Jökull


Jökull - 01.12.1984, Side 87

Jökull - 01.12.1984, Side 87
Length On Fig. 10 all lineaments in the southern part of the area in Figs. 2 and 6 have been drawn from air photographs. Field observations have shown that most of these lineaments are dykes. Nevertheless, a few of the lineaments have been identified as normal faults, as indicated in Fíg. 10. N S Fig. 7. Stereoplot of the poles to 388 dykes in the research areas. Mynd 7. Pólar 388 ganga á rannsóknarsvœðun- um. Fig. 8. Thickness distribution of 365 dykes in the research areas. The average thickness is about 4.3 m. Mynd 8. Pykktardreifing 365 ganga á rannsóknar- svœðunum. Table 2 gives the inferred length and the calcu- lated length/width ratio of some well-defined dykes in the research areas. When estimating the length of a dyke, I followed the rule to count as a single dyke any lineament that could be traced as a single line, though in places it might be offset or otherwise discontinuous. When tracing the dyke across fjords, the criteria used were lithology, thickness and direction. The lengths of the dykes, as given in Table 2, are presumably minimum lengths. In eastern Iceland where I traced a few dykes along their length (Gudmundsson 1983a) some were longer than estimated from air photo- graphs. Also, because only a few thickness measurements were made on each dyke in north- western Iceland, whereas tens of measurements were made on each of the traced dykes in eastern Iceland, the length/width ratios given in Table 2 are less accurate than those of the dykes in east- ern Iceland. The dykes seen as lineaments on the air photo- graphs are much fewer than the dykes observed in the corresponding mountain sections. In gene- ral, only dykes thicker than 3—4 m are seen on the air photographs, whereas in well-exposed sections along cliffs most or all the dykes are seen. Form The dykes vary in shape; some are nearly straight with parallel edges while others are irregular and have nonparallel edges. Some dykes are sinuous and vary little in thickness, others have an apparent pinch-and-swell struc- ture. Often a dyke thins when passing through a thick lava flow. The observations show that the dykes use subvertical joints in the lava flows as channels (Gudmundsson 1984). Some thin dykes when dissecting thick lava flows deviate consider- ably from the vertical where they follow inclined joints. In general, the columns of the dykes are nearly horizontal, that is, they are perpendicular to the walls of the dyke-fracture. But in a few dykes the columns in the middle part are bent upwards. This is taken to indicate that the magma was moving upwards in the still mobile middle part of the dyke after the outer parts had become solidi- fied and immobile. No lateral bending of columns was observed. Most of the dykes are multiple in the sense that they are composed of many parallel rows of JÖKULL 34. ÁR 85
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Jökull

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