During 14-20 December, HVO reported that the lava lake circulated and periodically rose and fell in the deep pit within Kīlauea's Halemaʻumaʻu Crater, remaining below the inner ledge (75 m below the crater floor). Almost daily measurements indicated that the gas plume from the vent continued to deposit variable amounts of ash and fresh spatter nearby.

Incandescence was visible along the 21 September fissure on the SE flank of Puʻu ʻŌʻō cone, from a skylight on the lava tube, and from small spatter cones on the E and S edges of the Puʻu ʻŌʻō crater floor. Pāhoehoe flows that were 300-400 m wide, fed by lava tubes from the fissure, continued to be active about 6.8 km SE of Puʻu ʻŌʻō and entered the ocean W of Kaʻiliʻili. The lava delta now extends 15-20 m into the ocean at a point 6.4 km W of the Chain of Craters Road. During 17-20 December the lava flow branched, with lobes advancing NE and W into the ocean. Infrequent plumes were observed from the ocean entry during 18-20 December.

Source: US Geological Survey Hawaiian Volcano Observatory (HVO)

Geological Summary. Kīlauea overlaps the E flank of the massive Mauna Loa shield volcano in the Island of Hawaiʻi. Eruptions are prominent in Polynesian legends; written documentation since 1820 records frequent summit and flank lava flow eruptions interspersed with periods of long-term lava lake activity at Halemaʻumaʻu crater in the summit caldera until 1924. The 3 x 5 km caldera was formed in several stages about 1,500 years ago and during the 18th century; eruptions have also originated from the lengthy East and Southwest rift zones, which extend to the ocean in both directions. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1,100 years old; 70% of the surface is younger than 600 years. The long-term eruption from the East Rift Zone between 1983 and 2018 produced lava flows covering more than 100 km², destroyed hundreds of houses, and added new coastline.