Volcano Watch — Exploring the deep source of Hawaiian volcanoes
Throughout January 2014, the USGS Hawaiian Volcano Observatory (HVO), in cooperation with Hawai‘i Volcanoes National Park, University of Hawai‘i at Hilo, and Hawai‘i County Civil Defense, will offer public talks across the island. For more information about each talk, go to HVO's website.
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 relation 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 moving slowly 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 over 6,000 km (3,700 mi) 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 that 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 O‘ahu and the Island of Hawai‘i. If you look at the map of the volcanoes that make up the islands of Hawai‘i, Maui, Lāna‘i, Kaho‘olawe, and Moloka‘i, you'll see that those volcanoes follow two parallel trends. The northern trend begins with Kīlauea and progresses to the northwest through Mauna Kea, Kohala, Haleakalā, West Maui, and East Moloka‘i. The southern trend starts with Lō‘ihi, the youngest volcano in the Hawaiian chain, and continues northwest through Mauna Loa, Hualālai, Māhukona, Kaho‘olawe, Lāna‘i, and West Moloka‘i.
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, 2,900 km (1,800 mi) 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 Hawai‘i at Mānoa 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 towards 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 Hawai‘i may not be as big as you think.
Until then, you're invited to attend our Volcano Awareness Month talks about Kīlauea and Mauna Loa volcanoes. Details are posted at hvo.wr.usgs.gov. You can also email askHVO@usgs.gov or call 808-967-8844 for more information about this week's talks.
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 was relatively steady. The lava lake level was about 45 m (148 ft) below the rim of the Overlook crater on Thursday, January 9.
On Kīlauea's East Rift Zone, the Kahauale‘a 2 flow continued to advance slowly into the forest northeast of Pu‘u ‘Ō‘ō. The active front of the flow was about 7.5 km (4.7 miles) northeast of Pu‘u ‘Ō‘ō when mapped on January 9.
There were no earthquakes reported felt on the Island of Hawai‘i in the past week.
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