December 1, 2010
L is for Lava
From Mt. Vesuvius to Hawaii, molten rock continues to leave its mark.
By Bill Langer
Lava (origin): lave, Italian, ‘a streame or gutter suddainly caused by raine’ (John Florio, 1611, A Dictionary Italian & English).
It is ironic that when Mt. Vesuvius erupted in 1737, there was no word for lava. Shortly after, though, Francesco Serao applied the term ‘lave’ (which means stream) to describe molten rock, likening it to ‘rivers of liquefied stone’ in his book ‘Istoria dell’incendio del Vesuvio’ (History of the Eruption of Vesuvius – circa 1738). During 1761, lave was anglicized to lava in a description of Vesuvius probably based on Serano’s work; ‘On the 21ſt ult. and ſeveral dayſ following, all the neighbourhood of Mount Veſuvius was overflowed by a deluge of burning bitumen, called lava’ (R. & J. Dodsley, 1761, The Annual Register, or a View of the History, Politicks, and Literature of the Year 1760, London).
Indeed, lava flows are streams of molten rock like those described by Serano and Dodsley. The surfaces of lava flows commonly take on two basic shapes and are described in Hawaiian terms. `A`a (pronounced ah-ah) is the term for lava flows that have a rough rubbly surface composed of broken lava blocks. The surface of a solidified `a`a flow is covered with sharp, spiny fragments. If you are looking for a way to remember the term, just think what you would say if walking barefoot on these rocks.
Pahoehoe is lava with a smooth, hummocky, or ropy surface covered with bizarre rounded shapes that are referred to as lava sculpture. Pahoehoe typically advances as small lobes that break out from a cooled crust.
When a lava flow stops moving and solidifies, it forms a variety of igneous rocks according to the composition of the lava, including andesite, basalt, dacite, or rhyolite. These are descriptive geologic terms, and for simplicity, commercial stone producers use the term traprock when referring to fine-textured, dark-colored igneous rocks such as andesite and basalt. The term traprock is derived from the Swedish word trappa, meaning step, because some lava flow rocks have a step-like appearance.
People have used traprock in architecture for centuries. For example, the ancient Romans used basalt known as Pietra Aniciana to build the roads and monuments of the Roman Empire. That same basalt was used to pave the sidewalks of Rome in the 1920s. Today, Pietra Aniciana is quarried and cut into veneers for buildings or is cut, polished, honed, brushed, or hammered for high-end uses such as stone tile flooring and slab countertops.
More recently, most quarried traprock is crushed for use in asphalt, concrete, and railroad ballast. Crushed traprock makes up about 6 percent of the U.S. crushed stone production and is produced from more than 350 quarries in 28 states. Hawaii is one of those states; more than 75 percent of the crushed stone produced in Hawaii is traprock (basalt).
Think about it. Eventually, the lava flowing over the road in the photograph of pahoehoe will solidify and become the same type of rock that was crushed and used as aggregate in the asphalt pavement that the lava is destroying. How is that for irony?
Bill Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey and can be reached at 303-236-1249 or via e-mail at firstname.lastname@example.org.