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By Yuri GENSCHAFT, Dr. Sc. (Phys. & Match.), Otto Schmidt Joint Institute of Physics of the Earth, Russian Academy of Sciences
There have been several international symposiums held in the last few decades on the problems of the evolution, active tectonics and structure of Iceland. Why such a vivid interest in a rather small patch of land on an island situated in the North Atlantic halfway between the Old and the New worlds?
FRAGMENT OF A CONTINENT? A CONTINENTAL "FOAM" RELIC? VOLCANIC STRUCTURE?
Back in 1980 Sigurdur Steinth-orsson and Wolfgang Jakoby, editors of the Journal of Geophysics, an international periodical covering the Iceland symposiums, noted this in part: "The contrast between Iceland and the adjacent oceanic region is an essential question in studying the lithosphere and the upper mantle." On the one hand, the island belongs to intrapelagic structures, while on the other, its geological history and structure are so atypical as to puzzle even advocates of the acknowledged geotectonic theories.
Iceland lies at the juncture of two mid-Atlantic ridges-Reykjanes in the south and Kolbensey in the north. These are the youngest oceanic structures with a very thin earth shell. According to some geologists, Iceland is, in fact, a part of a submarine ridge rising above sea level. Yet our tectonics researchers, Corresponding Member of the USSR Academy of Sciences Vladimir Belo-ussov and Academician Yevgeni Mi-lanovsky, were of a different opinion:
"Surveying in Iceland with a geologist's pick in hand", they wrote, "we do not walk on the basalts that used to line the ocean floor..."
Perhaps the most important difference of the island from typical oceanic structures lies in the significant thickness of its lithosphere and in the substantial contribution of acid (persilicic) magmatic rocks both among intrusives (rock solidified underground) and effusives (rock flowing out on the surface). The presence of such large volumes of contrast magmatic rocks- basalts and rhyolites, gabbros and granitoids-impelled Robert Wilhelm Bunsen, a distinguished German scientist and foreign member of the Russian Academy of Sciences, to hypothesize 150 years ago about two primary magmas, the basic and the acid (granitic) ones. For a long time the earth shell was thought to contain a sialitic (siallite) layer (from the name of its components. Si and Al) which, by melting, was supposed to make a significant contribution to the acid rock.
Early in the 12th century the Swedish scientist Gerhard de Heer suggested Iceland was a fragment of a continent that had got stuck in the waters of the Atlantic. Even Alfred Wegener (1880-1930), a German geologist and the founder of the mobilistic (continental drift) theory, at first thought Iceland to be akin to spreading, or an ocean-floor extension structure; later on, however, he came to regard i as a relic of the continental "foam" left upon the separation of Greenland from continental Europe. Iceland was visualized as part of a vast volcanic land stretching all the way from Scotland to Greenland-an area formed by
the effluvia of plateau-basalts as early as the Tertiary time, or 60 million years ago.
And thus Iceland happened to be a touchstone for all the various geo-dynamic theories attempting to explain the formation of continental and oceanic structures. As shown by the experience of the past few decades, studying deep-mantle matter (deep-seated rocks and minerals) may furnish evidence in support of this or that theory in the earth sciences. It is in such studies that I was engaged during five field seasons (1986 to 1991) within a geological and geophysical expedition of the USSR Academy of Sciences working in Iceland.
A RESEARCH LANDING PARTY
That expedition, set up at the initiative of Vladimir Beloussov, was in fact the fourth landing party of Russian geologists and geophysicists in Iceland. The data of the previous works at sea and on dry land were published in a series of monographs under the general title Iceland and the Mid-Oceanic Ridge; these studies provided factual evidence on the salient features of Iceland's plutonic structure and magmatism. Our expedition, which was supervised by Corresponding Member of the USSR Academy of Sciences Lev Rykonov, had in it a small geological party (Alexander Krasnov, Arthur Saltykovsky and myself) and a larger one made up of geophysicists. They, the geophysicists, were involved with a seismic experiment and, as a source of elastic waves, first used explosions and then started registering seismic noises produced for the most part by subterranean thermal springs (hydrothermae) found in Iceland in great abundance. In this mode they were making a detailed study of the fine structure of the uppermost parts of the earth's crust, especially in thermal regions.
