Satellite photos show the lava covered about 300 sq km (115 sq miles).
According to Addis Ababa University, Monday's eruption prompted a minor earthquake, but there were no reported injuries or damage to infrastructure.
When Mount Erta Ale, Ethiopia's only active volcano, erupted in 2005, it displaced thousands people.
It is in the Danakil Depression, one of the lowest and hottest places on earth, which is known for its salt mines.