Chemical analyses shed light on possible origins of island ash deposits

  • USGS Hawaiian Volcano Observatory scientists examine an outcrop of reddish-brown Hilo ash during a recent study to investigate the origins of volcanic ash deposits on the Hawaii Island. Age dates of lava flows above and below the Hilo ash deposit indicate that the ash was erupted between 3,000 and 14,000 years ago. USGS photo by Z. Tamashiro.

The origin(s) of volcanic ash deposits on Hawaii Island have been an enigma, especially those found on and between Kilauea and Mauna Loa. We know that ash is from explosive eruptions, but the question has been “from which volcano?”

Previous workers concluded that the most probable source of the ash was Mokuaweoweo, the summit caldera of Mauna Loa. Others suggested that the ash came from the vicinity of Puu iki north of Hilea, mauka (upslope) of Punaluu Bay in Ka‘u. More recent work has shown that some of these deposits are from Kilauea.


The age and origin of ash deposits on Mauna Loa’s southeast flank still need to be determined. Based on recent geologic mapping of Mauna Loa (, Sheet 2), explosive eruptions that produced the ash date back to at least 49,000 years.

Zion Tamashiro, a University of Hawaii at Hilo Pacific internship Programs for Exploring Science intern, recently worked with a USGS Hawaiian Volcano Observatory geologist to investigate outcrops (visible deposits) from some of these ash-producing eruptions. Their study concentrated on three broad regions: Hilo, Pahala, and Kalae (South Point).

The ash deposits range in age from 3,000 to 49,000 years. The Hilo ash is bounded by lava flows that are around 3,000 and 14,000 years old. Ashes near Pahala are beneath a flow dated at 9,000 years. Kalae ash deposits are 13,000, 26,000, 28,000 and more than 49,000 years old.

The ash deposits also vary in thickness from 0.1 feet to 18 feet. Their consistencies range from friable (crumbly) to indurated (solid). Hilo ash is soft and mud-like, while Pahala outcrops consist of dry layers of ash with varying degrees of consolidation. in the Kalae region, ash layers are so thin, it was difficult to avoid cross-contamination between them when collecting samples.

Ash samples were collected in the field for geochemical analyses to determine the volcanic source of the deposits. Pressed pellets of ash were carefully prepared and then analyzed using an energy-dispersive X-ray fluorescence (ED-XRF) instrument at the University of Hawaii at Hilo. For quality control, a USGS basalt standard BHVO-2 was analyzed.

The ED-XRF analyses yielded chemical abundances of oxides, metals, and rare earth elements. Hilo and Kalae ash samples vary in composition, while the chemical composition of Pahala samples is consistent. Hilo ash samples have the least sodium, potassium, calcium, nickel, and strontium. Kalae ash samples are enriched in sodium, magnesium, nickel, and chromium.

Weathering of ash outcrops also varies. The degree of alteration was determined by sodium content in the ash samples because that element is easily weathered from the deposits. in areas with higher rainfall amounts, ash is subject to more intense weathering. So, Hilo samples had less sodium compared to ash from the drier Pahala and Kalae areas. However, some Kalae samples have excess sodium, most likely from seawater spray.

Where Hawaiian volcanoes are located relative to the hotspot beneath the Hawaii Island result in distinct geochemical trends. Hualalai, Mauna Loa, and Loihi are on what is referred to as the “LOA trend,” whereas Kohala, Maunakea, and Kilauea are on the “KEA trend.” Volcanoes on the same trend have similar magma chemistry that differs from the chemistry of the other trend.

Using the ED-XRF geochemical data, Tamashiro and his HVO mentor determined the chemical trend from which the ash deposits were erupted. Although their study is far from comprehensive, they discovered that the chemistry of ash samples from Pahala is comparable to Kilauea or Maunakea (KEA trend). Kalae ash has a more complex history, with some samples suggesting an origin from Kilauea or Maunakea, but others suggesting they were probably erupted from Mauna Loa or Hualalai (both KEA and LOA chemical affinities). Hilo ash deposits are too altered to reliably indicate magmatic origin.

HVO is just beginning to scratch the surface on the origin of Hawaii island’s ash deposits, with much work still to be done. Analyzing chemical compositions of the ash was useful to the ongoing research, and we greatly appreciate Tamashiro’s contributions to the study while interning at HVO. With continued investigations, we hope to shed additional light on the origin of ash deposits on the flanks of Mauna Loa.

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Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.

Volcano Activity Updates

Kilauea Volcano is not erupting. its USGS Volcano Alert level remains at NORMAL ( Updates for Kilauea are now issued monthly.

Kilauea monitoring data over the past month showed no significant changes. Rates of seismicity were variable but within long-term values. Sulfur dioxide emission rates were low at the summit and below detection limits at Puu Oo and the lower East Rift Zone. The water lake at the bottom of Halema‘uma‘u continued to slowly expand and deepen.

Mauna Loa is not erupting. its USGS Volcano Alert level remains at ADVISORY. This alert level does not mean that an eruption is imminent or that progression to an eruption is certain.

This past week, 44 small-magnitude earthquakes were recorded beneath the upper elevations of Mauna Loa; the strongest was a magnitude-2.2 on Feb. 16. Deformation indicates continued slow summit inflation. Fumarole temperature and gas concentrations on the Southwest Rift Zone remain stable.


Mauna Loa updates are issued weekly. For more info, visit

No earthquakes were reported felt in the Hawaiian Islands this past week.

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