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Volcano Watch: Revisiting the 1919–1920 Mauna Iki eruption

Fast-moving lava flows erupted from Mauna Iki were hand-colored in this black-and-white photo taken May 17, 1920. (Historic photo courtesy of Roger and Barbara Myers)

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Shortly after dark on Dec. 16, 1919, a bright meteor streaked over Kilauea like a fireball and exploded southwest of the summit. The concussion — a portent of things to come on the volcano’s southwest rift zone — was heard throughout most of East Hawaii.

The year 1919 was a lively time at Kilauea. Halemaumau was a lava shield — not the large crater we see today — and formed a high point on Kilauea’s caldera floor. In early July, the lava column began rising slowly, marking an increase in activity, and the Halemaumau shield was soon topped by a handful of overflowing lava lakes.

Lava, erupting from the north flank of the shield, swept around the eastern caldera floor below today’s location of the Volcano House Hotel and pooled at the base of Byron Ledge.

This activity continued through the autumn, with lava fountains on the lava lakes, skylights along a lava tube, and active lava flows on the caldera floor. Kilauea’s incandescence illuminated the night sky.

But this ended on the morning of Nov. 28, when the lava column dropped more than 180 m (600 ft) in just a few hours. The resulting crater was 365 m (1,200 ft) across, with nearly vertical walls and a small lava lake churning at its bottom.

Recovery was quick, though, and a great flood of lava unlike any previously described at Kilauea began to refill the crater at a rate of about 12 m (40 ft) per day. By Dec. 15, the lava lake was a mere 6 m (20 ft) below the rim of the new crater, and its surface roiled violently with thousands of small dome fountains.

Southwest of Halemaumau, the caldera floor began to heave and groan and, just before noon, ripped open in a long fissure. Lava poured through the crack and spread quickly across the southwestern caldera floor.

The next evening, as the meteor flashed overhead, the lava lake began to rise again. The rumble of lava fountains was clearly heard by visitors on Uwekahuna Bluff, where the Hawaiian Volcano Observatory now sits, and the caldera walls were lit by the lava lakes’ brilliant glow. Unbeknownst to them, lava was pushing its way underground toward the southwest.

On Dec. 21, upon seeing smoke in the distance, scientists journeyed into the Ka‘u Desert along Kilauea’s southwest rift zone. There they discovered that lava from the summit had traveled even farther down the rift. Lava could be seen deep in cracks and was found welling from the ground 6.5 km (4 miles) from the caldera. These flows, however, were short-lived, dying out by the next day.

On Dec. 23–24, lava broke to the surface again along Kilauea’s southwest rift zone.

Fissures 10 km (6 miles) and 14.5 km (9 miles) from Kilauea’s caldera erupted lava that quickly piled up on the rift zone, forming low shields topped by lava ponds.

The main shield complex — the one nearest the caldera — burst open on Dec. 30, releasing a torrent of lava that fed a fast-moving aa flow. The more distant fissure was inactive by early January 1920, but the main shield, which became known as Mauna Iki (Little Mountain), continued to grow and feed lava to the advancing aa flow. This flow traveled downslope to the southwest, eventually reaching within 6.5 km (4 miles) of the coast before stopping on Jan. 12.

Mauna Iki continued to erupt and, by the end of January, the shield was about 3 km (2 miles) long and 30 m (100 ft) high. Over the following months, the lava ponds atop Mauna Iki overflowed repeatedly, and spiny aa flows seeped from its flanks.

This activity persisted until mid-June, when the Mauna Iki eruption began to wane simultaneously with the lowering of the lava column at Halemaumau.

By July 21, pits had begun to form on top of Mauna Iki as lava withdrew. No eruptive activity was seen after Aug. 15.

The lava lake at Halemaumau began to rise again shortly afterward, but the southwest rift eruption did not resume. After eight months of activity, Mauna Iki was dead.

Mauna Iki remains a prominent landmark on Kilauea’s southwest rift and can be reached easily by hiking the Ka`u Desert Trail in Hawaii Volcanoes National Park.

What better way to appreciate a lava shield created 80 years ago than to stand at its summit?

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