Guest “How about that geology fans?” by David Middleton

And it’s not Mauna Loa…

UH researchers reveal largest and hottest shield volcano on Earth
Posted on May 13, 2020 by Marcie Grabowski

In a recently published study, researchers from the University of Hawai‘i at Mānoa School of Ocean and Earth Science and Technology revealed the largest and hottest shield volcano on Earth. A team of volcanologists and ocean explorers used several lines of evidence to determine Pūhāhonu, a volcano within the Papahānaumokuākea Marine National Monument, now holds this distinction.

Geoscientists and the public have long thought Mauna Loa, a culturally-significant and active shield volcano on the Big Island of Hawai‘i, was the largest volcano in the world. However, after surveying the ocean floor along the mostly submarine Hawaiian leeward volcano chain, chemically analyzing rocks in the UH Mānoa rock collection, and modeling the results of these studies, the research team came to a new conclusion. Pūhāhonu, meaning ‘turtle rising for breath’ in Hawaiian, is nearly twice as big as Mauna Loa.

“It has been proposed that hotspots that produce volcano chains like Hawai‘i undergo progressive cooling over 1-2 million years and then die,” said Michael Garcia, lead author of the study and retired professor of Earth Sciences at SOEST. “However, we have learned from this study that hotspots can undergo pulses of melt production. A small pulse created the Midway cluster of now extinct volcanoes and another, much bigger one created Pūhāhonu. This will rewrite the textbooks on how mantle plumes work.”

[…]

University of Hawai‘i at Mānoa School of Ocean and Earth Science and Technology

Despite being barely visible above water, Pūhāhonunis huge, larger than the Big Island.

Pūhāhonu (AKA Gardner Pinnacles) is part of the Northwestern Hawaiian (AKA Leeward) Islands.

Pūhāhonu is estimated to be 14 million years old (Middle Miocene). Interestingly (to me, anyway), this coincides with what may have been the coldest part of the Miocene Epoch, when atmospheric carbon dioxide was at a multi-million year low.

The phrase “largest and hottest shield volcano on Earth” seemed odd, because Pūhāhonu is very extinct. The “hottest” bit is the explanation as to why it is so large.

We considered four testable mechanisms to increase magma production, including 1) thinner lithosphere, 2) slower propagation rate, 3) more fertile source, and 4) hotter mantle. The first three of these have been ruled out. The lithosphere was old (∼88 Myrs) when Pūhāhonu was formed, and thus, too thick and cold to allow for greater extents of partial melting. The propagation rate was relatively fast when it erupted (87 km/Myr), so this is another unlikely reason. Source fertility was Kea-like and no more fertile than for other much smaller NWHR volcanoes. A hotter mantle remains the best mechanism to produce the large magma volumes and is consistent with the high forsteritic olivine phenocryst compositions (up to 91.8%) and the calculated high percent of melting (24%). Thus, the gargantuan size of Pūhāhonu reflects its high melting temperature, the highest reported for any Cenozoic basalt. A solitary wave within the Hawaiian plume is the probable cause of Pūhāhonu’s higher melting temperature and the resulting increased volume flux given the absence of a more fertile source for Pūhāhonu basalts, as found for many basalts from the Hawaiian Islands.

Garcia et al., 2020

References

Garcia, Michael O., Jonathan P. Tree, Paul Wessel, John R. Smith,
Pūhāhonu: Earth’s biggest and hottest shield volcano,
Earth and Planetary Science Letters, Volume 542, 2020, 116296,
ISSN 0012-821X,https://ift.tt/2T937bb.

Kürschner, Wolfram M., Zlatko Kvaček, David L. Dilcher. “The impact of Miocene atmospheric carbon dioxide fluctuations on climate and the evolution of terrestrial ecosystems”. Proceedings of the National Academy of Sciences. Jan 2008, 105 (2) 449-453; DOI: 10.1073/pnas.0708588105