How the Sartorius Craters Formed on Mount Etna: A Chronicle of the 1865 Eruption
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The Sartorius craters are a series of pyroclastic cones located on the northeastern side of Mount Etna, whose formation dates back to 1865. They take their name from the German scientist Wilhelm Sartorius, one of the first geologist to conduct in-depth research on the nature and origins of Etna’s eruptions.
To better understand the genesis of the Sartorius pyroclastic cones and the associated lava flows, we need to step back and examine the geological setting in which this eruption developed.
The Fracture System of the Northeastern Side of Mount Etna
The Sartorius cones lie within a complex system of fractures (Figure A), faults, and rift zones, including:
The Pernicana Fault,
The Northeast Rift,
The northern rim of the Valle del Bove,
The Ripa di Naca and Ripa di Piscio fault systems.
In past geological periods, this fractures network generated several lateral eruptions in the Sartorius area, forming volcanic edifices such as Monte Baracca, Due Monti, Monte Zappinazzo, Monti Conconi, and Monte Frumento delle Concazze (Figure A).
The differing locations of these eruptions are due to a progressive southeastward migration of the fault system.
The 1865 eruption that formed the Sartorius cones originated from the same structural system, specifically from the Main Eruptive Fracture System and a Secondary Eruptive Fracture System.
The Beginning of the 1865 Eruption
The eruption began on January 29, 1865, from a pre-existing fissure vent at the base of Monte Frumento delle Concazze.
Already the day before, villagers living on Etna’s slopes had been alarmed by seismic tremors and deep subterranean rumblings, like the echo of giant footsteps reverberating through underground caverns.
The sequence of Events
It was the evening of January 29. Winter cold kept people indoors, gathered around stoves and fireplaces.
At around 11:00 PM, a strong earthquake forced frightened residents out of their homes. Outside, they immediately noticed a fiery red glow coming from the volcano.
A volcanic eruption had just begun, with the emission of incandescent pyroclastic material from the base of Monte Frumento delle Concazze. This was followed by the formation of three lava fountains, which poured molten lava onto a sparse pine forest below, destroying it completely.
This initial phase lasted only a few hours. As magma continued to push up, it fractured the ground downslope, propagating the fissure system and replaicing lava fountains with explosive activity that lasted for seven days
On January 30, while three vents were in full Strombolian activity [Figure B (Shield cones)], a 400-meter-long fissure opened, along which eight lava fountains forming spatter cones [Figure B (Spatter cones)].
Although spectacular, this phase was short-lived and ceased after one day.
As the fissure propagated eastward, the eruption vents migrated downslope, leading to the formation of five pyroclastic cones [Figure B (1, 2, 3, 4, 5)], all active simultaneously between February 4 and 5, 1865.
At the same time began the emission of lava from the base of the pyroclastic cones in four different stages:
In the first stage began on January 29, an intense paroxysmal event generated the main lava flow [Figure A, Main (A)], which flowing down for 6.5 km, reaching an elevation of 800 meters. Some days after from a new emission point a new branch developed: the Monte Crisimo lava flow [Figure A, Lava Flow (B)], that widening and pull back the lava front.
In the second stage started on February 9, was produced the Monte Ragamo flow [Figure A, Lava Flow (C)], characterized by lower eruption rates. The reduced output led to the expansion of the lava field, with overlapping flows and the formation of ephemeral vents producing pāhoehoe lava, recognizable by its smooth, ropy texture.
In the third stage on the first week of March a sudden increase in magma rising from the fracture triggered a new lava flow: the Monte Chiovazzi flow [Figure A, Lava Flow (D)], emitted from vents located at the base of the last two downstream cones. This lava flow reached its maximum length on April 4.
The fourth and final stage was marked by a gradual decline in the emission rate, along with further widening and thickening of the lava field.
The eruption officially ended on June 28, 1865.
Which one volcanic structures formed during the Eruption of 1865 are still visible today?
Shield Cones (Figure B)
These were pre-existing cones reactivated during the eruption, today partially visible in the east flank of Monte Frumento delle Concazze. They produced volcanic bombs exceeding one meter in diameter, thrown up to 500 meters away. The eastern flank of Monte Frumento delle Concazze was covered by a layer of lapilli up to 15 cm thick.
Spatter Cones (Figure B)
Along the 400-meter fissure formed on January 30, eight lava fountains created small mounds hills with rounded tops. Today, these are no longer visible, having been buried by later volcanic deposits.
Pyroclastic Cones (Figure B)
The present-day Sartorius Mountains formed during the intermediate phase of the eruption. Their distribution reflects the progressive elongation of the eruptive fissure downslope. They consist of five irregular shaped volcanic edifices perfectly visible today, with one or more elliptical craters in the bottom part. Many of these craters are now partially or completely filled due to erosion and debris accumulation, transforming them from crater with open canal into close-bottomed depressions.
One cone, located within the second crater from the top, appears morphologically younger. Its well preserved features suggest a brief renewal of volcanic activity after the main eruption, forming a new crater into an older one.
Lava Flows (Figure A, A-D)
The 1865 eruption produced an extensive composite lava field, consisting of multiple overlapping lava flows. These flows were generated by three eruption types:
Paroxysmal eruptions: short duration, high emission rates
Intermediate eruptions: moderate duration and emission
Quiet eruptions: long duration, low emission rates
This calssification use the R ratio (Frazzetta & Romano, 1984), that defined vulcanic event based on the ratio between eruption duration (in days) and average emission rate (in cubic meters per second).
How and when to Visit Sartorius Craters
Thanks to their location and elevation, the Sartorius craters are accessible year-round via several trails:
The Sartorius craters trail: a beautiful path that winds through pine forests, vast lava fields, and the Quaranta Ore lava channel, home to Etna’s only surface stream. The trail continues up to Monte Frumento delle Concazze and the rim of the Sartorius craters. This route is ideal for an exciting snowshoe hike in winter or a spring trek on Mount Etna, when the melting snow reveals the magic of an alpine stream flowing across a volcano.
The Monte Baracca Trail: a charming and panoramic route that begins in a birch forest before approaching the base of Monte Frumento delle Concazze. From there, a steep path leads up to the craters, offering a stunning view of the Bay of Taormina and the sea, before descending through the striking, otherworldly shapes of an arid lava field. Is strongly recommended for visiting the Sartorius craters a guided excursion of mount Etna.
Access via the Mareneve Regional Road: along the road, there is an access point to the last crater. From here, a short walk leads to the rim of the volcanic cone, offering a spectacular view of the Bay of Taormina. A jeep tour on Mount Etna is ideal for visiting the Sartorius craters from this perspective.
Bibliography
CARVENI P., BENFATTO S., IMPOSA S., MELE G., SALLEO PUNTILLO M., STURIALE G. (2013), Geologia dell’ambiente - I Monti Sartorius, sul medio versante nord-orientale dell’Etna: un geomorfosito importante per il patrimonio geologico della Sicilia
FRAZZETTA G. & ROMANO R. (1984), The 1983 eruption: event chronology and morphological evolution of the lava flow. Bull. Volc., 47, 1979-1096.
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