Jökull - 01.12.1981, Page 3

JOKULL
31. ÁR 1981 NO. 31
JÖKLARANNSÓKNAFÉLAG ÍSLANDS . JARÐFRÆÐAFÉLAG ÍSLANDS
The Development of bonded Discontinuities in Basalt,
and their Significance to Freeface weathering
G. R. DOUGLAS
Menntaskólinn við Hamrahlíð, Reykjavík
ABSTRACT
A system of small scale discontinuities m basalt lava
flowsfrom Icelaiid and Ireland is described. These are
believed to have originated as brittle fractures under
low stress conditions. The fractures have subsequently
been bonded by secondary minerals including clay
minerals. These are thought to have formed at a low
temperature hydrothermal stage. Further changes
occur within the bonded discontinuities under sub-
aerial weathering condilions, which include new
fracture propagation, and failure along these produces
fragments infreeface locations. A combination of stress
conditions and inherent rock properties such as men-
tioned, are considered to be more important than many
environmental factors.
INTRODUCTION
The freeíace environment (i. e. a near ver-
tical rock wall), is often considered to be one
where frost weathering is the predominant
present day process. The Icelandic climate is
sometimes regarded as being typical for this
kind of activity (Tricart 1970). An accumulat-
mg literature demonstrates the importance of
the freeface environment to present day
denudation in a variety of countries (Rapp
1960, Bjerrum and Jorstad 1968, Luckman 1976,
Selby 1980, Douglas 1980). There is also grow-
ing evidence that many factors relating to the
rock itself are more important than the en-
vironment (Whalley 1974, McGreevy 1980). For
example cracks and microcracks in igneous
rocks have only lately received attention as a
means of explanation for many weathering
processes (Whalley et al. in preparation).
This paper deals with a system of discon-
tinuities found to be widespread in extrusive
basaltic rocks where they are exposed at
freeface locations both in Iceland and Ireland.
The essential features and possible origin of
these discontinuities is described and their
relevance to present day weathering discussed.
The rocks studied contain a wide variety of
discontinuities which include faults, joints and
microfractures as well as vesicles and other
non-Iinear cavities at crystal level. Those
described here are planar in nature and can be
recognised on cut surfaces or in thin section as
thin dark lines or as dark coatings on freshly
broken rock. They occur in the rock prisms
which are the result of the cubical or columnar
joint sets so common of this freeface environ-
ment.
JÖKULL 31.ÁR 1