Today our fancy takes us to the NE of Spain. Just to the corner where the Pyrenees mountain range meets the Mediterranean Sea, to the province of Girona in Catalonia. For millions of us European tourists the Mediterranean Costa Brava with its golden beaches is a household name. As in so many other touristic spots in the world, one could ask, how many of the sun worshippers would have known that, some 60 km inland, there is a volcanic field to be visited?
Almost halfway between the Costa Brava and the Pyrenees is the beautiful volcanic landscape of the “Zona Volcánica de la Garrocha” or, in Catalan language and officially: Garrotxa. Having erupted in the Holocene, the Garrotxa is an active volcanic field. It is part of the Catalan Volcanic Zone which includes the plain around Olot and stretching down towards north and west of Girona and the river Ter.
In the 1980s, when it became all too obvious that the volcanic landscape was under dire threat to be mined away, the “Parc Natural de la Zona Volcànica de la Garrotxa” was created which today includes the most important parts of the Garrotxa Volcanic Field (GVF). Here, pleasant forested volcanic hills alternate with lush green valleys, dotted with the historical stone buildings of Catalan villages. To the north of the river Fluvià, however, the landscape changes radically. Abrupt cliffs and narrow gorges mark the entrance to the pre-Pyrenees, here called the Alta Garrotxa. Those rough mountains are mostly sedimentary rocks folded up by compressive forces in the collision of the Iberian and Eurasian plates.
WHY ARE THERE VOLCANOES IN SPAIN?
To understand why there are volcanoes here in Spain of all places, we’ll have to start with the collision of the Iberian and Eurasian Plates during the Palaeogene. This caused the folding up of the Pyrenees, as well as manifold stretching and breaking of the continental crust. The lithosphere responded with rifting, perpendicular to the collision pressure, and thus formed the European Cenozoic Rift System. The great grabens and basins of the ECRIS run between Southern Spain and the North Sea, branching off to France and Eastern Europe. Examples of the resulting tectonic basins are the Rhine Grabens in Switzerland/Germany or the Limagne Graben in France. The part affecting the Catalan Volcanic Zone is the Valencia Trough, off-shore of NE Spain.
In north-eastern Spain the propagation of magma is controlled by the main NW-SE trending faults – in this case the Amer and Llorà faults. Most of the Garrotxa eruption centers lie between those two lines. Eruptions occurred in places of weakness, where smaller faults and fissures cut across the larger faults. It is thought that the upwelling of mantle materials lead to a rising of the Mohorovičić discontinuity (Moho, the crust-mantle boundary) to a depth of only 25 km to 30 km. Mafic magmas could then rise and erupt through the stretched, thinned, lifted and broken crust.
THE GARROTXA VOLCANIC FIELD
The Garrotxa Volcanic Field is the youngest of four in Spain. It was active from the Middle Pleistocene (590–700 ka Sant Joan les Fonts lava flow) to the latest dated eruption (Croscat 11,500 [±1,500]) years ago. However, stratigraphic evidence suggests that more recent eruptions have occurred.
The northern GVF around the town of Olot has the most volcanic edifices. A few larger and more complex volcanoes lie more to the south towards Girona city. Olot itself is entirely built on volcanic deposits. In the pre-volcanic past, the area around Olot was a mountainous territory with deep V-shaped valleys, part of the Pre-Pyrenees. With the eruptions, volcanic materials filled up the river valleys, so that in some places the volcanic deposits exceed a thickness of 150 m. This way several of these valleys have been blocked which, over time, resulted in more than a dozen barrage lakes (“barratge”) and wetlands. Overall, the areas between cones, mountain peaks and ridges have become rather flat, referred to as the plain of Olot. A considerable part of this area – some 25 km² – is covered by lava flows. More volcanic activity can be seen SE of the plain, along the river Ser in the tectonic valley below the fault escarpment of the Corb/Finestres mountain ranges, and the valleys of the rivers Llémena and Adri.
What makes the Garrotxa volcanoes so interesting is the great variety of volcanic edifices and deposits. They are as diverse as were the geologic and hydrologic conditions at the times of eruptions. The entire field has over 50 volcanic cinder and scoria cones, many craters, tuff rings, maars, and more than 20 basaltic/basanitic lava flows. The cones were built during short-lived monogenetic eruptions, i.e. lasting a few days to some months. The eruptions were mostly of low intensity but, besides larger lava flows, also emitted lots of scoria, lapilli and volcanic bombs. Phreatic or phreato-magmatic eruptions produced the maars and tuff rings as well as large amounts of tephra.
The average total volume of extruded magma in each eruption was quite low, though, between 0.01 and 0.2 km³ DRE (dense-rock equivalent). Petrological and geochemical studies find that the region consists of a suite of intra-continental rocks: leucite basanites, nepheline basanites and alkali olivine basalts. These compositions are very similar to those in other European Cenozoic volcanic areas and are quite homogeneous throughout the entire GVF.
