Rit (Vísindafélag Íslendinga) - 01.06.1967, Page 199

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Calculations for the entire postglacial time, viz. the last
11 000 years or so, or a time span 10 times larger than the
historical one, will necessarily he more rough. We know,
however, approximately the total amount of produced rhyo-
litic lava and tephra. The volume of the biggest postglacial
tephra layer, H 3, is about 2.5 km3 (12 km3 as fresh fallen),
that of other rhyolitic prehistoric Hekla layers (H 4, H 5,
Selsund pumice) ahout 3 km3. Other prehistoric rhyolitic
tephra layers (from Snaefellsjökull, Öraefajökull, Grákolla,
Hrafntinnuhraun area) I estimate at 1.5 km3 and the acid
lava flows about 1 km3. Altogether 8 km3. The volume
of the intermediate prehistoric Hekla lavas I estimate at
10 km3 and intermediate Hekla tephra of the same period
2 km3. The amount of the intermediate lavas outside Hekla
is not known, but a reasonable guess is that they amount
to 2 per cent of the basaltic ones. The total amount of
basalt lava and tephra produced in postglacial times can be
roughly estimated. The volume of the higgest shield volcano,
Skjaldbreidur, is about 15 km3, or about the same as that
of the biggest lava flow, the oldest Thjórsárhraun (Kjart-
ansson 1960). The volume of the 10 higgest shield vol-
canoes is about 55 km3, that of all postglacial shield volca-
noes 60 to 65 km3. Calculating with 10000 to 10500 km2
as the area covered by postglacial lava flows, and assuming
an average thickness (outside the shield volcanoes) of 25 m,
the volume of postglacial lava flows in Iceland is 250 to 260
km3, and adding the volume of the shield volcanoes we get
310 to 325 km3. The estimate of the volume of postglacial
tephra depends to some extent on where we put the limit
between the last glacial and the postglacial in Iceland. There
is hardly a doubt that the great amount of thick hardened
tephra which in many places is found immediately ahove
the groundmoraine is due to eruptions through the retreat-
ing inland ice. I estimate the volume of basaltic tephra dur-
ing the last 11 000 years to be about 40 km3.