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The panoramic parapet just opposite the church of the Metropolis in Chora is a good vantage point for comprehending the nature of the volcanic caldera of Santorini. The caldera is the physical incarnation of the island’s history. It extends below and to the west, covering an area of around eighty five square kilometres, surrounded by a broken, roughly elliptical rim. You are standing on its eastern edge at a height of c. 250m above sea level; it sweeps slowly round in two arms to north and south, and its fragmented western rim is formed by the island of Therasia across the water to the west and the small, steep rock of Aspronisi, visible just to the south of due west in the middle of the breach between the southern tip of Therasia and the western end of Santorini. These landmarks describe the contour of the vast crater which is now filled by the sea, whose surface hides water-depths of as much as 400m. In the centre of the area, two (relatively new) land masses of up-thrust magma have appeared— the islets of Palaia and nea Kameni—which from this angle appear as one low, deserted island of black rock. This whole desolate and beautiful scene is the remnant of what was probably the largest volcanic explosion in human history. The island was in all probability once a mountainous, roughly circular land mass, similar in shape and size to Ios : but violent volcanic activity over many millennia prior to the final eruption in Minoan times had already given rise to a collapse of the central cone, allowing the sea to pour in through a massive breach to the west and creating a central, water-filled caldera. The final eruption, in historic times in the middle of the second millennium BC ejected almost sixty cubic kilometres of incandescent pumice and ash into the atmosphere, and left the outlines and the island-form that we see today.
The depth of the water immediately below the cliffs is so great that it does not permit boats to anchor: they have to tie-up to floating buoys instead, which are chained to the floor of the sea at a depth of nearly 200m. The floor of the caldera lies generally between 200m and 400m be low sea level, reaching its deepest points to the north and east of Aspronisi, and directly west and below the crest at Imerovigli where the dry land rises in a cliff to 320m: this represents a total drop between clifftop and seabed of over 700m.
The black and barren islands of the Ka[i]meni (meaning burnt) in the centre of the caldera are the result of the slow and continuing up-thrust of solidified magma. They first appeared in the 2nd century bc, and have grown in circumference, subsided, partially disappeared, re-appeared, split and joined in a constant ballet of movement over the centuries. They represent the focus and dome of the active volcano today, and are currently growing in height and size by a slow process of extrusion from below.
Santorini Island is part of the Cyclades Island Group, Greece.