Region of Volcanoes
Volcanism in the Thermen- & Vulkanland Styria
Volcanoes are not only found around the Mediterranean or in Hawaii – Austria, too, has been shaped by volcanic activity, even though its volcanoes have been dormant for millions of years. Plenty of traces of this fiery past remain, and Styria’s thermal spas are among those that benefit from the region’s volcanic heritage.
The volcanoes in this region were formed as a result of the collision between the African and European tectonic plates. According to the latest scientific findings, the volcanoes of eastern Styria were active between 18 million and 1.8 million years ago. Their eruptive activity eventually came to an end, although this does not mean that they could never become active again.Two distinct periods of volcanic activity can be identified: the older Miocene volcanism and the more recent Pliocene volcanism.
Traces of Volcanic Activity
Austria is home to numerous examples of past volcanic activity. Some are well hidden from the untrained eye and even difficult for geologists to identify, as the rocks have already been transformed by mountain formation or weathering. Other examples reveal how magma repeatedly penetrated existing mountains throughout Earth’s history, solidifying close to the surface, or how lava flows were later buried beneath layers of sediment.
These volcanic rocks are often barely recognisable in the landscape, and it is only through quarrying that their hidden structures are revealed. Other, less obvious traces include layers of volcanic ash and the clay minerals formed from them. These provide evidence that volcanic eruptions, sometimes occurring far away, deposited ash even in Austria. Some volcanic formations, however, remain visible from a distance and can still be recognised with a little guidance, despite being heavily weathered. After all, the last volcanic eruptions in Austria took place at least several million years ago.
The hotspots of this volcanic activity are found in Burgenland and, particularly, in Styria. Here, striking, rugged rock formations occasionally rise from the gently rolling landscape. Many of these prominent elevations became sites for castles and fortresses – not least because of their commanding appearance and excellent views of the surrounding countryside. Kapfenstein and, above all, Riegersburg are well-known examples beyond the region.
Although only fragments of the original volcanic formations remain in these locations, a little time and imagination can bring the spectacular natural events of the distant past back to life.
Volcanic activity in Austria is considered to have been declining for a very long time, with elevated geothermal gradients remaining as reminders of its volcanic past. The people of southeastern Styria have learned to make excellent use of this natural resource. Today, the region’s numerous thermal spas and wellness resorts have become an integral part of the Thermen- & Vulkanland Styria.
The Older (Miocene) Volcanism
The Volcanoes of the Earlier Volcanic Period
The earlier period of volcanic activity in the region took place approximately 17 to 12 million years ago. The most significant examples of this older (Miocene) volcanism include the Gleichenberg Hills (Gleichenberger Kogeln) and the volcanic cones in Ilz/Kalsdorf, Mitterlabil and Walkersdorf, which formed between 17 and 12 million years ago.Other examples include the volcanoes near Bairisch Kölldorf in the Schaufelgraben valley, where the rare rock quartz trachyte can be found, as well as Weitendorf-Wundschuh (14 million years ago), Gossendorf (13 million years ago), Oberpullendorf and Pauliberg (11 million years ago).Of particular interest are the post-volcanic alterations of the original rock, including opalisation, alunitisation and the formation of montmorillonite and kaolinite. This was followed by a long period of volcanic inactivity until approximately 3 to 2 million years ago, when the ground in eastern Styria began to tremble once again. Glowing masses of lava, ash and smoke were once more ejected from the Earth. A new geological era began, leaving behind traces in the form of volcanic cones that remain visible in the landscape to this day.
As the African tectonic plate moved beneath the European plate, rocks began to melt. Rising magma from deep within the Earth carried fragments of African rock to the surface, forming the Gleichenberg volcano. This process began around 17 million years ago and continued for approximately five million years. For millions of years, the volcano stood in a subtropical sea, erupting and ejecting masses of glowing molten rock. Over time, most of the volcanic cone was buried beneath layers of sediment, leaving only its peaks protruding above the ground today.
Significant volcanic alterations of the rocks occurred in the northern part of the volcanic area. After the flow of magma had ceased, rising hot gases and fluids transformed the trachytes and trachyandesites into opal, trass and Gossendorf Fango, a type of therapeutic clay.
The Klausen Quarry near Bad Gleichenberg consists of a mass of red and greenish trachyandesites, intersected by a reddish breccia layer that weathers to a brownish colour and is composed of angular fragments of andesite. This layer is, in turn, overlain by grey trachyandesites. On one side, the entire formation is overlain by trachytes, indicating that the trachyandesites are older than the trachyte eruptions. At the Stahlquelle spring, near the convalescent home, large deposits of semi-opal have been discovered within the andesite.
