Volcano Watch — The 1840 Kīlauea eruption was big, spectacular, and made the Sand Hills

The eruption occurred from vents spread along 35 km (21 miles) of the rift zone--an unusually great length. A wide range of lava types was erupted, from basalt without crystals high on the rift zone to basalt rich in olivine crystals low on the rift zone. The eruption was the largest on the lower east rift zone in the past 2,000 years. Twenty-six square kilometers (ten square miles) of forest were covered. About 265 million cubic meters (350 million cubic yards) of lava remained on land after 26 days of eruption, more than twice that erupted from Pu`u `O`o in a year. An unknown amount of lava went into the water.

Missionaries described the eruption, and the U.S. Exploring Expedition, led by Captain Charles Wilkes and including the noted geologist Janes D. Dana, visited the eruption site soon thereafter. Thus, the eruption was the first well-documented outbreak of Kīlauea. A quotation from the experienced volcano watcher and scientist Rev. Titus Coan captures the excitement just prior to the eruption.

"There had been activity such that no man had seen. It was reported that, for several days before the outburst, the whole vast floor of the crater [Halemaumau] was in a state of intense ebullition; the seething waves rolling, surging, and dashing against the adamantine walls, and shaking down large rocks into the fiery abyss below. The heat was such that no one dared to venture close for a clear look."

A few earthquakes were felt near Hilo in May, before the outbreak. Precursory activity included 3 days of "severe earthquakes" and an increase in lava-lake height and activity at Halemaumau, which slowly filled in the early months of 1840. The lava lake attained a maximum height of 75 m (250 ft) above the crater floor in May.

On May 30, people in Puna noticed smoke, apparently the beginning of the eruption. Lava appeared in `Alae Crater, 8 km (5 miles) southeast of the summit, erupting from a fissure high on the northwest wall of the crater (now buried under Mauna Ulu lava flows). The erupted lava cascaded to the crater floor, forming a lava lake 90 m (300 ft) deep.

Once the Wilkes expedition arrived, a party visited the eruption site. They found that `Alae had drained and eruptive activity had migrated downrift, where a short fissure opened on the slope of Kane Nui o Hamo, 2 km (1 mile) from `Alae.

Several fissures erupted farther downrift, between Makaopuhi and Napau Craters. Lava formed a pond with a surface "cut by many cracks laden with sulfur and fuming profusely. Pumice as large as a man's head was seen." Lava covered about 3.7 square kilometers (1.4 square miles) of the surrounding forest, forming a flow field 0.5 km (1500 ft) wide and extending 4.8 km (3 miles) to the east-northeast. Hikers to Napau cross this flow, and the ruins of the Pulu Factory are on it.

The fissure system propagated 6 km (4 miles) eastward from Napau to Pu`u Kahaualea, erupting lava that covered 3.6 square kilometers (1.4 square miles). Finally, on June 1, the eruption migrated downrift 20 km (12 miles) to just southeast of Pahoa, where most of the activity centered, 8 km (5 miles) from the sea.

On June 3, the fifth day of the eruption, lava entered the ocean near Honolulu Landing. The flow built the coastline outward about 400 m (1,300 ft). Three littoral cones, now called the Sand Hills, were formed by fallout as steam explosions blasted bits of shattered lava skyward. The cones originally stood 30-50 m (100-150 ft) high, but later erosion has changed their shapes and lowered their heights.

During, or just preceding, the eruption, the level of lava in Halemaumau fell some 150 m (500 ft). After the eruption ended on June 2, Halemaumau was nearly inactive for awhile but within 6 months had returned to a state of, in Dana's words, "full ebullition."

Volcano Activity Update

Eruptive activity at the Pu`u `O`o vent of Kīlauea Volcano continued unabated during the past week. Surface activity is mainly visible in the westernmost section of the pali flow field. The Kohola arm of the Mother's Day flow has some breakouts on the coastal flat. The breakouts, small and sluggish, are increasingly confined to the west and east sides of the flow. The east-side lobe of the main Mother's Day flow also remains visible as a series of incandescent patches from the top of Pulama pali out onto the gentle slope below. No lava is entering the ocean.

Three earthquakes were felt during the week ending at noon on August 28. A small earthquake, of magnitude 2.5, was reported felt at Puna Geothermal Venture at 2:08 p.m. on August 21. It was located about 2 miles east of Pu`ulena Crater at a depth of 2.4 miles. The largest earthquake since April 2, 2000, was felt across the island at 8:24 p.m. August 26. The earthquake had a magnitude of 5.0 and came from about 6 miles under the south flank of Kīlauea, some 6 miles northwest of Ka`ena Point and 8 miles southeast of HVO. The third felt earthquake was recorded at Waikoloa, a magnitude 2.2 shock at the same location and depth as the large earthquake.

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains low, with 3 earthquakes located in the summit area during the last seven days.