This most famous and former largest spouting hot
spring of the world probably was created during an extended earthquake
period at the end of the 13th century.
According to one of
the annals, two large hot springs were created in the Haukadalur
Valley in 1274, and many were ruined and disappeared. Silica
deposits have created a mound around Geysir and its bowl measures
about 18 metres across. The main natural shaft of the spring is 18 m
deep and 2 m across.
For centuries on end, Geysir erupted and
attracted tourists, but after the turn of the 19th century
its activity diminished and after 1916 it has been very irregular.
Scientists concluded that its cooling surface was too large and
reduced it in size with good results for a spell. The highest measured
eruptions reached over 80 m and before it erupted it rumbled and small
tremors were felt. The hot spring area surrounding Geysir is about 500
m long and 100 m wide.
The ancient silica deposits cover a much larger
area, close to 200 thousand square meters, which confirms the
existence of a much larger hot spring area in the past. There are
dozens of hot springs within the present active area and some of them
spout as Geysir did. The most prominent one is Strokkur (The Churn),
which spouts at intervals of 3-5 minutes. The hot spring area is
fenced off and was declared inviolate in 1953.
(Saga trail South Iceland)