When magma moves into the rift zone of a Hawaiian volcano, there is understandably a lot of excitement and apprehension. How far will the magma go, and will it erupt? But even long after the activity stops — regardless of whether it erupts — the subsurface magma continues to have a noticeable impact on the landscape.
Hawaiian volcanoes are rightly famous for eruptions along their rift zones — geologic structures that are undergoing extension and through which magma can move over great distances. Kilauea and Mauna Loa each have two major rift zones that radiate away from the volcano summits, and rift zone eruptions are common. Mauna Loa’s 2022 eruption, and Kilauea’s 1983–2018 Pu‘u‘o‘o eruption and the 2018 Ahu‘Aila‘au eruption, all occurred from rift zones.
Magma moves through Hawaiian rift zones as dikes, which are a tabular sheets of magma that move laterally and vertically beneath the ground. When a dike reaches the surface, it often erupts as a linear curtain of lava, as several recent eruptions on the Island of Hawaii have demonstrated.
As dikes form in a rift zone, they wedge the zone apart, creating lots of surface displacement. Sometimes, the amount of horizontal and vertical deformation that occurs in the vicinity of a dike intrusion can exceed several feet (about a meter).
Once the dike stalls and any eruption (if one occurred) ends, one might think that is the end of the story. Not so. The signs of magma beneath the surface can be detected for years thereafter.
One rather obvious sign of recently active magma below the ground is thermal energy. Especially where a dike reached shallow levels, the ground will remain hot. Vapor emissions are common in those areas because rainwater that circulates below ground is warmed by the still-hot rock and rises to the surface as steam.
The ground can also continue to deform long after the dike activity has ended. As magma cools, it solidifies from its molten state and continues to drop in temperature as solid rock. During this process, it contracts or shrinks, like most materials (except water). Because magma underground cools very slowly (it is very well insulated down there!), it can take years to decades, and perhaps even centuries, before reaching background temperatures. Thermal contraction is fastest during the initial, most rapid phases of cooling, and that is reflected in the way the surface responds.
As the magma cools and contracts, the ground above the shallow parts of a dike subsides (sinks). This deformation is especially easy to see by comparing satellite radar images over time — a technique known as interferometric synthetic aperture radar, or InSAR. The subsidence looks like a linear streak that occurs directly above the dike in InSAR data. In that sense, using InSAR to map subsidence after a dike intrusion can help geologists understand where exactly a dike is located, how much magma is in the dike, and how quickly it is cooling.
The linear pattern of subsidence seen in InSAR images is apparent after nearly all dike intrusions at Hawaiian volcanoes. For example, subsidence occurred for a few years above the so-called “Fathers Day” dike that intruded between Maunaulu and Makaopuhi Crater in June 2007, feeding a very small eruption on Kanenuiohamo. Subsidence also marks the lower East Rift Zone of Kilauea above the trace of the dike that erupted in 2018. Streaks of subsidence are likewise apparent above the dikes that erupted from Mauna Loa’s Northeast Rift Zone in 2022 and from Kilauea’s Southwest Rift Zone in 2024.
There is obviously a lot to measure when magma moves through the rift zones of Hawaiian volcanoes, and it is a time of special vigilance, given how many residents live on the flanks of our active volcanoes. But the action doesn’t stop just because the eruption or intrusion ends. There’s still much that can be learned about the characteristics of magma within Hawaiian rift zones from mapping deformation patterns after the magma has stopped moving!
Volcano activity updates
Kilauea has been erupting episodically within the summit caldera since Dec. 23, 2024. Its USGS Volcano Alert level is WATCH.
The summit eruption at Kilauea volcano that began in Halema‘uma‘u crater on Dec. 23 continued over the past week. Episode 14, which began the morning of March 19, ended the afternoon of March 20. Low-level spatter fountains and lava flows marked the start of Episode 15 on March 25.
The lava fountaining phase of Episode 15 began the morning of March 26 and continued for the next nine hours, with fountains reaching over 1,000 feet (305 meters). Since the end of Episode 15, the summit region has showed inflation suggesting another episode is possible. Sulfur dioxide emission rates are elevated in the summit region during active eruption episodes. No unusual activity has been noted along Kilauea’s East Rift Zone or Southwest Rift Zone.
Mauna Loa is not erupting. Its USGS Volcano Alert Level is at NORMAL.
Two earthquakes were reported felt in the Hawaiian Islands during the past week: a M3.0 earthquake 12 km (7 mi) NE of Pahala at 30 km (19 mi) depth on March 26 at 3:37 a.m. and a M3.2 earthquake 31 km (19 mi) WSW of Kailua-Kona at 30 km (18 mi) depth on March 25 at 2:36 p.m.
Please visit HVO’s website for past Volcano Watch articles, Kilauea and Mauna Loa updates, volcano photos, maps, recent earthquake information, and more. Email questions to askHVO@usgs.gov.
