What Does Water in Halema‘uma‘u Mean?
The slowly deepening pond of water on the floor of Halema‘uma‘u, the first in recorded history, has captured the interest of media and the public, both locally and nationally.
Many questions are being asked. The two most frequent are, where is the water coming from and what is its importance?
Two potential sources of the water are recent rainfall and groundwater. At this writing, either remains a possibility. Circumstantial evidence, however, favors groundwater.
The local water table, below which rocks are saturated with water, is at an elevation of about 590 m (1936 feet; the elevation changes slightly with time), as measured in a deep hole drilled in 1973 about 800 m (about half a mile) south of Halema‘uma‘u. The elevation of the floor of Halema‘uma‘u is about 1,706 feet, 230 feet lower than the nearby water table.
Before the 2018 collapse of Kīlauea Volcano’s summit, geophysical data suggest that the water table near Halema‘uma‘u was at about the same elevation as in the drill hole, but it was apparently drawn down during the collapse. The water table is likely recovering now, and as it rises, water inundates low areas such as the crater floor.
So far, the surface of the pond is rising slowly and steadily, consistent with a rising water table. The pond level should rise in jumps during downpours if rain is directly responsible for feeding it. Unfortunately, Halema‘uma‘u has experienced no heavy rain since the pond was first observed on July 25, 2019. It would be best to sample the water and date it using isotopic means; rain would have today’s age, groundwater an older age.
How deep is the water? In the surface pond, no more than a couple of meters (yards). But the visible pond could be just the top of the saturated zone, which could conceivably be several tens of meters (yards) deep.
There is probably a bottom to the standing water, because heat in the plugged magma conduit below the floor of Halema‘uma‘u would boil away water at some depth. But as the conduit cools, the floor of standing water could move downward, deepening the water body from below as well as at the surface.
This may seem academic, but the total thickness of the water body impacts potential hazards. A mere puddle would scarcely affect the next summit eruption. But, if rising magma had to penetrate several tens of meters (yards) of water, an explosive scenario that has played out in the past could repeat.
Given a thick water body, the rate at which magma rises through the water becomes crucial. Slowly rising magma will simply evaporate the water and emerge on the surface as a lava flow or even eventually form a lava lake.
Magma that rises rapidly does so because it is being powered by expanding gas bubbles within it. A classic example is a lava fountain, which is already fragmenting because of gas expansion before even reaching the ground surface.
If such rapidly rising, fragmenting magma meets water, the fragments transfer heat to the water far more efficiently than a continuous surface of magma (as with slowly rising magma). The result is that the water rapidly boils, creating steam that expands and adds to the explosive energy of what would be a lava fountain under dry conditions.
We are quite sure that this kind of explosion has happened repeatedly in Kīlauea’s past. Detailed study of textures of glass fragments in deposits some 400 years old provide evidence of water quenching. Chemical analyses of this glass show that the amount of dissolved water and sulfur is intermediate between that of magma before eruption and that in lava fountains, the result of water quenching the magma before most of the gas could escape.
If the water body is thin, even rapidly rising magma would not create large explosions because of the small amount of steam generated. If, however, the water is several tens of meters (yards) deep, locally powerful explosions could ensue, probably not large enough to diminish public safety but perhaps big enough to create a nuisance ash fall during unfavorable wind direction.
We have no way to anticipate when magma will begin to rise up the Halema‘uma‘u conduit, much less if the rate of rise will be slow or fast. At present, monitoring data show no signs of impending eruption, and it could be years down the road before the next summit eruption happens.
Volcano Activity Update
Kīlauea is not erupting and its USGS Volcano Alert level remains at NORMAL. Reflecting this level, HVO is now issuing monthly updates for Kīlauea. For definitions of the alert levels, see https://volcanoes.usgs.gov/vhp/about_alerts.html.
Monitoring data for deformation have shown no significant changes in Kīlauea activity over the past week. Rates of seismicity across the volcano remain low. Sulfur dioxide emission rates are low at the summit and below detection limits at Pu‘u ‘Ō‘ō and the Lower East Rift Zone (LERZ).
At or near the 2018 LERZ eruptive fissures, elevated ground temperatures and minor releases of gas (steam, tiny amounts of hydrogen sulfide, and carbon dioxide) persist. These are typical post-eruption conditions and are expected to be long-term, as they were after the 1955 LERZ eruption.
The water level at the bottom of Halema‘uma‘u continues to slowly rise. HVO is monitoring the pond closely. Under the current conditions, its presence in the crater has not increased the risk to public safety.
Hazards remain at the lower ERZ and summit of Kīlauea. Residents and visitors near the 2018 fissures, lava flows, and summit collapse area should heed Hawai‘i County Civil Defense and Hawai‘i Volcanoes National Park closures and warnings. The 2018 lava flows are primarily on private property, and people are asked to be respectful and to not enter or park on private property.
Mauna Loa is not erupting. Its USGS Volcano Alert level remains at ADVISORY. This alert level does not mean that an eruption is imminent or that progression to an eruption is certain. A similar increase in activity occurred between 2014 and 2018 and no eruption occurred.
This past week, approximately 46 small-magnitude earthquakes (all less than M2.0) occurred beneath the summit and upper Southwest Rift Zone. Deformation measurements show continued summit inflation, suggestive of recharge of the volcano’s shallow magma storage system. No significant changes in volcanic gas release on the Southwest Rift Zone were measured, and fumarole temperatures there and at the summit remain unchanged.
Mauna Loa updates are issued weekly. For more info on the status of the volcano, please go to: https://volcanoes.usgs.gov/volcanoes/mauna_loa/status.html
Four earthquakes with three or more felt reports occurred in Hawai‘i this past week: a magnitude-4.5 quake 5 miles NE of Papa‘ikou at 26 miles depth on Aug. 12 at 4:41 a.m. HST; a magnitude-3.2 quake 17 miles SE of Honoka‘a at 12 miles depth on Aug. 11 at 10:02 a.m. HST; a magnitude-3.7 quake 8 miles south of Volcano at 5 miles depth on Aug. 10 at 1:19 p.m. HST; and a magnitude-3.2 quake 7 miles SE of Volcano at 4 miles depth on Aug. 9 at 9:13 a.m. HST.
HVO continues to closely monitor both Kīlauea and Mauna Loa for any signs of increased activity.
“Volcano Watch” is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.