[2] The immediate precursors to the 1984 eruption consisted of an abrupt increase in small earthquakes and volcanic tremor recorded on seismic stations located near Moku‘āweoweo caldera.

Just before 1:00 a.m. on March 25, the tremor amplitude increased to the point that the astronomical telescopes on Mauna Kea, 25 mi (40 km) to the northwest, could not be stabilized due to the constant ground vibration.

[2] At 1:25 a.m., a military satellite recorded a strong infrared signal from the summit of Mauna Loa, indicating that the eruption was underway.

Eruptive fissures migrated rapidly down the southwest rift zone to the 12,750 ft (3,890 m) elevation and across the southern half of Moku‘āweoweo (flow A on map).

[2] At 10:30 a.m., intense steam emissions began along a fracture 0.6 mi (0.97 km) long farther down the northeast rift zone between the 10,690 and 10,400 ft (3,260 and 3,170 m) elevation, but no eruptive fissure formed in this area.

This fissure rapidly migrated both uprift and downrift, so that by 6:30 p.m., a line of lava fountains slightly longer than 1 mi (1.6 km) was active.

Smoke from burning vegetation, loud explosions caused by methane gas along the advancing flow front and the intense glow at night all contributed to a growing concern among Hilo's residents.