Hawaiian Volcanic Products, Landforms,
|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 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.