lava

Lava entered Italian from the Neapolitan dialect, where it meant a stream of water caused by sudden rain — a torrent washing down a slope. The word comes from Latin lavāre (to wash). When Vesuvius erupted, Neapolitans applied their word for a rushing stream of rainwater to the rushing stream of molten rock. The analogy was visual, not geological: both lava and rainwater flowed downhill in channels, both were unstoppable, both reshaped the landscape. One was hot.

The word entered scientific vocabulary through descriptions of Vesuvius, which has erupted dozens of times in recorded history. The 1631 eruption killed approximately 3,000 people and prompted detailed written accounts that used lava for the molten material. Francesco Serao's description of the 1737 eruption is among the first published scientific uses. By the eighteenth century, lava was the standard geological term in multiple languages.

The distinction between lava (molten rock at the surface) and magma (molten rock below the surface) was formalized in the nineteenth century. The same material changes name when it crosses the Earth's surface — underground it is magma, above ground it is lava. The chemistry is identical. The vocabulary is not. Lava is magma that has been washed out of the Earth.

Different types of lava received Hawaiian names in the nineteenth century because Hawaiian volcanoes — Kilauea and Mauna Loa — are the most accessible active volcanoes on Earth. Pahoehoe (smooth, ropy lava) and ʻaʻā (rough, jagged lava) are Hawaiian words used by geologists worldwide. The Neapolitan word and the Hawaiian words coexist in geology: lava from Latin, the lava types from Polynesian.