Assembling Hawai‘i Island's volcanoes: Does size really matter?

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In this second of four Volcano Watch articles addressing the "big" questions faced by volcanologists studying Hawaiian volcanoes today, we will focus on some notions about how the Island of Hawai‘i may have been constructed.

Assembling Hawai‘i Island's volcanoes: Does size really matter?...

Colors ranging from red to purple indicate the water depth around the Island of Hawai‘i, while shades of gray show land topography above sea level. Red shows lava flows erupted over the past 200 years. The Puna Ridge represents the submarine extension of Kīlauea's east rift zone. The Hilo Ridge, originally thought to be a rift zone of Mauna Kea, may, in fact, be connected to Kohala.

(Public domain.)

In some ways, piecing together the Island of Hawai‘i can seem quite straightforward. The island is made up of five volcanoes—Kohala, Hualālai, Mauna Kea, Mauna Loa, and Kīlauea. A sixth volcano, Māhukona, is located beneath the water off the island's northwest coast (Lō‘ihi, off the southeast coast, is not part of Hawai‘i Island). The volcanoes grew sequentially, beginning with Kohala, the oldest exposed volcano, which hasn’t erupted in over 60,000 years. The youngest, Kīlauea, is currently erupting from two vents.

Calculating the size of the entire island is easy to do using maps of land- and sea-floor elevations. But how do you calculate the sizes of the individual volcanoes, which overlap with one another?

Consider Mauna Kea, which reaches 4,205 m (13,796 ft) in elevation and is the tallest volcano on the island (Mauna Loa is only about 35 m, or 120 ft, shorter). But is it really as "big" as it seems?

Off the southeast coast of Mauna Kea is the Hilo Ridge—a submarine ridge that extends 50 km (30 mi) offshore, much like Kīlauea's east rift zone and its submarine extension, the Puna Ridge. Early interpretations argued that the Hilo Ridge was a rift zone of Mauna Kea. The mapping of the ridge and determining its chemical composition, however, suggests a different association—that it very well may be a rift zone of the older Kohala volcano.

This alternative interpretation suggests that Mauna Kea is a smaller volcano perched on top of the older, more massive Kohala. In fact, Kohala may be as much as three times larger in volume than Mauna Kea. Ponder that on the next clear day, when the tallest point on the island can be seen from Hilo, Waimea, Waikōloa, and all points in between.

Likewise, there is disagreement over the size of Kīlauea. Submarine mapping of the south coast of Kīlauea has revealed rocks from Mauna Loa! This finding implies that Kīlauea has grown on the flank of the much larger Mauna Loa. But is Kīlauea merely a small bump on Mauna Loa's side or does it deeply cut into Mauna Loa's edifice, effectively wedging apart the older and larger volcano? The answer to this question has direct bearing on the eruptive activity that we should expect from Kīlauea in the future since volcano size and eruption rate are related.

We know that Kīlauea has been erupting vigorously for about 100,000 years. Yet its eruption rate over that period must be low if Kīlauea is considered to be a small volcano, since size and eruption rate are related. In this sense, the last several decades of continuous eruptive activity may be unusual (and might not continue far into the future).

If, on the other hand, Kīlauea is considered to be a large volcano, the eruption rate over the past 100,000 years must have been high, and the current eruptive activity would be normal. The same holds true for other volcanoes of Hawai‘i Island—if we know their size, we can deduce their long-term eruption rates. This information is important, not only for understanding the evolution of Hawai‘i's volcanoes, but also their hazard potential.

Clearly, scientists are still struggling to understand how Hawaiian volcanoes grow. How the volcanoes come apart is also an important issue and will be the topic of next week's Volcano Watch!

Until then, you're invited to attend our upcoming Volcano Awareness Month talks in Hawai‘i Volcanoes National Park and in Kona on January 21 and 22, respectively. Details are posted at hvo.wr.usgs.gov. You can also email askHVO@usgs.gov or call 808-967-8844 for more information.

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Volcano Activity Update

A lava lake within Halema‘uma‘u produced nighttime glow that was visible via HVO's Webcam during the past week. Summit tiltmeters recorded minor variations, but overall the tilt level remains relatively steady. No deflation–inflation cycles (DI events) have occurred since mid-December. The lava lake level was about 44 m (144 ft) below the rim of the Overlook crater on Thursday, January 16.

On Kīlauea's East Rift Zone, the Kahauale‘a 2 flow continued to advance slowly into the forest northeast of Pu‘u ‘Ō‘ō. Satellite imagery showed the active front of the flow to be 7.8 km (4.8 miles) northeast of Pu‘u ‘Ō‘ō on January 14.

There were no earthquakes reported felt on the Island of Hawai‘i in the past week.