Kīlauea is one of the most consistently active volcanoes on Earth. Its frequent, generally non-explosive eruptions are driven by a continuous supply of magma and a highly efficient internal plumbing system. The volcano’s continuous activity has shaped the Hawaiian landscape for hundreds of thousands of years, allowing scientists to monitor its complex dynamics.
Kilauea’s Geological Foundation
Kīlauea’s existence is a direct result of the Hawaiian Hotspot, a fixed plume of abnormally hot mantle material located deep beneath the Pacific tectonic plate. As the Pacific Plate slowly drifts northwestward over this stationary heat source, magma generated by partial melting rises to form the Hawaiian Island chain. Kīlauea, along with its neighbor Mauna Loa, currently sits above this plume, giving it a near-constant supply of fresh magma.
The resulting structure is a shield volcano, characterized by its broad, gently sloping profile. This shape is a consequence of its eruptive style, where highly fluid lava flows easily and spreads out over great distances before solidifying. The volcano has been active for an estimated 210,000 to 280,000 years and is currently in the shield-building stage of Hawaiian volcanism.
The Unique Magma Plumbing System
Kīlauea’s eruptive behavior is governed by a highly dynamic internal plumbing system that stores and transports magma from the deep mantle to the surface. Magma rising from the hotspot first accumulates in a shallow reservoir, located approximately 1 to 4 miles beneath the summit caldera, a broad, shallow depression at the volcano’s peak. Magma pressure cyclically builds and releases within this reservoir.
Radiating outward from the summit reservoir are two subsurface corridors of weakness known as rift zones: the East Rift Zone (ERZ) and the Southwest Rift Zone (SWRZ). These zones contain numerous fractures, dikes, and sills that serve as pathways for the pressurized magma to travel laterally away from the summit. Eruptions occur when the pressure in the central reservoir exceeds the strength of the surrounding rock, forcing magma to intrude into and then erupt from these rift zones.
The East Rift Zone is significantly more active than the Southwest, hosting the majority of Kīlauea’s recent flank eruptions. During a rift zone eruption or intrusion, a sharp and measurable deflation of the summit region occurs as the magma drains quickly from the shallow reservoir into the rift.
Characteristics of Hawaiian Eruptions
The style of Kīlauea’s eruptions is categorized as “Hawaiian,” which is predominantly effusive rather than explosive. This non-explosive character is due to the low viscosity of its basaltic magma, which is exceptionally fluid. The low viscosity allows gases dissolved in the magma to escape easily as it rises, preventing a catastrophic pressure build-up that would lead to a violent, ash-producing eruption.
When the magma reaches the surface, it often erupts in jets called fire fountains, or as a “curtain of fire” along a fissure. The low-viscosity lava flows rapidly downslope, sometimes traveling many miles from the vent in well-insulated channels or lava tubes. Once solidified, the basaltic lava forms two distinct types of flows, both named with Hawaiian terms: pahoehoe and a‘ā.
Pahoehoe Lava
Pahoehoe lava is typically hotter and less viscous. It forms a smooth, billowy, or ropey surface as it cools.
A‘ā Lava
In contrast, a‘ā lava is cooler or moving at a higher flow rate. It fractures into a rough, jagged, and clinkery surface of angular fragments. A pahoehoe flow can transition into an a‘ā flow as it loses heat and gas, but the reverse rarely occurs.
Analyzing the 2018 Eruption Sequence
The 2018 eruption sequence marked the volcano’s largest lower East Rift Zone eruption in at least two centuries. The event began with the collapse of the Puʻu ʻŌʻō vent, a long-lived eruptive site on the middle East Rift Zone, signaling a major pressure shift in the system. Magma then began to migrate rapidly eastward and downrift, traveling approximately 25 miles from the summit.
This shift resulted in the opening of multiple eruptive fissures in the lower East Rift Zone, within the residential area of Leilani Estates, starting on May 3rd. As the magma was forced out of the flank fissures, the summit magma reservoir began to drain significantly, leading to a drop in the Halemaʻumaʻu lava lake. The lack of support beneath the summit caldera floor caused a series of near-daily collapse events.
These collapse events were accompanied by earthquakes equivalent to a magnitude 4.7 to 5.4, which deepened and widened the Halemaʻumaʻu crater. The volume of magma that erupted in the lower East Rift Zone, estimated at about 0.8 cubic kilometers, was roughly equivalent to the volume decrease observed at the summit.