Jökull - 01.12.1965, Page 10

Fig. 7. Map showing (with Roman figures) the locations o£ seismic soundings in the Grímsvötn
area in June 1955 ancl their results. Figures to the left are ice thickness, figures to the riglit
elevation of bedrock. Figures without brackets are regarded as “good”, tliose within brackets
as “fair” and those with questionmarks as “doubtful” (cf. Table II). Arrows show the direction
of the placement of geophones. — Kort er sýnir slaði, þar sem seismiskar mælingar voru gerðar
í júni 1955. Tölur til vinstri sýna ísþykkt, tölur lil hægri hæð unclirlags. Tölur án sviga teljast
öruggar, lölur innan sviga sæmilegar, en tölur með sþurningamerki óábyggilegar. Orvar sýna i
hvaða átt geófónum var raðað.
thickness figure 560 m, quoted above, as shown
on Fig. 8, which is a section through the Gríms-
vötn area, following roughly the course the
hlaupwater from Grímsvötn is likely to take.
The section is based on the topographical maps
of the area and the seismic soundings.
This section is incomplete as regards the bcd-
rock profile, because of tlie lack of successful
soundings in the area N and E of Svíahnúkur
eystri. However some conclusions may be drawn
trom it.
If we assume that the ice in the Grímsvötn
caldera is lloating — an assumption which
seems the likeliest one to explain the regular
rising and sinking of the water and firn level
and the constant height difference, 21 to 22 m,
between the water and firn surface — then the
thickness of the floating cake of ice-firn does
hardly excecd 220 m (cf. above) which means that
even immediately after a jökulhlaup there must
be a great amovint of water left in the caldera.
But no more than Joset was Martin able to
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