Polarity Reversals and Oxidation State in Rocks

The correlation between reversed polarity in rocks and strong oxidation of magnetic minerals is briefly re-examined, with especial reference to a well-documented Columbia Plateau basalt suite, recently described in this journal. The correlation appears to vary in quality from place to place and some reasons for this are suggested. Attention is particularly drawn to recent studies which suggest that for palaeomagnetic purposes there may be two groups of lavas, divisible on the basis of whether their magnetic minerals became oxidized under high or low temperature conditions.
A sizable body of opinion now holds (not necessarily correctly) that self-reversals are a negligible factor in the correlation between oxidation and reversed polarity, and that some compositional effect is most probably involved. Virtually the only known petrographic criterion which can (statistically) distinguish reversely from normally magnetized rocks is their (statistically) higher oxidation states. An obvious implication is that the water content of magmas, and perhaps of high-grade metamorphic rocks, is somehow increased during episodes of reversed polarity, so that a slightly larger proportion of rocks is more strongly oxidized.
Since motions of the Earth's core, upper mantle processes, and field reversals are apparently all related, compositional changes originating in the upper mantle could well be also related to field reversals. Alternating linear zones of more and less altered baslat recently reported from one place near the mid-Atlantic ridge might have arisen in such a way.
The importance of relating palaeomagnetic measurements to detailed mineralogical studies of magnetic minerals is again stressed.     

Magnetic Properties and Normal and Reversed Natural Magnetization in the Mull Lavas.

The previous paper by Dr J. Ade-Hall has shown a petrological distinction between normal and reversed lava specimens from the Isle of Mull, Scotland. An investigation of several magnetic properties shows, surprisingly, that only the behaviour of the natural moments shows any distinction between normal and reversed specimens. The implication is that microscopic investigation is a necessary tool for the proper interpretation of many palaeomagnetic results, since it can reveal variations of properties unnoticed in a purely magnetic investigation. In particular, the problem of self-reversal versus field reversal demands petrological as well as magnetic investigation.     

The Petrology, Curie Points and Natural Magnetizations of Basic Lavas

Working out the history of the Earth's magnetic field will require a better understanding than now exists of the ways that rocks in fact obtain and sometimes change their fossil magnetizations. The contribution in this paper consists of finding correlations between the petrology, Curie points, and polarities of natural magnetization of basalt lavas.
The evidence contradicts previous evidence on which was based the hypothesis of exsolution of ülvospinel and magnetite from a titano-magnetite solid solution. It suggests strongly that this exsolution does     

Azimuthal variation of the P phase in Icelandic receiver functions

A curious observation has been made on radial receiver functions calculated from teleseisms recorded by 29 broad-band seismometers distributed over Iceland. The arrival time of the direct P phase of the radial receiver functions depends critically upon the azimuth of the teleseismic source. For a seismic station in West Iceland, the direct P  phase of the radial receiver function arrives consistently later for easterly source azimuths than for westerly source azimuths. The reverse applies for stations in East Iceland. In the original seismograms, the delayed P phase of the receiver function appears up to 450 ms later on the radial than on the vertical component. The seismometer locations in East and West Iceland are separated by the Neovolcanic Zone, a constructive plate boundary. The delayed P phases occur for seismic rays travelling across this zone. However, it is not obvious how wave propagation across the plate boundary zone could cause the observed delays. The tentative explanation proposed here involves the regional dip of the Icelandic lava sequences towards the Neovolcanic Zone. A dipping interface at shallow depth results in a P–S converted phase arriving shortly after the P phase. These phases cannot be separated in the radial receiver functions, given the bandwidth of the observed signals. However, a calculation of receiver functions from estimates of the P , SV and SH wavefields clearly reveals a P–S converted phase at about 500 ms for easterly source azimuths in West Iceland and for westerly source azimuths in East Iceland. The amplitudes of the direct P phase and the P–S phase converted at a dipping interface would be expected to vary strongly with azimuth in accordance with the observed behaviour.     

Nationalist heritage,sublime affect and the anomalous Icelandic landscape concept

Olwig, K.R. 2015. Nationalist heritage, sublime affect and the anomalous Icelandic landscape concept. Norsk Geografisk Tidsskrift–Norwegian Journal of Geography Vol. 69, 277–287. ISSN 0029-1951.

The idea of landscape has undergone change, as has the physical landscape, under differing historical, technological, ideological and representational contexts. The article is concerned with the way modern imaginaries of landscape have been read back into the past in order to naturalize, essentialize and legitimize a particular imagined political, economic and physical national heritage. The image of Icelandic landnám, understood as the settlement of the naturally bounded, wild, sublime, virginal nature of Iceland encountered by rugged and free Nordic Vikings, speaking a pure Nordic language, played a foundational mythic role in particularly 19th century Nordic and Germanic national romanticism. This was because Iceland provided an apparent means of ground-truthing the nationalism, coupled with economic liberalism, that the national romantics promoted. The article examines the ways that such romantic nativist ideas of national character, as something embedded in a people through the esthetic experience of raw sublime nature, continue to operate on Iceland, and far beyond the boundaries of Norden, in a modern world of freebooting financiers, offshore neoliberal economics and renewed nationalistic fervor. The modern international role of the physical landscape in this ideology is exemplified by a case from Mexico, whereas Iceland exemplifies the continued vitality of this ideational landscape.     

The Icelandic Laki volcanic tephra layer in the Lomonosovfonna ice core, Svalbard

The largest sulphuric acid event revealed in an ice core from the Lomonosovfonna ice cap, Svalbard, is associated with the densest concentration of microparticles in the ice core at 66.99 m depth. Electron microscope analysis of a volcanic ash particle shows it has the same chemical composition as reported for debris from the eruption of Iceland's Laki fissure in 1783 and confirms the identification of the tephra. Most of the particles in the deposit are not ash, but are common sand particles carried aloft during the eruption event and deposited relatively nearby and downwind of the long-lasting eruption. The tephra layer was found 10–20 cm deeper than high sulphate concentrations, so it can be inferred that tephra arrived to Lomonosovfonna about 6–12 months earlier than gaseous sulphuric acid precipitation. The sulphuric acid spike has a significant cooling impact recorded in the oxygen isotope profile from the core, which corresponds to a sudden drop in temperature of about 2°C which took several years to recover to previous levels. These data are the first particle analyses of Laki tephra from Svalbard and confirm the identification of the large acidic signal seen in other ice cores from the region. They also confirm the very large impact that this Icelandic eruption, specifically the sulphuric acid rather than ash, had on regional temperatures.