Náttúrufræðingurinn - 2011, Side 36

Náttúrufræðingurinn
140
Þakkir
Rannsóknir þessar hafa verið styrktar af Náttúrustofu Norðurlands vestra,
Náttúrustofu Suðurlands, Náttúrufræðistofnun Íslands, Veðurstofu Íslands
og CNRS GEOLAB í Clermont-Ferrand í Frakklandi. Sérstakar þakkir fá
landverðir Vatnajökulsþjóðgarðs í Skaftafelli fyrir ánægjuleg samskipti og
lipurð í að leysa úr málum okkar. Einnig hafa fjölmargir aðilar lagt leið sína
með okkur að bergflóðinu og er þeim öllum þakkað kærlega fyrir aðstoðina.
Sérstakar þakkir fær Dr. Erwan Roussel við Blaise Pascal-háskólann í
Clermont-Ferrand í Frakklandi fyrir aðstoð við GIS-vinnu.
Heim ild ir
Þorsteinn Sæmundsson, Esther Hlíðar Jensen, Halldór G. Pétursson, 1.
Decaune, A., Roberts, M., Ingvar A. Sigurðsson & Helgi Páll Jónsson
2008. Berghlaupið við Morsárjökul, 20. mars 2007. Vorráðstefna JFÍ, 30.
apríl 2008. Ágrip erinda og veggspjalda. Jarðfræðafélag Íslands. Bls.
63–65.
Þorvaldur Thoroddsen 1895. Ferð um Austur-Skaptafellssýslu og Múla-2.
sýslur sumarið 1894. Andvari. 20 bls.
Sigurður Þórarinsson 1952. Svigður á Morsárjökli. Jökull 2. 22–25.3.
Ives, J.D. 2007. Skaftafell in Iceland. A thousand years of change. Orms-4.
tunga, Reykjavík. 256 bls.
Oddur Sigurðsson 2005. Variations of termini of glaciers in Iceland in 5.
recent centuries and their connection with climate. Bls. 241–255 í: Iceland
– Modern processes and past environments (ritstj. Caseldine, Russell,
Harðardóttir and Knudsen) Elsevier, London.
Oddur Sigurðsson 1998. Glacier variations in Iceland. Jökull 45. 3–25.6.
Jóhann Helgason & Duncan, R.A. 2001. Glacial-interglacial history of the 7.
Skaftafell region, southeast Iceland, 0–5 Ma. Geology 21. 179–182.
Jóhann Helgason 2007. Berggrunnskort af Skaftafelli. Jarðfræðistofan 8.
Ekra.
Ólafur Björgvin Jónsson 1957. Skriðuföll og snjóflóð I. bindi – skriðuföll. 9.
Norðri, Akureyri. 586 bls.
Ólafur Björgvin Jónsson 1976. Berghlaup. Ræktunarfélag Norðurlands, 10.
Akureyri. 623 bls.
Árni Hjartarson 1982. Berghlaup á Íslandi. Týli 12. 1–6.11.
Halldór G. Pétursson & Þorsteinn Sæmundsson 2009. Skriðuföll úr 12.
móbergsmyndunum. Haustráðstefna JFÍ, 23. október 2009. Ágrip erinda.
Jarðfræðafélag Íslands. Bls. 19–23.
í ágúst 2011 hafði ísinn framan við
skriðuna rýrnað um 44 m umfram
það sem undir skriðunni er. Þrátt
fyrir að mikið farg hafi lagst yfir
jökulinn við bergflóðið virðist hvorki
skriðhraði hans né hophraði fram-
brúnar jökulsporðsins hafa breyst
mikið frá því að hrunið átti sér stað.
Eftir hrunið sáust miklar sprungur
efst í brotsári bergflóðsins. Ekki er
hægt að útiloka að frekara hrun
geti orðið úr því á næstu árum
og er þeim tilmælum því beint til
ferðamanna að þeir fari ekki undir
brotsárið.
Svipaðar aðstæður og við Morsár-
jökul er að finna víðar við jökla
landsins, þar sem mjög hröð jökul-
hörfun hefur orðið á undanförnum
árum og áratugum. Full ástæða
virðist til að kanna þau mál nánar,
sérstaklega í ljósi þeirrar miklu hörf-
unar og bráðnunar jökla sem spáð
hefur verið í kjölfar áframhaldandi
hlýnunar,32 t.d. með einhvers konar
kortlagningu og hættumati á þeim
stöðum þar sem t.d. ferðamönnum
eða mannvirkjum í nágrenni getur
stafað hætta af hruni eða berg-
flóðum.
Summary
The rock avalanche which fell
on the Morsárjökull outlet
glacier, 20th of March 2007
On the 20th of March 2007 a large rock
avalanche fell on Morsárjökull, one of
the outlet glaciers from the southern
part of the Vatnajökull ice cap, in south
Iceland. This is considered to be one of
the largest rock avalanches which have
occurred in Iceland during the last dec-
ades. It is believed that it fell in two sep-
arate stages; the main part fell on the
20th of March and the second and
smaller one, on the 17th of April 2007.
The Morsárjökull outlet glacier is about
4 km long and surrounded by up to
1000 m high valley slopes. The outlet
glacier is fed by two ice falls which are
partly disconnected from the main ice
cap of Vatnajökull, which indicates that
the outlet glacier is mainly fed by ice
avalanches.
The rock avalanche fell on the eastern
side of the uppermost part of the
Morsárjökull glacier and covered about
1/5 of the glacier surface, an area of
about 720,000 m2. The scar of the rock
avalanche is located on the north face of
the headwall above the uppermost part
of the glacier. It is around 330 m high,
reaching from about 620 m up to 950 m,
showing that the main part of the slope
collapsed. It is estimated that about 4 mil-
lion m3 of rock debris fell on the glacier,
or about 10 million tons. The accumula-
tion lobe is up to 1.6 km long, reaching
from 520 m a.s.l., to about 350 m a.s.l. Its
width is from 125 m to 650 m, or on aver-
age 480 m. The total area which the lobe
covers is about 720,000 m2 and its mean
thickness 5.5 m. The surface of the lobe
shows longitudinal ridges and grooves
and narrow flow-like lobes, indicating
that the debris mass evolved down gla-
cier as a mixture of a slide and debris
flow. The debris mass is coarse-grained
and boulder-rich. Blocks over 5 to 8 m in
diameter are common on the edges of
the lobe up to 1.6 km from the source. No
indication was observed of any deforma-
tion of the glacier surface under the de-
bris mass. It is evident that the glacier
has retreated considerably during the
last century and during the last decade
the melting has been very rapid.
Therefore it is thought that undercutting
of the mountain slope by glacial erosion
and the retreat of the glacier are the main
contributing factors leading to the rock
avalanche. The glacial erosion has desta-
bilized the slope, which is chiefly com-
posed of palagonite and dolerite rocks,
affected by geothermal alteration.
Hence a subsequent fracture formation
has weakened the bedrock. The exact
triggering factor, however, is not known.
No seismic activity or meteorologi-
cal signal, which could be interpreted
as triggering factors, such as heavy
rainfall or intensive snowmelt was re-
corded prior to the rock avalanche.
From 2007 considerable changes have
been observed on the glacier. The ice-
front has retreated considerably and the
debris lobe of the rock avalanche has
moved downward along with the gla-
cier ice about 80–90 m per year. The
rocky material, by insulating the ice, has
reduced its melting, leading to a relative
“thickening” of the ice beneath the rock
avalanche debris up to 11–15 m per year.
After four melting seasons the debris
mass was about 44 m above the sur-
rounding ice surface.
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