Volcano Watch — A geologic tour of the Hawaiian Islands: Kaua‘i and Ni‘ihau

Throughout January, the USGS Hawaiian Volcano Observatory (HVO), in cooperation with Hawai‘i Volcanoes National Park and the University of Hawai‘i at Hilo will offer public talks around the island. For the complete schedule and details, please visit HVO's website (http://hvo.wr.usgs.gov).

As in years past, our January "Volcano Watch" articles will explore a theme associated with Hawaiian volcanism. The subject for this month's series is the Hawaiian island chain itself. Each week we will explore the volcanic history of an island or island group, beginning with Kaua‘i and Ni‘ihau.

But first, let's review the basic model for hotspot island formation—a model first proposed by Harold Stearns, a USGS geologist who mapped most of the Hawaiian Islands in the 1930s and 40s.

Stearns recognized four stages to the growth of Hawaiian volcanoes: (1) preshield (or submarine), when a volcano first starts to slowly grow underwater; (2) shield, when eruptive activity is frequent and the volcano rapidly builds above sea level; (3) postshield, when volcanic activity starts to wane and erosion outpaces resurfacing by lava; and (4) rejuvenated, when infrequent, isolated, and small eruptions might occur up to millions of years after the postshield stage ends. All Hawaiian volcanoes experience preshield and shield stages, but, for reasons that are still unclear, not all go through the postshield and rejuvenated stages.

The stages of Hawaiian volcanism are defined by the vigor of eruptive activity, as well as the chemical composition of the erupted lava, which changes over time as the volcano is rafted away from the hotspot. New data and insights continually refine this model, but the overall tenets remain largely unchanged since first proposed by Stearns over 70 years ago.

Unlike other Hawaiian islands, both Kaua‘i and Ni‘ihau are single volcanoes rather than amalgamations of overlapping volcanoes, and the two islands were never connected. Ni‘ihau probably formed first, since it is farthest west in the chain and volcanoes to the east are younger, but data indicating the exact onset times of the two volcanoes do not exist. We only know that both volcanoes formed about 6 million years ago.

Like all Hawaiian islands, both Ni‘ihau and Kaua‘i experienced periods of massive landslides throughout their histories, the evidence of which is preserved on the ocean floor as jumbles of rocky debris. Ni‘ihau was once much larger but the bulk of the island collapsed around 5 million years ago. The submarine collapse deposits were subsequently covered by lava during the growth of Kaua‘i.

The geology of Kaua‘i is complex. There is evidence for a huge caldera in the east-central part of the island, but it appears to be mostly a collapse feature that was then filled by lava. Geophysical data suggest that the main center of volcanism was more or less beneath the LIHUE Basin, which subsided (by either collapse or faulting) between 3 and 4 million years ago. The basin was subsequently filled by marine sediment and lava as it alternately subsided below sea level and then grew above sea level with lava inundation.

Vigorous shield-stage volcanism on Ni‘ihau and Kaua‘i ended by about 4 million years ago, after which, the islands' spectacular canyons and cliffs began to form. Interestingly, rejuvenated volcanism has been long-lived on both islands. Elsewhere along the island chain, rejuvenated volcanism is minor. The gap between shield and rejuvenated eruptions on Ni‘ihau was about 2 million years, with the most recent eruption occurring about 350,000 years ago.

On Kaua‘i, rejuvenated volcanism has occurred more or less continuously for the last 3.5 million years. The most recent eruption was only 150,000 years ago in the south part of the island, where black rock around the blowhole near POIPU and cinder cones around KOLOA look similar to young volcanic rocks on the Island of Hawai‘i. In fact, these eruptions are young enough to suggest that rejuvenated volcanism on Kaua‘i is not yet over, but the odds of future eruptions in our lifetimes are small.

Next week we explore the geological development of O‘ahu. Between now and then, Volcano Awareness Month talks will be held at Hilo's Lyman Museum on January 11 and in Hawai‘i Volcanoes National Park on January 12. Please join us!

Volcano Activity Update

Kīlauea continues to erupt at its summit and East Rift Zone. During the past week, the summit lava lake level varied between about 28 and 35 m (95–115 ft) below the vent rim within Halema‘uma‘u Crater. On the East Rift Zone, scattered lava flow activity remained within about 6 km (4 mi) of Pu‘u ‘Ō‘ō and is not currently threatening any nearby communities. 

Mauna Loa is not erupting. Seismicity remains elevated above long term background levels. GPS measurements continue to show deformation related to inflation of magma reservoirs beneath the summit and upper Southwest Rift Zone of Mauna Loa.

Two earthquakes were reported felt on the Island of Hawai‘i this past week. On Thursday, December 31, 2015, at 4:06 p.m., HST, a magnitude-2.6 earthquake occurred 4.7 km (2.9 mi) southwest of Kea‘au at a depth of 16.4 km (10.2 mi). On Friday, January 1, 2016, at 9:38 a.m., HST, a magnitude-2.8 earthquake occurred 16.2 km (10.1 mi) southeast of Waimea at a depth of 15.5 km (9.6 mi).