The Peresteria composite cone once stood high on the horizon of the Aegean. It was a prominent feature on the landscape and graced the stratosphere with its peak. Peresteria was the first subaerial volcanic feature to appear in the northern part of Thera on a major fault system known as the Colombo Line. Since its construction, it has been blown up by four caldera forming eruptions, and covered by numerous lava flows. In its exposed interior, Peresteria shows the geologic history of the island and holds the key to Santorini’s complex past.
Wednesday as a class we hiked from Peresteria to the northernmost town on Santorini called Oia. Along the way we saw the early geologic history of the Colombo line’s volcanism laid out in the rock record.
The five volcanic centers on the South Aegean Volcanic Arc are situated on oblique strike slip faults that trend Northeast-Southwest. These major fault lines are composed of smaller faults that are connected and grow bigger with each earthquake. Strike slip faults move laterally past each other, but these strike slip faults are oblique, meaning: As the footwall and hanging wall move laterally they also dip downward and have normal motion. Oblique faults create a lot of space for magma to squeeze through.
Santorini is situated on two strike slip oblique faults, the Kameni line and the the Colombo line. When volcanism began on Santorini it began at the Kameni line with doming in the South on Akrotiri Peninsula. The first subaerial volcanism to sprout up on the Colombo line was Peresteria the composite cone volcano.
Peresteria was an andesitic composite cone. Composite cones erupt in two different styles due to the intermediate magma that composes them. Composite cones can explode out the top due to dissolved gases in the magma becoming depressurized as they rise to the surface. This type of eruption, called a Plinian eruption, results in immense clouds of ash billowing up in the air and pyroclastic flows that run down the slopes of the volcano. Composite cones also effuse lava out of the sides and bottom of the volcano. Every time the composite cone erupts it deposits a layer onto itself adding to its immense size.
Composite cones can take hundreds of thousands of years to form, building themselves higher and higher with each eruption. Peresteria consists of andesitic lava flows and tuff, silicic andesite lava flows, and basaltic flows (1). Peresteria was at one point so large its was four kilometers in diameter, and its peak was 350 meters above sea level (1). Peresteria is now a fraction of its original size. On our hike we could see the interior of the volcano and all its layers. It was hard to imagine what it use to look like.
Peresteria took roughly 100,000 years to form starting 530,000 years ago. About 200,000 years after its construction, the volcano was blown up by Santorini’s first caldera forming eruption, the Lower Pumice eruption (200-180 KA). The Lower Pumice eruption greatly diminished Peresteria’s size and formed an ancient caldera wall that incorporated Peresteria’s many deposits.
Following the Lower Pumice eruption, mafic magma found its way up through dikes and cracks in the rock and oozed out on top of blown-up Peresteria forming the Megalo Vouno shield volcano. Shield volcanoes are formed by a succession of basaltic lava flows that create broad flat slopes. Megalo Vuno covered Peresteria until another caldera forming eruption occurred 76,000 years ago called the Upper Scoria eruption. The Upper Scoria eruption blew up the volcanic deposits that made up Megalo Vuno adding another layer to the caldera wall.
After the Upper Scoria eruption another shield volcano formed 67,000 years ago called Skaros. Skaros was created by successive basaltic lava flows that filled up the inside of the caldera like a bathtub. The previous caldera wall made up of Peresteria’s deposits, Lower Pumice Tuff, and Megal Vouno, stood strong against the incoming basaltic flows. The lava flows that made up the Skaros Shield volcano buttressed against the existing caldera wall, the viscous flowing lava didn’t stand a chance against the fortified caldera wall. The caldera wall was there first, the lava flows came after, and this is clearly shown in the rock record. This ancient caldera wall is still present today.
On our hike it was hard to imagine what the old rim looked like so I created an image (below) to highlight the caldera wall. Before Skaros, the caldera rim ended at the barrier.
Geologists can see that Santorini had other caldera forming eruptions besides the Minoan because of the evidence in the ancient caldera wall. Lava continues to flow across flat ground until it runs into something. The Skaros Lava flows ran into the ancient caldera wall and stopped abruptly, they wouldn’t have stopped if the caldera wall wasn’t there. This evidence is preserved in the current caldera wall. The Cape Riva caldera forming eruption and the Minoan eruption further destroyed the ancient caldera wall and the Skaros volcano creating the new caldera rim that we see today.
(1) Druitt, T. H. In:Memoirs of the Geological Society of London, 1999, Vol. 19.