As a 3-6 mile thick slab of earth oceanic crust moves downward, some sixty miles below Santorini, sliding through resistant sticky mantle material, it shudders and quakes as it descends to create swarms of earthquakes on the surface.
Santorini Island, Fira, or ancient Thera?
The 400 meter high brown and red cliffs of Santorini, rise up out of the deep blue waters of the Aegean as observers approach the island by sea. The ridge-tops appear encrusted with crenelations of white and blue, as if a cake decorated by Zeus, with snow from Olympus and blue from the Aegean. But as the vessel approaches closer, these decoration are revealed to be clusters of whitewashed buildings with scattered blue painted domes, all clinging to the very edge of the near perpendicular, stratified cliffs.
The view from seaward has existed in some form since the 12th century BC when Thera, King of Sparta first arrived to colonize this circular island with its placid well-protected bay. In its long history the island had several names. Once called “Kallisti” (Καλλιστι) “the beautiful one”, and later, the “circular one” (Στρονγιλλι) Strongilli. Today, its official Greek name is Fira (Φιρα), most likely a demotic Greek corruption of the 12th century “Thera”.
All of Fira’s several names are descriptive and appropriate. But today, a new appellation might well be applied: “the trembling one” or Ο τρεμοντας. (Tremontas). This most recent term: the result of a recent series of thousands of continuous and weak earthquakes which began in January 2025.
At first these temblors were minor, being recorded at 1 to 2 on the Richter Scale. By February 7 of this yearthey continued to occur in the thousands, and increased in magnitude. Greek seismologists officially classed them: earthquake swarms. They are presently being recorded at an intensity to 4 -5 on the Richter Scale, a level which can be felt by most people, but cause little structural damage, though loose objects in building interiors may tumble from shelves and shatter. (In typical Greek efficiency, shop owners, displaying vases or other fragile objects, often connect these items to the ceiling by a thin inconspicuous wire or monofilament.)
Fira (Φιρα) or Kallisti is indeed a beautiful island. Beautiful enough to attract almost 4 million tourists each year, and it is circular. Nearly 20 thousands years ago, Fira was once a symmetrical steep volcano, with a circular base. Like the iconic Japanese volcano: Mount Fuji it was a typical stratocone volcano, smaller but in appearance much like Fujiama.
The original Fira, probably formed nearly 200,000 years ago which rose to a height of 1.6 km (@5,250 ft) at its peak. During the Greek Bronze Age, about 1600 BC, a massive eruption blew away three fourths of the pyramidal top causing a tsunami or eruption caused sea wave. The massive eruption created a tsunami which generated a standing ocean wave estimated at 150 feet (@ 46 meters) high which devastated the Minoan city of Knossos on Crete.
The massive eruption left behind only the base of the volcano which survives today as a near circular archipelago of several small islands. Santorini island itself is the large eastern segment of the base, while the island of Therasia, is located northwest of Santorini @ 7 km across the caldera bay, and the tiny “white island” of Aspronisi is located about the same distance away in the southwest. Within the 12 km long and 7 km wide caldera are two small volcanic islands (the older) Palea Kamini, and a more recent (1950) island known as (Νεα Καμινι) Νea Kamini—both arose from the still active magma chamber at depth below its center. On a cruise to this bay, my students and I anchored off of Palea Kamini and swan through crystal blue waters (72F) to Nea Kamini’s black, volcanic-mud shore. While there we bathed in the supposedly therapeutic mud. After a cleaning swim we explored the small island, to examine and photograph the still active, fume-producing hydrogen sulfide gases and fumeroles.
The 1600BC nearly obliterated an early Greek Bronze Age, Minoan-related site known as Akrotiri an important copper trading depot of that era. The site of Akrotiri is in the extreme southwest peninsula of Santorini itself.
Let’s review a few ideas.
In the 1950-60s geologists learned that the Earth’s continents were moving around on its surface…amazing!! One-hundred fifty million years ago Africa and South America were part of one mega continent (Pangaea). A crack in the crust (where the center of the Atlantic Ocean is today) began to spread as it produced new ocean crust. The Atlantic is the newest world ocean. But how and why?
The Earth’s 4-20 mile thick rocky crust is a rigid solid, but below the crust, its interior (mantle) is hot, plastic and in some places molten. That hot molten material (magma) can flow upward through elongate fissures or cracks. As the cracks fill with magma they produce elongate puckering ridges in the Earth crust. These elongate upwellings are Ocean Ridges. It is at the ocean ridges where molten material from below may rises to the surface, cool and solidifies into new ocean crust in the ridge structure. The thin wedge of newly solidified (an inch or two thick) rock tends to force the older crust apart—spreading it. As a result these long ridges are also referred to as spreading centers.
