Náttúrufræðingurinn - 2018, Blaðsíða 16

Náttúrufræðingurinn
16
LOKAORÐ
Hvernig fóru landsvæðin Rangár-
vallasýsla og Vestur-Skaftafellssýsla
út úr þessum miklu atburðum á fyrstu
öldum Íslandsbyggðar?
Í Vestur-Skaftafellssýslu urðu mestu
umhverfisbreytingar sem orðið hafa af
völdum eldgosa í Eldgjárgosinu á 10. öld.
Þetta stærsta gos Íslandssögunnar gerði
svæðið frá Mýrdal austur á Síðu nánast
óbyggilegt um tíma vegna gjóskufalls,
hraunrennslis og jökulhlaupa. Afréttir
stórskemmdust, einkum Álftaversaf-
réttur, Skaftártunguafréttur og Síðu-
afréttur. Svæðið sem verst varð úti var
tæpir 4.000 km2, um helmingur af öllu
flatarmáli sýslunnar og ¾ af landi utan
jökuls. Ekkert annað landsvæði á Ís-
landi hefur orðið fyrir viðlíka áfalli (sem
svo endurtók sig 1783), nema ef til vill
Litla hérað í Öræfajökulsgosinu 1362.
Rangárvallasýsla slapp furðulega vel.
Ef vindátt hefði verið af norðaustri eða
austnorðaustri í Heklugosinu 1104 hefði
láglendi í Rangárvallasýslu hulist 10–40
cm þykkri gjóskukápu. Í eindreginni
austanátt hefði Eldgjárgjóskan orðið
yfir 10 cm þykk í Fljótshlíð og meira en
metraþykk í Þórsmörk og á afréttum. Ef
Eldgjárgossprungan undir jökli hefði
legið vestar inn í Kötluöskjuna hefði
jökulhlaup komið undan Entujökli í
Markarfljót – stóru forsögulegu hlaupin
kaffærðu stór svæði í Landeyjum.
Því má segja að miðað við hverjir ná-
grannarnir eru hafi Rangárvallasýsla
sloppið vel – og miklu betur en ætla
mætti af lýsingum Herberts munks.
SUMMARY
Volcanic eruptions on the East-
ern Volcanic Zone during the first
centuries of settlement in Ice-
land and their effects within the
districts of Rangárvallasýsla and
Vestur-Skaftafellssýsla.
The volcanic zone that extends
from the Bárðarbunga and Grímsvötn
volcanoes below Vatnajökull glacier to
the Westman Islands off the south coast
of Iceland is the most active volcanic
zone in Holocene time, both in terms
of eruption frequency and volumes
erupted. It lies mostly with in the
districts of Rangárvallasýsla and Vest-
ur-Skaftafellssýsla. In the first three
centuries since the Norse settlement
in Iceland up to 20 eruptions, large and
small, took place on this volcanic zone.
These eruptions and their effects on
the settlers and the environment are
hardly mentioned at all in Icelandic
documents.
A large, predominantly explosive
eruption took place on the Vatnaöldur
volcanic fissure around the year 877,
producing about 5.0 km3 of basaltic
tephra and 0.5 km3 of silicic tephra as
well as a few lava flows. The tephra,
also known as the landnámslag or
Settlement tephra, was deposited over
half the country. Around 920 an er-
uption in the Katla volcano deposited
0.27 km3 of tephra over the southern
lowlands and at the same time a small
eruption took place in the Eyjafjalla-
jökull volcano. Both eruptions were
accompanied by jökulhlaups. Around
the year 939 another large but predom-
inantly effusive eruptions occurred on
a 75 km long fissure extending from
the Katla caldera below Mýrdalsjökull
through the Eldgjá fire chasm almost
to the edge of Vatnajökull. In addition
to 18–19 km3 of lava that flowed down
to the inhabited lowlands some 4.5 km3
of basaltic tephra was erupted, mostly
on the subglacial part of the fissure.
This eruption damaged larger areas
and caused more drastic environmental
changes than any other eruption of
historical time in Iceland.
Shortly after the Eldgjá eruption a
major earthquake hit Rangárvallasýsla.
The effects were most severe in the
Fljóthlíð area, where soil on hill sides
was ripped up, possibly as a result of
slope failures.
Some 10 explosive basaltic eruptions
in the subglacial Grímsvötn volcano or
the associated fissure swarm left tephra
layers outside the Vatnajökull glacier
during the first three centuries. Most of
them were relatively small.
Hekla volcano was quiet during the
first two centuries. In 1104 the first
and largest Hekla eruption in histor-
ical time took place, producing highly
silicic tephra that reached 60% of
Iceland. The tephra was carried towards
north and damage in the district of
Rangárvallasýsla was relatively small. In
the following smaller eruptions of 1158,
1206 and 1222 the tephra fall was mostly
confined to the highlands northeast and
southeast of Hekla.
The earliest descriptions of a volcanic
eruption in Iceland, referred to as earth
fire (jarðeldur), are found in Landnáma-
bók. These are two short comments, 2–3
sentences each, describing the largest
eruption in Iceland and its effects on
the settlers. Icelandic references to the
first Hekla eruptions are even shorter.
However, much more detailed if
somewhat exaggerated descriptions of
Icelandic eruptions were written down
and preserved overseas.58
In the Eldgjá eruption about 50% of
the district of Vestur Skaftafellssýsla
was severly affected by tephra fall, lava
flows and jökulhlaups. The district of
Rangárvallasýsla fared much better
despite the location of Hekla volcano
just beyond the hayfields of some of
the farms and certainly much better
than expected from descriptions in De
inferno Hyslandie that depict Hekla
as standing in perpetual blaze which
spreads over the mountain and even
beyond it (from Liber miraculorum,
written in 1178–1180).58