A deep geothermal vent has sprung into life near Rotorua after laying dormant for more than 30 years.

The Mud Rift in the Waimangu Volcanic Valley, about 25km south of Rotorua, erupted last month, releasing steam and water into the surrounding area.

GNS Science Volcanologist Brad Scott said the eruption seemed to be mainly steam and fine sands.

"There is lots of evidence of fluids flooding in to the rift, especially at the west end. Vegetation nearby is stressed and browning off, but is not damaged by the eruption. There is also some evidence of collapse around the edges of the two smaller vents."

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Mr Scott said it had appeared to be a short-lived eruption.

The Mud Rift is a 36m long, 5-6m wide, and 15m deep geothermal vent in Raupo Pond Crater, formed in the June 1886 Tarawera eruption.

The Mud Rift formed in 1906 and last erupted in 1981.

GNS have recorded 47 earthquakes within 10km of Waimangu in the past year. In the last six years, 388 quakes have been recorded, which is typically 64 per year.