Náttúrufræðingurinn - 1955, Blaðsíða 36

98
NÁTT0RUFRÆÐINGURINN
öskumagnið í síðasta Kötlugosi vera 698 millj. teningsm (Ýmislegt
smávegis viðvíkjandi Kötlugosinu 1918, Eimreiðin 1919, bls. 212—
222). Reikningsgrundvöllur Samúels verður ekki ræddur hér, en telja
má vafalítið, að öskumagnið 1918 hafi verið allmiklu minna en 1721.
Sú aska, er féll á landi í öskjugosinu 28. og 29. marz 1875 var um
0.8 teningskm, en meira en annað eins féll utan landsins (Tefrokrono-
logiska studier pá Island, Geograf. Annaler 1944, bls. 101). Ösku-
magnið, sem kom úr Heklu á fyrsta degi Heklugossins 1947, hefur
mér reiknazt vera um 180 millj. teningsmetra (The Eruption of
Hekla, 11,3). Þýzki eldfjallafræðingurinn Karl Sapper telur þau eld-
gos, er framleiða meira en 1 teningskílómetra ösku, í fyrsta stærðar-
flokki gosa, en aðeins fá gos af þeirri stærð koma á öld hverri á gjör-
vallri jarðarkringlunni.
Það var happ í óhappi, að 1721-askan féll á tiltölulega góðum tíma.
Henni hefur skolað og feykt burt að nokkru, áður en grænka tók að
ráði. Kötlugosið 1918 var á enn heppilegri tíma. En hvenær bryddir
næst á Barða?
SUMMARY:
Ashfull so thut footprints wcrc traceable — and tlic eruption of Katlu 1721.
In annals and other original sources of our knowledge of volcanic eruptions that
have occurred in Iceland since the time of settlement the thickness of the volcanic
tephra (ash) layers is seldom given in figures, hut one frequently meets with the
expression: “ashfall so that footprints were traceable”. Informations such as: “the
ashlayer was so thick that it covered the pasterns of sheep” are also common. The
author discusses the quantitative value of such informations. During an ashfall from
Hekla on May 9th 1947 the author found by experiment that a thickness of 0.035 mm
was needed to make footprints traceable on grassland (cf. Fig. 2). The ash in que-
stion was fine-sandy. Fig. 3 was taken when the ashlayer had reached a thickness
of 0.04 mm.
In order to exemplify the use of information of the types abovementioned the
author deals with the available accounts of the tephrafali from the subglacial Katla
volcano in 1721. This eruption was one of Katla’s biggest in historical time. The
most thorough account of this eruption is anonymous, but the author thinks it likely
that it was written by Sigurdur Stefánsson (bom ab. 1698), who later became she-
riff in the county Austur-Skaftafellssýsla. Having critically examined the accounts
and the information they yield concerning the tephrafall the author calculates that
the tephra produced by the Katla eruption 1721 amounted to at least 1.5 X 10° m3
or 1.5 km3, corresponding to ab. 0.55 X 10° m3 of dense rock of the same chemical
composition as the basaltic tephra.