As to the geological party, it was to search for and collect inclusions of deep-seated rocks and minerals in the volcanic material; subsequently we expanded our research area and started collecting samples of intrusives (typhoons).
From the previous record of our field work on midland young volcanism caused by the development of rift structures (some analogue of median continental ridges) we knew this: alkali-basalts typical of such regions usually contain minerals and rocks formed in the lower parts of the shell and in the upper mantle. This matter is ofultrabasic composition-peridoties, pyroxenites (occasionally containing magnesiogar-net-pyrope) as well strongly metamorphosed rocks-granulites and eclogites.
Among minerals of most frequent occurrence are the so-called "high-pressure megacrystals"-large homogeneous high-alumina and low-calcium monoclinic pyroxenes, soda-potash feldspars as well as high-titanium amphiboles: kaersutites, mag-nesioilmenites, and so on. And since alkali-basalts occur frequently in Iceland too, we were hoping to find inclusions characterizing the matter of the lower lithosphere and of the upper mantle. Only then we could be more certain on what foundation the Icelandic geological structure was formed-on the continental or on the oceanic one.
Unfortunately we were in for poor luck and misfortunes in the first year of our work: just two weeks after the start our small party had a bad car accident from which we could recover only a month later. To add insult to injury, Alexander Krasnov, a member of several preceding expeditions to Iceland and an active organizer of ours, died suddenly on one of the first field routes.
MORE QUESTIONS THAN ANSWERS
When we were beginning our work, there were two different seismic models of Iceland's structure. What they differed on was not the location of plutonic boundaries or the characteristics of identified strata (as is often the case) but data interpretation. After a series of seismic studies along profiles crossing the island and the adjoining water area we identified four strata in the upper part of the earth's interior-strata characterized by a steady increase in the velocities of longitudinal waves in depth from 2 to 7.4 km/s. The latter value (7.4 km/s) is not at all typical of normal rock of the upper mantle and is above the normal rate in the deep mantle.
It was the seismically anomalous fourth stratum that elicited a good deal of controversy. The Icelandic geophysicist Gudmundur Palmason and other Western scientists postulated: it belonged to the uppermost part of the mantle where some of the matter is molten. A layer of high electric conductivity lying at about the same depths (10 to 20 km) likewise indicated that. And thus a "classical" model of Iceland's shell appeared in the literature: three strata, with the total thickness of the shell not exceeding 15 km:
Yet, according to the latter-day "Russian" model suggested by staff researchers of our Institute Drs. Sergei Zverev and Ninel Pavlenkova (with other Russian geophysicists contributing to it), the lithosphere of the island extends to a depth of 30 km at least. Besides, the anomalous fourth stratum is said to belong to the lithosphere as well.
Which of these two postulates was correct? And would it be possible to find hard proof in favor of one particular theory? Such were the questions
on our mind as we turned to our field work. The body of evidence amassed by the time seemed to argue for the thin shell hypothesis (the "classical" model). Indeed, Iceland is a hot spot on earth, a source of active contemporary vol-canism. Exceedingly high heat flows accounting for a temperature gradient near the surface up to 160C/km are indicative of a possible heat build-up to 1,200C at a depth of ca. 15 km. But this is the temperature of lavas, a temperature at which basaltic magma is fused from the mantle's peridotite matter. And then no rock older than 16 million years has ever been found in Iceland, a fact that shows it to be a very young geological formation. This evidence agrees best of all with the ideas postulating the island's birth on the ocean floor within a mid-Atlantic ridge.
But if it were as simple as that! As Sigurdur Steinthorsson and Wolfgang Jakoby said, Iceland gives "more questions than answers" the more you study it. Iceland, they mused, "looks like a chance phenomenon in nature". Not only lying on the junction of two submarine ridges-the island saddles two "bridges" linking continental Europe and Greenland: the Island- Faroe Sill in the east and the Greenland-Iceland Sill in the west. And what the Russian geophysicists have shown is this: in its structure Iceland is very like these continental "bridges".
