Hazards associated with potential volcanic activity in American Samoa include ground shaking and related hazards, local tsunami, volcanic gases, low-level localized explosions of lava, lava flows, lava ocean entry hazards, and hydrovolcanic and phreatomagmatic activity.
It is unclear if the current earthquake unrest in American Samoa is related to volcanic activity and, if so, whether it will escalate to a volcanic eruption. Currently, the primary hazard of concern is earthquake shaking, although no damaging earthquakes have occurred as part of this swarm. If activity escalates to an eruption, it could pose significant hazards to residents of American Samoa.
Not all earthquake swarms result in a volcanic eruption. When they do occur, volcanic eruptions can range in size and vigor. Hazards associated with potential volcanic activity in American Samoa depend on the type of activity. Potential types of activity include earthquake activity, eruptions on land, eruptions involving water (which could occur near or offshore), and submarine eruptions. Hazards associated with these potential types of volcanic activity include ground shaking and associated hazards, local tsunami, volcanic gases, low-level localized explosions of lava, lava flows, lava ocean entry hazards, and hydrovolcanic and phreatomagmatic activity. These different types of activity and associated hazards are described below.
Earthquakes can be caused by a number of subsurface processes, such as ground movement along faults, geothermal activity, or movement of magma beneath a volcano. The word swarm is used to identify when a sequence of smaller earthquakes is occurring over time in a localized area. Most earthquakes in a swarm are small and cannot be felt by people, but magnitude 3 and larger earthquakes can be more widely felt. Shaking can be locally intensified by water-saturated soils that amplify earthquake ground motions, and can result in other hazards such as ground movements and local tsunami.
Ground shaking, ground cracking, landslides, local tsunami
Strong earthquakes endanger people and property by shaking structures and by causing ground cracks and ground settling. On steep slopes, intense shaking may cause such soils to fail, resulting in landslides and mudflows. Large offshore earthquakes can displace large volumes of water to form tsunamis, a series of sea waves that can be far more damaging than any of the direct seismic hazards. Strong earthquakes have the potential to also damage infrastructure such as buildings, roads and bridges, and water, sewer, and utility lines. Earthquakes can occur at any time and the exact location and timing of earthquakes cannot be predicted.
For information on how to prepare for an earthquake, see: https://www.usgs.gov/faqs/what-can-i-do-be-prepared-earthquake.
If you feel shaking and are not near the coast, immediately drop, cover, and hold on until the shaking stops.
If you are at the coast, heed the natural tsunami warning signs. If you feel a strong or long-duration earthquake, see a sudden rise or fall of the ocean, hear a loud roar from the ocean, or see a large aerial plume from an eruption, a tsunami may follow, and you should immediately move to higher ground.
Eruption on land
During an eruption on land, volcanic gas is emitted that is hazardous to breathe and can create volcanic air pollution (vog) that can affect broad areas downwind. Eruptions on land can form lava flows, spatter ramparts, and scoria cones. Lava flows destroy everything in their path and generate additional hazards when they enter the ocean. Small-scale explosive activity, such as lava fountains, shreds and blasts molten rock into the air, forming lava fragments called tephra that solidify and fall back to the ground.
Volcano Gas Emissions
Volcanic gases—primarily water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)—are continuously released during the eruptions. As SO2 is released, it reacts in the atmosphere with oxygen, sunlight, moisture, and other gases and particles, and within hours to days, converts to fine particles. The particles scatter sunlight and cause the visible haze that can be observed downwind, and is known as volcanic air pollution or vog (volcanic smog). Vog creates the potential for airborne health hazards, damages agricultural crops and other plants, and affects livestock operations.
Lava flows are streams of molten rock that typically erupt from a shield volcano's summit or along rift zones on its flanks. A lava flow moves down the flank of a volcano, towards the ocean, under the influence of gravity, similar to how water flows downhill through gullies, channels, and valleys. 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. Lava flows can be smooth and ropey (pāhoehoe) or rubbly and rough (‘a‘ā).
Lava Ocean Entry
When lava enters the ocean, a number of hazards can occur, including laze (lava haze) plumes, unstable deltas, and hydrovolcanic explosions.
Laze plumes form when molten basaltic lava and sea water interact, forming hydrochloric acid, hydrofluoric acid, and halide precipitate. Laze plumes are a health risk and it is not known how far downwind harmful constituents drift in a laze plume of this size. At times, onshore winds carry the laze plume an unknown distance inland.
