Today we spent about an hour at Deildartunguhver, a tiny thermal area northeast of Reyjavik. It consists of a single, wide fissure about 50 meters long that pours out boiling water from 10 major vents and numerous smaller cracks. It has been culturally modified to provide hot water for heating districts to the west.
(For identification purposes, I've tried to follow the numbering found in Niemann at Volcanic Springs. Unfortunately, he does not provide a map or photos of all the vents. I counted 10 vents, and based on his photo of #9, I think #10 became active after his visit.)
The vents are located in a wall at the base of Hverahóll, in two parallel lines. Some are about 1/2 meter up the side of the wall. Others are pools in the runoff channel, which eventually flows into the thermal district's intake.
The vents seem to be split into two groups, with five in each. Several The northnermost of the southern group, #5, is a pool in which I saw only occasional splashes. The rest of them spout continuously up to two meters (maybe even three) high from a crack in the wall.
The southernmost of the northern group, #6 is a geyser. It is a hole in the wall which spouts about 25-40cm most of the time. Every two to three minutes the activity increases in strength and the height approaches two meters. This continues for only a few seconds, then all activity and overflow ceases. The water in the vent drops from sight for a few seconds, then splashes start up down in the vent and the cycle continues.