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Earth of fire

Actualité volcanique, Articles de fond sur étude de volcan, tectonique, récits et photos de voyage

Publié le par Bernard Duyck
Publié dans : #Historical eruptions, #Volcanic news

One of the most important finds ever made in the western region of Lesvos and its petrified forest has been unearthed as part of excavations carried out in the area of ​​the intersection leading to the Natural History Museum and the petrified forest of Sigri.

"A fossil has been identified which is one of a kind. It is an entire tree with its branches almost intact, as when it died. We also found numerous leaves around the main find as well as its root system. The whole was protected by volcanic ash from an eruption dated 23 million years ago "says the director of the Sigri Forest Museum.

Lesbos - recently found petrified tree, 23 Ma old - photo Stonisi.gr

Lesbos - recently found petrified tree, 23 Ma old - photo Stonisi.gr

The island of Lesvos is located in the northeast of the Aegean Sea. Its surface area is 1,632 km², including 320 km of coastline. It is the third largest Greek island. The Strait of Mytilene separates it from Asia Minor and the Turkish coasts, to which it was attached until the end of the Paleolithic, before the end of the last Ice Age.

Lesvos is a hilly island with higher peaks: Lepetimnos (968 m), Olympos (967 m) and Psilokoudouno (914 m).
It is characterized by volcanism in the Miocene and was created from two volcanoes that are no longer active. The original volcanic cones have disappeared under the water, leaving two bays: the largest bay is the Gulf of Kalloni and measures approximately 20 x 8 kilometers and to the southeast the smaller bay, the Gulf of Gera, measures approximately 10 x 5 kilometers. Researching with satellite images, scientists discovered 5 additional craters, near Stipsi, Vatousa, Antissa, Agra and Mesotopos.

During drilling, they determined that the geothermal heat at depths of up to 1,000 m was about 100 ° C. This geothermal energy is used, among other things, to heat greenhouses. There is still latent volcanic activity on Lesvos. Hot springs rise to the surface in five places, in Eftalou, Thermi, Gera, Polichnitos and Lisvori.

The island is largely made up of volcanic rocks. Only the area to the south, above Plomari, has pre-volcanic rocks: slate, gneiss and quartz, about 300 million years old.

The island of Lesbos, and its two gulfs - photo eol.jsc.nasa.gov

The island of Lesbos, and its two gulfs - photo eol.jsc.nasa.gov

Lesbos geosites (Doc. Geomorphology of Dodecanese)

1: Petrified Forest park; 2: Nissiopi park; 3: Sigri park; 4: Plaka park; 5: Skamiouda park; 6: fossiliferous site of Antissa; 7: Mesotopos fossil site; 8: fossiliferous site of Eressos; 9: Rougada fossil site; 10: fossiliferous site of Molyvos-Petra; 11: Lapsarna fossil site; 12: Chamandroula fossil site; 13: Gavathas fossil site; 14: fossiliferous site of Vatera; 15: Metohi fossil site; 16: Akrohiras fossil site; 17: Mavros Lofos fossil site; 18: Sarakina fossil site 19: Lepetymnos caldera; 20: Vatoussa caldera; 21: Agra caldera; 22: dome of Mesotopos; 23: dome of Ipsilou; 24: volcanic neck of Petra; 25: basaltic organs of Panagia; 26: Alifada's dyke; 27: Filia dyke; 28: basalt columns of Anemotia; 29: dome of Eressos; 30: basaltic organs of Pelopi; 31: basaltic organs of Molyvos; 32: Achladeri ignimbrites; 33: Gera thermal spring; 34: thermal spring of Thermi; 35: Lisvori thermal spring; 36: thermal spring of Polichnitos; 37: thermal spring of Argenos; 38: thermal spring of Eftalou; 39: spherical erosion forms of Vatoussa; 40: cliffs of Lapsarna; 41: gorges of Voulgaris; 42: Antissa cave; 43: Taxiarhon cave; 44: cave of Mihos; 45: cave of Alifada; 46: Faneromeni folds; 47: fault in the Gulf of Gera; 48: Larsos fault; 49: Agia Paraskevi fault; 50: Olympos tectonic window; 51: Vatera fault; 52: old Moria quarry; 53: mines north of Lesvos; 54: magnesite mines.

Lesbos geosites - Doc. Dodecanese geomorphology

Lesbos geosites - Doc. Dodecanese geomorphology

Lesbos, Sigri petrified forest - photo Lesvosgeopark

Lesbos, Sigri petrified forest - photo Lesvosgeopark

The petrified forest of Lesbos :

The Sigri pyroclasts, the densest in the west of the island, are covered by several layers of Polichnitos ignimbrite, dated to around 17.0 Ma.

The fossilized tree trunks of the Petrified Forest appear in the thick "Sigri pyroclastic sequence".

This formation consists of several horizons of pyroclastic flows, mainly pumice flows, mud flows, debris flows and stream conglomerates interspersed with pyroclastic deposits.

The magma intrusion caused the western flank of the Vatoussa volcano to collapse in an avalanche of debris, leaving a horseshoe-shaped caldera.

The explosive phases seem to have been very strong, as shown by the distribution of the eruptive products. The pumice phase was probably ejected to a great height before falling on the sides of the volcano and its surroundings. A blast of hot gas preceded the avalanche of debris, flattening thousands of trees which were later covered with pyroclastic material.

This caused the amputation of the majority of the trees one or two meters above the ground surface. Today, the upper parts of tree trunks lie horizontally, buried in pyroclastic layers. The comparable catastrophic 1980 eruption of St. Helens in the United States helps to understand the mechanism of the Vatoussa volcano eruption and to give some idea of ​​its effects.

The rapid covering of tree trunks, branches and leaves led to their isolation from atmospheric conditions. Along with the volcanic activity, hot solutions rich in silica penetrated and permeated the volcanic materials that covered the tree trunks. Thus, the major process of fossilization began with an exchange, molecule by molecule, of the chemical constituents of the plant, from organic to inorganic (quartz), while maintaining the tissue structures.

 

Sources:

- Stonisi - Lesvos environment - discovery of a fossilized tree, with its branches, 20 million years old

- Géomorphologie - Assessment, protection, and promotion of geomorphological and geological sites in the Aegean area, Greece - N. Zouros.

- The Plaka petrified forest park in western Lesvos - by Zouros N. & al.

Lesbos - Pelopi basalt organ formed due to the rapid cooling of lava intrusions in soft pyroclastic materials on the flanks of the Leptymnos volcano (geosite n ° 19) - photo Lesvosgeopark

Lesbos - Pelopi basalt organ formed due to the rapid cooling of lava intrusions in soft pyroclastic materials on the flanks of the Leptymnos volcano (geosite n ° 19) - photo Lesvosgeopark

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