Why Two Volcanoes in Hawaii Are So Close, but So Different

SCIENCE

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Discussion Ideas

All Hawaiian volcanoes, including Mauna Kea and Mauna Loa, formed over the mantle plume responsible for the Hawaiian hot spot. What is a mantle plume? Read through the “mantle plume” section in our resource on the mantle for some help.
- According to our resource, “A mantle plume is an upwelling of superheated rock from the mantle. Mantle plumes are the likely cause of hot spots, volcanic regions not directly created by plate tectonics. As a mantle plume reaches the upper mantle, it melts into a diapir. This molten material heats the asthenosphere and lithosphere, triggering volcanic eruptions.”
- According to the New York Times, “Scientists have seismic evidence that the deep part of the mantle is a graveyard where long ago slabs of earth were subducted, or thrust underneath one another, creating separate regions with different chemical compositions that eventually made their way to the surface in a hot mantle plume, or upwelling, as the core heated the rock into magma.”

Scientists have known about the the double-track volcanism of Mauna Kea and Mauna Loa since 1849. How did volcanologists know the volcanoes were on different tracks?
- The rocks of Mauna Kea and Mauna Loa have very different chemical compositions. Loa-type basalts have lower amounts of lead, and higher amounts of silicates, for instance. Mauna Loa also “contains higher proportions of an igneous rock called pyroxenite, which comes from subducted ocean crust.” (Subduction, so awesome.)

How does the same mantle plume produce different chemical compositions for Kea and Loa? Read the New York Times article and take a look at the diagram above for some help.
- The two volcanoes are releasing magma from different parts of the magma plume. Rocks formed from magma at Mauna Loa are from a shallower, lower-pressure zone of the plume than thosed formed at Mauna Kea. These different zones produce different chemical signatures. The authors particularly point to the presence of “secondary pyroxenite [which] dominates the low-pressure melt region beneath Loa-track volcanism” but does not dominate Kea-track volcanism.

How did the movement of the Pacific plate help create different chemical signatures of Kea and Loa three million years ago? Are the twin tracks going to stay the same, grow further apart, or reconnect in the future?
- Three million years ago, the Pacific plate rather abruptly shifted course north. “This shimmy rearranged zones of magma that are heated under different pressures in the shallower part of the mantle … Previously stacked on top of one another, the movement of the plates exposed now geographically separates magma zones that fed the volcanoes individually.”
- Geologists think that the mantle plume has undergone a degree of “vertical rebound” and the double-track volcanism of the Hawaiian hot spot will eventually return to single-track volcanism.

Can geologists observe and test their theory of double-track volcanism on other volcanoes or volcanic island chains?
- Yes, scientists are looking forward to modeling and analyzing the chemistry of volcanoes that are part of the Hawaiian-Emperor seamount chain, and elsewhere in the Pacific.
  - Hawaii-Emperor seamount. The chemistry of Hawaii’s older volcanoes, on the northwestern islands of Kauai and Oahu, have both Kea and Loa chemistries. The Pacific plate’s shift in direction dramatically impacted the younger volcanoes on Molokai (which exhibits Kea volcanism), Lanai (which exhibits Loa volcanism), and even other volcanoes on the “Big Island” of Hawaii itself. The active volcano Hualalai exhibits Loa-type volcanism, while the active volcano Kilauea exhibits Kea-type volcanism. The infant Loihi Seamount, the next great Hawaiian volcano, exhibits Loa-type volcanism.
  - elsewhere in the Pacific. According to the research, “all volcanic chains on the Pacific plate that have displayed persistent volcanism for the past 5 million years show this trend.” For example, double-track volcanism exists over hot spots below the Galapagos, Samoan, Society, Easter, and Marquesas islands.