Jökull - 01.01.2005, Page 153

George P.L. Walker
of that kind in the world. The routine measurements
of the samples, plus a number of smaller related stud-
ies, took several years to complete. However, prelimi-
nary results already published by Dagley et al. (1967),
along with reliable radiometric dating in the following
year, showed that the geomagnetic field had reversed
its polarity at least 65 times in a period of some 11
million years. This important direct conclusion still
stands, but has not been heeded by geomagnetists;
they have since 1968 preferred basing their “polarity
time scale” on interpretations of ocean-ridgemagnetic
lineations, a very indirect procedure involving many
assumptions. As a result, the number of geomagnetic
reversals, at least during the last 15 M.y., has always
been underestimated by a factor of two or more.
Coming back to Walker’s profile drawings (Fig-
ure 2), I have rarely received stratigraphic diagrams
of this quality with comparable speed from other ge-
ologists. One exception was John Preston (b. 1922)
of Queen’s University of Belfast who carried out
mapping of strata in the Arnarfjörður-Patreksfjörður
area in 1970–1973. His previous research had in-
cluded formations in Finland and Spitzbergen, but
from around 1960 he also studied the Tertiary igneous
rocks of Northern Ireland. Preston’s background was
therefore quite similar to that of Walker; they were ac-
quainted, and he later wrote review papers on the vol-
canic geology of Northern Ireland covering Walker’s
contributions (e.g. Preston, 1982). We met a few times
in 1972–1973 and collaborated on a paper containing
his valuable observations. He mentioned to me in a
letter in 1971, that authorities in Iceland would not
permit him to investigate the central volcano north of
Arnarfjörður. I expect that the reason for their deci-
sion was a desire to “preserve” such centers for future
investigations by Icelandic scientists. As it happens,
subsequent generations of geologists have not shown
much interest in this or other extinct central volca-
noes of Iceland: few detailed studies on them have
been initiated since the mid-1970’s and almost none
have been published. Only in the last few years has
stratigraphic work been resumed in the region south
of Arnarfjörður.
1978–1996
Walker (pers. comm., 1980) told me that he would
have liked to have more time to prepare the mapping
for the 1964–1965 expedition, and to revise it later in
the light of the paleomagnetic results. The distances
between successive profiles were in two cases more
than 50 km, and in a few other cases of the order of
10 km. It turns out that magnetic polarities in pro-
file segments that were supposed to overlap in age
did not always agree. An attempt was made to re-
sample the four profiles in Norðfjörður in the early
1990’s, in part by a French team and in part by a group
including Walker, but the results from these efforts
were reported in the literature to only a limited ex-
tent (see Herrero-Bervera et al., 1999). The sampling
of another composite section through Eastern Iceland
employing additional methods such as Ar-Ar dating,
geochemical and isotope analysis, may well be worth
considering.
Walker maintained an interest in investigations
of lower crustal structure. Following my visit to
him and paleomagnetists in Hawaii in 1989 (Fig-
ure 4), he wrote to me on 13 July 1990 describing
his ideas about those intrusive complexes which he
termed high-intensity ones, i.e. with more than 40%
content of dikes and sheets. Walker considered these
to be an important component of the crust at central
volcanoes, and even of oceanic areas in general. In
SE-Iceland, he added, there were fine and very ac-
cessible examples of intrusive complexes “just crying
out to be studied by a whole range of techniques”.
An accompanying map indicated favorable localities
for such studies, including one exceptionally so in
an unnamed tributary valley west of Kálfafellsdalur
(64◦10’N, 16◦06’W). In areas where the basement
consists of low-density rocks such as hyaloclastites
and vesicular pahoehoe lavas, he expected that a co-
herent complex might form close to the surface. I
am not aware whether Walker or others did embark
on a project to investigate Icelandic intrusive com-
plexes in the way he envisaged, which would have
required considerable expenditures on field and lab-
oratory work.
A final point in these disconnected reminiscences
concerns a reprint I received from George Walker ten
JÖKULL No. 55, 2005 153