Volcano Watch — A look back at powerful Halemaumau eruptions of 1924

Precisely 74 years ago today, the final chapter of one of Kīlauea's most alarming displays of volcanic power came to a close. Halema`uma`u, the fire pit nestled in Kīlauea's summit caldera, ended a 10-day-long outburst of violent steam explosions on May 24, 1924.

For nearly two decades prior to 1924 an immense lake of molten lava churned and bubbled at the bottom Halema`uma`u Crater. Travelers from across the world were drawn to the rim of Halema`uma`u to witness the spectacular array of volcanic pyrotechnics. In February 1924, however, a curious thing happened. A giant molten whirlpool formed on the lake's surface. Over two days' time, lava drained away like water in a bathtub, leaving behind a dully glowing pit 112 m (370 feet) deep and 520 m across (1,700 feet). Halema`uma`u remained in this ominously quiet state for almost two months.

In April 1924, attention was diverted from the unusual happenings at Halema`uma`u by an earthquake swarm that rattled the summit of Kīlauea. The swarm migrated 45 km (28 miles) down the volcano's East Rift Zone to the eastern tip of the island near Kapoho. On April 22 and 23, Kapoho residents felt more then 200 earthquakes. Cracks opened, and a stretch of land 6.4 km (4 miles) long and 1.6 km (a mile) wide subsided. The area just north of Cape Kumukahi dropped 4 m (14 feet) and the sea washed inland a kilometer (half a mile) past the shoreline. Such swarms typically herald the start of an eruption, but no eruption came-or so it seemed.

A few days later, attention again turned to Halema`uma`u. The floor of the pit began to sink rapidly. Incandescent slabs of rock peeled from the walls and crashed down into the pit. Hot ash and pebbles swirled out over the rim as the rocks hit bottom. Undaunted by ash storms that "stung like hail," some 400 visitors from a Thomas Cook steamship tour were thrilled by the awesome sight.

On the morning of May 11, a ranger from Hawai`i National Park noticed several hot boulders on the rim of Halema`uma`u. Evidently, a small explosion had occurred in the pit overnight. The park superintendent, Thomas Boles, put up roadblocks 0.8 km (a half a mile) from the crater and ventured out to investigate with two other observers. Boles was within 3 m (10 feet) of rim when he heard a "thud" followed by a "prolonged whooosh." Thousands of red-hot boulders shot up amidst a fury of black ash. The ash column rose 915 m (3000 feet) above the crater as the party ran for cover. Fortunately, all three made it back to their vehicle, sustaining only a few cuts and bruises. They found that a boulder weighing nearly 45 kg (100 pounds) had sailed over the vehicle during the explosion, landing more than 600 m (2,000 feet) from the crater. Superintendent Boles pushed the roadblocks back 2.2 km (2 miles) from the crater.

Similar events followed, with each explosion more intense than the last. At night, the white-hot rocks that were hurled from the crater looked like rockets trailing sparks.

The largest explosion occurred on May 18. With a resounding B‘Ō‘ōM, an ash column shot up 6.5 km (4 miles) in the air while a hurricane-force rush of gas and ash spread across the caldera floor. To Superintendent Boles, the dark, mushrooming column "loomed up like a menacing genie from the Arabian Nights." Static electricity generated between ash particles produced streaks of blue lightning and condensed steam mixed with the ash to create a rainstorm of gray mud.

A young man from a nearby sugar plantation had slipped past the road blocks set up by the Superintendent and was within 600 m (2,000 feet) of the rim when the explosion occurred. He was hit by a boulder and severely burnt by the falling ash. Rescuers hurried in to the caldera when the explosion ended some 20 minutes later, but the unfortunate man died on the way to the hospital.

After the deadly blast of May 18, the explosions continued, but with waning intensity. When the dust finally settled on May 24, Kīlauea caldera was littered with huge boulders. Rocks weighing as much as 8,000 kg (8 tons) were found 500 m (1,600 feet) from the rim of Halema`uma`u. The pit itself was almost twice as wide as it had been and eight times deeper.

The cause of the 1924 explosions can be deduced from seismic and geodetic measurements made by HVO scientists before, during, and after the event. April's earthquake swarm indicates a massive draining of magmafrom Kīlauea's summit reservoir into the East Rift Zone. The considerable ground cracking and subsidence in Kapoho suggests that magma moved out into the submarine portion of the rift where it very likely fed an eruption on the ocean floor.

The summit of the volcano sagged inward and cracked as the magma drained. Groundwater rushed through the developing crack system where it encountered incandescent rock at temperatures close to 980°C (1,800°F). The water flashed violently to steam at the sudden encounter, releasing sufficient power to excavate over a thousand-million kilograms (a million tons) of rock.

HVO scientists estimate that approximately 400 million cubic meters (520 million cubic yards) of magma shuttled down the east rift zone conduit in 1924. That's enough magma to fill 265,000 Olympic swimming pools!

Volcano Activity Update

The eruption of Kīlauea continues without significant change.

There were no felt earthquakes last week.