Spreading centers are the cause of much that happens on the Earth surface. Besides pushing continents apart and creating new ocean crust the spreading centers or ocean ridges are also responsible for creating mountain ranges, volcanoes, island arcs and volcanic arcs, and earthquakes!.
The thin wedge of “new” l rock does not form very rapidly…only at about the rate at which your fingernails grow..perhaps on average about an inch (2.5 cm) a year. Its a slow process but the Earth has plenty of time for change. It took 150 million years or so to produce the Atlantic Ocean and push North America from Eurasian and South America from Africa.
Making new crust on the earth-sphere, which obviously already had its “well fitting” crust can cause “spatial problems”. New crust in the form of a whole new crust for the Atlantic Ocean, created excess crust that had to go somewhere. Some of the new crust formed drove against existing continents cause them to buckle up to generate linear mountain chains, such as the cordillera of North and South America’s west coasts. In other places spreading crusts may push other continents together to for the Himalayas, or the Alps. But even accounting for those areas, there was still too much Atlantic Ocean crust to account for.
We now know that much of that “excess” Atlantic crust must have been simply recycled by sliding down into the mantle and remelting. In these places the down-sliding ocean crust drags near by ocean crust downward as well to form the world’s OceanTrenches. These linear “deeps” occur at the Aleutian, Japan, Philippine, and Indonesian trenches where ocean crust (2-6 miles thick) is sliding downward in what are termed “subduction zones”. (Subduction means to “slide under”). At Subduction Zones one crust (the heavier one) tends to slide under a lighter crust. As it sinks, it forms a trench or “deep” in the adjacent ocean bottom. And as the slab sinks to depth it begins to melt.
Often some of that melted rock is composed of lighter or less dense materials than the mantle rock itself. This mateiral tends to rise upwards and melt to form liquid rising flows that may melt overlying rock and burst through to the surface as a volcano. Most often, since the entire slab of crust is subducted and tends to sag lower in its central region, it often forms arcs of volcanoes. It is noteworthy to mention here that this melting process which occurs at a depth of about 60 miles from the subduction zone produces a “curving” volcanic arc at right angles to and between 60 to 180 miles from the subduction zone.
The origin or cause of the earthquake swarms presently affecting Greece are presently unknown. Though Greek seismologist, report that the swarms are most likely not related to volcanic activity. They are also not the result of short term violent displacements along a fault zone which generally produce a major shock and a series of smaller aftershock sequences. The present swarms are not related to earthquakes related to the movements in the main Santorini magma chamber at depth below the caldera. When magma refills the magma chamber small quakes result. But these swarms are not located directly below Santorini caldera. Earthquake sensors seem to put the origin of the activity to the northwest of the caldera at an undersea volcano (Kolumbo) about 7km northwest of Santorini. This would place it in the area where subduction tends to produce volcanic arcs. Thus the earthquake swarms are more likely the result of tectonics—the movement of plates—than actual volcanism.
Santorini itself is a mostly submerged caldera - a crater formed as a result of volcanic activity over the past 180,000 years, with its last eruption in the 1950s. Earthquakes can be connected to volcanic activity - specifically, the movement of magma beneath the surface.
Greek seismologists report the following: Santorini Island (Fira) is located about 159 miles north of the Hellenic Subduction Zone and the associated Calypso Trench. At this place the African Plate is being subducted below the Hellenic Micro Plate—the Hellenic Plate is moving south at a rate of about 3.5 cm per year (@1.4 inches). The Calypso subduction zone has formed a volcanic arc which includes the ancient Santoriicn (Fira) volcanic site which is about 159 miles north of the subduction zone. so we can assume that the African plate, which is sinking down and sliding under the the Island of Crete and perhaps as deep as 60 miles belwo the island of Santorini is the cause of these swarms. Will they continue? Yest very likely perhaps for weeks or months. Are they dangerou..probably not at this stage. Though as the plate melts at depth it may activate an existing submarine volcano northeast of Santorini.
So the tourist season of 2025 on Santorini may well be disrupted by these disturbing temblor swarms. Millions of visitors come to see this beautiful circular, and yes, trembling earthquake-prone island each year. It is likely they will be able to do so in the future too.
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