Jökull - 01.12.1966, Blaðsíða 11

depressions. For Iceland we take Stokkseyri and
for Scandinavia a point with the same relative
distance from the centre, on the section Gedvnia-
Moskenesöy. This point is near Nyköping south
of Stockholm, we shall calf it „Nyköping". We
construct a curve of rise for this point, Fig. 7,
using Fig. 3. There are two alternatives, tak-
ing absolute sea-level frorn Si or from S2. The
rise at „Nyköping“ at a convenient time, 9000
B.P., is found to be 40 m and 45 m per 1000
years respectively. This gives the viscosities
(127 + a)-900/40 and (137 + a) • 900/45 in
arbitrary units, where a is the present amount
of downwarping. To give consistency with the
present uplift, about 3 m/1000 vr, a must be
about 10 m, and we get the viscosities 3080
and 2940. For Stokkseyri we get 30 • 240/29 =
248. The ratio of the viscosities is then 12.4
and 11.9. The above value for Stokkseyri is
independent of the time chosen, and this would
also be the case for Nyköping if the curve had
been constructed on the same principle. But
it has simply been drawn smoothly through 3
points and it gives a slightly less viscosity for
the latest time than for 9000 B.P.
We conclude that it must be a good approx-
imation to say that viscosity for the Iceland
region is one order of magnitude less than for
Scandinavia, as earlier estimated.
It would be tempting to assume that the low
viscosity for Iceland is due to its position on
the mid-Atlantic Ridge. But it has been point-
ed out (Broecker 1966) that the uplift of
Scandinavia may lead to an abnormally high
calculated viscosity, and the value for Iceland
is then closer to what might be a normal value.
If the great mobility for Iceland were due
to a special thin fluid layer, then the models
for Iceland and Scandinavia would be quite
different and the above comparison would be
invalid. But it is clear that in this case the
viscosity of the thin layer would have to be
less than the value we found.
Another centre of Pleistocene glaciation are
the Faeroe Islands. The radius of the glacier
can be taken as 50 km. If the maximum thick-
ness was 500 m, and the thickness followed the
formula H = 500 (1 — (x/50)2), where x is the
distance from the centre in km, then the
centre woulcl be depressed 24 m while it would
rise isostatically 22 m on account of lowering
of the surrounding sea, thus giving a net sink-
ing of only 2 m (Found by method given in
Einarsson 1953). Under these circumstances
raised beaches would not occur, which is in
keeping with observations. (Rasmussen personal
communication.) This small glacial centre, there-
fore, does not lend itself to viscosity studies,
which shows the limit to the application of the
above considerations.
Finally, it may be remarked that when gener-
al sea-level dropped by 85—100 m during the
last glaciation it dropped by that amount at
the Faero Islands. A quite considerable area
was thereby laid dry and unless it was entirely
covercd bv the glacier it presented a possible
refugium for plants and lower animals.
ÁGRIP
RIS ÍSLANDS í I.OK ÍSALDAR
OG SEIGJA DJÚPLAGA UNDIR JARÐ-
SKORPUNNI.
Eftir Trausta Einarsson.
Ris Islands eflir að jökulfarginu létti af þvi
var mun hraðara en hliðstcett ris Norðurlanda.
A grundvelli þess áœtlaði ég 1953, að seigja
djúplaga væri hér eitthvað um 1 /io af gildi
hennar undir Norðurlöndum og stceði þetta
lága gildi vcentanlega i sambandi við legu Is-
lands á Mið-Atlantshafshryggnum. Siðan hafa
verið gerðar ýmsar aldursmcelingar, sem snerta
ris Islands, og er nú hcegt að gera málinu fyllri
skil.
Fyrst er athugað, hvort hinn alrnenni sjávar-
botn muni hafa sigið, er 100 m þykkt sjávarlag
bcettist á hann við bráðnun jökla i lok isaldar.
Niðurstaðan er óviss, en sýnt, að viðunandi
nákvcemni fœst, ef gert er ráð fyrir óhreyfðurn
sjávarbotni.
Rýrnun jökla á Norðurlöndum undir lok is-
alclar er borin saman við mcelingar á hcekkun
almenns sjávarborðs og reynist í stórum drátt-
um í góðu samrcemi við þcer. Er hún því einnig
notuð sem mcelikvarði á rýrnun jökla hér.
Strandlinur frá ýmsum tímum eru þekktar
við Eystrasalt, og sýna þcer, svo og nútíma ris
svceðisins, að sigdceldin hefur grynnkað sem
heild án þess að skreppa saman, og rishraði á
hverjum stað liefur verið í réttu hlutfalli við
dýpið. Sömu reglu má ganga út frá hér, en af
JÖKULL 165