Getting in the “Rift” Zone: Why & How They Erupt

Kīlauea Volcano, on the Island of Hawai‘i, has two rift zones. The East Rift Zone is longer, with 50 km (about 31 miles) on land plus another 80 km (about 43 miles) below sea level. The Southwest Rift Zone, which is historically less active, is 40 km (about 20 miles) long with only a small portion underwater. The rift zones extend from Kīlauea Caldera and, like the summit region, are prone to volcanic activity. But why are the rift zones so active?

Rift zones are areas of weakness in the volcano which form early in its lifetime, likely due to spreading of the volcano as it settles. This linear area that is being rifted, or pulled apart, remains active through most of the volcano’s building stages. Volcanic rift zones provide the easiest pathways for magma to travel underground from the summit storage region, with successive eruptions from the rift zones building up the volcano’s flanks.

The youngest Hawaiian volcanoes typically have two or three rift zones depending on whether they are built up against a neighboring volcano. In the case of Kīlauea, there are only two rift zones because the volcano is buttressed against the southeastern slope of Mauna Loa. Kīlauea’s two rift zones are nearly parallel to Mauna Loa’s rift zones reflecting this buttressing and the rift zones separate the relatively stable northern flank from the more mobile southern flank of the volcano. When magma intrudes into the rift, the northern flank remains stable against Mauna Loa Volcano to the north, and Kīlauea’s southern flank is forced southward to accommodate the additional magma.

Lava fountains erupting from fissure 22 (center) with heavy degassing (upper right) during Kīlauea’s lower East Rift Zone eruption in 2018. A narrow channelized lava flow from the fissure drains into a large, pre-existing ground crack. Weak spattering from the fissure 20 vent is visible just beyond to two sources of fissure 22 fountaining. Kapoho Cone, formed during an older rift zone eruption is visible on the horizon, downrift of the active fissures (upper left). USGS photo taken on May 20, 2018 by L. DeSmither.

As pressure builds within the summit magma plumbing system, rift zone intrusions, like the 2018 intrusion into the lower East Rift Zone (LERZ), can occur. Intrusions are typically accompanied by increasing numbers of earthquakes as the magma strains and fractures the ground along its path. The earthquakes are concentrated at depths of 2–4 km (about 1.2–2.5 miles) below the ground surface, and periods of increased seismicity can last several hours to days as the intrusion progresses. In addition to seismicity, ground deformation also occurs during a rift zone intrusion. Inflation above the intrusion is measured by tilt and GPS stations showing upward and outward motion as the stations move away from the swelling rift zone.

As the magma ascends and forces its way through the rock, fracturing is mirrored on the ground surface with many parallel cracks above the intrusion. These cracks continue to widen as the rift is forced open, and the surface block above the intrusion subsides—forming a graben. If the intrusion reaches the surface, one or more fissures will open and erupt lava. Long curtains of lava fountains or spatter form as the lava erupts through cracks in the ground. As a fissure evolves, it typically transitions from erupting along a line to focusing at a single—or few—principal vent(s). This in turn can cause increased pressurization within the erupting system resulting in higher lava fountains.

Lava fountains are driven by the rapid formation of gas bubbles as magma rises to shallow depths, which then burst to create the pressurized lava at the surface. The bubbles form because pressure at shallow depths is low enough for the gas dissolved within the magma to escape, like bubbles forming when you open a carbonated drink. Besides lava flows, fissure fountains can produce spatter build-up adjacent to the vent in linear (spatter ramparts) or conical (spatter cone) formations. Spatter and tephra cones—a common feature along Kilauea’s rift zones—are likely to build when eruptive activity persists. The vent itself may also be enlarged by thermal erosion (slow melting of the vent walls by erupting lava) during prolonged activity at a vent. [DLG1]

When an eruption ends, the intrusion’s un-erupted magma drains back into the rift zone where it can remain molten for decades. In fact, lava with a chemical composition similar to the 1955 eruption was produced during the first week of the 2018 LERZ eruption, suggesting that the early fissures were supplied by stored magma. This illustrates that rift zones are not only essential for the transportation of magma within the volcano, but are also storing magma that could feed future eruptions.

Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates. 

Volcano Activity Updates

Kīlauea Volcano is not erupting and its USGS Volcano Alert level remains at NORMAL. For definitions of USGS Volcano Alert Levels, see https://volcanoes.usgs.gov/vhp/about_alerts.html.

Rates of deformation, gas release, and seismicity on Kīlauea have not changed significantly over the past week.

Monitoring data have shown no significant changes in volcanic activity over the past week. Rates of seismicity across the volcano remain low. Real-time sulfur dioxide emission rates are low at the summit and are below detection limits at Puʻu ʻŌʻō and the lower east rift when last measured on June 19 and June 13 respectively.

Since early March, GPS stations and tiltmeters at the Kīlauea summit have recorded deformation consistent with slow magma accumulation within the shallow portion of the Kīlauea summit magma system (1-2 km or approximately 1 mile below ground level). However, gas measurements have yet to indicate significant shallowing of magma. HVO continues to carefully monitor all data streams at the Kīlauea summit for important changes.

Further east, GPS stations and tiltmeters continue to show motions consistent with slowed refilling of the deep East Rift Zone magmatic reservoir in the broad region between Puʻu ʻŌʻō and Highway 130 over recent weeks. While the significance of this pattern is unclear, monitoring data do not suggest any imminent change in volcanic hazard for this area. HVO continues to carefully monitor all data streams along the Kīlauea east rift zone for important changes.

No earthquakes with three or more felt reports occurred in Hawaiʻi this past week.

Hazards remain in the lower East Rift Zone eruption area and at the Kīlauea summit. Residents and visitors near the 2018 fissures, lava flows, and summit collapse area should heed Hawaii County Civil Defense and National Park warnings. Hawaii County Civil Defense advises that lava flows and features created by the 2018 eruption are primarily on private property and persons are asked to be respectful and not enter or park on private property.

The USGS Volcano Alert level for Mauna Loa remains at NORMAL. A slight increase in detected earthquakes was noted over the past month. GPS instruments show slow inflation of the summit magma reservoir. Gas and temperature data showed no significant changes the past month.

HVO continues to closely monitor both Kīlauea and Mauna Loa for any signs of increased activity.