On June 1, 1950, a fissure erupted high on Mauna Loa's Southwest Rift Zone, and within 3 hours, ‘a‘ā lava flows had crossed the main highway on the west coast of the Island of Hawai‘i. The flows soon inundated the coastal village of Ho‘okena-mauka and reached the ocean, creating billowy clouds of steam that rose 10,000 feet into the air. All the villagers reached safety unharmed, but lava flows destroyed about two-dozen structures and cut the highway in three places before the eruption ended on June 23.
Precursory activity included several earthquake swarms
Magma pressure in the volcano remained high following an eruption from the summit of Mauna Loa in 1949 eruption. Notable earthquake swarms occurred during three intervals from May 27 to June 14, July 30 to August 7, and December 1 to 15, 1949, but none ended in an eruption.
At 5:43 a.m. on March 25, 1950, a strong earthquake was felt across the island. On May 29 at 3:17 p.m., two days before the eruption, a magnitude-6.4 earthquake occurred beneath the volcano's west flank. Kona residents reported damage to water tanks and stonewalls. The late May earthquake and aftershocks signaled volcanic unrest and an imminent eruption.
Another swarm directly preceded and accompanied the eruptive outbreak. Volcanic tremor appeared on seismographs at the Hawaiian Volcano Observatory at 9:04 p.m. on June 1 as fissures opened from the uppermost Southwest Rift Zone.
Eruption begins as a fissure
At 9:25 p.m., the eruption glow appeared on the skyline of the southwest rift zone. Observers reported that a 4.0-km-long (2.5-mi-long) fissure opened near the pit crater, Lua Hou (3,840 m, 12,600 ft), and propagated downrift to near Sulphur Cone (3,350 m, 10,990 ft). A lava flow from this fissure headed westward down the mountain. This initial flow moved toward Ho‘okena but advanced only 8 km (5 mi) to the 2,740-m (8,990-ft) elevation.
By 10:23 p.m., a bright glow appeared in a fume cloud 13 km (8 mi) downrift from the previous vents. The illuminated area quickly expanded, which meant that the fissures were rapidly propagating. Vigorous lava fountains jetted into the air 45 to 60 m (148 to 197 ft), with occasional bursts to 90 m (295 ft). This flow moved south-southeastward toward Punalu‘u. By daybreak on June 2 it crossed Kahuku Ranch's upper pasture road. The flow entered the upper reaches of the Ka‘ū Forest Reserve after flowing approximately 16 km (10 mi) to the 1,670-m (5,480-ft) elevation.
Honokua flow blocks the highway
Two parallel fissure systems were erupting. The upper system extended from 3,200 to 2,590 m (10,500 to 8,500 ft) along 9.6 km (6 mi) of the rift zone. The second, lower fissure system, reached from the 2,500- to 2,380-m (8,200- to 7,810-ft) elevation. Lava from the fissures flowed westward in braided streams.
At 12:20 a.m. on June 2, the Honokua flow was approximately 1.5 km above Highway 11. By 12:30 a.m., ‘a‘ā flows it crossed Highway 11 at the town of Pāhoehoe, taking out a gas station, the post office, and several homes. The ‘a‘ā flow continued toward the ocean and reached it at 1:05 a.m. This flow covered the 24-km journey in less than 3 hours. By daylight the activity was greatly diminished, and by noon the flow had ceased at this locality.
Ka‘ohe flow was the second to cross the highway
The second flow across the highway, the Ka‘ohe flow, originated from the upper fissure system between the 2,740- and 2,440-m (8,990- and 8,005-ft) elevation. The Ka‘ohe flow crossed the highway at 5:00 a.m. on June 2, approximately 2 km (1.2 mi) south of the Honokua flow near the Magoon Ranch. This ‘a‘ā flow destroyed several homes and a coconut grove at the coast as it traversed toward the sea, entering the ocean at around noon. Twenty-four hours later, activity in the Ka‘ohe flow ceased.
While the main Ka‘ohe lobe was active, a new ‘a‘ā flow broke off from it. The offshoot traveled farther south and reached within 500 m (1,640 ft) of the highway but never crossed it.
Ka‘apuna barreled downslope and into the sea
Late in the evening of June 1, the fissurecontinued to propagate farther downrift. This lowest fissure system created at the 2,500- to 2,380-m (8,200- to 7,810-ft) elevation, produced another ‘a‘ā flow—the Ka‘apuna flow.
At 12:20 p.m. on June 2, observatory personnel, police and lava enthusiasts were waiting on Highway 11 at the ‘Ōhi‘a Lodge, anticipating the arrival of the Ka‘apuna flow through the forest. At 12:30 p.m. a National Guard plane making an overflight recommended that the ground observers evacuate their current location for positions farther south, as the flow was headed in their direction.
The Ka‘apuna flow crossed the highway at 2:00 p.m., 400 m south of the ‘Ōhi‘a Lodge and 6.4 km (4 mi) south of the Ka‘ohe (middle) lobe. Observatory personnel estimated lava-flow velocities in the ‘a‘ā channel at 16 to 48 km per hour (10 to 30 mph). By 3:30 p.m. the Ka‘apuna flow entered the sea. The lava flowed directly into the sea, unimpeded by the cooling effects of the water. A line of steam 800 m (2,625 ft) out to sea demarked the submarine front of the flow.
Kahuku flow was the last new lava flow
Late in the evening of June 2 or early on June 3, a new lobe broke out from the 2,530-m (8,300-ft) elevation. This flow moved southward toward Kahuku Ranch. On June 8, it ceased after reaching the 1,920-m (6,300-ft) elevation, a distance of 9.3 km (5.8 mi) from its vent.
After the Kahuku outbreak, the activity stabilized, with the Ka‘apuna source and flow becoming the eruptive center. Lava coursed through the lava channels toward the sea. Occasionally lava breached the channels, causing the Ka‘apuna flow to widen and thicken.
The 1950 eruption lasted for 23 days. It erupted 376 million cubic meters (492 million cubic yards) of lava, the largest outpouring of lava from the southwest rift zone of Mauna Loa since written records have been kept. This is equivalent to about 3.5-4 year's output form the 1983–2018 Pu‘u ‘Ō‘ō eruption of Kīlauea. This comparison illustrates how much larger eruptions of Mauna Loa can be compared with those of Kīlauea.
Flows from this eruption made their way to the sea quickly. For example, the Honokua flow covered the 24-km (15-mile) journey, from vent to the ocean, in less than 3 hours.
Was this eruption unusually large and rapid? Yes and no. The eruption of 1859 produced about the same volume of lava but took more than ten times as long to do it. The summit eruption starting in 1872 supplied nearly twice as much lava in approximately 1200 days, not 23. These three eruptions are by far the largest of the past 200 years. So, the 1950 eruption was voluminous though not record breaking, and its discharge (the volume rate of eruption) was by far the greatest since records have been kept.
See the table summarizing Mauna Loa activity over the past ~200 years here.