February 17, 2000
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Why is the water in `Ahalanui pond so warm?
Annual rainfall totals on the windward slopes of Mauna Loa and Kilauea average up to nearly 10 meters (300 inches), yet there are no perennial streams on either volcano. Where does all the water go? A small amount is taken up by plants and transpired into the air, but most of it infiltrates the ground and percolates down until blocked by some barrier. Generally, the barrier is a layer of salt water that saturates the base of the island.
Fresh water is less dense than salt water, so it floats on the layer of salt water and forms a layer of its own. The top of this layer is called the water table, and the layer of fresh water is called the basal ground water lens. Hydrologists are very familiar with this reservoir of fresh water and know that for every meter (foot) above sea level where the water table is located, there are 40 meters (feet) of fresh water below. So, if a drill hole encounters the water table at a depth 10 meters (feet) above sea level, the fresh water layer beneath it would be 400 meters (feet) thick. This 40 to 1 ratio is often called the Ghyben-Herzberg principle, named after the two men who discovered it.
The basal ground water moves outward toward the edge of the island and discharges into the ocean near the shoreline. The fresh water layer assumes a lens-like shape, because the water does not move freely through the rocks. The water table must rise toward the interior of the island to provide a hydraulic head for the water to move seaward. The higher the fresh water table rises, the lower the salt water layer is depressed, thus resulting in a lens-like shape for the fresh water layer with a gentle rising top and downward bulging bottom.
In volcanic areas, the basal ground water often moves through rocks heated by magma. The heat is transferred to the water, and the water can become very hot. If the water flows away from the heat source, the temperature drops, especially when cold water is mixed in. Eventually this warm fresh water makes it to the sea, where it is discharged along the shoreline. Warm springs are commonly found along the Puna coast from Cape Kumukahi to `Opihikao. Bathers have their favorite warm spots in the Kapoho Beach Lots, `Ahalanui Park, Pohoiki, Isaac Hale Park, and `Opihikao.
Old-timers remember the most famous of these warm waters - the Kapoho Warm Springs, which was frequently the scene in movies and tourist brochures. The Kapoho Warm Springs was formed when the downthrown block of the Kapoho fault slipped below the water table and exposed the warm waters, probably heated by a magmatic body intruded in 1840. Unfortunately, this favorite picnicking and swimming spot was covered by lava during the 1960 Kapoho eruption.
At sea level pressures, the maximum temperature of water is 100? Celsius (212? Fahrenheit), but when water is placed under high pressures, it can be heated to more than 300? Celsius (570? Fahrenheit). These high-temperature waters are found at depth near magmatic bodies.
The weight of the column of water above the heat source provides the pressure necessary to allow the high water temperatures. When a drill hole intercepts a superheated body of water, the water can be made to flash to steam by reducing the pressure above it. This is accomplished by pumping out the column of cold water above the hot water. The reverse is also true. If you want to stop a well from producing steam, you can pump cold water down it until the weight of the cold water column raises the pressure sufficiently to stop the hot water from flashing.
The lower east rift zone of Kilauea Volcano is recognized as having many of these high-temperature water bodies. The geothermal plant in Pohoiki is presently using this source of natural energy to produce over 25 percent of this island's electrical energy
Eruptive activity of Kilauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a network of tubes toward the coast. The only flow entering the ocean is at Lae`apuki. The flow located near Waha`ula is not entering the ocean at this time but is filling low areas mauka of the shoreline and is also inflating earlier active flows. The public is reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying unpredictable collapses of the new land. The active lava flows are hot and have places with very thin crust. The steam clouds are highly acidic and laced with glass particles.
Three earthquakes were reported felt during the week ending on February 17. Residents of Kona felt an earthquake at 5:19 p.m. on Friday, February 11. The magnitude-3.8 temblor was located 34 km (20.4 miles) west of Keauhou at a depth of 16.8 km (10.1 miles). A resident of Leilani Estates subdivision reported an earthquake at 2:18 p.m. on Thursday, February 17. The magnitude-3.5 earthquake was located 9 km (5.4 miles) south of the summit of Kilauea Volcano at a depth of 31.5 km (18.9 miles). A minute later at 2:19 p.m., an earthquake was felt throughout the island. No damages were reported from the magnitude-4.5 temblor which was located 12 km (7.2 miles) south of the summit of Kilauea Volcano at a depth of 32 km (19.2 miles).
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Updated: 23 Feb 2000