The island is also unique in the abundance of magmatic rocks. Besides acid (persilicic) rocks, there occur all their basic petrochemical varieties, such as tholeiites, alka-liolivine and high-ferriferous basalts (or else their transition type), calcareous-alkaline rocks. It was known from literary data: highly typical of Icelandic magmatism are the differentiation of melts through crystal fractionation (formation of crystals at low depth) and the intermixing of melts of different composition. As shown by geophysical studies, magmatic chambers (hearths) do not lie deep under Icelandic volcanoes (such as Hekia and Krafla)-not deeper than a few kilometers anyway.
The oldest, Miocenic rocks of Iceland (ca. 25 million years old) crop out in the west and east, while the latest Quaternary volcanites are concentrated in the contemporary neovolcanic zone. In all its characteristics the island is a young superficial rift in which subhorizontal tensile stresses predominate. In lava strata they create extended faults
which are most effective at Lake Tigvallavatn near Reykjavik.
It was in a fault valley, under bluffy basalt murals that Icelandic parliament, the Alting, was founded a thousand years ago; on the ledge, where now the national flag is fluttering, the land's first laws were promulgated. Just as impressive are the basalt beddings-torn by tensile forces-in the country's north, in the Asbirgi wildlife sanctuary. Those horseshoe-like murals about 100 meters tall look like ocean vessels, all set to plow the main...
In the elevated eastern and western parts of the island the Tertiary basalt sheets are cut by intrusive veins and typhoons. The latter bear traces of magmatic digestion and of the actually simultaneous intrusion of two contrast magmas-the basalt and the granite ones. The most striking pictures of these processes may be seen in the example of best-studied typhoons on Iceland's eastern coast, at Vesturhorn and Austurhorn.
THE LAND OF VOLCANOES AND GEYSERS
Relying on the experience of the previous searches for deep-mantle inclusions, we zeroed in on volcanic centers whose lavas and scoriae, in their chemical composition, relate to alkali- basalts. These are the young slag cones and lava streams in the west (Snaefellsness Peninsula) and in the south (the islands of Vestmannaeeyar and Surtsey). The island Surtsey is the youngest land area formed as a result of the submarine eruptions of 1963 to 1967. This island is named after the Norwegian epic hero Surtur: as legend has it, he was to start a fire on earth on Doomsday. Today Surtsey is a nature preserve, off limits to visitors.
As a matter of fact, Iceland is famous for its volcanic events. The earth's largest fissure eruption took place here at the end of the thirteenth century when as much as 12 km3 of lava covered around 600 km2 of the land surface. The aftermath was devastating: mighty discharges of sulphur dioxide killed the crops and the vegetative cover. A fifth of the island's population died of famine because of the mass mortality of cattle. In 1973 the ashes, spewed by a volcanic eruption on the island of Heimaey, buried the community of Vestmannaeeyar. But its residents had been evacuated to safety, and later on the town was cleaned of volcanic products. Staying as a reminder of that disaster are a hardened lava stream that had reached the town's fringe, and a ruined house nearby.
Iceland sees periodic subglacial eruptions of the active volcano Grimsvotn in the northwestern part of the Vatnajokull glacier, Europe's largest. The last grandiose eruption of this volcano occurred in the fall of 1996.
Owing to vigorous hydrothermal activity, Iceland is by right considered a classical country of geysers. The world's greatest hot water gusher, Big Geyser, is there, in Iceland (true, it has to be activated with the aid of... liquid soap, for the geyser's natural power is as good as gone). But the resounding ejections of water and steam from its younger brother, Strockur, every five minutes are a breath-taking sight.
Speaking of the natural wonders of Iceland, we should mention the Dettifoss waterfall, Europe's largest, in a lava gorge of the neovolcanic zone. Next to the Hekia volcano is the highest local waterfall, Hauifoss, precipitating from a 122-meter steep mural, formed by lava and slag deposits accumulated over many centuries from volcanic discharges...
