Volcanic Activity
It is difficult to summarise briefly the chaotic succession of volcanic phenomena which
repeatedly convulsed the region. An attempt will be made in this section, however, to give
a schematic outline of the phases into which the course of events during the various
geological periods can be divided, in order to convey at least a broad general
understanding of the palaeogeographical development of the region.
- First phase
In Oligocene times the landscape consisted of mountain ridges of crystalline rocks, mainly
granitic, which sloped gradually down from north-west to south-east to form the bottom of
a large salt lake. The main orographic features were the southern outliers of the
Kırşehir massif, articulated into two long ridges, one consisting of İsmail Tepe,
Kargın Dağ and Kabak Tepe, the other of Idış Dağ, with a broad valley between the two
chains. In the Miocene period the salt lakes were filled up and, in the warm moist climate
then prevailing, the slopes of the mountains were exposed to intensive weathering and
erosion, producing the rounded contours we see today. There thus developed at the foot of
the mountain ranges a large area of marshy plain covered with tropical vegetation and
inhabited by large proboscideans, carnivores, giraffids and equids. The plain was
interrupted by local outcrops of the crystalline foundation, forming low hills which
relieved the rather featureless landscape. Meanwhile a number of lake basins were being
formed: a process which can be observed in the sedimentary rocks to the west of Avanos and
the east of Ürgüp.
- Second phase
The Alpine orogenic movements which led to the formation of the Taurus range, just to
the south of the region with which we are concerned, gave rise to deep fractures in the
crystalline foundation, with a pattern of lozenge-shaped joints, accompanied by
subsidences of varying extent. The main fracture lines ran from north-west to south-east
and from north-east to south-west. Typical examples of stepped subsidences still to be
seen are the Salt Lake (Tuz Gölü) depression, the Kayseri and
Yeşilhisar depressions, and the depression which later became the valley of the
Kızılırmak.
This deep and intense fracturing process also created the conditions in which the
underlying magma could force its way to the surface, leading to the formation of the large
eruptive cones of Erciyas Dağ, Develi Dağ, Melendiz Dağ,
Keçiboydoran Dağ and Hasan Dağ. We must thus picture the
development of a chain of volcanoes, alternating between periods of quiescence and
explosive phases when they emitted clouds of fiery gas, cinders, volcanic ash, scoriae and
immense lava flows which surged down the sides of the newly emerged cone, swept into the
valleys and plains, and spent their force in the lake basins. All this completely changed
the face of the landscape, which on the one band was diversified by the contorted
structures of the volcanic cones, while on the other the matter ejected in the eruptions
filled up the depressions formed by the existing valleys, giving the area something of the
aspect of a plateau. The volcanic ash and cinders deposited in the lake basins altered the
depth and contours of the lakes.
- Third phase
In this phase the activity of the main eruptive centres continued, though still with
periods of quiescence. The cones continually increased in size, forming a volcanic chain
running parallel to the Taurus. This activity came to an end in the historical period with
various minor emissions from fumaroles. As an illustration of the scale of this activity
we may take the volcanic eruption of Erciyas Dağ.
The early Miocene period, as we have already noted, saw the beginning of an andesitic
eruption which eventually deposited a cone 30 km. in diameter. At the end of the Pliocene
there was a phase of extreme violence during which clouds of burning gas resulting from
explosions in the highly acid magma burst out of the crater with great force, ejecting
enormous masses of volcanic matter which spread over an area of 10,000 km². Between the
Pliocene and the Pleistocene there were large-scale emissions of molten lava (basalts).
Then towards the beginning of the historical period the activity became steadily less
intense and the eruptive matter tended to be vitreous and andesitic. The period of
quiescence was preceded by considerable emissions from fumaroles.
- Fourth phase
The plateau formed by the pyroclastic deposits from the major volcanic cones was the
scene of other forms of volcanic activity of lesser intensity, localised along Fines of
fracture, which produced continual changes in the structure of the landscape. Among the
volcanic cones -involved in this minor activity were those at Nevşehir, Yeşilhisar,
Tekke Dağ and Topuz Dağ, Bayramhacılı, Kara Dağ and Acıgöl.
After this succession of eruptions the lake basin of Ürgüp increased considerably in
size, became shallower and acquired a new shoreline passing by way of Ürgüp, Ayvalı and
Mustafapaşa. Earth and clay from the disintegration of the previously existing structures
were deposited in the lake, filling in the depressions in the substratum and covering the
land round the shores. Meanwhile, after various oscillations and changes of course, the
Kızılırmak had settled down into its present bed, broadly following one of the lines of
fracture running past the base of Idış Dağ. The lowering of the river bed led to the
emptying of the residual lake basins, and a period of intense erosion began.
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