E dall'oceano spuntò l'isola del terzo millennio
di CLAUDIA DI GIORGIO

ROMA - C'è una nuova isola nell'oceano Pacifico. Per adesso, è poco più di un cilindro di polveri, un cono vulcanico appena emerso dal mare, che sputa verso il cielo fiamme e vapori a qualche migliaio di chilometri dall'Australia. Lungo poco più di 200 metri e largo meno di 40, il neo-isolotto si trova nel territorio delle Tonga, un arcipelago di origine interamente vulcanica le cui 170 isole di tanto in tanto aumentano di numero in seguito a una nuova esplosione sottomarina. L'ultima arrivata si è manifestata una decina di giorni fa, con una grossa colonna di fumo a nordovest della capitale, Nuku'alofa. E da allora non ha smesso di crescere, attentamente sorvegliata da aerei della Difesa nazionale.

Ma non è detto che ce la faccia a diventare adulta. Molto spesso, infatti, le correnti oceaniche erodono velocemente la polvere di cui sono fatti questi isolotti, nati dall'improvviso emergere di un vulcano sottomarino al di sopra della superficie oceanica. Perché queste "isole temporanee", come le chiamano gli esperti, divengano permanenti, è necessario che le ceneri siano coperte da strati di lava indurita, capaci di resistere all'incalzare delle onde. Quindi non tutte le eruzioni, molto frequenti nella zona delle Tonga, riescono a donare all'arcipelago nuove fette di territorio, che l'oceano tenta continuamente di riprendersi. Il nuovo isolotto dovrà letteralmente conquistarsi l'esistenza contendendola ai flutti, ed è troppo presto per sapere se ce la farà, o se scomparirà tra le onde entro un paio d'anni, il tempo medio di vita degli atolli più instabili.

Secondo alcuni, però, questa è un'isola più fortunata delle altre perché si affaccia in superficie a meno di dodici mesi dal Duemila. Grazie alla linea internazionale del cambiamento di data, infatti, che passa più o meno per le Tonga, l'arcipelago polinesiano rivendica il titolo di prima nazione al mondo dove sorgerà l'alba del nuovo millennio. E c'è già chi ha detto che la nuova isola è un segno del cielo, addirittura il regalo del terzo millennio a chi lo vedrà arrivare per primo.
(18 gennaio 1999 - Repubblica ) 


vedere anche l'articolo (in inglese) su Eva - Your Guide to Tonga n. 45

Un fenomeno simile a quello descritto sopra era stato riportato alla fine dell''Ottocento sulla rivista "Scientific American" :
cliccare sulle immagini per ingrandirle

Una relazione sull'isola emersa nel 1895, chiamata Falcon Island, fu pubblicata anche dalla Royal Geographical Society.

A Visit to the newly emerged Falcon Island, Tonga Group, South Pacific.
By J. J. LISTER, M.A., H.M.S. Egeria.

On Wednesday, October 2nd, 1889, H.M.S. Egeria left Nukualofa (Tonga) to visit Falcon Island. It lies in the south-west part of the Tongan group, nearly in a line between the high volcanic islands of Tofooa, and Kao to the north, and Hongatonga and Hongahapai to the south. Tofooa is some 35 miles away and is seen in clear weather with the high conical top of Kao, 3030 feet high according to the chart, looking over the middle of it, as though it was part of the same island. Hongatonga and Hongahapai, two remnants of an old crater, are nearly always visible, pale purple or grey with distance, 15 miles to the south.
At the present time (October 1889) the island consists of two distinct parts. 
1st. The remains of a very wide-based conical hill, the side of which slopes gently up, at an angle of about 6° to the highest part and then ends abruptly in a cliff whose base is washed by the sea at high water. Captain Oldham informs me that the present height of the island is 153 1/2 feet. In a bird's-eye view the outline of this part of the island is a nearly symmetrical oval-the cliff presenting a convexity to the sea, and the base of the slope of the hill, where it joins the level, a convexity in the opposite direction.
2nd. A flat, extending away from the base of the hill in a northerly direction. This is about 10 to 12 feet above high tide level and is traversed by tide ridges, which run in a general way parallel with the shore of the flat and present a steep side towards it, and a more gradual slope in the other direction.
Except for some few seedling plants half-a-dozen of which were found during our visit, the island is entirely destitute of any vegetation. It is just a bare brown heap of ashes, round which the great rollers break and sweep up the black shores in sheets of foam.
The structure of the hill is seen in the cliff section. It is composed of fine-grained, dark greenish-grey material, arranged in strata. The strata are marked partly by slight differences in colour but chiefly by the salts, some white, some yellow, which have crystallised at the surface, more abundantly from some layers than others, and form pale bands. The strata are thickest in the highest part of the hill and thin out as it becomes lower. Volcanic bombs are scattered over the slope of the hill. They are largest and most numerous at the highest part. There are two main varieties among them: one a dark whitish-grey stone with white crystals, which presents all stages of the vesicular structure; the other a coarse conglomerate. The blocks are generally rounded, and some of them present a spiral twist in their surface ridges. Though so abundant on the hill-side, none or very few of these bombs appear in the face of the cliff-which shows, I think, that they were ejected at the close of the eruption.
The flat consists of the same material as the hill, except that there are no bombs on it, and the very finely divided elements are absent. In the lower lying parts there are beds of very fine grey mud which has been washed down by rains.
As one walks over the hill-side there is a distinct whiff of sulphur in the air, and the distant parts are seen through a very thin blue haze.
The island is still hot beneath the surface. Two pits were dug, one near the top of the slope and one on the flat, and the heat was measured at successive depths.
On the slope the results were as follows:

