excursions and trips

In the Reykjanes Peninsula, in the Brennisteinsfjöll volcanic system located further east of the 2021-2022 eruptive sites, an interesting and unique composite spatter cone, discovered in 1974, has been open to tourism since 2012: Þríhnúkagígur (Thríhnúkagígur).

Direct observations of shallow magmatic plumbing systems of basalt fissures as well as associated, intact eruptive vents are rare.

Thríhnúkagígur presents an opportunity not only to see both the shallow intrusive structures and the extrusive components of a small volume basalt fissural eruption, but allows the study of the connection between the intruding magmas, the rock of the region (ie buried tephra) and eruptive products.

Descend into a volcano in Iceland: Thríhnúkagígur
Functional diagram of the formation of the caves during the eruption of Thríhnúkagígur - [C], the unconsolidated tephras were carried by the dike (dyke) and the flow was concentrated on two vents. Incorporating Tephra created a vacuum under the vent in the duct. At the end of the eruption, lava reflux left the south vent evacuated and open to the surface. Rockfall from the ceiling of the cave filled much of the area where the tephra had been incorporated and moved the cave upwards from its original location in the conduit to the overlying country rock [D] . -

Illustration by Natalie Renier, WHOI Creative Studio ©Woods Hole Oceanographic Institution.Rasbourg University PressPage 262

A scientific article, published in "Volcanica", and titled "Conduit formation and crustal microxenolith entrainment in a basaltic fissure eruption: Observations from Thríhnúkagígur Volcano, Iceland ", by MichaelR. Hudak & al.” (references in sources) explains the origin.

The excerpt below:

“The Thríhnúkagígur volcano in Iceland is a composite spatter cone and lava field characteristic of basalt fissure eruptions.

Lava drainage at the end of the eruption left about 60 m of evacuated conduit and a 40,000 m³ cave formed by the erosion of unconsolidated tephra by the feeder dyke.
Field relationships in the shallow plumbing system provide three-dimensional insight into conduit formation in fissural systems.
Both petrographic estimates and relative volumes of cave and erupted lava indicate that xenolithic tephra comprises 5–10% of the erupted volume, which cannot be reproduced by geochemical mixing models. Although the entrainment of crustal xenoliths is not geochemically significant, we postulate that this process may be common in the Icelandic crust.

The Thríhnúkagígur eruption illustrates how ubiquitous, poorly consolidated tephra or hyaloclastite can act as a mechanically weak pre-existing structure that provides a preferential pathway for magma ascent and can influence vent location."

An elevator, similar to those used in mining, has been installed and visitors are now allowed to take guided tours into its depths.
Its popularity comes not only from the uniqueness of the visit and the scale of the chamber, which could easily contain Reykjavik's Hallgrímskirkja church, but also from the incredible coloration of the mineral deposits, vivid shades of red, yellow , green and blue.

Tours only operate in the summer.

Reaching Þríhnúkagígur requires a short but uphill hike, which can be done by anyone with a reasonable level of fitness (see video).

Sources:

- Hudak, M. R., Feineman, M. D., LaFemina, P. C., Geirsson, H. and Agostini, S. (2022) “Conduit formation and crustal microxenolith entrainment in a basaltic fissure eruption: Observations from Thríhnúkagígur Volcano, Iceland”,

Volcanica, 5(2), pp. 249–270. doi: 10.30909/vol.05.02.249270.

- The websites: Inside the volcano and Guide to Iceland.

When we talk to you about Mongolia, this Asian country as big as two and a half times France, wedged between Russia and China, how do you see it?

A country of immense green and open expanses, the steppe, with a few herds, small horses, yurts, nomadic herders... or even a desert and inhospitable mountains.

If you refer to history, a name comes immediately: Genghis Kahn, gathering Mongol tribes and founder of the Mongol Empire.

But you don't think VOLCANOES!

My friend, Pierre Gondolff, who is coming back from there (80 days voyages), told me about the existence of a few volcanoes in this immense country. Let's discover them with his photos.

