Lava flows on the Island of Hawai‘i are renowned for their frequent occurrence, relatively easy access, land-building ocean entries, and for Hawaiian names that describe their surface textures—rubbly ‘a‘ā or smooth and sometimes ropey pāhoehoe. Hawaiian lava flows generally advance slowly and can be easily avoided by people. However, lava flows can destroy everything in their paths—rainforests, buildings, roads, utility and communication systems, and whole communities. During the ongoing eruption of Kīlauea Volcano at the Pu‘u ‘Ō‘ō vent, several people have died because of the hazards associated with lava entering the ocean.
The likelihood that future lava flows will interfere with human activity and infrastructure increases as communities and other development encroach on Hawaii's active volcanoes. USGS scientists characterize long-term lava flow hazards based on where and how often past lava inundation has occurred. When a new lava flow erupts, scientists use different methods to estimate where and how quickly it will travel downslope.
The first USGS map showing volcanic hazard zones on the Island of Hawai‘i was prepared in 1974 and revised in 1987 and 1992. The latest map divides the island into 9 zones, with lava flows most likely to occur in Zone 1 and least likely in Zone 9. The zones are based on the mapped locations of vents and lava flows, frequencies of past eruptions, written eruption accounts, and oral traditions of Hawaiians (chants and stories about eruptions). The zones also take into account larger topographic features that will affect the paths of future flows. The hazard boundaries are approximate, and the change in the degree of hazard is generally gradual rather than abrupt.
The probability of future lava flows is not the same for all areas of the Island of Hawai‘i. The long-term lava-flow threat is greatest on Kīlauea and Mauna Loa, the two most active volcanoes, followed by Hualālai.
Hawaiian Volcano Observatory (HVO) scientists use two methods to forecast the path of a lava flow: broad inundation zones based on the mapped pathways of previous lava flows, and steepest descent paths based on a digital elevation model of a volcano's topography or surface. Estimates of lava flow travel times are based on known advance rates of active flows and earlier flows from within the same area.
HVO scientists have mapped more than 500 individual lava flows on Mauna Loa Volcano. These flows originated from vents in the summit area, rift zones, and flanks of the volcano. Their locations help identify the approximate pathways for future flows that erupt in similar locations. Mauna Loa is divided into 9 inundation zones, each including potential future vent locations and the corresponding downslope areas that could be buried if future lava flows reached the ocean. When an eruption begins, the zones can be used to quickly identify communities, infrastructure, and roads that could be affected by lava flows from the erupting vent(s). However, it is likely that only part of a zone will be buried in a single eruption.
A lava flow moves down the flank of a volcano under the influence of gravity, similar to how water flows downhill through gullies, channels, and valleys. Such low areas can be described as steepest descent paths for flowing water. In the absence of obvious water drainages on active Hawaiian shield volcanoes, scientists calculate steepest descent paths for flowing lava using computer software and digital topographic maps called digital elevation models (DEMs); the paths are shown as lines on a map. DEMs are not perfect representations of the earth's surface, however, which means that steepest descent paths are only approximate. Steepest descent paths in the Puna District, Island of Hawai‘i, published in 2007 were used to forecast paths for a lava flow that spread eastward from the Pu‘u ‘Ō‘ō vent on the East Rift Zone of Kīlauea Volcano in 2014-2015.
Travel time estimates for a lava flow are based on the actual advance rate of the flow and, if known, the overall advance rate of earlier lava flows that passed through the same location. However, this method of forecasting travel time is highly uncertain, because factors that control flows are constantly changing—eruption rate, ground slope the flow is moving over, and complex behavior of ‘a‘ā and pāhoehoe flows.
Measured advance rates on the Island of Hawai‘i are as fast as 9.3 km (5.8 mi) per hour for an ‘a‘ā flow erupted from Mauna Loa in 1950, which is slightly slower than typical human jogging speed. Pāhoehoe lava flows typically move more slowly, less than a few hundred meters (or yards) per hour or day. The distance a flow travels ultimately depends on the eruption rate and duration, slope, and evolution of the flow field, including whether and how lava tubes, lava channels, or subsequent breakouts develop and progress.