Temperature in shade on surface 74° F.
Temperature 3 inches deep in soil exposed to sun 77.5° 

Depth in Feet Reading of
Thermometer
before placing it in Soil
Temperature Difference
1   77.5 0.0
2   78.0 0.5
3  74.0 80.5 2.5
4 74.0 85.0 4.5
5 74.0 93.5 8.5
6 76.5 96.5 3.0
7 76.0 100.0 3.5

On the flat, at 2 feet depth, the thermometer registered 85°, and at 6 feet 6 inches it registered 106.5°.
I expect that the reason why the temperature at corresponding depths is higher on the flat than on the hill is, that the flat is permeated by seawater which rises and falls with the tide and assists in communicating heat to the parts near the surface. In a depression between two of the tide ridges on the flat there is a pool at sea-level in which the water is salt and rises and falls with the tide. The small pebbles in the bottom of this were coloured red with iron. The temperature of the water in the hottest part of the pool was 113° F., and in a hole dug among the pebbles close by, the temperature of the water was 121° F.
At one place on the cliff were three small jets of steam, the surface round each whitened with deposited salts.
Landslips are of frequent occurrence along the cliff, ten or twelve were seen in one afternoon, after a wet day. They occurred at the time of high water. A slice from the face of the cliff went sliding down, with clouds of dust and steam-leaving a paler newly exposed surface, and every wave which broke on the freshly fallen heap caused it to steam again. All along the edge of the cliff above there are cracks running parallel with the edge which make it dangerous to approach.
Two young coco-nut trees, not in a very flourishing condition, are established on the flat, and specimens of three other plants were obtained: one leguminous; another possibly a seedling candle-nut, which is now in a flourishing condition in Captain Oldham's cabin; the third is a grass. There were several dry stranded fruits on the flat: Barringtonia, Pandanus, and others.
The only bird that I saw was a sandpiper-I think Actitis incana (Gmel.)-which flew on ahead as I walked along the shore. A small moth was the only other living thing that I saw-but the black sandy shore was bored with the vertical burrows of some creature.
Three small pieces of coral were picked up on the flat. I spent one afternoon in a boat, dragging an arrangement of hemp swabs over the north-east edge of the bank which extends some distance out to sea on the northern side of the island. There is a depth of 10-12 fathoms over the greater part of it, and the water deepens quickly at the edge. This was with the view of bringing up coral if there was any established there. I found none-a piece of seaweed was the only living thing that came up.
There is a considerable shoal area, about three fathoms in depth over the greater part, north of the island-i. e. opposite the highest part of the cliff. When the wind is fresh there are breakers over parts of this, and its position is nearly always marked by a patch of turbid yellow water.
The island was formed by a volcanic eruption which occurred four years ago. It was visited during the eruption by several of the residents in Tonga, who all say that the centre from which the materials were ejected lay entirely on one side of the heap that was being formed-and was bounded on that side by cliffs. In other words, the materials thrown out were all carried to leeward by the wind-or at least enough did not
fall to windward to form a mound above the level of the sea. I have not been able to learn what was the direction of the wind at the time of the eruption, but judging of the position assigned to the focus, it appears to have been the usual east to south-east trade.
Accepting this evidence, a diagram may be drawn showing the relation of the island as it is at present, to the original formation when the eruption ceased.
The area occupied by the part of the original cone which has been washed away, is indicated by the shoal water to the south of the island.

Diagram to show the relation of Falcon Island in Oct. 1889 to the original island when the eruption ceased in 1885.
indicate the centre of eruption ; a' a a, supposed outline of the original island (1885). a' b b, the remnant of it in Oct. 1889.
 c c, outline of the flat extending from the base of the cone.

Considering how rapidly the island is being carved down by the action of the waves, it is evident that ins few years, unless a fresh volcanic outburst occurs, it will have entirely disappeared beneath the surface of the ocean. Some distance to the east of it lie two islands, Nomuka-eki and Mango, which have since been visited by the Egeria. They are formed by stratified volcanic material deposited under water, and are now surrounded by broad coral reefs.* In them we may read the possible future history of Falcon Island -or if no elevation takes place, the volcanic stones and debris will give a resting-place to a host of marine animals and plants; banks of the shells of Foraminifera, Pteropods, and other pelagic organisms will accumulate in sheltered places; coral reefs will grow and reach the surface, sandbanks may be formed to which the seeds of coco-nuts, Barringtonia, Scaevola, and Tournefortia, and other shore-loving plants will be drifted by the waves, and another green island be added to these summer seas.

* These islands have apparently been elevated before any considerable thickness of coral grew on them, as I was unable to find any trace of a raised reef on either.

Falcon Island, South Pacific, thrown up by submarine volcanic eruption, October 1885 - three views


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