Volcano Watch — Kamakaiʻa Hills: what are they and why are they there?

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Visitors to the Jaggar Museum and Ka‘ū Desert in Hawai‘i Volcanoes National Park, struck by the appearance of three dark, symmetrical volcanic cones on the western slope of Kīlauea Volcano, often ask "what are they?" and "why are they there?"

KAMAKAIA Hills: what are they and why are they there?...

Shown here are three of the main volcanic cones (center and right) that make up the Kamakaiʻa Hills on Kīlauea Volcano's Southwest Rift Zone. Findings from a study of the Kamakaiʻa Hills that is currently underway by the USGS Hawaiian Volcano Observatory will likely revise the known geologic history of this remote area on Kīlauea and provide new insights on past and future eruptions along the volcano's Southwest Rift Zone. USGS photo.

(Public domain.)

The cones are the Kamakaiʻa Hills. Their Hawaiian name means "the eye of the fish," possibly because the cones, each dimpled with a cup-shaped crater, reminded early Hawaiians of the eyes on prized fish, such as ulua.

The Kamakaiʻa cones are merely the largest of a series of vents, including older, more eroded cones, spatter ramparts, and large ground cracks, that spewed lava clots and blocks of older rock for short periods. Multiple eruptions have originated along this three-mile long "mini-rift" over a period spanning at least 500 years.

The two largest eruptions, each probably lasting weeks to a few months, produced far-travelling flows and rootless lava shields similar to those that have grown around Kīlauea's East Rift Zone Pu‘u ‘Ō‘ō vent over the last three decades. The big Kamakaiʻa cones developed during explosive phases of the eruptions, producing fields of volcanic bombs, rubbly scoria, and spatter. The largest bombs in these ejecta beds exceed 1 m (3 ft) in diameter.

Unusual eruption products at the Kamakaiʻa Hills correspond with a form of lava that has no equivalent elsewhere on Kīlauea—pasty pāhoehoe with a distinctively stretched "skin" that resembles the grain one might find on pieces of old driftwood. This lava is also chemically distinctive. 

Preliminary analyses of the lava indicate that at least some of it contains much more silica than ordinary Kīlauea basalt. In fact, it is similar to basaltic andesite, a type of lava abundant in the Coast Range of Oregon and northern California. This suggests that the magma beneath the Kamakaiʻa Hills was stored for a long time before it erupted to the surface, which allowed it to evolve to a greater degree than lava found anywhere else on Kīlauea. Such chemical evolution might also explain its explosiveness.

Initial efforts to establish the ages of the Kamakaiʻa Hills lava were based on palemagnetism—measurements of ancient orientations of Earth's magnetic field preserved in the basalt. That study concluded that the Kamakaiʻa flows were older than the Footprints Ash from the A.D.1790 explosive eruption, which marked the onset of historically recorded eruptions at Kīlauea.

However, current investigations of the Kamakaiʻa Hills show that two flows are younger than the 1790 Footprints Ash. These flows probably erupted sometime between 1790 and 1823, when the first Euro-Americans visited the volcano. Reverend William Ellis, leader of that first expedition, commented that he observed "smoking chasms" in the vicinity of the Kamakaiʻa cones—highly suggestive of recent volcanic activity there.

Geologists also recently discovered sets of fossil human footprints in the Footprints Ash deposit within the Kamakaiʻa Hills. This significantly extends the area in which people are known to have been moving immediately following the 1790 eruption.

To explain why the Kamakaiʻa Hills exist requires combining several critical strands of research, including geophysics, structural geology, and geochemistry. Findings from a study presently underway at the USGS Hawaiian Volcano Observatory will likely revise the known geologic history of Kīlauea's Southwest Rift Zone.

The Kamakaiʻa Hills are positioned where the Koa‘e Fault System, which National Park visitors can easily view along the Hilina Pali Road, merges with Kīlauea's Southwest Rift Zone. They terminate westward at a bend in the rift zone near Pu‘ukou, a site of ongoing shallow earthquake activity.

Seismicity over the past few decades suggests that magma periodically intrudes from Kīlauea's summit reservoir southward to the Koa‘e Fault System, then bends to follow the Southwest Rift Zone into the area beneath the Kamakaiʻa Hills. Pu‘ukou could act as a "log jam," causing long-term storage of the trapped magma beneath the Hills.

Repeated eruptions in the Kamakaiʻa Hills might occur because occasional intrusions of fresh magma drive the older, more evolved magma to the surface. Continuous southward sliding of Kīlauea's seaward slope might also keep the Kamakaiʻa Hills corridor open and volcanically active.

Past events provide important insights into Kīlauea's future. Knowing this, we certainly expect that eruptions of unusual character are likely to break out again in this remote and interesting area on the volcano.

Volcano Activity Update

Kīlauea continues to erupt at its summit and East Rift Zone. This past week, the summit lava lake level varied between 20 m and 36 m (66–118 ft) below the vent rim within Halema‘uma‘u Crater. The 61g lava flow continued to enter the ocean near Kamokuna, with active breakouts about 2 km (1.2 mi) inland from the ocean entry. The lava flow does not pose an immediate threat to nearby communities.

Mauna Loa is not erupting. Seismicity remains elevated relative to the long-term background rate, with small earthquakes occurring mostly in the volcano's south caldera and upper Southwest Rift Zone at depths less than 5 km (3 mi). Global Positioning System (GPS) measurements show deformation related to inflation of a magma reservoir beneath the summit and upper Southwest Rift Zone, with inflation occurring mainly in the southwestern part of the volcano's magma storage complex.

One earthquake was reported felt on the Island of Hawai‘i this past week. On September 23, 2016, at 9:29 a.m., HST, a magnitude-3.2 earthquake occurred 14.2 km (8.8 mi) west of Kīlauea's summit at a depth of 8.2 km (5.1 mi).