(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)
Molten lava flows downhill. This simple fact has spurred many residents on the Island of Hawaii, where Kilauea Volcano has been erupting for more than 27 years, to think about the likelihood of an eruption occurring upslope of their homes.
But what about residents on the other Hawaiian islands? Should they worry about lava flows, too?
For the most part, probably not. The exception is Maui, the only other island with a volcano classified as “active.”
Haleakala, the volcano that forms East Maui, erupts every 200 to 500 years. This estimate is reached by averaging the number of eruptions over the past 10,000 years.
Because little has changed about the volcano during this time, future eruptions should be expected. In fact, the probability of renewed eruptive activity is sufficiently large that the need for lava flow hazard zones on East Maui cannot be ignored.
Volcano hazard assessment always fails to answer the one question raised most frequently by island residents and visitors, namely, when and where the volcano will erupt next. In the absence of volcanic or seismic unrest, scientists can offer only statistical information — probabilities — about future volcanism.
On East Maui, the most likely site of the next eruption will be along a line of vents that ascends the southwest rift zone from La Perouse Bay and crosses the length of Haleakala Crater. The volcano’s east rift zone is generally less active, but an area of vents upslope of Waianapanapa and the Hana airport has matched the extent of eruptive activity in the Crater area and southwest rift zone.
Each of these areas has seen at least five eruptions in the past 1,500 years and, on this basis, are assigned to Maui lava flow Hazard Zone 1. No eruptions have occurred in the last 450 years ago, however.
Lava invades the other three hazard zones even less frequently. Zone 2, which encompasses the north and south flanks of Haleakala’s southwest and east rift zones, includes land that was covered by lava at least once in the past 13,000 years. Keanae Valley and the Kaupo area are also in Zone 2, because they are downslope of lava flows that might erupt within Haleakala Crater.
Maui zones 3 and 4 — most of East Maui — encompass areas with essentially no hazard under most lava-inundation scenarios. In large part, Haleakala Crater gets the credit for sheltering so much of East Maui because it blocks rift-zone lava flows from any but a few paths downslope. Also, new vents pop up infrequently within zone 4.
The numbers assigned to lava flow hazard zones are unique to each island. East Maui’s eruptive potential pales in comparison to that of many places on the Big Island. In terms of eruptive frequency, Maui’s Zone 1 is most like Hawaii Island’s Zone 4, a zone that corresponds to Hualalai volcano.
But even this correlation is imperfect because, when Hualalai erupts, lava flows tend to coat substantially larger areas. In the past 1,500 years, Hualalai eruptions (on Hawaii) have paved twice the area covered by Haleakala lava flows (on Maui) in the same interval of time — 174 versus 89 square km; or 67 versus 34 square miles.
All parts of Hualalai’s rift zones seem capable of eruptive activity, whereas some parts of Haleakala’s rift zones have been inactive for more than 50,000 years, a fact that further diminishes the likelihood of future lava inundation on East Maui.
None of this discussion is intended to raise or lower concern for Maui residents. Haleakala is currently inactive and shows no sign of disturbing the peace in the near future. However, Maui residents should know that Haleakala has experienced lava-producing eruptions in the past and will do so in the future. Our safety is always heightened by knowledge of the environment in which we live.