The Hawaiian Volcano Observatory said the vent opened along the middle of Kilauea Volcano's east rift zone, sending lava shooting up to 65 feet high.
The vent opened when the floor at the Pu'u O'o crater collapsed around 5 p.m. on Saturday.
Kilauea has been in constant eruption since Jan. 3, 1983.
At the summit, lava receded rapidly late Saturday but seemed to slow Sunday. Also, about 150 small earthquakes were recorded within Kilauea in the past 24 hours.
Because of the latest activity, the Hawaii Volcanoes National Park has closed Chain of Craters Road and all east rift zone and coastal trails. Kulanaokuaiki campground was also closed until further notice.
No homes were threatened.
The Associated Press contributed to this report.