The Origins of Pu`u Mahana

The rise of Pu`u Mahana on the southeast coast of the island of Hawai`i is a landmark and a popular attraction. The literal translation of Pu`u Mahana is “warm hill.” People who are familiar with this region note that it is a favorite destination of fisherman, hikers, and swimmers. Amongst geologists, the origin of the cone is up for debate.

Some claim that Pu`u Mahana is a littoral cone, meaning that it was formed by the interaction of flowing lava and the sea on or near the shore. When hot lava and water are mixed at just the right ratio, the combination can be very explosive. Under ideal conditions, such as favorable wind directions, the accumulating lava fragments can build a large cone.

What happens when lava comes into contact with shallow seawater? The seawater turns into super-heated steam. The steam expands rapidly and tears the lava apart, resulting in disintegration of the flow. The size of the fragments is a direct indication of the efficiency of the mixing process. A high degree of fragmentation indicates a very explosive interaction of lava and seawater. Several examples of littoral cones include Sand Hills in Puna at Nanawale, Pu`u Hou near South Point (Ka Lae), and Na Pu`u a Pele at the end of Road to the Sea.

Pu`u Mahana sits right at the water’s edge and forms a nice bay for swimming. Why is there a possible question about its origin? It is easy to envision that the lava would have had easy access to the sea to build the cone.

The key to Pu`u Mahana`s formation lies in its age. Radiocarbon analysis of charcoal collected reveals that the cone formed more than 49,000 years ago. Charcoal is created when plants, growing prior to an eruption, are carbonized by flowing lava. Radiocarbon does have limitations, however. Its useful range for age determination is approximately 150 to 49,000 years before the present.

Where was sea level approximately 49,000 years ago? We know from long-term studies that the island of Hawai`i is sinking at a rate of 2.4 millimeters, or 0.008 feet, per year. This may seem like a very small amount. After 10 years, the island has subsided by 24 millimeters, or approximately 1 inch. After 100 years, it will subside 240 millimeters (or 0.8 feet); after 1,000 years, 2,400 millimeters (or 7.9 feet); and after 10,000 years, 24,000 millimeters (or 78.7 feet). So when Pu`u Mahana was formed, it really was at an elevation of 117,600 millimeters (117.6 meters or 387 feet) above sea level.

This elevation is approximately the elevation of Komohana Street above Hilo or, for Kona residents, twice the elevation of the Kuakini wall. To further complicate the story, oceanographers report that 50,000 years ago, we were in a glacial period, where sea level was 70 meters (229 feet) lower than it is today. So Pu`u Mahana was really 180 meters (600 or more feet) above sea level. So can Pu`u Mahana be the result of the interaction of surface flowing lava and seawater? Probably not!

Another plausible explanation is that Pu`u Mahana is a primary vent. A primary vent is a vent where magma comes to the surface from depth and erupts like Pu`u `O`o, the currently active cone on Kilauea’s east rift zone. However, the character of the deposits at Pu`u Mahana is different than those at Pu`u `O`o and is the result of magma interacting with water in the subsurface environment. A prominent example of this type of formation is Kapoho Cone, the home of Green Lake.

Pu`u Mahana is Green Sand Beach just east (Na`alehu side) of South Point. An olivine-rich magma produced the cone. As the pu`u erodes, olivine is released. Due to its density, the olivine accumulates at the base of the cone and gives the beach its green color. Next time you are at Pu`u Mahana, consider that this cone formed at more than 300 feet above sea level.

Activity Update

During the past week, the count of earthquakes located beneath Kilauea remains at low levels. Inflation continues, but has slowed over the past week.

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater and on the southwest side of the cone. From October 19-23, several meters of subsidence along the south side of the Pu`u `O`o crater caused two spatter cones on the crater floor and a portion of the south crater wall to collapse. Lava continues to flow through the PKK lava tube from its source on the flank of Pu`u `O`o to the ocean, with only a few surface flows breaking out of the tube. In the past week, flows were active on the steep slope of Pulama pali, and visible at night (weather permitting) from the end of Chain of Craters Road.

As of October 27, lava is entering the ocean at East Lae`apuki, in Hawai`i Volcanoes National Park. Small bench collapses continue to occur at the ocean entry. Large cracks cross both the old and new parts of the bench. Access to the ocean entry and the surrounding area remains closed, due to significant hazards. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field. Daily eruption updates are posted on the HVO website.

There were two earthquakes reported felt on Hawai`i Island within the past week. A magnitude-4.0 earthquake occurred at 10:48 a.m. on Sunday, October 23, and was located 4 km (2 miles) southeast of Pu`u `O`o crater at a depth of 4 km (2 miles); it was felt in Papa`aloa. A magnitude-2.8 earthquake occurred at 9:18 p.m. on Tuesday, October 258, and was located 7 km (4 miles) northeast of Ke`anae, Maui, at a depth of 7 km (4 miles); it was felt in Haiku, Maui.

Mauna Loa is not erupting. During the past week, the count of earthquakes located beneath the volcano remains at low levels. Inflation continues but has slowed over the last month.

This article was written by scientists at the U.S. Geological Survey’s Hawaii Volcano Observatory and is republished by with permission.

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