But back to our field work. Much to our surprise, we discovered single-type inclusions of rocks and minerals in both alkali-basalt volcanoes and in tholeiites where phe-nocrysts (crystal inclusions) are not typical. But we found no ultrabasic rocks and minerals of the upper mantle. Yet there was a great abundance of inclusions of various gabbroids
(granophyres) and big crystals of calk plagioclase, black pyroxene and olivine. In actual fact in their composition the minerals and rocks found by us fully corresponded to inclusions in lavas.
Our Islandic colleagues wondered why we could find no mantle inclusions. That puzzled us too at first, until we realized: it was because of the specifics of local magmatism and eruptions. As said before, magmatic chambers (hearths) under nearly all volcanoes are not deep-seated in the earth shell. Besides, local lavas are not saturated with gases, and the entire shell must be filled with dikes and intrusions formed in the process of sluggish invasion of magmatic melt into its upper storeys. A combination of such conditions is never conducive to eduction of deep matter from the upper mantle or from plutonic horizons of the lithosphere.
The likeness of mineral inclusions and typhoon rocks prodded us to look into the abundance of intrusives in Iceland, and so we tried to thoroughly select appropriate samples. Apart from the detected and mapped outcroppings of such massifs in different parts of the island, we found much of the gabbroid pebble in alluvial and fluvial deposits. That suggested the idea that intrusives were of common occurrence on the island and played an important role in the structure and composition of its earth shell.
LITHOSPHERE, A FOUR-LAYER PIE
Our expedition collected a large number of samples-inclusions in volcanites and typhoon specimens; all that was studied at our Institute and at the geological research centers of Baku and Yerevan. And thus we obtained a database on mineralogi-cal-geochemical and petrophysical characteristics of Icelandic rocks. Next, we compared our evidence with other available data. To begin with, we divided our collection of samples into seven groups. Of the utmost importance to us were three of these groups: amphibolized gabbros and gabbro-norites, magnetite gabbros and dolerites as well as cumulative inclusions formed during crystallization and agglutination of minerals in a magmatic hearth (chamber).
A study of the electric characteristics of our samples showed Iceland to have no extended and thick stratum of acid rock in the bowels of the earth; that is to say, the sialitic (siallite)
layer must be absent there. The mean temperature in the third stratum of the lithosphere is equal to 600-800C. This evidence is all-important for our knowledge about the composition of the fourth stratum, the stumbling-block for the two models of the island's plutonic structure-the "classical" and the "Russian" models. If the temperature in that stratum is equal to 1,200C or is higher, it cannot be composed of basalts (gabbros), or else they would have melted. In that case we can say almost certainly: the fourth stratum belongs to the uppermost parts of the mantle. True, according to our estimates lower temperatures are characteristic of this stratum and, therefore, solid gabbroids may persist there.
Studying the elasticity and density of our samples, we came to other interesting conclusions which confirmed the hypothesis of the essentially intrusive nature of deep shell horizons in Iceland.
In the mid-1990s, after we had completed our field work in Iceland, the Western literature published novel seismic data that corroborated the "Russian" model of the shell structure of Iceland. Surprisingly, the Icelandic geophysicist Olafur Flovenz, formerly an ardent adherent of the "classical" model, happened to be among the authors of these publications. The new evidence argued in favor of a "cold" lithosphere (compared with the older data on plutonic temperatures)-on its lower boundary in the third stratum the temperature does not exceed 900C. Deep in the fourth stratum there is a seismic reflection boundary about 25 km underneath, where the rates of elastic waves are as high as 7.7 km/s.
And what concerns the petro-geochemical side of the problem, it was shown that the acid melts widespread in Iceland must have been formed as a result of the submelting of gabbroids within the lithosphere. Consequently, a number of processes must be essential for the evolution of its composition and structure: mantle magmatism (with the accumulation of intrusives and efflusives); intermediate magmatic hearths formed in the subsurface (magma is differentiated in these hearths through the fractional crystallization of minerals); and the remelting of the basalt crust at different depths.
