Jökull - 01.12.1987, Side 34

and fossil, is associated with the hyaloclastite ridges of
Vesturháls and Sveifluháls and a fault running through
Austurengjahver just east of Sveifluháls (Fig. 1). Ge-
ologically the field is conveniently divided into two areas
on the basis of the distribution of surface manifestations,
the Sveifluháls area, including Austurengjahver, and
the Vesturháls area. Steam-heated pools are confined to
the Sveifluháls area. Except in Sog, acid surface alter-
TABLE 1. Gas analyses from fumaroles in Krfsuvfk (mmoles/kg steam)
Location co2 h2s H2 °2 CH4 N2 Ar
1 Kóngsnáman3 302.0 41.33 11.02 0.16 0.115 3.80 0.104
2 Kóngsnámanb 271.9 39.00 7.70 0.15 0.088 4.26 0.104
3 Seltunc 306.0 42.60 16.28 0.04 0.153 2.42 0.092
4 north of Seltún 253.0 32.78 16.36 0.04 0.156 1.65 0.093
5 Krfsuvfk, farm 279.4 15.32 0.38 0.03 0.027 1.34 0.063
6 Austurengjahver 205.7 20.49 9.06 0.06 0.100 3.31 0.150
7 north of Auste.hver 258.0 28.50 14.71 0.00 0.286 74.84 1.29
8 Hvammar 279.2 12.92 30.91 0.00 0.939 9.80 0.197
9 Nedri-Hveradalurc 245.6 16.85 9.24 0.10 0.044 1 .49 0.052
10 Nedri-Hveradalurc 258.0 15.43 8.82 0.13 0.052 1.76 0.044
11 above Nedri-Hverad. 278.9 17.72 6.76 0.05 0.029 1.61 0.060
12 Sveifluhálsb 282.9 16.18 7.12 0.04 0.059 5.42 0.144
13 Sveifluhálsc 258.4 16.08 0.78 0.10 0.072 4.18 0.120
14 Sveifluháls 272.5 23.30 7.09 0.07 0.067 2.50 0.069
15 Efri-Hveradalur 268.8 19.63 6.56 0.02 0.053 4.42 0.118
16 Efri-Hveradalurc 276.0 14.64 7.52 0.09 0.222 7.61 0.263
17 Efri-Hveradalur 286.5 19.15 4.78 0.02 0.049 1.79 0.091
18 Efri-Hveradalur 317.3 7.74 0.60 0.94 0.098 11.54 0.327
19 Efri-Hveradalur 241.3 19.48 4.50 0.07 0.025 1.79 0.069
20 Arnarvatnb 429.2 13.72 3.36 0.07 0.390 11.20 0.326
21 Ketillc 111.0 2.58 0.32 0.08 0.034 2.46 0.052
22 Sog 202.8 2.18 0.46 0.01 0.333 8.52 0.378
23 Hverinn Eini 965.7 82.51 8.71 46.95 - 336.5 5.81
Superscripts for sample sites designate replicate samples as follows:
a = 7, b = 3, c = 2.
