Volcano Watch — Kīlauea's not Hawaii's only active volcano

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The volcanic hazards posed by Kīlauea Volcano are obvious to everyone who sees the active eruptions and visits Hawai`i Volcanoes National Park, where the starkness of much of the volcanic landscape is a testament to the recency of activity. Mauna Loa Volcano erupts frequently enough for people to remember that it is a young, active, and dangerous volcano. However, other volcanoes in the Hawaiian Islands also pose volcanic hazards to those who choose to live on their slopes. They are less notorious than Kīlauea and Mauna Loa only because they erupt less frequently, and therefore have not erupted during the lifetime of anyone alive today.

Hualālai Volcano, which looms above Kailua on the Kona coast of Hawai`i, last erupted in A.D. 1801, whereas East Maui (Haleakalā) Volcano last erupted in about 1790. Mauna Kea Volcano on Hawai`i cannot be considered extinct, as it last erupted about 4,000 years ago—a long time in human history, but a relatively short time in geologic history. Today, we will focus on the eruptive history of, and hazards posed by, Hualālai Volcano.

Hualālai is a shield-shaped volcano that occupies the west-central part of the island of Hawai`i and makes up most of the North Kona District. It covers an area of about 290 square miles and rises 8,271 feet above sea level. The oldest lava on the surface is the huge cone at Pu`u Wa`awa`a and the 900-foot thick flow that issued from it about 105,000 years ago. This flow and cone consist of a very unusual lava type, called trachyte, that is rich in volatiles, making the eruptions explosive and low in temperature, making the lava far less fluid than most Hawaiian lavas (hence the extraordinarily thick flow).

However, most of the surface is much younger, and roughly 95 percent is covered by basaltic lava flows less than 10,000 years old. Lavas younger than 1,500 years old cover about 17 percent of the surface. The last eruption occurred in A.D. 1800 to 1801; lava flowed from five main vent areas along the northwest rift zone, which is easily identified by the abundant eruptive cones that occur there. Two of these flows reached the ocean; the younger Hu`ehu`e flow is that upon which the Keahole airport is built. This flow erupted from a series of spatter cones just below the Mamalahoa Highway and is four miles wide at the shore. The upper, and older, of the two main vents, located between 5,500 and 6,000 feet above sea level, sent an `a`a flow, the Ka`upulehu flow, to the sea, just southwest of Kiholo Bay, that flowed down the slope rapidly enough to engulf several Hawaiian villages on the shore and nearly entrap some villagers.

Before the A.D. 1800-1801 eruption, the next well-documented eruption occurred above 750 years ago; it forms the north side of Keauhou Bay and underlies the shopping center at Keauhou. This `a`a flow, Waha Pele, is about one mile wide at the shore and extends out beneath the sea about 1.25 miles. Prior to the eruption 750 years ago, there was a series of eruptions between about 900 and 1,150 years ago; none of the resultant flows reached the sea. A major sequence of eruptions occurred between about 2,000 and 3,000 years ago, covering 38 percent of the surface and sending six major flows to the shore. The two largest of these flows were more than three miles wide at the coast. Another cluster of intense eruptive activity occurred about 4,700 years ago, but numerous additional eruptions intervened between 3,000 and 4,700 years ago.

The eruptive history forms the basis for classifying Hualālai Volcano in lava flow hazard zone 4. The volcano is not subdivided into more and less hazardous zones based on proximity to the likely vents because the distance from the vents to the shore is short and the slopes steep. With an eruptive recurrence interval of several hundred years and steep slopes allowing lava to flow quickly into inhabited areas, Hualālai must be considered a hazardous volcano. The main question is not whether Hualālai will erupt again, but when.

Although we cannot forecast when Hualālai will next erupt, we can be fairly certain that the episode 51 eruption of Kīlauea Volcano, whose vents are located on the west flank of Pu`u `O`o, will be erupting off and on in the weeks to come. The eruption had stopped last Sunday, but again became active during the day on Wednesday.