Contrary to conventional geological thinking, the Hawaiian mantle plume has gotten hotter by about 250°C (480°F) over the past 47 million years. This discovery, led by Earth scientists at the University of Hawai'i at Mānoa, reverses the long-held idea that hotspots start out very hot and progressively cool with time. The study, published recently in Earth and Planetary Science Letters , also found that heat surges produced the two largest volcanoes along the Northwestern and Main Hawaiian Island chain.
The Hawaiian Ridge consists of 65 volcanoes spread over 3500 km (2179 miles) that were formed during the past 47 million years as the Pacific plate slowly moved over the Hawaiian hotspot, where a plume of molten material flows out of Earth's crust. A team of researchers, led by Michael Garcia, Emeritus Professor of Earth Sciences in the UH Mānoa School of Ocean and Earth Science and Technology , set out to evaluate hypotheses that might explain the 100-fold fluctuation in lava volume along this ridge. By examining olivine basalts from 16 volcanoes, the researchers assessed four testable causes, including changes in lithosphere thickness, magma source composition, rate of the Pacific plate movement, and plume temperature.
"It was a major surprise to find such a strong, direct correlation between mantle temperatures and volcano size," said Garcia. "Other potential explanations simply failed to explain the data."
To uncover this connection, the team developed a cutting-edge "geothermometer" to estimate the temperature of the lava when it first formed deep within the Earth's mantle. They paired these temperatures with recent, comprehensive ocean floor surveys to calculate more accurate estimates of volcano size along the Hawaiian Ridge.
The results show an overall increase in lava temperature with two heat surges: One at 14-20 million years ago (making the largest and longest lived shield volcano in the last 60 million years, Pūhāhonu) and the other making the Hawaiian Islands (0-6 million years ago).
"The development of the new geothermometer allows one to better understand the history of volcanoes and why some are larger than other volcanoes," Garcia added.
The researchers suggest that the overall increase in the Hawaiian mantle plume temperature is possibly related to drifting of dense, hot material in the lowermost mantle.