Also, it has been calculated that magmas ascended very fast: from the source to the surface there was no time for magmatic differentiation. The study of ultrabasic xenoliths contained in the ascending magma allowed to estimate that the rising magma had a velocity of 0.2 m/s. Taking into account that crust thickness in this area is around 30 km, the ascent of the magma from the lower crust to the surface took only one and a half day.
Diversity in Eruptive Styles
All the volcanic edifices show an unusual diversity of eruptive styles, which makes the GVF something special compared to other monogenetic fields. There is evidence of vent migration during the same eruption, and they had alternating Strombolian and phreato-magmatic phases. This may be due to variations in the stratigraphy and structure of the geological basement beneath each volcano: The volcanoes erupted through sediments of limestone, mudstone, sandstone and/or conglomerates. Each of such layers would have different characteristics regarding amounts and hydraulics of the available groundwater. Which, in turn, would have an impact on the particular way the rising hot magma came to eruption.
This is nicely demonstrated (for example) in the eruption sequence of the Can Tià maar: It began with a short phreatomagmatic eruption, the deposits of which contain rock fragments (lithics) from the Bellmunt Formation, ~300 m below the surface. The deposits of the following magmatic phase contain grey sandstone lithics from the Rocacorba Formation, the top layer of the basement in the graphic above. Still further up in the Can Tià, sandstone fragments from the Folgueroles Formation can be seen. Towards the end of yet another phreatomagmatic phase, lithics from the Bellmunt Formation dominate again.
Throughout history small watch or defense towers have been built on top of the cones to survey the surrounding terrain, churches or hermitages have found shelter within their craters. Unfortunately, many of the cones were not only used to build towers on their tops, but up to the early 1990s they were also widely quarried to extract construction materials. Moreover, the quarried cones were then often used as rubbish dumps and land fills.
Eventually this had sparked a flood of protests in the 1970s, demanding to protect the volcanic field. Now several restoration projects take advantage of the former quarries: they have been cleaned up and made publicly accessible. Not only is this a bonus for the tourist industry – volcanologists also find here a great opportunity to literally see the interior of volcanic edifices! This has already been used to study internal structures and associated volcanic processes in situ.
A shining example of a quarry turned “window into the past” is the Croscat volcano, which had been industrially quarried. Like a slice of cake, a wedge from top to bottom has been cut into its NE side to extract pyroclastic gravels. There is a small visitor information near the quarry that promotes a visit with the slogan “Croscat, the open volcano” (“El Croscat, un volcá obert!”). Also, a lava tongue emerges from the volcano on which one can see different morphologies associated with flowing lava. The Croscat is the largest volcano in the Iberian Peninsula, with a height above sea level of 786 m and ~170 m elevation from the plain. It is also the youngest, since its last eruption is the one dating back to 11,500 years ago.
The Croscat volcano had mainly Strombolian eruptions; to the greatest part its cone is made up of thick coarse lapilli deposits. These are overlain by a phreatomagmatic deposit that extends for several kilometres to the E. Its last activity was the effusive eruption of a basanite lava flow that breached its western flank. The flow runs for 10 km to the W and has an average thickness of 10 m. Today this rough lava flow with its numerous “blisters” (tossols, see below) is covered by the famous beech forest known as “La Fageda d’en Jordà”.
Volcà Santa Margarida
The Santa Margarida cone with the (originally) Romanesque chapel in its crater is probably the most visited and photographed volcano in the GVF. With its 350 m wide and 70 m deep circular crater the shape seems to conform best with the visitor’s idea of a perfect volcano.
However, the Sta. Margarida just masquerades as a cinder cone. Large parts of it are tephras from the nearby erupting Croscat cone. It is not even formed exclusively of volcanic materials, its southern inner rim contains pre-volcanic Eocene rocks. This is because phreatomagmatic explosions have started the eruption deep within the ground, where the rising magma had encountered an aquifer. So, technically, the Santa Margarida volcano is a maar or diatreme-maar.
Until recently it was thought that the Sta. Margarida stemmed from a different eruptive period then the other volcanoes around it. Now it has been found that this volcano, together with the Croscat and a third cone (La Pomareda), represent just different phases of the same eruption. All three of them lie along a 3 km long NW-SW oriented eruption fissure, where the activity progressed from the S end northwards.
The eruption began with the vent-opening blast that created the large explosion crater of Santa Margarida in the Eocene basement. This also generated a massive pyroclastic flow deposit (on which later the Croscat volcano was built up). This phreato-magmatic phase was followed by short Strombolian activity.
