The following essay is reprinted with permission from The Conversation, an online publication covering the latest research.
Where did that molten rock originate?
We asked Gabi Leske ,, a geophysicist from the University of California-San Diego, to explain. He was the one who led the first project to map the deep plumbing which feeds the Hawaiian Islands’ volcanoes.
Where is the magma surfacing at Mauna Loa coming from?
The magma that comes out of Mauna Loa comes from a series of magma chambers found between about 1 and 25 miles (2 and 40 km) below the surface. These magma chambers serve as temporary storage for magma and gases and do not represent the original source of magma.
The origin is much deeper in Earth’s mantle, perhaps more than 620 miles (1,000 km) deep. Some scientists even postulate that the magma comes from a depth of 1,800 miles (2,900 km), where the mantle meets Earth’s core.
The crust of the Earth is made up tectonic plates, which are slowly moving at about the speed of a fingernail. Volcanoes are usually found where these plates push beneath one another or move away from eachother. But volcanoes can also be in the middle of plates, as Hawaii’s volcanoes are in the Pacific Plate.
The crust and mantle of the Pacific Plate crack at different points as it moves northwestward. Magma can travel upward through cracks beneath Hawaii to feed different volcanoes. The same thing happens at Maui’s Haleakala, which last erupted about 250 years ago.
How does the molten rock travel deep within Earth’s mantle? And what is a mantle plumb?
Scientists believe that mantle is not composed of uniform rock. Instead, differences in the type of mantle rock make it melt at different temperatures. Mantle rock can be solid in some places and melt at others.
The partially molten rock becomes buoyant, and it ascends towards the surface. A mantle plume is made up of ascending mantle rocks. The rock melts faster as it ascends because the pressure above it decreases. This eventually leads to the formation of the magma chamber. If there is enough volcanic gas in the magma chamber and a sufficient opening at the surface, the magma will be forced to the surface by a volcanic eruption.
Seismic imaging by research teams I’m involved with has shown that Hawaii’s mantle plume comes from deep inside the mantle.
But, the plume isn’t a straight pipe like some concept figures suggest. Instead, it has twists and turns, originally coming from the southeast, but then turning toward the west of Hawaii as the plume reaches into the shallower mantle. The magma is channeled upwards through cracks in the Pacific Plate to the magma chamber below the island of Hawaii.
Why does Hawaii experience less dramatic eruptions than other places?
Hawaii lies in the middle an oceanic plate. It is actually the most isolated volcanic hot spot in the world, far from any plate boundary.
Oceanic Magma is very distinct from continental magma. It has a different chemical structure and flows much easier. So, the magma is less prone to clog volcanic vents on its ascent, which would ultimately lead to more explosive volcanism.
Thermal imaging shows the Mauna Loa eruption, which began around 11: 30 p.m. local time on Nov. 27, 2022. The Celsius temperature scale is shown. Credit: USGS
How do scientists find out what’s beneath the surface?
Volcanic activity can be monitored using many instruments.
GPS is perhaps the most simple to understand. GPS is a different way of using GPS than the everyday life. It can detect tiny movements of just a few centimeters. Any upward movement detected by GPS on volcanoes indicates that something is pushing below the surface.
Even more sensitive are tiltmeters, which are in essence the same as bubble levels that people use to hang pictures on a wall. Any tilt change on a volcano slope is an indication that the volcano “breathes,” again due to magma moving below.
Seismic activity monitoring is a very important tool.
Volcanoes such as Hawaii’s are closely monitored by a large network seismographs. Any movement of magma below will cause tremors that are picked up by the seismometers. Scientists noticed that the tremors were coming from ever lower depths a few weeks before the eruption at Mauna Loa. This indicated that magma was rising, and that an eruption was possible. This allowed scientists to warn the public.
Other ways that volcanic activity is monitored includes chemical analysis of gases coming out through fumaroles–holes or cracks through which volcanic gases escape. If activity or composition changes, it is a sign that the volcano has changed.