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Assembling Hawaii Island’s volcanoes: Does size really matter?


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

In some ways, piecing together the island can seem quite straightforward. It is made up of five volcanoes — Kohala, Hualalai, Mauna Kea, Mauna Loa and Kilauea. A sixth volcano, Mahukona, is located beneath the water off the island’s northwest coast — Loihi, off the southeast coast, is not part of Hawaii Island. The volcanoes grew sequentially, beginning with Kohala, the oldest exposed volcano, which hasn’t erupted in more than 60,000 years. The youngest, Kilauea, is currently erupting from two vents.

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

Consider Mauna Kea, which reaches 13,796 feet in elevation and is the tallest volcano on the island — Mauna Loa is only about 120 feet 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 30 miles offshore, much like Kilauea’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: 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, Waikoloa and points in between.

Likewise, there is disagreement over the size of Kilauea. Submarine mapping of the south coast of Kilauea has revealed rocks from Mauna Loa. This finding implies that Kilauea has grown on the flank of the much larger Mauna Loa. But is Kilauea 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 Kilauea in the future since volcano size and eruption rate are related.

We know that Kilauea has been erupting vigorously for about 100,000 years. Yet its eruption rate over that period must be low if Kilauea 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, Kilauea 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 Hawaii 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 Hawaii’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 Hawaii Volcanoes National Park on Tuesday and in Kailua-Kona on Wednesday. Details are posted at hvo.wr.usgs.gov. You can also email askHVO@usgs.gov or call 967-8844 for more information.

Kilauea activity update

A lava lake within Halemaumau produced nighttime glow 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 have occurred since mid-December. The lava lake level was about 144 feet below the overlook vent rim Thursday.

On Kilauea’s east rift zone, the Kahaualea 2 flow continued to advance slowly into the forest northeast of Puu Oo. Satellite imagery showed the active front of the flow to be 4.8 miles northeast of Puu Oo Tuesday.

There were no earthquakes reported felt on Hawaii Island in the past week.

Visit hvo.wr.usgs.gov for Volcano Awareness Month events and Kilauea, Mauna Loa and Hualalai activity updates, recent volcano photos, recent earthquakes and more; call 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov.

Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.