|
|
Project
>
Geological History of Sites
>
The Volcanic History
The Volcanic History
The Basal Volcanic Complex (ca. 160,000 to 100,000 years)
The Early Caldera Stratovolcano (ca. 100,000 to 30,000 years)
Lakki Pyroclastics and Avlaki Complex
Lacustrine and Pyroclastic Formations
The Melisseri - Evangelistra - Afionas Complex
The Emborios Complex
The Ellinika - Kyra - Lies Pyroclastic Complex
The Argos - Stavros Complex
The Main Caldera Formation (30,000 to 15,000 years ?)
The First Caldera Phase (?30,000 to 20,000 years ?)
The Second Caldera Phase (20,000 to <15,000 years ?)
The Second Caldera Phase (20,000 to <15,000 years ?)
A pumice fall ("The Upper Pumice"), surge and flow sequence consisting
of a massive to slightly stratified basal fall layer (up to 8 m thick) is covered
by a sequence of wet and dry surge and ash flows. Lag breccia deposits are present
in the uppermost part including lithics of scarn and of hydrothermally highly
altered lavas.
Fresh granular andesitic fragments are abundant (5-6%) as well as rare mixed
pumices. The estimated magma volume of this explosive event is similar to that
of the lower pumice deposits (2-3 km3). Field evidence indicates that the present
caldera depression was formed contemporaneously with the upper pumice eruption.
The upper pumice deposits cover large areas of the northern and northeastern
part of the island. On the circum-caldera road between Nikia and Emborios, it
seems to directly overlie the porphyritic rhyodacite flows of Nikia. The unit
occurs mainly as air-fall but also surge deposits.
The air-fall deposits are often bedded and occasionally inversely graded by
size. The upper felsic pumice, locally exceeding 100 m in thickness, is dacitic
to rhyodacitic in composition and most probably represents the climax that resulted
in the caldera collapse.
The "Post-Caldera Profitis Illias Domes and Lava Flows" built up the
now 698 m high Mt Profitis Illias and cover two thirds of the caldera on its
western side. Typical outcrops occur along the trail that leads from the Monastery
of Evangelistra to the hydrothermal field.
They range in composition from dacite to rhyodacite and cover two thirds of
the caldera floor in the form of six steeply sloping domes, some flown-out the
western rim of the caldera and extruded along NNE-trending fractures. The presence
of sharp protrusion structures, a fresh scoria cover and the absence of soil
and lateral scree indicate that the northeast (Boriatiko) and south-west (Karaviotis)
domes and flows are the most recent products of Nisyros volcano.
|
|