A MANTLE PLUME UNDERNEATH
Iceland's magmatic rocks are essentially different from those forming mid-oceanic ridges. This is particularly conspicuous when
comparing their isotopic composition. Icelandic volcanites are characterized by the enhanced presence of strontium and of lead isotopes, and by the lower concentrations of neodymium isotopes compared with the tholeiites of proximate oceanic ridges. In contrast, Icelandic volcanites are rich in strontium, light rare earths (lanthanum, samarium), radioactive rubidium, thorium, uranium. These differences are attributed to so-called "plumes"-the hot flows of matter rising from the bowels of the earth (possibly, from its core); the composition of this matter is different than that of the old mantle poor in basaltoid components. Anyway, such plumes have a strong effect on magmatic sources of the upper mantle. Lately the existence of plumes was confirmed by geophysical methods-namely, by the method of seismic tomography that helped detect in the upper mantle columnar districts where the velocities of elastic waves are reduced. The substance of a plume should be 150-300C hotter than the material of the surrounding mantle.
There have been many publications off the press attempting to substantiate the existence of an Icelandic plume in terms of geophysics and geochemistry. This plume must have been born about 65 million years ago in the North Atlantic, and it is thought to have had a crucial effect on the tectonic and geological history of the region. Most likely the plume is asymmetrical in profile; north of Iceland it has an escarpment towards the ocean and does not interact with the rocks of the mid-oceanic ridge Kolbensey. As shown by seismic tomography data, there is indeed a structure 175 km in diameter with clear lateral marginslying 150 to 400 km deep beneath central Iceland. This structure acts upon the ambient mantle, as seen in the growing concentration of many elements in rock (including potassium, niobium, zirconium and strontium) along the Reykjanes Ridge towards the island.
But has the mantle plume always played the same role in the geological history of the region? Even primitive petrochemical diagrams suggest the idea of differences in plutonic magmogenesis under Iceland in the Tertiary and Quaternary times. At the initial stages the magma formation conditions were strikingly similar to those for midland structures. Our colleague Anna Sholpo has shown that gabbroids from different tectonic structures of the earth (folded belts, old continental structures, insular-arcuate systems, ocean floor) have petrochemical features of their own, i.e. one can proceed from their composition for paleo- tectonic identification of structures under study. With reference to Icelandic rocks, this method has made it possible to determine a definite sequence in the tectonic history of the island.
Working together with Arthur Saltykovsky and Natalia Titayeva of the Moscow State University, we reviewed isotopic and geochemical data published in the literature; and here's what we saw: over the past 16 million years the nature of plutonic magma sources under Iceland has undergone significant changes. At first that was a mantle pool enriched with radioactive rare-earth elements and characteristic of continental structures. Thereupon a mantle source typical of mid-oceanic ridges became magmatical-ly active. And finally, at the ultimate stage, it was plume matter that acted upon the composition of the mantle source of Iceland's basalts and the Reykjanes Ridge.
Our data have clearly shown this: simple schemes of the evolution of the earth shell-like those of "pure" spreading (extension of the ocean floor)-are not enough for gaining an understanding of many things in the region. Such schemes should be augmented by a comprehensive approach, namely: while examining endogenic plutonic processes we should likewise consider the interaction of earth mantles located at different depths and identify different stages of tectogenesis. And we have shown on the example of Iceland that processes proper to this contemporary geological structure were also characteristic of the earth's distant past as well.
Trends in magma composition variation in the process of fractional crystallization (what we first detected in Iceland) could be observed in the Pre-Cambrian metamorphosed rocks of pristine shields in other regions as well-at Anabar and Aldan on the Siberian platform, at Voronezh on the East European platform and elsewhere. And here cumulative rocks (formed from minerals crystallized in magma), prove to be actually nonmagnetic; but those formed upon the cooling of residual melts are magnetized. This regularity, identified first in Icelandic intrusive rocks, is well reproduced in pristine continental complexes.
So, Iceland is the place where different geodynamic theories have been verified and tested. Small wonder that large research collectives, international among them, go there for regular and comprehensive explorations. Such studies have made it necessary to rethink the conventional notions about the island's plutonic structure. Well, Iceland is likely to spring many surprises in the future.
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