To the northwest of the capital Oula-Bator, the natural park of Uran Toggo is home to the extinct volcano Uran Uul. This small cinder cone rises 1,686 meters above sea level. From the top of its crater, 500 meters wide and 50 meters deep, you can see a small lake in the hollow of thick forests.

Of an age not determined with certainty, it would be closer to the Khanuy Gol volcanic complex referenced by the GVP, according to Jacques-Marie Bardintzeff, volcanologist accompanying the group.

In the Arkhangai Province, after crossing vast steppes populated by nomadic herders and where yaks and other animals graze, the group reaches the Khorgo – Terkhiin Tsagaan Nuur natural park which is located at an altitude of 2,060 meters above sea level. above sea level.

This natural park is home to two jewels of Mongolia: the lake Terkhiin Tsagaan and the extinct volcano of Khorgo.

The Terkhiin Tsagaan Lake, "the great white lake of the Terkh River", results from the volcanic eruption of Mount Khorgo. The Horgo lava flow was radiocarbon dated to around 4,930 years ago and dammed the Chulutu River, forming Lake Terkhin-Tsagan-Nuur.

Surrounded by craters of other extinct volcanoes, it is 16 km long and 4 to 10 km wide. Its area is 61 km², its average depth 20 meters.

Khorgo, an extinct volcano, rises 2,240 meters above sea level east of Lake Terkhiin Tsagaan. The summit of this volcano is covered in basalt, which was active 8,000 years ago. Its crater is 200 meters in diameter and 70 to 80 meters deep, and is surrounded by small groves of trees. It has been protected since 1965.

Sources:
- Photos by Pierre Gondolff, whom I thank.
- Friendly hello to J.M.Bardintzeff, S. Chermette and P. Thiran, who were on the same trip.
- Global Volcanism Program: Khanuy Gol - link
- Global Volcanism Program: Taryatu-Chulutu - link

In the northeast of the Italian boot, not far from the Dolomites, a fortuitous mobile home breakdown led to a volcanic discovery: our friends Dominique and Jean-Michel had to park for a few days to repair, and discovered in the south of Venice, a isolated group of steep hills of volcanic origin:

                           The Euganean Hills (the Colli Euganei)

The Euganean Hills are a series of emergences of volcanic origin, which occupy an elliptical area of ​​nearly 22,000 hectares.
The multiple aspects of these reliefs result from the meeting of the lava resulting from the fractures of the old ocean floor with the sedimentary rocks, which were raised and partially eroded.

First geological phase - sedimentary phase.

The oldest sedimentary rock is called Rosso ammonitico, it was formed between 150 and 135 million years ago, in the Upper Jurassic. This rock has many ammonite fossils from which it takes its name.

The next rock by genesis was formed between 135 and 90 million years ago, in the Lower Cretaceous, and is called Biancone. It is a white limestone composed almost entirely of calcareous shells of small microscopic organisms that have settled on the bottom of the open and deep sea.

Below we find the Scaglia Rossa, which dates back 90-55 million years, between the Upper Cretaceous and the Lower Eocene. It is a clayey limestone that often contains fossils like sea urchins and shark teeth (the sea temperature was around 25 ° C, like that of the tropics).

The last sedimentary rock formed is the Euganean Marne, 55 to 33 million years ago, from the Upper Eocene to the Lower Oligocene. It is a very clayey rock, greyish green in color, which is mainly found in the region of Teolo.

During this last sedimentary phase, about 43 million years ago, a first eruptive cycle took place.

To be continued ...

Sources: Gruppo Mineralogico Euganeo (GME), Collieuganei.it / geologia

In Yellowstone National Park, the "Old Faithful" geyser in the Upper Geyser Basin received its unique name in the 19th century due to the regularity and predictability of its eruptions.

