Náttúrufræðingurinn - 01.06.1968, Blaðsíða 57

NÁTTÚRUFRÆÐINGURINN 169
the river courses of Krossá and Markarfljót down to the sea, a distance of
35 km. Only this last phase of the Steinsholt hlaup was observed by people
while it was still going on, and only in two places at that.
The time of the beginning of the hlaup was determined by an earth tremor
registered on the seismograph at Kirkjubæjarklaustur 75 km east of Steinsholt.
According to information given by Ragnar Stefánsson, seismologist of the
Geophysical Section of the Meteorological Office in Reykjavik, the tremor
consisted of microseismic surface waves of a rare character. It began at 13 h.
47 min. 55 sec. and lasted for 2 minutes. The time of the tremor is in such
a good agreement with the timing by eyewitnesses of the advancing flood in
the Markarfljót (see Table 1) that it is concluded that the tremor was synchrono-
us with, and was started by, the rockslide in Steinsholt.
Along the first section of its course, Innstihaus — Þórólfsfell, the hlaup
advanced with a velocity averaging at 8.8 m/sec, and along the second section,
Þórólfsfell — Markarfljótsbrú, at 2.8 m/sec.
At Markarfljótsbrú a section of the hlaup track was investigated and measured
by S. Rist, hydrologist of the State Electric Authority. Applying the Manning
formula, he found that the maximum discharge of the hlaup had been
2100 m3/sec. and estimated the total volume of the hlaup water to have been
1.5xl0°-2.5xl0° ms (the water of the Markarfljót, in great flood, having
been subtracted).
Most of this water was certainly derived from Steinholtslón. This lake has an
area of 2xl05 m2, and it was probably deep enough (8—13 m on an average)
to account for the volume of the hlaup water.
However, another source of water may be taken into consideration. Actually,
1.5X107 m3 of rock with a density of 2.7 and falling 150 m will generate
sufficient heat to melt 175.000 m3 of water from ice. In the very special case
of the Steinsholt rockslide, which, falling onto a glacier, crushed and pushed
forward an enormous volume of ice, it may be assumed that a considerable
proportion of the released energy, possibly about 1/10, was consumed by the
melting of ice. Further, the side of the valley that had been overrun by the
air-launched hlaup had everywhere a pattern produced by water flowing
downhill. As this was also the case above the Steinsholtslón lake, the hlaup
must have contained a considerable quantity of water before getting there.
This water was probably released from the glacier by the energy of the rockslide.
But as this source of water is quite insufficient and, in addition, not needed to
account for the volume of water in the flood at Markarfljótsbrú, we must
conclude that Steinsholtslón lake was the only substantial source of water for
that flood.