Jökull - 01.12.1982, Page 98

could not account for such grouping of polarities by
self-reversal, local field anomalies, tectonics, or
temporary phenomena. Tests of stability, within-
sample consistency, and magnetic behaviour on
heating, produced positive results. Alongwith bak-
ed-contact studies which began elsewhere in the
early to middle fifties, Hospers’ work provided some
of the strongest evidence to convince earth scientists
of the reality of global geomagnetic reversals (see
e.g. conference report in Nature, 17 Nov 1956).
Hospers estimated the length of time covered by
each polarity group to be 0. 25-0. 5 M. y., and he
demonstrated their possible use in stratigraphic
correlation (Hospers 1954c). However, it must be
remembered that Hospers generally measured only
one sample per lava flow in the laboratory, without
any demagnetization treatment. Someofhis results
from Snaefellsnes were later revised by Doell (see
below).
R. A. Fisher (1953) had developed normal statis-
tics on a spherical surface, to use in analysis of
directional measurements. The first application of
these was to Hospers’ Icelandic data. To quote Sir
Ronald himself (in a letter to T. Einarsson, dated 21
May 1955), they provided a “tolerably expeditious
method by which the observations can be combined
and summarized”, and they have in fact been used
for all within-unit calculations of directional dis-
persions to the present day. Fisher’s statistics have
also been widely used for between-unit paleo-
magnetic dispersion (i.e. secular variation) esti-
mates, although there is some doubt as to their
applicability in that case. For historical back-
ground, see J. Hospers’ book review in Tecton-
ophysics vol. 5, p. 345-352, 1968.
T. EINARSSON, T. SIGURGEIRSSON,
A. BRYNJOLFSSON
Hospers’ work was soon to be continued by Ice-
landic scientists who realized its potential as a geo-
logical mapping aid in the volcanic pile. Their
studies began in 1953. Einarsson and Sigurgeirsson
11955) and Einarsson (1957a, b) pioneered the
measurement of magnetic polarity in rocks by
means of a compass in the field, and measured
thousands of lava flows across Iceland using this
method. It was soon found in this way that the
intensity of primary remanence decreased with
increasing age and alteration, so that measure-
ment could be difficult, or this primary remanence
might be masked by secondary or induced magnet-
ization. These problems could, however, be mini-
mized by sampling from the bottom contact zone
of flows.
The polarity measurements were important in
Einarsson's stratigraphic mapping of long sections
through the volcanic pileofSW-Iceland andcentral
W-Iceland, and E-Iceland where about 30 polarity
zones were mapped (Einarsson 1957a). There were
also more detailed regional studies, e.g. in Snaefells-
nes, central N-Iceland, and Tjörnes (Einarsson
1958a, b, 1959,1962,1963). Einarsson found that
the average thickness of magnetic polarity zones
was 200-350 m, and he initiated a numbering
system for these (Nl, Rl, N2,...). The conclusion of
Einarsson and Sigurgeirsson (1955) that normal and
reverse polarity zones have approximately equal
mean thicknesses, has been confirmed by subsequ-
ent work. Einarsson’s polarity results from lavas
have also been found to be very reliable, but he has
expressed reservations about the value of such mea-
surements on clastics and sediments (Einarsson and
Sigurgeirsson 1953, Einarsson 1957c).
Due to dating difficulties, Einarsson’s polarity
zones have only to a limited extent been correlated
with other geomagnetic polarity timescales. There
are also ambiguities in long-distance correlations of
these zones within Iceland because of the very diff-
erent rates of buildup of the lava pile in different
parts of the country.
Sigurgeirsson (1957) and Brynjolfsson (1956, 1957) de-
signed a sensitive shielded 5-Hz rotation magneto-
meter (Fig.3) for the measurement of remanence in
hand samples of rock, and one-axis a-c demagne-
tization apparatus that reached 140 Oe peak field.
This equipment was used for accurate direction
determination in several hundred lava samples.
Those measurements that were published in detail
included a study of a lava section through a gradual
geomagnetic polarity transition in the mountains of
the Hvalfjörður area. It was suggested by their
results that a polarity zone corresponded to 0. 1-1
M. y. [Einarsson 1958a), that the reversal process
took of the order of 1000 years, and that the geö-
magnetic field was likely to have weakened con-
siderably during these transitions. The Hvalfjördur
location was one of the very few known sites in the
world for a decade or more to yield details on geo-
magnetic transitions, but their existence was very
important evidence on the nature and long-term
behaviour of the geomagnetic field.
Various theoretical and experimental work on
the magnetization and demagnetization processes
94 JÖKULL 32. ÁR