What volcanoes are there on Oahu

Largest shield volcano on earth discovered

Hidden giant: A good 900 kilometers northwest of Honolulu is the largest and hottest shield volcano on earth - the underwater volcano Pūhāhonu. Although only the top tip protrudes from the water, the entire fire mountain covers a volume of 148,000 cubic kilometers, as scientists have determined. This makes the Pūhāhonu twice as large as the previous record holder Mauna Loa. Its magma is also the hottest in the entire Hawaiian chain.

Hawaii is known for its large, active volcanoes and is considered a textbook example of hotspot volcanism. The 6,000-kilometer chain of islands and submarine volcanoes was created when the earth's crust migrated over a mantle plume - a hot upflow of magma in the earth's mantle. To this day, it provides the lava supply for the mighty Hawaiian volcanoes. The Mauna Loa was even considered to be the largest shield volcano on earth.

Twice the size of the Mauna Loa

But now there is a new record holder: the Pūhāhonu. This mountain of fire is located a good 900 kilometers northwest of the island of Oahu and is part of the chain of small islets into which Hawaii ends in the northwest. From the Pūhāhonu, only Gardner Island can be seen above water, a 52-meter-high rock with two smaller neighboring peaks. But as early as 1974 geologists suspected that the rest of this volcanic mountain hidden in the sea could be enormous.

Now this has been confirmed. Researchers led by Michael Garcia from the University of Hawaii in Manoa have re-measured the Pūhāhonu and analyzed its rock composition and structure. The result: "At 148,000 cubic kilometers, this 12.5 to 14.1 million year old volcano is twice the size of Mauna Loa, which was previously not only considered the largest volcano in Hawaii, but also the largest shield volcano on earth," said the scientists.

Thus the Pūhāhonu replaces the Mauna Loa as the largest shield volcano on earth.

Hottest lava of the modern earth age

But the undersea giant has another special feature: chemical analyzes of the volcanic rock have shown that 91.8 percent of the lava in this mountain of fire consists of forsterite, a magnesium silicate. "This is the highest forsterite content that has ever been determined for Hawaiian lava," report Garcia and his team. Typically, these transparent to brownish-white crystals arise when silicate-rich rock melts quickly cool and crystallize.

The high forsterite content means that the lava of the Pūhāhonu only melts when it is very hot. As a result, the underwater shield volcano even has the highest melting point of all lava known from the Modern Era (Cenozoic), as the researchers report. They assume that this particularly hot magma reached the earth's crust and the surface during a particularly strong pulse of activity in the Hawaiian hotspot.

Pulsating coat plume

“Our study showed us that hotspots can run through pulses of melt production,” says Garcia. “A smaller pulse of this kind produced the group of the now extinct Midway volcanoes, another, far stronger one created the Pūhāhonu. That means that we have to rewrite the textbooks on the behavior of mantle plumes. ”Instead of causing volcanic activity in the crust for one to two million years and then gradually cooling, the Hawaiian hotspot apparently shows waves of stronger and weaker activity.

This also explains why the Pūhāhonu is so huge: Not because the lithosphere was thinner at this point, it received more supplies or it grew more slowly - the researchers excluded all of this in their investigations. Instead, at the time of its formation, the earth's mantle was particularly hot at this point and the mantle plume was therefore more active. "The hotter mantle is the most plausible mechanism for producing these large volumes of magma," said Garcia and his team. (Earth and Planetary Science Letters, 2020; doi: 10.1016 / j.epsl.2020.116296)

Source: University of Hawaii at Manoa

May 15, 2020

- Nadja Podbregar