Vesicular porphyritic olivine basalt from the upper Holocene of Hawaii (~6.5 cm across at its widest)

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.

This interesting piece of lava is from the 13 January to 19 February 1960 Kapoho Eruption at the easternmost point of Hawaii. The former town of Kapoho was located in the Kapoho Graben (Puna Rift Zone) segment of Mt. Kilauea’s East Rift Zone. Kapoho was famously burned, buried, and destroyed by aa lava flows erupting continuously for a little over a month in early 1960, despite the hasty construction of barriers. The lava flows overtopped the barriers and even “burrowed under” the barriers.

The rock is a vesicular porphyritic olivine basalt (more technically, it can be called vesicular tholeiitic picrite basalt). It has abundant subrounded to stretched vesicles and common large greenish forsterite olivine phenocrysts. The basalt matrix is lustrous and cryptocrystalline, with very small crystals of plagioclase feldspar and pyroxene. The sample has an obvious black-and-white speckled xenolith of gabbro from the lower oceanic crust.

This lava specimen comes from the post-30 January 1960 phase of the Kapoho Eruption. Starting the 31st of January, observers noted that large olivine phenocrysts were becoming common in the newly erupted lavas.