A sample of mineralized (silicified) wood was discovered in his mouth, as evidenced by a Journal published over 60 years ago. Its radiocarbon dating has just been confirmed in a study that was just published in October 2020, on 41 dates: 1233-1362 CE.

This date corresponds to a series of severe regional droughts, which lasted for many decades, at the end of the Middle Ages climate anomaly, before the onset of the Little Ice Age.

Murray pines (Lodgepole pine) were able to thrive in the mouth of an active natural geyser only during a period of great drought that affected much of the United States, and affected the native cultures of Anasazi, Fremont and Lovelock . The remains of trees were then preserved by mineralization in an alkaline water rich in silica ... to reveal to us today the mysteries of the "Old Faithful".

Natural geysers are rare on the planet, because they require special conditions to form: heat input from active or recent magmatism, and adequate geometry of rock fractures underground to ensure episodic discharge. These parameters can change following earthquakes modifying the fractures, or by regional variations in precipitation, which feed underground reservoirs.

Climate change forecasts, which include regional droughts for the middle of the 21st century, suggest less frequent eruptions of these geysers, and even the end of their activity.

Happy are those who were able to see them in great shape!

Sources:

- USGS - Caldera chronicles - A time when Old Faithful wasn’t so faithful

- Geophysical Research letters - Yellowstone's Old Faithful Geyser Shut Down by a Severe 13th Century Drought

In the state of Minnesota in the United States, on the north shore of Lake Superior, stand cliffs 90 meters high whose geology is linked to volcanism: Palisade Head and Shovel Point.

This rock formation is part of the MRS, the Midcontinent Rift System, one of the largest continental rifts in the world.

1,100 million years old, it stretches from Kansas to Michigan, and contains magmatic and hydrothermal mineral deposits.

MRS rocks are exposed at surface only in the Lake Superior region, a member of the group of five lakes located on or near the Canada-United States border. This group of freshwater lakes consists of Lake Superior, Lake Michigan, Lake Huron, Lake Erie, and Lake Ontario.

Palisade Head and Shovel Point are formed by a rhyolitic lava flow, extruded 1.1 billion years ago.

Regional volcanism, under the influence of molten magma rising from a deep mantle plume, has resulted in large intrusions and surface eruptions of basalt and rhyolite flows, largely confined to elongated and sagging basins formed along of the rift system, in a relatively short interval estimated at 26 million years.

At the end of the major period of volcanism, around 10 million years ago, the basalt sections in the central basins of Lake Superior had a thickness of about 20 km.

After this major period of volcanism and rifting, the thermal and mechanical gradient of the mantle plume began to decrease, ending crustal extension and rifting.

The SRM then underwent regional cooling and subsidence, evolving into a series of sagging basins along the central axis of the rift system. Flowing waters and glaciers eroded the highland MRS rocks along the sides of the central basins and flowed to the center of the rift where they deposited clastic sediment. These clastic sediments, such as sand, gravel, and cobbles, accumulated over millions of years, creating layers of solidified sedimentary rock, eventually reaching a thickness of about 7 kilometers (4.3 miles) near the center of the rift basins in Lake Superior.

Around 1080 Ma, compression of tectonic origin shortened the width of the fault. The compression caused the central basins to heave along the faults bordering the basins. In parts of the rift system, this reverse movement placed older basalt on top of younger sedimentary rocks ... a huge upheaval.

Sources:

- USGS - America's volcanic past - Minnesota

- USGS - Mineral deposits of the Midcontinent Rift System

- Department of Natural resources - Tettegouche State Park

The Columbretes archipelago, located in the Mediterranean between the east coast of Spain and the Balearic Islands, 30 nautical miles from Grao de Castellón, is made up of four groups of volcanic islands: La Columbrete Grande, La Ferrera, La Foradada and el Carallot.

These islands are installed on funds 80 meters deep and cover a space of three nautical miles.

The name of the archipelago comes from Roman and Greek navigators, who noticed that the main island was inhabited by a large number of snakes (Colubraria, on Latin maps).

