Exploring the deep source of Hawaiian volcanoes


Welcome to Hawaii Island’s fifth annual Volcano Awareness Month. Throughout January, the USGS Hawaiian Volcano Observatory, in cooperation with Hawaii Volcanoes National Park, University of Hawaii at Hilo and Hawaii County Civil Defense, will offer public talks across the island. For more information about each talk, go to HVO’s website, hvo.wr.usgs.gov.

Last January, in this column, we focused on what scientists have learned about volcanoes during some noteworthy eruptions. This year, we will explore the exact opposite — what scientists don’t know about how Hawaiian volcanoes work.

Perhaps the most basic of these unanswered questions concerns the relationship between the volcanoes at the surface and their source deep within the Earth.

Most scientists agree that Hawaiian volcanism results from a plume of hot rock that originates deep within the Earth and ascends through the crust, creating the Hawaiian “hot spot.” Because the tectonic plates that comprise the crust are slowly moving over the hot spot, eruptions fueled by the hot spot created a chain of volcanoes stretching across the Pacific Ocean. This Hawaiian-Emperor seamount chain extends more than 3,700 miles from the youngest Hawaiian volcanoes to 80-million-year-old extinct and submerged volcanoes in the northwest Pacific.

Sounds simple, but why is there a bend in this chain of volcanic islands and seamounts? Some scientists have suggested that this bend — believed to have formed around 50 million years ago — is a sign that the hot spot is not stationary. This notion implies that the plume feeding the hot spot moves — perhaps like the flame of a candle. Other evidence, however, suggests it was the Pacific Plate that changed direction while the hot spot remained relatively fixed. Although the bend is an obvious feature of the Hawaiian hot-spot track, its origin remains uncertain.

Closer to home, the hot-spot track appears broader between Oahu and Hawaii Island. If you look at the map of the volcanoes that make up the Hawaii, Maui, Lanai, Kahoolawe and Molokai, you’ll see that those volcanoes follow two parallel trends. The northern trend begins with Kilauea and progresses to the northwest through Mauna Kea, Kohala, Haleakala, West Maui and East Molokai. The southern trend starts with Loihi, the youngest volcano in the Hawaiian chain, and continues northwest through Mauna Loa, Hualalai, Mahukona, Kahoolawe, Lanai, and West Molokai.

This dual chain was first recognized in the mid-1800s and was referred to as the “Loa” and “Kea” trends after the tallest volcanoes in each line. Scientists studying the composition of the volcanoes have also found that the trends are chemically distinct. This means that a geochemist can identify whether a rock is from a Loa- or Kea-trend volcano based solely on its composition.

Why does this dual chain exist? As with the bend in the hot-spot track, there are multiple theories. One possibility is that the plume taps a region at the boundary between the mantle and core, 1,800 miles beneath the surface, that has two distinct compositions, and that each composition is preserved along the plume’s path to the surface. Another idea, advanced by University of Hawaii at Manoa scientists, is that the plume coming from the deep Earth stalls during ascent and creates a zone of hot rock a few hundred kilometers beneath the Hawaiian Islands. Differences in melting of this region before the magma rises toward the surface may thus create the dual chain.

The nature of the Hawaiian hot spot and mantle plume remain important topics for future study. Despite the lessons learned over the past century about how Hawaiian volcanoes work, our understanding of the ultimate origin of the magma that feeds them remains largely theoretical.

Next week, we’ll explore why the highest volcano in Hawaii may not be as big as you think.

Until then, you’re invited to attend our Volcano Awareness Month talks about Kilauea and Mauna Loa volcanoes. Details are posted at hvo.wr.usgs.gov. You can also email askHVO@usgs.gov or call 967-8844 for more information about this week’s talks.

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 was relatively steady. The lava lake level was about 148 feet below the rim of the overlook vent Thursday.

On Kilauea’s east rift zone, the Kahaualea 2 flow continued to advance slowly into the forest northeast of Puu Oo. The active front of the flow was about 4.7 miles northeast of Puu Oo when mapped Thursday.

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

Visit the HVO website for Volcano Awareness Month events and current 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.