When lava enters the ocean, it can create a lava delta that extends the pre-eruption coastline. Lava deltas can be unstable and have the potential to collapse without warning. When lava deltas collapse, explosions caused by the interaction of seawater with hot rock can occur.
Hydrovolcanic explosions can occur when large amounts of molten lava suddenly come in contact with seawater, and can send fragments of lava and hot water 80 to 300 feet (25 to 100s) of meters high. This situation can occur during collapse of a delta or lava conduit and has been observed as far as 100 meters offshore on the Island of Hawai‘i.
Tephrafall: bombs, lapilli, and ash
Tephra are small fragments of rock that are produced when magma or rock is explosively ejected during a volcanic eruption. The largest fragments, blocks and bombs (>64 mm, 2.5 inches diameter), can be expelled with great force but are deposited near the eruptive vent. Lapilli-sized material (6-64 mm, 0.24-2.5 inches diameter) can be carried upward within in a volcanic plume and downwind in a volcanic cloud, but fall to the ground as the eruption cloud cools. Around lava fountains, these fragments land within a few hundred meters from the vent, typically forming a spatter rampart or a scoria cone. The smallest material, volcanic ash (<2 mm diameter) is both easily convected upward within the plume and carried downwind for very long distances; as it falls out of suspension it can potentially affect communities and farmland across a great distance.
Eruption involving some water
Eruptions involving some water can be significantly hazardous as they can be explosive. These types of eruptions can occur on land near the coast (where groundwater is available), in the shallow marine environment just offshore, or in the farther offshore environment beneath shallow ocean. An example of a deposit from this type of eruption in American Samoa is Faleasao tuff cone on Ta‘ū Island, near the village of Faleasao. In addition to volcanic gas and vog hazards, described above, eruptions involving water have additional hazards of hydrovolcanic explosions and pyroclastic surges, described below.
Hot lava interacting with water forms steam and fragments of rock that can be ejected to great heights. Near the vent, large projectile-like rocks travel on cannonball-like paths outwards in all directions. This is a zone of extreme hazard. Farther from the vent, small particles are carried downwind, where they can affect transportation, infrastructure, and utilities. These types of explosive eruptions create a hazard to aircraft and can deposit ash on downwind areas.
Pyroclastic surges and density currents are highly destructive, generally fast-moving clouds of volcanic gases and fragments of magma and older rocks. They are generally hot and form when eruption columns become unstable and collapse back around the vent. Surges typically travel tens of meters per second, but certain types can move as rapidly as 100– 300 meters per second (200–700 miles per hour). Surges usually extend radially from the vent in all directions, but can be influenced by topography, thickening in valleys and thinning over topographic highs. Owing to their high heat and speed, pyroclastic surges are among the most dangerous and destructive of volcanic hazards. There are very few survivors among those caught in the path of a hot surge, and property damage is severe. The direction that surges may travel is difficult to predict.
A submarine eruption takes place deep beneath the surface of the ocean. Submarine eruption products mostly stay underwater and therefore pose little to no hazard, though the eruption area should be avoided by mariners and air traffic. A submarine eruption could impact the surrounding ocean in the form of local volcanic gas pollution and increased water temperature, both of which can affect marine life.
Additional resources on volcanic hazards:
USGS Earthquake Hazards Program website: https://earthquake.usgs.gov/
HVO webpage on earthquake hazards: https://www.usgs.gov/observatories/hawaiian-volcano-observatory/damaging-earthquakes-common-hazard-hawaii
- USGS Volcano Hazards Program webpage on lava flow hazards: https://www.usgs.gov/programs/VHP/lava-flows-destroy-everything-their-path
- HVO webpage on lava flow hazards: https://www.usgs.gov/observatories/hawaiian-volcano-observatory/lava-flow-hazards-zones-and-flow-forecast-methods-island
USGS Volcano Hazards Program webpage on volcanic ash: https://volcanoes.usgs.gov/volcanic_ash/
Hawaii Interagency Vog Information Dashboard page on the health impacts of volcanic ash: https://www.ivhhn.org/information/health-impacts-volcanic-ash
Hawaii Interagency Vog Information Dashboard website: https://www.ivhhn.org/information/health-impacts-volcanic-gases
HVO Frequently-Asked-Questions about Volcanic Gas: https://www.usgs.gov/observatories/hvo/monitoring-volcanic-gas-hawaii
HVO Frequently-Asked-Questions about Vog: https://www.usgs.gov/observatories/hawaiian-volcano-observatory/frequently-asked-questions-about-volcanic-smog-vog