These islands were visited only by fishermen and pirates until the middle of the 19th century, and the destruction of snakes by arson.

Colonization took place between 1856 and 1860, with the construction of a lighthouse to guide boats navigating near the islands.

The Columbretes Islands are now part of a nature reserve, and the number of daily visitors has been limited for this reason to 78 people, who must take the guided tour in groups of 20 and at specific times.

The volcanic origin:

The archipelago is located in the Gulf of Valencia, which is a rift formed before the oceanic opening of the north-western Mediterranean basin during the Lower Miocene.

Volcanism corresponds to two cycles of different ages and nature:

The first, from the Upper Oligocene to the Middle Miocene, is characterized by calc-alkaline materials (rhyolites, trachytes, dacites and rhyodacites).

The second cycle is alkaline and more recent, in the Quaternary (1 to 0.3 Ma on Grossa Island).

The interest, in addition to its difficulty of access and its low attendance, is mainly petrological and geochemical. It constitutes a unique and representative example of alkaline volcanism in the western Mediterranean. Basanites and phonolites predominate, accompanied by tephrites, tracytes, syenites, and intermediate terms.

At the morphological level, the archipelago reveals volcanic stratigraphy, coastal erosion and sedimentation, forms of wind erosion, landslide marks, and craters.

Sources:

- Instituto Geologico y minero de Espana - Volcanismo Cuaternario de las Islas Columbretes - link

- Reservas Marinas de Espana - Islas Columbretes

- Structure and volcanism of the Valence Pit - Agnès Maillard and Alain Mauffret / 1991 / Bulletin of the Geological Society of France

- Specially protected areas in the Mediterranean - SPAMI 2010 / SP7

A glance at the inhabited areas, clinging to the walls of the caldera, we discover small white volumes ... which have their roots in the cliff, in the layers of rocks or pumice emitted by volcanic eruptions.

Various conditions pushed the inhabitants to build their houses in troglodytes or semi-troglodytes. The declivity of the land, the nature of the soil, and the proximity of the materials (black rock, red rock and pumice) - without much wood - and the lack of means of transport have favored this type of habitat, named Yposkafa.

They are houses all in length, with a narrow facade. The exterior volumes are covered with flat roofs, or dome of different shapes and sizes.

The homes designed by native people meet the needs and bioclimatic requirements: the walls are thick, thermally inert, and therefore cool in summer and warm in winter; the habitat is largely buried, with a volume calculated at the fairest and at minimal openings and positioned according to the prevailing winds to ensure good ventilation, ... everything contributes to thermal comfort. The white lime paint reverberates the sun's rays.

The colors are mainly white, and blue, which symbolize Santorini.

This habitat is found in towns that are open on the caldera, Fira, Imerovigli and Oia, but also in the small fortified towns of the interior, such as Emporio, Pyrgos Callisti, and Megalochori, where a troglodyte habitat remains under a current house.

In the cellar, the house has two small vaulted rooms, with utilitarian cavities, lit by a tiny opening and the entrance door.

Other typical structures complete the architecture, many small churches with colorful dome, mills, now converted into rent, and Kapetanospita, Captains' houses on the upper and more spacious neighborhoods, neoclastic 19 ° century favorable to the navy.

Sources:

- Petit Futé, Santorini guide

- Ankyra - Travel, architecture and other discoveries - Santorini - link

In Akrotiri, at least one frescoed room has been found in every house. This characteristic is a testimony to the high standard of living of the inhabitants. They describe sports games or cultural scenes, or everyday life, with landscapes of Santorini or Egypt.

The execution is neat, the details are precise, the natural color spectrum extended. The paintings are started on a coating) wet lime, but finished on a dry support.

The pigments used are ocher and jarosite for yellow, ocher or hematite for red, and Egyptian blue. The contours are drawn in graphite. The colors are used in a united way, without gradients.

Frescoes found in the "House of the ladies" :

The storied house was named after the discovery of frescoes with female characters and papyrus, a motif borrowed from the Egyptian cult of the goddess Hathor.

