An active lava lake partially fills the crater of Pu`u `O`o. The flat terrace surrounding the
lake is about 50 m below the north rim of
the crater (right), where most ground observations of the lake are made. On
September 12, magma from beneath Pu`u `O`o and the summit caldera moved into
the upper east rift zone. A dike formed between Pauahi Crater and
Mauna Ulu. Two days later, a small pond of lava appeared in the crater. It slowly rose, and by September 16 the lake was about
92 m below the east rim of the crater and 50-60 m in diameter.
In the next week the lava rose to about 63 m below the east rim
and covered much of the crater floor. The lake remained at
this approximate level through October, episodically
rising 10-15 m and occasionally overflowing onto the
top of the adjacent terrace and resurfacing it.
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