Tachylite from the upper Holocene of Hawaii. (field of view ~4.2 cm across)

Mt. Kilauea is the world’s most active volcano. It sits atop the Hawaii Hotspot in the central Pacific Ocean. It had a decades-long eruption from 1983 to 2018. Kilauea lavas are basaltic in composition, but the physical appearance of Kilauea’s output varies tremendously. Many of Kilauea’s ongoing basaltic eruptions have occurred along its East Rift Zone, a fracture system extending ~eastward from the summit vent area.

The rock shown here is from the thin ceiling of a lava tube that was active during the 1969 to 1974 eruption of the Mauna Ulu Vent. The Mauna Ulu Lava Field is located along Kilauea’s East Rift Zone. This lava tube ceiling specimen ranges from 1 to 3.5 cm thick.

The photo depicts the interior (lower) surface of a lava tube ceiling. Unlike the external surface (see elsewhere in this photo album), the inside roof of the lava tube doesn’t display stretched textures. The heat inside the lava tube was sufficient to partially re-melt already solidified lava, resulting in a relatively smooth and lustrous surface, somewhat like chocolate cake batter. The blackish-colored basaltic glass is called tachylite.