The dynamics of Hawaiian lava in surface flows and tubes

U.S. Geological Survey, Hawaiian Volcano Observatory,

Close observation of long-lived eruptions in Hawai`i over the past several decades has advanced our understanding of the emplacement of basaltic lava. Lava flows take on a wide range of morphologies from pahoehoe to `a`a indicative of their emplacement temperature and history, supply rate, and ground slope. Pahoehoe advances at the slowest rate - typically not faster than 0.2 km/hr. `A`a flows generally advance at faster rates – the fastest lava flow ever observed in Hawai`i was an `a`a flow advancing 9 km/hr from the southwest rift zone of Mauna Loa in 1950. Lava travels even faster through lava tubes and channels flowing at speeds between 5 and 60 km/hr. Tubes typically form within pähoehoe flows, although they can also form within ÿaÿä flows. Tubes are well insulated and allow lava to be efficiently supplied to the front of the flow, which will advance farther than a flow fed only by surface channels. Channelized `a`a flows are common on moderate to steep slopes. If supply persists, the lava in the channel will convert to pahoehoe, the channel will roof over forming a tube, and the `a`a levees will eventually be buried by pahoehoe breakouts from the tube. Downcutting by lava flowing within a tube is now well documented at rates up to 10 cm/day for as long as several months. The downcutting seems to slow down after this time. Lava falls commonly develop within tubes. Rather than the falls cutting back, as in the case of water falls, lava fal ls develop by accelerated downcutting of the plunge pools.