The ground is shaking and swelling at Mauna Loa, the legendary volcano on the Big Island of Hawaii and the largest active volcano in the world.
Scientists say the shaking indicates it could erupt, though they don’t expect that to happen right away. Regardless, officials on the island are telling residents to be prepared.
Here’s what you need to know:
What is Mauna Loa?
Mauna Loa is one of five volcanoes that together make up the Big Island of Hawaii, which is the southernmost island in the Hawaiian archipelago.
The volcano is not the tallest – that title goes to Mauna Kea – but it’s the largest and makes up about half of the island’s land mass.
Mauna Loa sits immediately north of the Kilauea volcano, which is currently erupting from its summit crater. Kilauea is well-known for a 2018 eruption that destroyed 700 homes and sent rivers of lava spreading across farms and into the ocean.
When did the volcano last erupt?
Mauna Loa last erupted 38 years ago. In written history, dating to 1843, it’s erupted 33 times.
What would an eruption look like?
Mauna Loa’s eruptions differ from Kilauea’s in part because it is taller. Its greater height gives it steeper slopes, which allow lava to rush down its hillsides faster than Kilauea’s. Its enormous size may allow it to store more magma, leading to larger lava flows when an eruption occurs.
Frank Trusdell, research geologist at the Hawaiian Volcano Observatory, which is part of the US Geological Survey, said data indicated that Mauna Loa has a much larger magma reservoir than Kilauea, which may allow it to hold more lava and rest longer between eruptions than Kilauea.
Where would Mauna Loa erupt from?
Scientists will not know until the eruption begins. Each eruption since 1843 started at the summit. In about half of the eruptions, the volcano later also began erupting from vents at lower elevations.
Scientists can’t tell far in advance when and where Mauna Loa will open new vents and erupt. Vents generally form along the volcano’s rift zone. That’s where the mountain is splitting apart, the rock is cracked and relatively weak and it’s easier for magma to emerge.
An eruption from vents on the south-west rift zone could hit residential communities, coffee farms or coastal villages on the west side of the island. Lava could reach homes in just hours or days.
The west side’s most populous town would be protected from any Mauna Loa eruption by the presence of another active volcano. The broad flanks of that volcano, Hualalai, sit between Mauna Loa’s south-west rift zone and Kailua-Kona and would block any lava heading toward the coastal community.
An eruption from the north-east rift zone could send lava toward the county seat of Hilo or other towns in east Hawaii. It could take lava weeks or months to reach populated areas on this side of the mountain.
Scott Rowland, a geologist at the University of Hawaii at Manoa, said there was no pattern when it comes to where an eruption will occur. “Just because the last one was on the north-east rift zone does not mean the next one will be down the south-west rift zone,” he said.
Will Mauna Loa have an explosion eruption?
Fifty-seven people died in 1980 when Washington state’s Mount St Helens erupted and blasted more than 1,300ft (400 meters) off the top of the mountain. Steam, rocks and volcanic gas burst upward and outward. A plume of volcanic ash rose over 80,000ft (24,000 meters) and rained down as far as 250 miles (400km) away.
Hawaii’s volcanoes tend not to have such explosive eruptions. That’s because their magma is hotter, drier and more fluid, said Hannah Dietterich, a research geophysicist at the US Geological Survey’s Alaska Volcano Observatory.
The magma in Mount St Helens tends to be stickier and traps more gas, making it much more likely to explode when it rises. The gas in the magma of Hawaii’s volcanoes tends to escape, with lava flowing down the side of their mountains when they erupt.
Hawaii’s volcanoes are called shield volcanoes because successive lava flows over hundreds of thousands of years built broad mountains that resemble the shape of a warrior’s shield.
Volcanoes like Mount St Helens are called composite or stratovolcanoes. Their steep, conical slopes are built by the eruption of viscous lava flows and rock, ash and gas.
How do scientists monitor the volcano?
The Hawaiian Volcano Observatory has more than 60 GPS stations on Mauna Loa taking measurements to estimate the location and the amount of magma accumulating beneath the surface.
Scientists use tiltmeters to track long-term changes in the tilting of the ground, helping them identify when the ground is swelling or deflating. A rapid change in tilt can indicate when an eruption will occur.
There is also a thermal webcam at Mauna Loa’s summit that will identify the presence of heat. And satellite radar can keep track of ground swelling and deflation.