Another fresco depicts the preparation of a ceremony by the mistress of the house; A lady wears heavy breasts, belonging, according to Marinatos, to a married matron.

These female figures, of different ages, represent a female feminine community of Akrotiri, during a ceremony of offerings to the goddess.

Women wear either a colorful skirt down to the ankles and a blouse with sleeves to the elbow, or a short-sleeved dress, low-cut well below the breasts. The clothes are woven and often adorned with bands.

Frescoes of "Potnia Theron", the Mistress of animals and crocus collection in bloom.

Decorating a wall on the second floor of the house Xeste 3 / room 3, a girl, the skull partly shaved (gray head) leaving a few locks of long hair and festive clothes, picks crocus flowers on a hill.

This collection shows a significant economic activity, and the trade of saffron. This fresco represents the human level of society ... the divine level is depicted in another part of the floor, with the offering of the pistils to the great goddess, sitting on a podium and holding a winged griffin on a leash. A monkey offers him a handful of crocus, while a girl prepares another basket with flowers. No direct contact between humans and the goddess. We find monkey and griffin on murals of Knossos.

This house is considered a place of initiation and rites of passage, because of the layout of the place and the various frescoes.

Fresco of the room of the blue monkeys (room B6)

The theme of the fresco, which decorates the Beta room, is the celebration of nature.

In a landscape depicting the nature of Thêra -  water in the bottom, rocks, and trees - the monkeys are represented in a composition, in various poses, with an impression of movement corresponding to their natural attitude, suggesting that the painter was able to observe directly these monkeys, imported from the eastern Mediterranean.

In Knossos we find a similar fresco, proof that both islands were occupied by men and women of the Minoan civilization.

Sources:

- Akrotiri - Thera and the Mediterranean - by Nanno Marinatos / Edit. Militos

Historical volcanism has today created two islands in the center of the caldera, Palea Kameni and Nea Kameni.

Their construction probably began shortly after the Minoan eruption; they are the subaerial expression of activity, with the summit 500 meters above the calderal floor. The pyroclastic cone broke through the water level in 197 BC, and the last eruption of Nea Kameni dates back to 1950.

The vents are located on a NE-SW tectonic line, which controls the regional rise of magma : the Kameni line.

The evolution of these islands was made during nine subaerial eruptions, whose products are all of dacitic nature: domes, lava flows channeled or with levees, block lava flows, plumes of ash, ballistic products.
Bathymetric data reveal subaqueous / lava flows in cushions, which makes the total issued at 4.85 +/- 0.7 km³.

In the diagram below, the location and the flows cast.
Thumbnail n ° 1: 197 BC - formation of the Iera pyroclastic cone
Thumbnails 2-3 about Palea Kameni, with:
- 46-47 AD - extrusive activity and formation of Palea Kameni
- 726 AD - explosive activity in the northern part of Palea Kameni, responsible for a lava lobe in blocks near Agios Nikolaos.

The activity of Nea Kameini ranges from 1570 to 1950:

- 1570 - (1573): extrusion of the lava dome Mikri Kameni

- 1707 -1711: effusive / explosive eruptions forming the northwestern part of Nea Kameni

- 1866 - 1870: effusive activity concerning the south of Nea Kameni

- 1925 - 1928: extrusion of domes Daphne and Nautilus and castings, explosions at the summit crater and plumes of ash. Growth to the north and east fills the bay between Mikri and Nea Kameni. A plume linked to phreatomagmatic activity rises to 3,300 meters, then the activity becomes phreatic.

- 1939-1941: Extrusion of the Triton, Ktenas, Fouqué, Smith-Reck, and Niki domes, and lava flows, summit explosions and ash plumes. Typically phreatic explosions precede lava extrusion.

- 1950: extrusion of the small dome Liatsikas preceded by phreatic explosions. The activity lasted a month.

