The eruption started on 3 January 1983 when ground fissures opened up and "curtains of fire" (long thin fountains of molten lava) issued from a 7-km long discontinuous fissure system in the remote rainforest of the middle east rift zone of the volcano. From this "typical" sort of beginings a very atypical rift zone eruption unfolded.
By June 1983, the activity had localized to one central vent and there it stayed for the first 3.5 years (and 44 eruptive episodes) of the eruption. At times, spectacular lava fountains (up to 460 m) high could be seen throughout east Hawaii. During this period, the eruption settled into a regular pattern of brief eruptive episodes, each lasting less than 24 hrs (with the exception of the-384-hr-long episode 35A) and each separated by repose periods averaging 25 days. The fallout of cinder and spatter from the towering lava fountains built a cone at the Pu`u `O`o vent that was measured to be 255 m high by HVO. During this time, a shallow magma reservoir developed beneath the Pu`u `O`o vent. The reservoir was estimated from ground deformation to be 1.6-km long, 2.6-km deep but only about 3-m wide. This reservoir was open at the top, which promoted rapid cooling of the magma (believed to be at a rate of ~5% in 20 to 30 days).
In July 1986, the locus of lava production abruptly shifted 3-km downrift to form the Kupaianaha shield, which was active nearly continuous until early 1992 (Kupaianaha means "mysterious one" in Hawaiian). With this vent shift came a new eruption style: continuous, quiet effusion from a lava pond replaced the episodic high fountaining. Overflows from the pond built a broad, low lava shield. In November 1986, lava from Kupaianaha reached the ocean, some 12 km away. It covered the small community of Kapa`ahu in its path. Over the next five years, much of the lava erupted from Kupaianaha streamed directly into the sea via a lava tube system that led from the lava pond to the coast.