VOLCANOES

Volcanoes are land or underwater natural formations, usually hills or mountains. They form when magma—molten rock located deep within the Earth—finds an exit to the surface. Once magma reaches the surface, it cools and solidifies into lava.

The formation and rise of magma toward the Earth's surface is favored in areas with tectonic disturbances. From the magma chamber, it ascends to the surface through volcanic conduits or feeder veins, leading to the volcanic vent, typically a funnel-shaped cavity known as the crater. During the intervals between volcanic eruptions, the only sign of volcanic activity is the release of fumaroles, that is, hot gases and steam from ground cavities.

Volcanic activity ranges between two extremes: in one type, magma low in gas reaches the surface calmly, flowing out of the vent to create lava flows when it is fluid, or lava domes when it is viscous. In the other type, gas-rich magma fuels
explosions. In this case, fragmented lava is ejected from the vent as ash, which is also known as pyroclastic material; its finest particles are called volcanic dust or ash. Some volcanoes remain active for long periods, such as the Stromboli volcano in the Aeolian Islands, which has been active for the past 3,000–4,000 years.

However, most volcanoes have short-lived episodes lasting from a few days to a few years, after which they become dormant for decades, centuries, or millennia. A dormant volcano may be completely quiet or may emit gases and steam, indicating it is still active. Today, volcanoes that have erupted in the past 10,000 years and are in tectonically active areas are considered active.

The Structure of a Volcano

Active Volcanoes in the Aegean Sea

About five million years ago, the Aegis land—a terrestrial Alpine mountain range occupying the present-day Aegean Sea—began to fragment and sink into the waters of a blue sea. The tectonic plate pressure from Turkey, which slides along the North Anatolian fault line, and the ascending hot currents caused by the subduction of the African lithosphere thinned and sank large parts of the Aegean continental crust. (image Diagram of the African lithosphere sinking under the Aegean.)

At the same time, volcanoes joined the struggle of the land against the sea. The magma generated under the southern Aegean due to the subduction of the African lithosphere found primary exits in the Saronic Gulf, where it formed much of Aegina and the Methana Peninsula, in the central Aegean, creating the island complexes of Milos and Santorini, and in the eastern Aegean, forming Nisyros with its surrounding islets and parts of western Kos. (image The volcanic fields of the southern Aegean )

Active volcanoes in the Aegean, with volcanic activity within the past 10,000 years, include Methana, Milos, Santorini, and Nisyros.

Diagram of the African lithosphere sinking under the Aegean.

The volcanic fields of the southern Aegean

Rocks of the Aegean Volcanoes

The original magma fueling volcanic activity in the southern Aegean is a calc-alkaline basalt—a fluid mixture of silicon, aluminum, magnesium, iron, calcium, sodium, titanium, and potassium oxides. When this magma reaches the surface, it produces basaltic lavas (SiO₂ <56%). If stored in the crust for a long time, it differentiates over time, with basalt gradually transforming into andesite (SiO₂ 56–63%), dacite (SiO₂ 63–70%), and finally rhyolite (SiO₂ >70%). (Images Classification chart of volcanic rocks in the southern Aegean/Image of basalt under a polarizing microscope with transmitted light, polarized (right) and unpolarized (left))/Image of andesite under a polarizing microscope with transmitted light, polarized (right) and unpolarized (left)). /image of rhyolite under a polarizing microscope with transmitted light, polarized (right) and unpolarized (left).

Primary Mineral Crystals in Aegean Volcano Rocks:

Feldspars: Clear, white crystals. ( Image of a feldspar thin section)
Pyroxenes: Dark green to black crystals. (Image of a pyroxene thin section)
Hornblendes: Dark green to gray crystals. ( Image of a hornblende thin section)
Olivines: Honey-colored to olive-green transparent crystals. (Image of an olivine thin section)
Biotites: Flaky, dark brown hexagonal crystals. (P6 Image of a biotite thin section)

Classification chart of volcanic rocks in the southern Aegean

Image of basalt under a polarizing microscope with transmitted light, polarized (right) and unpolarized (left)

Image of andesite under a polarizing microscope with transmitted light, polarized (right) and unpolarized (left)

Image of rhyolite under a polarizing microscope with transmitted light, polarized (right) and unpolarized (left)

Image of a feldspar thin section

Image of a pyroxene thin section

Image of a hornblende thin section

Image of an olivine thin section

Image of a biotite thin section

Black slag.

Pumice stones

Thin section images of skarn under a polarizing microscope with transmitted light, both polarized (right) and unpolarized (left)

Images of a thin section in a petrographic microscope with transmitted light, both polarized (right) and non-polarized (left)

Images of a thin section of skarn in a polarizing microscope with transmitted polarized light (right) and non-polarized light (left)

Native sulfur crystals in a fumarole

Artifacts from the prehistoric era made of Melian obsidian

Artifacts from the prehistoric era made of glass obsidian.

Gases play a critical role in the final texture of volcanic rocks. A fluid basaltic or andesitic magma with many gases creates rocks with a spongy appearance and typical black or dark red color, called scoria. A viscous, gas-rich dacitic or rhyolitic magma produces pumice—a lightweight white rock made of "foamed" glass with few crystals. Sulfur is often found near volcanoes, especially in areas with hydrothermal activity, where hot gases and fluids escape. These spots are known as fumaroles.

Geological Maps of Volcanoes
The accumulated knowledge of each volcano is depicted on a map showing all the area's rock types, grouped into "geological formations." Each represents rocks formed by a major eruption or a group of smaller eruptions within a short period and from a single volcanic center.

Milos

Thirasia

N. - P. Kameni

Santorini

Simplified geological map of Nisyros

Simplified geological map of western Kos

Geological map of Gyali and islets

Geological map of Nisyros

Information for Hikers

The routes recommended for the Aegean volcanoes should not be considered the most scenic. Selection is based on representative geological formations, good rock exposures, and accessibility for hikers in good physical condition. Essential gear includes sturdy shoes (light hiking boots), a hat, and sunglasses, as the sun in the Aegean is strong. Also necessary are sufficient water (over a liter per person) and light food (fruit, nuts, bread, and cheese).

Spring (April–June) and fall (September–October) are the best seasons for hiking, with nature in full bloom during the former and in maturity during the latter. However, the other seasons are also suitable, as the Aegean rarely lacks a cooling breeze in summer, and the winter rarely drops below 10°C during the day, even in February, the coldest month.

Finally, those who admire and respect nature should avoid any form of vandalism. They should refrain from chipping rocks, uprooting plants, and disturbing animals, leaving the smallest possible trace and contributing ideas and support to preserve the area’s delicate balance.

MILOS

The islands of Milos, Kimolos, Polyaios and the rocky islets of Ananes are almost entirelycomposed of volcanic products.

METHANA

The Methana Peninsula hosts the youngest volcanic products of the Saronic volcanic field, which also includes Aegina and Poros.

SANTORINI

Santorini, along with Nisyros, is one of the two youngest and most active volcanoes in the Aegean region. It began...

NISYROS

Nisyros is one of the Dodecanese islands, located on the eastern side of the South Aegean. It is a cone...