A warning has been issued after volcanic ash started falling near the volcano.
The outbreak began on Sunday.
The lava flow is still confined to the summit, but should conditions change, it could pose a threat to nearby communities, according to the US Geological Survey.
So far, no one has had to be evacuated, but the volcano summit and several roads in the region have been closed.
It is unclear how long the outbreak will last.
Hawaii has six active volcanoes, including Mauna Loa which has erupted 33 times since 1843. The last eruption in 1984 lasted 22 days.
At the time, the lava flow stretched seven kilometers from the city of Hilo, where 44,000 people live today.