The path through the Klause gorge in Bad Gleichenberg, starting at the spa administration office, passes through trachyte containing large phenocrysts of sanidine. To the east, this trachyte is overlain by basalt tuff from the more recent period of volcanic activity. The Konstantinhöhe inn still stands on the older Miocene volcanic formation, dating back approximately 17 to 12 million years. Along the path towards Albrechtshöhe, however, visitors encounter uphill-dipping layers of basalt tuff and basaltic tuffite, which formed around two million years ago.
The quartz trachyte quarry in the Schaufelgraben valley near Bairisch Kölldorf reveals how numerous lava flows accumulated on top of one another. The quartz trachyte contains phenocrysts of typical porphyritic quartz, sanidine and normally zoned plagioclase, measuring up to one centimetre, as well as titanium-rich biotite crystals measuring one to two millimetres. These are embedded in a fine-grained groundmass consisting of alkali feldspars and quartz. The rock is particularly unusual because of the combined occurrence of andesine, sanidine and quartz. Elliptical inclusions of clay-rich material, ranging in size from a walnut to a human head, are either rock fragments carried along by the lava or glassy portions that underwent secondary alteration. The most recent mineral formations are pyrite crusts several centimetres thick, found along fractures in the rock. This rock represents the youngest formation of this volcanic period, as demonstrated by the presence of trachyandesite inclusions.
The More Recent Volcanic Period (Around 2 Million Years Ago)
The Volcanoes of the More Recent Volcanic Period
Around 40 volcanic vents in southeastern Styria belong to this more recent Pliocene period of volcanic activity. Three major lava eruptions led to the formation of the Klöch Massif around 2.6 million years ago, the Stradener Kogel around 1.71 million years ago and the Steinberg near Mühldorf around 2.64 million years ago. Among the most striking remnants of Pliocene volcanism are the Kindsbergkogel near Tieschen (2.6 million years ago), the Forstkogel near Gossendorf, the volcanic cones near Pertlstein and Petersdorf, the Kaskogel near Gnas, the Kalvarienberg in Unterweißenbach and the Auersberg in Gniebing.
The volcanoes around Fürstenfeld and Jennersdorf, as well as the castle rock of Güssing, also belong to this period of volcanic activity. Embedded within the volcanic tuff is a remarkable geological treasure brought up from deep within the Earth: the olivine bomb. These volcanic formations can be found in the tuff deposits of the Kuruzzen Kogel near Fehring, in Kapfenstein and around Feldbach. Olivine provides valuable insights into the composition of rocks originating from depths of approximately 60 kilometres beneath the Earth's surface.
Around two million years ago, the Earth seemed to be in a state of unimaginable turmoil. In what is now southeastern Styria alone, approximately 40 volcanoes erupted, ejecting masses of glowing molten rock onto the Earth's surface. One of these was the Riegersburg volcano. Magma rose through fractures in the thin Earth's crust, leaving behind the volcanic cone on which Riegersburg Castle stands today. The ascent to the castle begins at the main parish church, which was built from blocks of basalt tuff. Between the first and second castle gates, metre-sized inclusions of Pannonian clay, fine-grained older tuffs, quartz pebbles and other rocks can be seen embedded in the basalt tuff. In the tuff wall in front of the fourth gate, known as the Lichtenecker Gate, layers of volcanic tuff can be seen lying diagonally on top of one another.
To the west of Riegersburg lies the Altenmarkt tuff area, where three main phases of volcanic eruption can be identified. The volcanic tuff found here was used as a building material for centuries.A striking example of the versatility of this easily worked stone is the stone cross carved from a single block of tuff, located at the road junction in front of the quarries near Altenmarkt.
Basalt tuff was once quarried at the old quarries in Auersberg, in Gniebing-Weißenbach near Feldbach. Large olivine bombs can be found here, both embedded in the basalt tuff and in the surrounding soil.
In Unterweissenbach, a basalt tuff quarry covering approximately 0.35 square kilometres is exposed. Two distinct phases of volcanic eruption can be clearly identified here.
The earlier phase of volcanic activity is characterised by unstratified, steeply fractured lapilli tuffs, overlain by layers of basalt fragments, clay inclusions, olivine bombs and stratified ash tuffs. The site also contains ejected fragments of crystalline rocks, trachyandesites and conglomeratic Tortonian Leitha limestones containing andesite pebbles.
