USGS

Hawaiian Volcanic Products, Landforms,
and Structures

 The volcanic mountains of Hawaii have been built by the accumulation of basalt flows erupted over hundreds of thousands of years, as the Pacific Plate moved northwestward over the hot spot. In contrast, the volcanic mountains in the zones where tectonic plates converge, such as Mount St. Helens and the other volcanoes of the Cascade Range, have been built primarily by pyroclastic debris. Even though they both form linear mountain ranges, the Hawaiian volcanoes differ greatly from the Cascade volcanoes in mode of origin and types of volcanic rocks.

Molten lava can solidify in a variety of ways, depending on eruption conditions and gas content of the erupting magma. Volcanic products of Hawaiian eruptions are mostly dark in color but vary widely in form and texture.

Lava flows

Lava flows form more than 99 percent of the above-sea parts of Hawaiian volcanoes. Pahoehoe (pronounced "pah-hoy-hoy") and aa (pronounced "ah-ah") are the two main types of Hawaiian lava flows, and these two Hawaiian names, introduced into the scientific literature in the late 19th century, are now used by volcanologists worldwide to describe similar lava-flow types. Pahoehoe is lava that in solidified form is characterized by a smooth, billowy, or ropy surface, while aa is lava that has a rough, jagged, spiny, and generally clinkery surface. In thick aa flows, the rubbly surface of loose clinkers and blocks hides a massive, relatively dense interior.
Center: An active clinkery aa lava flow advances over the smooth surface of earlier erupted pahoehoe lava during the 1972-74 Mauna Ulu eruption. (Photograph by Robert I. Tilling.) Right: Closer views of the surface of an aa flow (above) and a pahoehoe flow (below). (Photographs by J.D. Griggs and Taeko Jane Takahashi, respectively.) Aa lava flow Closeup of aa

Closeup of pahoehoe

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Updated 05.01.97