TABLE 2. Composition of gas from hot pools in Krfsuvfk (mole %)a
Location co2 h2s H2 °2 ch4 n2 Ar
24 Hvammar 70.9 0.7 13.6 0.00 0.53 13.4
25 north of Auste.hver 86.4 5.1 6.6 0.00 0.05 1.8
26 north of Auste.hverb 79.4 2.8 14.3 0.23 0.00 5.3 0.18
27 Kleifarvatn 72.7 2.8 15.9 1.6 4.10 2.9
28 Seltún 86.5 3.9 6.6 0.10 0.86 2.2
29 Seltúnb 68.9 6.1 8.6 0.00 0.12 17.8 0.25
30 Seltún 89.5 1.8 3.3 0.50 2.20 2.7
31 Sveifluháls 92.2 2.0 2.9 0.40 0.05 2.8
32 Nedri-Hveradalur 90.8 2.1 4.9 0.30 0.02 2.7
33 Efri-Hveradalur 90.5 1.9 4.7 0.70 0.04 3.9
34 Sveifluháls (Hattur) 86.0 0.3 3.8 1.20 0.15 7.9
35 Arnarvatn 85.6 0.9 3.6 1.50 0.15 7.9
36 Ketill 88.6 1.3 0.8 1.10 0.10 8.1
37 Efri-Hveradalurc 86.9 3.3 8.4 0.0 1 -4
38 Krfsuvfkc , 88.2 7.0 4.1 0.0 0.7
39 Austureungjahverd 66.7 22.5 7.1 0.6 0.0 3.1
40 Austurengjahvere 85.0 7.0 6.6 0.2
41 well 14f 81.3 12.5 4.5 0.0
42 well 14f 82.8 9.8 7.0 0.0 0.1 0.6
43 well 15e 89.3 5.7 3.6 0.1
44 well 16e 83.9 9.6 5.4 0.1
45 Sog 81.7 0.4 1.1 1.70 0.50 16.5
46 Trölladyngja 94.2 <0.1 2.5 0.20 0.17 3.3
47 Köldunámur 91.3 2.1 1.1 0.10 0.32 1.1
aIf not otherwise specified the data are from ARNÓRSSON (1971).
bThis study. CBUNSEN (1847). dS0NDER (1941). eLÍNDAL (1951).
fELÍSS0N (1967).
ation is limited in the Vesturháls area. The thermal
manifestations in this area are characterised by dis-
persed steam emerging in permeable pillow breccias and
post-glacial lava fields making impossible collection of
steam samples free from air contamination. In fact sam-
pling at Oddafell and north of Trölladyngja (see Fig.l)
was unsuccessful. Today far the larger part of the natural
steam discharge in the Krísuvík field is confined to a
relatively small area in the vicinity of Seltún (Fig. 1).
The hyaloclastite ridges of Sveifluháls and Vesturháls
formed during sub-glacial fissure eruptions during the
last glaciation (Jónsson, 1978) and are separated with a
trough covered with post-glacial lava flows. The east-
ernmost ridge, Sveifluháls, is composed of four eruptive
formations and has possibly been built up during four
volcanic episodes (Imsland, 1973) suggesting that the
volcanic activity has for a period of time been concen-
trated on relatively narrow zones represented by the two
hyaloclastite ridges.
The faults and fissures of the fissure swarm dissecting
the Krísuvík field are much more dense in the two hyalo-
clastite ridges than in the lava covered trough in be-
tween. This may be due to longer exposure of the glacial
formations to the tectonic processes than the post-gla-
cial lava fields, but it is also possible that the tectonic
activity has truly been concentrated on the ridges during
and after the last glaciation.
Fumaroles and hot ground are very often located by
faults and fissures indicating that these near vertical
structures control the upflow of the geothermal fluid. It
may be that the two hyaloclastite ridges overlie hydr-
ologically separate geothermal systems.
Several explosion craters (maars) occur within the
Krísuvík field but not in the surrounding area. They are
often represented by small lakes (Fig. 1). Some magma
and gabbroic xenoliths were brought to the surface dur-
ing the formation of some of these craters, both in-
dicating the presence of intrusives at depth.
According to a conceptual model of the development
of volcanic centers in Iceland (Steinthórsson et al., 1986)
the Krísuvík field is in an early stage of evolution with no
shallow major magma chamber. Minor volcanic edifices
on fissures are abundant in the area of the Krísuvík field.
Their composition is extremely homogeneous, slightly
quartz normative tholeiites (Gunnlaugsson, 1975) in-
dicating very slight crustal assimilation of mantle de-
rived magma. By the model of Steinthórsson etal. (1986)
magmas which are characteristic of evolved centers
show frequently compositional features indicative of ex-
tensive reactions with crustal rocks. It may be that rela-
tively small dyke intrusions below the volcanic edifices
represent the heat source to the geothermal field. Such a
32