In his alphabetical ruminations on quarrying terms, Bill Langer focuses on lava, a term whose origins are ironically rooted as much in water as in fire.
Lava — (origin) lave, Italian, “a streame or gutter suddainly caused by raine” (John Florio, A Dictionary of Italian and English, 1611).
When Mount Vesuvius erupted in 1737, there was no word for “lava”. In 1738, Francesco Serao coined the term lave (for stream) to describe molten rock, likening it to “rivers of liquefied stone” in his book History of the Eruption of Vesuvius.
In 1761, “lave” was anglicised to “lava” in a description of Vesuvius 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, The Annual Register, London, 1761)
Indeed, lava flows are streams of molten rock. The surfaces of lava flows take on two basic shapes, and are described in Hawaiian terms. `A`a (pronounced ”ah-ah”) is the term for lava flows with a rough rubbly surface composed of broken lava blocks. The surface of a solidified `a`a flow is covered with sharp, spiny fragments. To remember the term, 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 solidifies, it forms igneous rocks according to the lava composition, including andesite, basalt, dacite or rhyolite. These are descriptive geologic terms. For simplicity, commercial stone producers use the term “trap rock” for andesite and basalt.
Trap rock is derived from the Swedish word trappa, meaning “step”, because some lava flow rocks have a step-like appearance.
Trap rock has been used in architecture for centuries. The Romans used basalt known as “Pietra Aniciana” to build the Roman Empire’s roads and monuments. That same basalt was used to pave Rome’s sidewalks in the 1920s. Today, Pietra Aniciana is quarried and cut into veneers for buildings, or is cut, polished, honed, brushed or hammered for stone tile flooring and slab countertops.
Now most quarried trap rock is crushed for use in asphalt, concrete and railroad ballast. It makes up six per cent of US crushed stone production and is produced in over 350 quarries in 28 US States, including Hawaii, where over 75 per cent of the crushed stone produced is trap rock (basalt).
Think about it. Eventually the lava flowing over the road in the pahoehoe photograph 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’s that for irony?
Bill Langer is a consultant geologist. Email bill_langer@hotmail.com or visit researchgeologist.com