Náttúrufræðingurinn - 1973, Qupperneq 42

34
NÁTTÚRUFRÆÐINGURINN
S U M M A R Y
Catastrophic floods in Jökulsá á Fjöllum
by
Haukur S. Tómasson,
Natural Energy Authority, Reykjavík.
The author describes a catastrophic flood wliich occurred in Jökulsá á Fjöll-
um some 2500 years ago. This flood is most comparable to the catastrophic
floods on the Columbia plateau in the western United States which were first
described and explained by Bretz in 1923. In this flood the huge Jökulsá
canyon was eroded and also formed waterfalls like Ásbyrgi and many others.
The total erosion in rock amounted to 0.6—0.7 km3.
The ílood has been dated by teprochronological methods performed by S.
Thorarinsson. He found that inside the flood channel the ash layer H3 from
Hekla which is 2900 years old is missing but it is found outside the channel
as well as older ash layers. Inside the channel the oldest ash layers are esti-
mated to be 2000 years old. This dates the flood as approximately 2500 years
old and at that time there was cold climatic period in Scandinavia which
suggests that glaciers were advancing.
The peak flow has been estimated as 0.4—0.5 million m3/sec and the total
volume at least 10 km3 of water but most likely several times more.
The origin of this water is somewhere between Kverkfjöll—Grímsvötn and
Bárdarbunga. It can either be originated in tlie calderas of Kverkfjöll or Gríms-
vötn by melting or more likely from an ice dammed lake south of Kverkfjöll.
The flood lias lasted only a few weeks. The enormous erosion which has
taken place during this short time occurred because caviation loosened the
basaltic rock in a similar manner as happens in blasting. This caviation process
is very effective in basalt due to its rather dense jointing pattern and often
horizontal cleavage.