In 2011-2012, a phase of instability marked the caldera, with numerous small volcano-tectonic earthquakes of M <3.3 at a depth of 1-6 km on an almost vertical plane of 6 km in length on the Kameni line, accompanied by inflation of up to ten cm. , which probably corresponds to an intrusion of 10-20 million cubic meters under the caldera at 3-6 km depth (magmatic, magma + fluids, or tectonics origin ?).

The present activity is fumarolic and marks the twin summit craters, and hot springs, with green waters (due to the presence of Fe 2+ / colloidal pyrite, and redheads (near-surface oxidation to Fe 3+).
 

Sources:

The morphodynamic evolution of Santorini volcanic complex - 09,2019 - Paraskevi Nomikou, Konstantinos Vouvalidis and Spyros Pavlides

Geological Society memoir n ° 19 Santorini volcano - T.H.Druitt & al.1999

A little jumpto north of Santorini, towards the Skaros Shield and the Therasia Dome Complex.
 

The Skaros Shield was built inside the Caldeira generated by Middle tuff eruptions (70,000 - 54,000 years ago) and covered it. The remains of the shield can be seen in Cape Tourlos (on Thêra) and on the island of Therasia.

The remains at Cap Tourlos consist of a basal complex of domes and dacitic flows, surmounted by a sequence 300 meters thick of basalts and andesites, and capped by a welded spatter agglomerate of Upper Scoriae 2, resulting from the of the development of the Skaros Shield and an explosive andesitic eruption.

Following the Upper Scoriae 2 episode, the extrusion of rhyodacites from many vents constructed the lava domes complex on the western flank of Skaros. The remains dominate the walls of the current caldera in Therasia, in a succession of more than 200 meters thick.

Thin flows of weakly phyric andesite to Oia (Oia andesites, etc.) occupy the same stratigraphic level and were probably erupted by mouths on the flank of the Skaros Shield.

The so called Cape Riva eruption occurred 21,000 - 18,000 years ago; Its products are largely dacitic or rhyodacitic, and chemically resemble to the lavas of the Therasia dome complex.

The initial phase was Pliny, with falls and deposits of pumice preserving much of the walls of the northern caldera.

The collapse of the Plinian column occurred at the end of eruption, and produced a distinctive red-brown weld ingnimbrite. The eruption then became more violent with discharge of pyroclastic flows and unfused ignimbrite. It ended with the installation of a second welded ignimbrite on the north of Thêra.

A distal distribution of tephra on the eastern Mediterranean, towards Lesbos and the Sea of ​​Marmara, is recognized as a y-2 bed of marine ash.

Let's go back to Cap Tourlos, which has been inhabited since 1207, after the integration of the island into the Venetian Duchy of the Aegean Sea. The natural rock fortress was chosen as the seat of the capital, and numerous seigniorial and religious buildings were built there. In 1480, the island gave to Pizanias Domenico, son of the Duke of Crete as dowry for his marriage to Princess Fiorentza, daughter of the Duke of Naxos. One hundred years later, the Duchy of the Aegean Sea passed into the hands of the Ottoman Empire.

Thomas Hope describes the colony, in his book "Images of the 18th Century", as a fortress to defend itself from pirate raids; A continuous facade of houses with a few small openings protected the village from the only possible access to the east, and doors accessed by a movable wooden bridge, could close in the event of a hostile invasion.

An earthquake in 1650 caused terrible damage, after which the inhabitants abandoned this narrow and difficult environment for Fira. The move was not final until the last decades of the 18th century, due to the continuing pirate raids.

Sources:

- The morphodynamic evolution of Santorini volcanic complex - 09,2019 - Paraskevi Nomikou, Konstantinos Vouvalidis and Spyros Pavlides

- Geological Society memoir n ° 19 Santorini volcano - T.H.Druitt & al.1999

- Santorini.net - Skaros, the capital of Thêra under the Venetian occupation - by Clairy Palyvou, architect, professor emeritus of the Aristotle University of Thessaloniki