The geologic evolution of Theracan be relatively easily studied because the steep and up to 300 m high cliffs of the caldera offer an excellent cross-section of the rock formations of the islands. The islands of the Thera group are composed dominantly of volcanic rocks, that have been erupted in the last half a million years. However, the island is underlain by much older rock formations of limestone and schist in the south-eastern part, dating from the Triassic to the Tertiary. Granite also occurs on the island, dating back some 9.5 million years.
		During the past four hundred thousand years, the volcanic activity on Thera has consisted of eruptions that produced either andesite and basalt lava flows, or pyroclastic deposits of pumce and ash from violent explosive eruptions. There have been twelve large explosive eruptions that have produced the distinctive white or yellowish pumice layers seen in the island’s cliffs. Four of these eruptions were of such violence and magnitude that the island collapsed and calderas were formed. A caldera is a wide (ca. 5 to 10 km diameter) and deep (ca. 500 m) depression resulting from emptying of the magma reserrvoir and collapse of the volcanic structure. During the eruption that produced the Lower Pumice, about 200,000 years ago, the first known caldera was formed. Similarly, following the eruption of the Middle Pumice about 100,000 years ago, another caldera was created. A third caldera formed after the Cape Riva eruption ca. 21,000 years ago, and finally, the present-day caldera was formed as a result of the Minoan eruption 3,600 years ago. In the periods between these explosive eruptions the volcano has built up shields or domes of basaltic and andesitic lava flows. They are easily recognized as darker or reddish brown layers in the cliffs of the caldera wall. After the Minoan eruption, about 3,600 years ago, Thera entered a period of dormancy, when little or no volcanic activity took place. In 197 BC, however, volcanic activity was resumed, when eruptions began in the middle of the Thera caldera and formed a new island occurred named Palaea Kameni. The eruptions were both explosive and lava flow eruptions and the island grew gradually in size. In 1707 another volcanic island formed just to the north-east of Palaea Kameni, and it acquired the named Nea Kameni. This new lava island has remained active, with notable eruptions in 1866, 1925, 1939 and most recently in 1950. Although it has remained dormant for over half a century, nea Kameni is not extinct, and will without doubt erupt at some time in the future.
		Santorini, showing the volcanic deposits. The thick light colored layer in the lower part is Lower Pumice, from an explosive eruption that took place about 200,000 years ago. The light grey layer at the top is from the Bronze-Age Minoan eruption, about 3,600 years ago. (Druitt et al, 1999)	The volcanic island of Nea Kameni erupting in the center of the Thera caldera in 1950.