Rit (Vísindafélag Íslendinga) - 01.06.1967, Page 135
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open fissures of lentical shape, 200—300 m long, about 1 in
wide in the middle, offset 20—50 m against each other. The
individual fissures make an angle of 6—10° with the prin-
cipal fracture line. Fig. 3 shows schematically two consecu-
tive fissures of the great Hrafnagjá at Vatnsleysa, SW-Iceland.
Each secondary fissure is about 400 m long and about 1 m
wide in the middle (the width is much exaggerated in the
drawing. Fig. 4 serves as an explanation of the formation of
such stepped fissures, based on experiments by Riedel (Nádai
1931). The upper layer C is soft clay that cracks when the
underlying blocks are moved as indicated along S-S. The
shearing cracks, 3—3, make an angle of about 12° with the
underlying “fault” S-S. (Only when the surface of the clay
is wet, and the tensile strength is zero, do tensile cracks form,
at 45° with the fault, 1-1. This second type of cracks I have
found in soil.) The angles which I have measured in the
field are mostly close to this value: 9—10° in
Hrafnagjá at Vatnsleysa, 10—15° in Réttargjá
and Strandagjá, Selvogsheidi, SW-Iceland, 11 ____.
—12° in Kelduhverfi, and 10—12° in earthquake
fractures near Hekla.
Fig. 4. System of cracks produced on surface of a plastic clay resting on
two rigid plates A and B which have been displaced tangentially relative
to each other (after Nádai 1931).