The eruption moved on to the central and northern parts of the fissure where it produced basaltic lavas, massive spatter and occasionally welded scoria agglomerates. These deposits formed the beginning of the Croscat and La Pomareda volcanoes. From then on, the eruption was concentrated in the central part of the fissure and changed from Hawaiian (fissural) to Strombolian (central conduit) activity. The Croscat and Santa Margarida fissure erupted a total volume of magma (DRE) in the order of 0.2 km³.
Castellfollit de la Roca
Two lava flows, one from the west and one from south, somehow got stuck in a tight place on their way down the valleys eastwards. They ended up one on top of the other, blocking the valley for all traffic. Later, the river Fluvià came from the west, the river Turonell from the south. Both wanted to reclaim their former beds and travel east, much like the old lavas did, to eventually reach the Mediterranean Sea.
However, seven km NE of the modern town of Olot, both rivers encountered these darn lava flows. The river Fluvià began gnawing on their northern edge to work itself through anyway, the river Turonell nibbled on the obstacle from its southern side. Both took advantage of the weak boundary between lava and softer Eocene basement rocks. Finally, they united and reached the sea with combined forces. Between them they had left standing a magnificent pointed ridge of double-layered basalt flow, cut out of the landscape, showing off on its northern vertical walls the exposed interior of two lavas.
The narrow, 1 km long and 50 m high basalt ridge proved a good strategic point to build a castle on in the 11 century. Later the locals found this a great place for their houses, until the surface was tightly packed. Today this is the very photogenic village of Castellfollit de la Roca.
Image left: Graphic of the Castellfollit de la Roca exposed cliff face. A lower and an upper lava flow can be made out clearly, as they are divided by a layer of soil/gravel (Palaeosoil) that has accumulated in dormant time between eruptions. (Annotations edited in English.)
The two lava flows were deposited on an old river terrace some 217 000 and 192 000 years ago; they are not the oldest, but today surely some of the most impressive. Originally, the ridge must have been much longer, as it has been receding for those many thousands of years. The ridge is suffering landslides and rockfalls every year due to erosion by the river Fluvià and frost weathering (freeze-thawing), which is all the more effective given the columnar jointing. Also, devastating earthquakes likely do their bit, like the two that demolished Castellfollit in 1426 and in 1428.
An interesting, rare, but in the GVF quite frequent feature are the “tossols”. These phenomena are found only in very few places in the world; to date they have only been described from Iceland and the northern USA. Tossols are between 4 and 15 m high mounds that form at the tips of still very hot lava flows when they encounter water-logged ground. The water goes into steam and “blisters” up the soft lava, often breaking its partially solidified surface.
Thus, very rough areas were created, dotted with the tossols and strewn with their numerous basalt fragments. Such areas have largely remained free of agriculture or human settlement, providing the ground for the lovely old beech forest. In the GVF more than 50 tossols have been counted in three lava flows. Technically, they are “rootless volcanoes” – like hornitos, they are not directly connected to a magma source.
WHY MONOGENETIC, AND CAN THEY ERUPT AGAIN?
This area in NE Spain has been tectonically extremely active during the last 12 million years. Volcanism began in the Empordà region near the coast to the east, then moved south towards the La Selva region and eventually north-west into The Gironès and Garrotxa regions. The various places of eruptions depended on the behaviour of the underlying fault systems. Wherever the crust gave after new faults appearing, high seismicity followed and magma worked its way up through cracks and fissures.
However, with faults propagating ahead, their older parts became inactive, cracks closed and volcanic activity there stopped. Instead the magma would find new pathways in the newly active region. This is why the old volcanoes will not erupt again, their magma supply is cut off forever. They erupted only once, and therefore are called monogenetic.
Nevertheless, the faults in the region are still active. Seismicity is still high and strong earthquakes have occurred in historical time. Scientists have calculated that an eruptive episode took place every some tens of thousands of years, up to the early Holocene. So, there is no reason to think that all volcanism has ended. The next eruptions could be nearby or further afield, in ten years or in tens of thousands.
Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.
Enjoy! – GRANYIA
SOURCES & FURTHER READING
– GVP, Olot Volcanic Field
– Volcano-structural analysis of La Garrotxa Volcanic Field[…] (2015, paywalled)
– Structural control of monogenetic volcanism in the Garrotxa volcanic field (2014, paywalled)
– La Garrotxa Volcanic Field of Northeast Spain (book, 2016)
– 7th Intl. Maar Conference Field Trip, Olot, 2018 (PDF)
– Evolution of the European Cenozoic Rift System:[…] (2004, paywalled)
– Ranges and basins in the Iberian Peninsula:[…] (2006)
– Geozona 210 Volcans Santa Margarida, Croscat i Rocanegra (<2012, PDF)
– Volcanoes-A Field Guide to La Garrotxa Volcanic Zone (2012, PDF)
– Museu dels Volcans, Olot
– La Garrotxa Volcanic Zone Natural Park