Particularly remarkable is the massive volcanic basalt formation near Stein, close to Bad Loipersdorf. The village of Stein lies amidst the rolling Pannonian hills, which are composed of fine sands and clays. Most of the basalt remaining from the former volcano was extracted through quarrying operations. There were once four quarries, but these are now heavily overgrown and difficult to locate. The northern quarry reveals a particularly impressive example of columnar basalt, with beautifully formed basalt columns.
The Steinberg near Mühldorf consists of several lava and tuff formations of different ages that have merged together. Most of the Steinberg originated from the earliest explosive eruption, which occurred approximately three million years ago. The main volcanic fissure was probably located in the southeastern part of the formation. The quarry walls are divided into several terraces. A layer of plate-like, grey-black, lightly speckled softer basalt is overlain by dark grey to black, columnar hard basalt. Above this lies another layer of softer basalt that breaks apart into prismatic formations. According to the latest dating results, further volcanic eruptions occurred approximately 2.64 and 2.38 million years ago.
Near the entrance to the village of Pertlstein, there is a quarry containing basalt tuff. The exposed rock face, measuring approximately 50 by 30 metres, reveals well-stratified layers of tuff, alternating between light-coloured sandy sedimentary material and dark basaltic material. Several decades ago, basalt tuff was a highly valued building stone and was used in the construction of several houses in the surrounding area.
The finely laminated illite clays found in the LECA extraction area near Fehring are believed to be maar deposits, meaning sediments that accumulated within a volcanic crater.
The nearby quarry in Burgfeld consists of basalt tuff, while the Kuruzzenkogel is known for its basalt tuffs containing large hornblende crystals. A distinctive feature of the tuff deposits at Kuruzzenkogel is the presence of large olivine bombs.
The Kapfenstein volcano formed around two million years ago. Glowing magma rose from deep within the Earth and came into contact with groundwater, triggering a massive explosion that blasted an enormous crater into the ground. Around this crater, the ejected magma accumulated, mixed with fragments of rock carried along by the eruption. The crater filled with water, and further explosions followed, once again ejecting magma from deep within the Earth and bringing rock material to the surface. The rim surrounding the crater lake gradually grew into an impressive volcanic mountain. A second volcanic vent then formed, violently ejecting magma and rock fragments and creating a second volcano.This process gave rise to maars – enormous, water-filled volcanic craters. Over time, most of the volcanic ash deposits were washed away, leaving only those sections that lay below the original ground level preserved to this day.
The Hochstraden volcano has a different geological history. Around two million years ago, an initial explosion blasted an enormous crater into the ground. Although this eruption ejected relatively little volcanic ash, vast quantities of lava flowed out and formed a lava lake. This is a typical example of a widespread lava flow. Over millions of years, uplift and subsidence of the landscape, together with erosion and the washing away of material, shaped the volcanic formation into its present appearance. The basalt formation of Hochstraden extends approximately eight kilometres from north to south and is two kilometres wide. The nephelinite layer reaches a thickness of 80 metres and is even thicker at the summit of the Stradner Kogel.
The legendary Waltrafelsen rock formation consists of hauyne nephelinite. Thousands of years ago, a large section of rock broke away from its front face. This collapse was probably triggered by a landslide along the underlying clay layer. The resulting rockfall also destroyed a prehistoric settlement, which remains buried beneath the fallen rock masses.
The volcanic activity that shaped today’s Königsberg near Tieschen began approximately 2.6 million years ago with explosive eruptions that ejected enormous quantities of volcanic ash, which accumulated in clearly defined layers. Further explosions transported these tuff deposits over greater distances. In the Seindl area and around Königsberg, vast quantities of lava flowed across the landscape, forming the basalt formations that can still be found in the south and at the centre of the volcanic complex. The Königsberg was formed by powerful volcanic explosions. Its volcanic cone developed from vesicular basaltic scoria, sheet-like lava flows, basaltic volcanic bombs and fragments of sedimentary rock carried up from deep within the Earth.
The southern part of the Klöch volcano consists of a large volcanic crater filled with solid basanite, scoria and tuff. Nepheline basanite is exposed in the quarry. Columnar hard basalt is overlain by softer basalt with shell-like and plate-like structures. Scoria and vesicular basalt can be found at the highest elevations. The volcanic formation covers an area of approximately 2.5 square kilometres.
In the Jörgen quarry near Tieschen, inclined layers of basalt tuff can be found, intersected by fault fractures and zones of fragmented rock.
East of Gnas, on the Gnaseggberg, Pliocene basalt tuffs are exposed in several quarries.
