THE SANTORIN GROUP.
[Illustration: Fig. 13.--Ideal Section through the Gulf of Santorin, to
show the structure of the submerged volcano.--_a._ Island of Aspronisi;
_b._ Island of Thera; 1. Old Kaimeni Island; 2. New Kaimeni Island; 3.
Little Kaimeni Island.]
(_a._) Before leaving the subject of European active volcanoes, it is
necessary to give some account of the remarkable volcanic island of
Santorin, in the Grecian archipelago. This island for 2000 years has
been the scene of active volcanic operations, and in its outline and
configuration, both below and above the surface of the Mediterranean,
presents the aspect of a partially submerged volcanic mountain. (See
Section, Fig. 13.) If, for example, we can imagine the waters of the sea
to rise around the flanks of Vesuvius until they have entered and
overflowed to some depth the interior caldron of Somma, thus converting
the old crater into a crescent-shaped island, and the cone of Vesuvius
into an island--or group of islands--within the caldron, then we shall
form some idea of the appearance and structure of the Santorin group.
_Form of the Group._--The principal island, Thera, has somewhat the
shape of a crescent, breaking off in a precipitous cliff on the inner
side, but on the outer side sloping at an angle of about fifteen degrees
into deep water. Continuing the curvature of the crescent, but separated
by a channel, is the island of Therasia; and between this and the
southern promontory of Thera is another island called Aspronisi. All
these islands, if united, would form the rim of a crater, in which the
volcanic matter slopes outward into deep water, descending at a short
distance to a depth of 200 fathoms and upwards. In the centre of the
gulf thus formed rise three islands, called the Old, New, and Little
Kaimenis. These may be regarded as cones of eruption, which history
records as having been thrown up at successive intervals. According to
Pliny, the year 186 B.C. gave birth to Old Kaimeni, also called Hiera,
or the Sacred Isle; and in the first year of our era Thera (the Divine)
made its appearance above the water, and was soon joined to the older
island by subsequent eruptions. Old Kaimeni also increased in size by
the eruptions of 726 and 1427. A century and a half later, in 1573,
another eruption produced the cone and crater called Micra-Kaimeni. Thus
were formed, or rather were rendered visible above the water, the
central craters of eruption; and between these and the inner cliff of
Thera and Therasia is a ring of deep water, descending to a depth of
over 200 fathoms. So that, were these islands raised out of the sea, we
should have presented to our view a magnificent circular crater about
six miles in diameter, bounded by nearly vertical walls of rock from
1000 to 1500 feet in height, and ruptured at one point, from the centre
of which would rise two volcanic cones--namely, the Kaimenis--one with a
double crater, still foci of eruption, and from time to time bursting
forth in paroxysms of volcanic energy, of which those of 1650, 1707, and
1866 were the most violent and destructive.[1] Of this last I give a
bird's-eye view (Fig. 14).
The only rock of non-volcanic origin in these islands consists of
granular limestone and clay slate forming the ridge of Mount St. Elias,
which rises to a height of 1887 feet at the south-eastern side of the
island of Thera, crossing the island from its outer margin nearly to the
interior cliff, so that the volcanic materials have been piled up along
its sides. The rocks of St. Elias are much more ancient than any of the
volcanic materials around; and, as Bory St. Vincent has shown, have been
subjected to the same flexures, dip and strike, as those sedimentary
rocks which go to form the non-volcanic islands of the Grecian
archipelago.
[Illustration: Fig. 14.--Bird's-eye View of the Gulf of Santorin during
the volcanic eruption of February 1866.--(After Lyell.)]
[Illustration: _Ground Plan of Rocca Monfina_
Fig. 15.--Rocca Monfina, in Southern Italy, showing a crater-ring of
trachytic tuffs, from the midst of which, according to Judd, an andesite
lava-cone has been built up. Compare with the Santorin Group.]
