Haleakala, the shield volcano forming over 75% of Maui, is classified as active but currently dormant, showing no immediate signs of unrest. A future eruption would represent the island’s most significant natural disaster. Although the U.S. Geological Survey (USGS) rates its threat potential as “Moderate,” its immense size and the proximity of its rift zones to major population centers mean an eruption would severely disrupt life. The possibility of an eruption is a matter of when, not if, making preparation a continuous necessity for Maui.
The Nature of a Potential Eruption
Based on its geological history, a future Haleakala eruption is expected to be effusive, characterized by lava flows, rather than a violent, explosive event involving large ash clouds. This is typical of Hawaiian volcanoes, which produce thin, highly fluid basalt lava. This low-viscosity lava flows easily and quickly downhill, posing a direct threat to anything in its path.
The magma is most likely to surface from the volcano’s rift zones, which are long fracture systems extending away from the summit. The Southwest Rift Zone and the East Rift Zone are the areas of highest probability, having been the sites of nearly all recent activity. These rift zones extend from the summit region down to the coastline, meaning lava could emerge anywhere along these paths and flow toward the sea. Eruptions would not necessarily occur from the summit crater, which is a feature created primarily by erosion.
Immediate Physical Hazards on Maui
The most immediate physical hazard is the lava flow itself, which could reach coastal areas relatively quickly given the steep slopes of the volcano’s flanks. The Southwest Rift Zone puts the communities of Wailea and Kihei at direct risk, as lava would flow straight down into these areas. The East Rift Zone poses a threat to the remote region around Hana, where flows have reached the sea in the past. These flows are unstoppable and destroy everything in their path, including roads, homes, and utilities.
A secondary, but more widespread, hazard is volcanic gas, primarily sulfur dioxide (\(\text{SO}_2\)). When \(\text{SO}_2\) reacts with oxygen, moisture, and sunlight, it forms a haze known as VOG (volcanic smog), which contains fine particulate matter. VOG can be carried across Maui and to other islands by prevailing winds, causing widespread air quality issues. Exposure to VOG can aggravate respiratory conditions like asthma and bronchitis, affecting sensitive populations across the entire island.
An eruption would be preceded by increased seismic activity, as magma forces its way through underground rock. Small-scale local earthquakes would be expected. These tremors, along with subtle ground deformation detected by GPS sensors, would be the earliest signs of the volcano’s reawakening.
Socioeconomic Disruption and Recovery
Lava flows along the rift zones would sever crucial infrastructure, immediately isolating large sections of the island. A flow down the Southwest Rift Zone could cut the Piilani Highway, a lifeline for South Maui residents. A breach of the East Rift Zone would close the Hana Highway, isolating the entire eastern side of the island.
Loss of these major arteries would complicate emergency response and large-scale evacuation efforts for residents and the approximately 70,000 visitors who may be on Maui. The destruction of utility lines, including power and water mains, would lead to prolonged service outages. Evacuation logistics would be a major challenge, requiring the rapid movement of thousands of people, likely through the Kahului Airport (OGG), and the establishment of temporary mass sheltering for displaced residents and tourists.
The economic impact would be severe and immediate, beginning with the shutdown of the island’s tourism industry. Haleakala National Park generates tens of millions of dollars annually, which would cease instantly. The loss of hotel rooms and businesses in the lava inundation zones, coupled with long-term damage to Maui’s image as a safe tourist destination, would lead to a deep economic depression. Rebuilding destroyed infrastructure and replacing lost homes would place a massive financial burden on the county and state for decades.
Current Monitoring and Preparedness
The Hawaiian Volcano Observatory (HVO) is responsible for monitoring Haleakala’s status. Scientists use a network of instruments, including seismometers, to detect small earthquakes caused by magma movement deep underground. Continuous GPS receivers measure ground deformation, indicating if the volcano’s flanks are swelling due to magma accumulation.
Gas sensors also play a role in monitoring, though the primary focus for Haleakala remains on seismic and ground movement data. The current Volcano Alert Level for Haleakala is “Normal,” and the Aviation Color Code is “Green,” indicating no signs of unrest have been detected. If activity were to increase, HVO would adjust these levels to reflect the growing threat.
Local preparedness falls to the Maui Emergency Management Agency (MEMA), which is tasked with planning for various disaster scenarios, including a volcanic eruption. MEMA uses a public alert system, including the MEMA Alerts notification platform and the Emergency Alert System (EAS), to communicate warnings and evacuation orders to residents and visitors. These systems are designed to provide timely, specific instructions to those in the path of potential hazards.