USGS: “Stratigraphic column of the Kulanaokuaiki Tephra sequence, and an in-the-field look at this section with closer photographs of subunits K-1, K-3, and K-5. At some localities, there are interbedded lava flows that help correlate units across larger distances on Kīlauea volcano.” (USGS photos by Abigail Nalesnik.)

(BIVN) – In this week’s Volcano Watch article, the USGS Hawaiian Volcano Observatory examines the Kulanaokuaiki Tephra as a means to better understand Kīlauea volcano’s explosive eruptions of the past.

From the article, written by HVO intern and University of Delaware graduate student Abigail R. Nalesnik:

My work has been focused on finding and sampling parts of the Kulanaokuaiki Tephra, which erupted from Kīlauea 1,000–1,600 years ago (400–1000 CE).

The Kulanaokuaiki Tephra is part of the Uēkahuna Ash, and it has been best studied on the south flank of Kīlauea. Five stratigraphically distinct subunits, K-1 through K-5, make up the Kulanaokuaiki Tephra. All of them are composed of tephra—rock pieces ejected from a vent during explosive volcanic eruptions.

After they are ejected from the vent, tephra travel through the air for a distance; where they fall is determined in part by their shape and mass. These particles may travel far when a large explosive eruption has a plume high enough to reach trade wind and even jet stream heights. The plume that produced some of the Kulanaokuaiki Tephra deposits on the south flank of Kīlauea is estimated to have reached 9–11 miles (14–18 km) high!

I am studying the Kulanaokuaiki Tephra deposits to learn more about where the tephra settled, how the eruptions happened and what the physical properties of the tephra itself can share about magmatic processes. It’s important to study a tephra unit with primary deposition, meaning that the deposit appears exactly as it was originally deposited.

Let’s examine the typical sequence of Kulanaokuaiki Tephra from the bottom (the first eruptions) to the top (the last eruptions). K-1 is usually the thickest of the units and is commonly found resting on the Kīpukanēnē lava flow field (350–190 BCE). It is made up of small 0.2–0.4 inch (0.5–1 centimeter) pumice from high lava fountains that lofted this lighter material for several miles (kilometers).

K-2 is reworked and appears similar to K-1 but is chemically distinct making it useful as a geochemical marker between field sites.

There is a stark color and grain size change in K-3. It’s mostly made up of dark scoria ranging from less than 0.04 inches (1 millimeter) to over 1.2 inches (3 centimeters) in size. K-3 is a loose deposit and if I poked an exposure of it with a paint brush, pieces quickly fall out of the outcrop. I sub-sample K-1 and K-3 at several localities, which will be useful to explore the properties of each unit from beginning to end.

K-4 is a bright orange, fine grained layer that feels like cement when trying to dig into it. You certainly know it when it is present, so like K-2, it is a useful marker in the stratigraphy.

Lastly, K-5 is a scattered scoria deposit. It’s not always present but it begins poking out when you dig down from the top to sample. It can sometimes also be found embedded in the top of K-4!

During my ventures on Kīlauea’s south flank, I’ve found that the thickness of subunits K-2 and K-4 are variable over short distances and are typically thicker in low-lying areas. Does this mean that more tephra truly fell in these localized areas (unlikely), or was some of it moved by wind or water in a process known as reworking?

We ask these questions because explosive eruptions like those that generated the Kulanaokuaiki Tephra have never been seen in the modern era at Kīlauea, so only their primary deposits can give us clues. The variable thickness of K-2 and K-4 suggests that the deposit was reworked, so my study of the Kulanaokuaiki eruptions focuses on units K-1, K-3, and K-5.

Farther down Kīlauea’s south flank, the Kulanaokuaiki Tephra deposit quickly changes. K-5 disappears, lost in the weeds of the soil and local vegetation. The bright orange K-4 becomes thinner, and the grains of K-3 become smaller. The once-abundant K-1 also disappears, as we are farther from the assumed source area of the summit.

As I follow these units to the coast, the only remaining discernible unit is K-3. This part of the deposit made it all the way to Halapē and beyond, ultimately settling in the ocean.

After making observations and sampling the Kulanaokuaiki Tephra deposit in the field, I’ll begin the work of analyzing samples in the USGS Hawaiian Volcano Observatory’s new state-of-the-art lab. My work will help to better interpret the explosive eruptions that produced the Kulanaokuaiki Tephra, giving us more insight into Kīlauea’s eruptive processes over time.