Why Mauna Loa and Other Hawaiian Volcanoes Are Different From Most
- December 3, 2022
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The following essay is reprinted with permission from The Conversation, an online publication about the latest research.
Hawaii’s Mauna Loa, the world’s largest active volcano, began November 27, 2022, when its first eruption in nearly four decades began shooting up fountains of red-hot rock and spilling lava from fissures.
Where does this molten rock come from?
We reached out to Gabi Laske, a geophysicist at the University of California-San Diego who led one of the first projects to map the deep pipelines that feed the Hawaiian Islands’ volcanoes.
Where does the magma that appears at Mauna Loa come from?
The magma that emanates from Mauna Loa originates from a series of magma chambers found between about 1 and 25 miles (2 and 40 km) below the surface. These magma chambers are only temporary storage sites for magma and gases and are not the original origin of the magma.
The origin is much deeper in the Earth’s mantle, perhaps more than 620 miles (1,000 km) deep. Some scientists even suggest that the magma originated from a depth of 2,900 km (1,800 miles) where the mantle meets the Earth’s core.
An illustration shows what Hawaii’s mantle cloud might look like. Photo credit: Joel E. Robinson, USGS
The earth’s crust is made up of tectonic plates that move slowly, about as fast as a fingernail grows. Volcanoes typically occur where these plates either move apart or where one slides under the other. But volcanoes can also lie in the middle of plates, like Hawaii’s volcanoes lie in the Pacific Plate.
The crust and mantle that make up the Pacific Plate fractures at various points as it moves northwest. Under Hawaii, magma can move up through the fissures to feed various surface volcanoes. The same is happening at Maui’s Haleakala, which last erupted about 250 years ago.
How does molten rock get out of deep in the Earth’s mantle and what exactly is a mantle cloud?
Scientists suspect that the mantle is not made of uniform rock. Instead, differences in the type of mantle rock make it melt at different temperatures. Mantle rock is solid in some places while in other places it begins to melt.
The partially molten rock becomes buoyant and rises to the surface. The rising mantle rock makes up a mantle plume. Because the overpressure decreases as the rock rises, it melts more and more and eventually collects in the magma chamber. If there is a sufficiently large opening at the surface and enough volcanic gases have accumulated in the magma chamber, a volcanic eruption will push the magma to the surface.
Seismic imaging by research teams I’m involved with has shown that Hawaii’s mantle plume originates deep within the mantle.
But the cloud isn’t a straight pipe as some concept figures suggest. Instead, it has twists and turns that originally come from the southeast but then turn towards the west of Hawaii as the cloud reaches into the flatter mantle. Fissures in the Pacific Plate then direct the magma upward toward the magma chamber beneath the island of Hawaii.
Why does Hawaii typically have fewer dramatic eruptions than other places?
Hawaii lies in the middle of an oceanic plate. In fact, it’s the most isolated volcanic hotspot on Earth, far from any plate boundary.
Oceanic magma is very different from continental magma. It has a different chemical composition and flows much more easily. The magma is thus less prone to clogging volcanic vents as it rises, which would ultimately result in more explosive volcanism.
Thermal images show Mauna Loa’s eruption that began on November 27, 2022 at approximately 11:30 p.m. local time. Temperatures are given in Celsius. Credit: USGS
How do scientists know what’s happening beneath the surface?
Volcanic activity is monitored with many different instruments.
Perhaps the easiest to understand is GPS. The way scientists use GPS is different from everyday use. It can detect the smallest movements of a few centimetres. With volcanoes, any upward movement on the surface detected by GPS indicates that something is pushing up from below.
Map showing the subaerial extents of historic lava flows from Mauna Loa. Lava flow hazard zones and Hawaii County counties are also shown. Photo credit: K. Mulliken, HVO. public domain
Even more sensitive are inclinometers, which are essentially the same as bubble levels that people use to hang pictures on the wall. Any change in slope on a volcanic slope indicates that the volcano is “breathing”, again due to magma moving beneath it.
A very important tool is the observation of seismic activity.
Volcanoes like Hawaii’s are monitored with a large network of seismographs. Any movement of magma below causes tremors, which are recorded by the seismometers. A few weeks before Mauna Loa erupted, scientists noticed that the tremors were coming from shallower and shallower depths, suggesting magma was rising and an eruption could be imminent. This allowed scientists to warn the public.
Other methods of monitoring volcanic activity include chemical analysis of gases escaping through fumaroles — holes or fissures through which volcanic gases escape. If the composition changes or activity increases, that’s a pretty clear indication that the volcano is changing.
This article was originally published on The Conversation. Read the original article.