Volcano Watch — "Things are not always what they seem; the first appearance deceives many..."

For example, there are numerous cones, or Pu‘u, strewn along the coast from the "Road to the Sea" to the Kahuku fault scarp. Are these primary vents? If so, this region would be in Lava Flow Hazard Zone 1 because, by definition, Hazard Zone 1 includes primary vents. Yet this region is not designated Lava Flow Hazard Zone 1.

The "cones" along the coast are littoral cones. Littoral cones are a type of volcanic landform created when lava flows come into contact with the sea. Littoral is a word that means "of or pertaining to the shore of a lake, sea, or ocean."

When seawater and lava interact, the water is converted instantly to steam. This transformation is so abrupt that steam-driven explosions occur. The explosions fragment the lava and propel lava bombs and tephra into the air. The fragments deposited on shore form the cones.

This fragmentation process has to continue over and over to generate enough volcanic debris to form cone-shaped mounds. Another condition necessary for the creation of littoral cones is a favorable wind direction. If the winds are blowing off shore, the explosive products will be carried seaward and disappear into the ocean.

The fragments in littoral cones are denser and contain fewer vesicles than primary vents along the rift zones. They are also coarser, more angular, less glassy, and contain a greater proportion of older rock. In addition, there are some tell-tale characteristics to look for on the ground. Littoral cones are usually bisected by a feeder channel or tube system and are usually arcuate but do not form a complete circle. The range of products—much more diverse than in primary vents—reflects the explosive nature of cone formation.

On the Island of Hawai‘i, there are more than 20 littoral cones. Many of them are located on the southern and western coasts of the island. Three eruptions in the past 200 years formed large littoral cones: in 1840 on Kīlauea and in 1868 and 1919 on Mauna Loa.
The 1840 eruption of Kīlauea entered the sea near Honolulu Landing in Puna, producing the Sand Hills. This eruption flowed into the sea for nearly a month and formed three littoral cones. Today, only the remains of one cone are preserved; the other two hills were swept away by the sea.

The Mauna Loa eruption of 1868 was spectacular for various reasons, but one prominent result of that eruption is Pu‘u Hou (New Hill). Pu‘u Hou is the largest littoral cone in Hawai‘i, standing 73 m (240 ft) tall. It is easily visible from Highway 11, just west of Ka Lae (South Point) at the coast.

It was once thought that littoral cones formed only when ‘a‘ā lava flowed into the sea. Geologists felt that ‘a‘ā provided more nooks and crannies for lava-water interaction than did pāhoehoe. The greater surface area increased the frequency of hydrovolcanic explosions. But the current eruption of Kīlauea has shown that littoral deposits can be formed from pāhoehoe flows, as well.

Nonetheless, eruptions that produced ‘a‘ā flows have developed the largest littoral cones and associated deposits. Their greater surface area enhances fragmentation. Furthermore, high effusion rates result in greater volumes of lava interacting with sea water, thereby inducing more explosions and providing more material to be fragmented. A taller cone indicates a more explosive interaction between lava and water.

Many other examples of littoral cones can be seen along Mauna Loa's south and southwest coastline. All of the Pu‘u, or cones, seaward (makai) of Highway 11, from Kahuku Ranch (the Kahuku fault scarp) to Kauna Point and northwest to Pu‘u Ohau, are littoral cones. Spectacular examples are found at Pohue Bay, "Road to the Sea," and ‘Au‘au Point.

According to the Roman poet Phaedrus, "Things are not always what they seem; the first appearance deceives many; the intelligence of a few perceives what has been carefully hidden...." Along Highway 19—between Makalawena and the resort community of Kūki‘o, makai of the highway— is a Pu‘u call Kuili. Is this a littoral cone? The answer will be revealed in next week's Volcano Watch.

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Volcano Activity Update

A prolonged deflation started at Kīlauea's summit on February 16, and the lava lake within the eruptive vent in Halema‘uma‘u dropped about 50 m (164 ft) as a result. Deflation switched back to inflation on February 21, and the lava lake has since risen back to its previous level 70–80 m (230–262 ft) below the floor of Halema‘uma‘u. A small collapse of the northwest rim of the vent during the deflation triggered a small explosive event that ejected molten spatter onto the Halema‘uma‘u crater floor. Superimposed on the general rise and fall of the lava surface during the deflation-inflation cycle were sporadic, short-lived lava high stands that changed the lava level by 10–15 m (30–50 ft). Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

At Pu‘u ‘Ō‘ō, the deflation-inflation cycle resulted in the cessation of lava flows on the crater floor. Flows resumed once Pu‘u ‘Ō‘ō began to inflate again and, by the time of this writing (Thursday, February 24), much of the eastern half of the crater floor had been covered by fresh lava. Despite the shut-down in activity at Pu‘u ‘Ō‘ō during the deflation-inflation cycle, modest surface flows remained active on the pali and coastal plain west of Kalapana throughout the week.

Two earthquakes beneath Hawai‘i Island were reported felt this past week. A magnitude-2.4 micro-earthquake occurred at 12:22 p.m. HST on Thursday, February 17, 2011, and was located 10 km (6 mi) north of Waimea at a depth of 24 km (15 mi). A magnitude-3.2 earthquake occurred at 5:00 a.m. HST on Wednesday, February 23, 2011, and was located 6 km (4 mi) south of Volcano at a depth of 3 km (2 mi).