Hawaii State Rock
Pahoehoe is a basaltic lava flow with a smooth, hummocky, or ropy crust that forms over liquid lava. The black surface of pahoehoe covers large areas of the Hawaiian Islands and takes on a variety of sculptural forms and textures—aptly described as toes, spirals, entrails, and blisters—generated by the movement of the underlying molten rock. Pahoehoe’s congealed surface is often glassy, with dangerously sharp edges where the material is broken. Older pahoehoe may become friable, crumbling underfoot.
Despite its exotic shapes, pahoehoe is made of ordinary basalt. Basalt is a gray to black, fine-grained volcanic rock composed primarily of feldspar (AlSiO2 with potassium, sodium, or calcium) and pyroxene (Al2O6 or Si2O6 with sodium, calcium, iron, or magnesium, among others). Additional components of basalt may include olivine and iron oxide. Because pahoehoe is lower in silicates than some lavas, it is more fluid, so it erupts more gently and spreads farther over land.
A STATE SYMBOL
While Hawaii has not designated an official state rock, pahoehoe is an obvious choice because of its importance to the state’s geology and natural scenery. The Hawaiian Islands are volcanic structures, made almost entirely of basalt. Each of Hawaii’s eight main islands has at least one volcano; two of these, Mauna Loa and Kilauea, are active, Kilauea notably being in a perpetual state of eruption since 1983. Hikers traversing the pahoehoe terrain of the Big Island know that the rock beneath their feet is still active beneath the surface. Pahoehoe is characteristic of Hawaii’s dynamic geology.
The Hawaiian Islands are the tops of undersea volcanic mountains that are very, very tall. If Mauna Kea is measured from its foot on the sea floor to its summit, it rises to 33,476 feet (10,203 m), more than 12 percent higher than Everest’s elevation of 8,848 meters. The reason for the mountains is believed to be a "hot spot" in Earth’s mantle under the tectonic Pacific Plate. As the plate rides northwest along the mantle, the part of the plate directly above the hot spot turns to magma and rises. This creates a volcano on the floor of the sea. Eruptions cause the volcano to grow taller until it may rise above sea level.
During this time, the plate inches northwest, carrying the volcano with it. Eventually, the volcano clears the hot spot and becomes extinct. In fact, the volcanoes of Hawaii have been found to be progressively older as one moves northwest along the chain; the currently active volcanoes are in the southeast. The youngest part of the islands is an active volcanic seamount, Loihi, southeast of the Big Island of Hawaii.
Pahoehoe formations occur throughout the Hawaiian Islands, although the rock on the older islands no longer flows beneath the surface. Pahoehoe is also found in India in the vast Deccan Traps, site of a colossal lava flooding event 65 million years ago.
Pahoehoe’s consistency is not suitable for building or carving, but other forms of basalt are widely used. Hawaiians long ago began quarrying quality basalt for toolmaking. Around the world, hard basalt has been used as a building and monument stone since ancient times in places where it is common. Because of the prevalence of basalt in the Middle East, Armenia, Greece, and Turkey, entire ancient cities were built of the stone. The impressive black gloss of finished basalt makes it popular for sculpture.
Seventeen quarries in the Hawaiian Islands produce crushed basalt for industrial applications. Roughly 6.5 million tons of crushed traprock (basalt) were quarried in 2006, with a value of $102 million. Pahoehoe slabs are also used as facing material for distinctively Hawaiian exterior walls.
The most famous examples of pahoehoe are, of course, Hawaii’s vast barren pahoehoe landscapes. Other forms of basalt have been fashioned into some of the iconic artifacts of ancient cultures. The remarkable jet-black stone statues and statuettes of pharaonic Egypt are carved of fine-grained basalt. The renowned huge statues of Easter Island are carved of basaltic tuff.
Chemical Formula: SiO2
Color: Usually grey or sometimes brown, depending on the iron content
|Author: World Trade Press|