(_b._) _Origin of the Santorin Group._--In reference to the origin of
the Santorin group, Lyell regards it as a remnant of a great volcanic
mountain which possessed a focus of eruption rising in the position of
the present foci, but afterwards partially destroyed and the whole
submerged to a depth of over 1000 feet. But another explanation is open
to us, and one not inconsistent with what we now know of the physical
changes to which the Mediterranean has been subjected since early
Tertiary times. To my mind it is difficult to conceive how such a
volcanic mountain as that of Santorin could have been formed under
water; while, on the other hand, its physical structure and contour bear
so striking a resemblance (as already observed) to those of Vesuvius and
Rocca Monfina that we are much tempted to infer that it had a somewhat
similar origin. Now we know that Vesuvius was built up by means of
successive eruptions taking place under the air; and the question arises
whether it could be possible that Santorin had a similar origin owing to
the waters of the Mediterranean having been temporally lowered at a
later Tertiary epoch. It has been stated by M. Fouque that the age of
the more ancient volcanic beds of Santorin belong, as shown by the
included fossils, to the newer Pliocene epoch. These are of course the
unsubmerged, and therefore more recent strata, and may have been
recently upheaved during one or more of the outbursts of volcanic
energy. But it seems an impossibility that the Gulf of Santorin, with
its precipitous walls and deep circular interior channel, as shown by
the Ideal Section (Fig. 13), could have been formed otherwise than under
the air. We are led, therefore, to inquire whether there was a time in
the history of the Mediterranean, since the Eocene period, when the
waters were lower than at present. That this was the case we have clear
evidence. The remains of elephants, hippopotami, and other animals,
which have been discovered in great numbers in the Maltese caves, show
that this island was united to Sicily, and this again to Europe, during
the later Pliocene epoch, so as to have become the abode of an
Europasian fauna. According to Dr. Wallace, a causeway of dry land
existed, stretching from Italy to Tunis in North Africa through the
Maltese Islands--an inference involving the lowering of the waters of
the Mediterranean by several hundred feet.[2] There is every reason for
supposing that the old volcano of Santorin was in active eruption at
this period; and its history may be considered to be similar to that of
Vesuvius until, at the rising of the waters during the Pluvial (or
Post-Pliocene) epoch, during which they rose higher than at present,
Santorin was converted into a group of islands, slightly differing in
form from those of the present day. This view seems to meet the
difficulties regarding the origin of this group, difficulties which
Lyell had long since clearly recognised.
(_c._) _Limit of the Mediterranean Volcanic Region._--With the Santorin
group we conclude our account of the active European volcanoes. It may
be observed, however, that from some cause not ascertained the volcanic
districts of the Mediterranean and its shores are confined to the north
side of that great inland sea; so that as regards vulcanicity the
African coast presents a striking contrast to that of the opposite side.
If we draw a line from the shores of the Levant to the Straits of
Gibraltar, by Candia, Malta, and to the south of Pantelleria and
Sardinia, we shall find that the volcanic islands and districts of the
mainland lie to the north of it.[3] This has doubtless some connection
with the internal geological structure. The immunity of the Libyan
desert from volcanic irruptions is in keeping with the remarkably
undisturbed condition of the Secondary strata, which seldom depart much
from the horizontal position; while the igneous rocks of the Atlas
mountains are probably of great geological antiquity. On the other hand,
the Secondary and Tertiary formations of the northern shores and islands
of the Mediterranean are generally characterised by the highly-inclined,
flexured, and folded position of the strata. Hence we may suppose that
the crust over the region lying to the north of the volcanic line, owing
to its broken and ruptured condition, was less able to resist the
pressure of the internal forces of eruption than that lying to the south
of it; and that, in consequence, vents and fissures of eruption were
established over the former of these regions, while they are absent in
the latter.
[1] Fuller details will be found in Daubeny's _Volcanoes_, chap. xviii.,
and Lyell's _Principles of Geology_, vol. ii. p. 65 (edition 1872). The
bird's-eye view is taken from this latter work by kind permission of the
publisher, Mr. J. Murray, as also the accompanying Ideal Section, Fig.
13.
[2] Wallace, _Geographical Distribution of Animals_ (1876). The author's
_Sketch of Geological History_, p. 130 (Deacon & Co., 1887).
[3] The _volcanic area_ lying to the north of this line will include
Sardinia, Sicily, Pantelleria, the Grecian Archipelago, Asia Minor, and
Syria; the _non-volcanic area_ lying to the south of this line will
include the African coast, Malta, Isles of Crete and Cyprus. The Isle of
Pantelleria is apparently just on the line, which, continued eastward,
probably follows the north coast of Cyprus, parallel to the strike of
the strata and of the central axis of that island.--See "Carte
Geologique de l'ile de Chypre, par MM. Albert Gaudry et Amedee Damour"
(1860).