the world are shield volcanoes. In northern California and Oregon, many shield volcanoes have diameters of 3 or 4 miles and heights of 1,500 to 2,000 feet. The floor of the ocean is more than 15,000 feet deep at the bases of the Hawaiian Islands. As Mauna Loa, the largest of the shield volcanoes (and also the world's largest active volcano), projects 13,677 feet above sea level, its top is over 28,000 feet above the deep ocean floor.

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