GEOPARK PORPHYRY LAND – On the trail of the supervolcanos

290 million years ago, during the Permian period, the area of today’s Geopark Porphyry Land was located close to the equator on the supercontinent Pangaea. Immense forces from deep within the earth unleashed their power here in two supervolcanic events. Multiple rivers of lava poured out of fissures and volcanic vents which constantly changed the landscape over a period of more than ten million years. Ash clouds obscured the sun. From volcanic calderas, hot mixtures of rock debris, ash and gases known as pyroclastic flows spread rapidly across the region. Their deposits solidified to form porphyry rocks (rhyolites, Greek for ‚flow‘ and ’stone‘). They have been used as versatile building blocks for thousands of years.

A caldera (Spanish for ‚cauldron‘) is a huge depression formed by the collapse of the roof over a (partially) emptied magma chamber. The Rochlitz and Wurzen calderas formed around 298 and 285 million years ago respectively over a magma reservoir up to 15 kilometres deep. In the millions of years of geological history that followed – most recently by Nordic glaciers in the Ice Age – both calderas were eroded down to the current level of the earth’s surface.

The motto „Saxony’s wealth of rocks“ refers to the heritage of the volcanoes in the Geopark Porphyry Land. Rock has characterised the architecture, industry and culture of the Mulde Valley region since the early Middle Ages. Rochlitz porphyry tuff is listed by the International Union of Geological Sciences as a globally unique building stone. During the warm and humid Cretaceous and Tertiary periods, the exposed volcanic rocks weathered to form white porcelain clay called kaolin the raw material for ceramics production. The ice ages in particular left behind gravel sands, clays and loess loams – more important raw materials in the Porphyry Land Geopark.

Captions

• Eruption columns of magma-gas mixtures rise kilometres high into the atmosphere. Volcanic ash often rains down to earth for years.
• Pyroclastic flows of rocks, ash, lava rock fragments and gases at temperatures of up to 400 degree centigrade are produced when eruption columns collapse.
• Viscous lava emerges from the earth’s surface at temperatures of up to 800 degree centigrade, often accompanied by lava rock and ash eruptions.
• Geological map of the Permian layers in the so-called Northwest Saxon Volcanic Complex illustrating the chronology of the two calderas.
• Diagram showing the development of a caldera with various volcanic formations within the caldera and in the earth’s crust as well as the current erosion level.
• The Rochlitz and Wurzen calderas are considered to be super-volcanic events in Earth’s history due to their surface area and the mass of volcanic ejecta they produced.
• The caldera collapse produces very thick volcanic rocks. In the Wurzen and Rochlitz calderas, boreholes have revealed porphyry layers at least 600 metres thick.
• Present-day bedding of the Permian volcanic rocks (volcanites) with younger cover layers of unconsolidated rocks and the position of important quarries.
• The granite porphyry in the Beucha quarry was formed from magma that cooled below the earth’s surface. It was used for the Leipzig Monument to the Battle of the Nations.
• Grey quartz porphyry (solidified pyroclastic flows) was once used to make paving stones. Today they are crushed into gravel and asphalt aggregates – here in the Lüptitz quarry.
• Reddish-brown Rochlitz porphyry tuff has been used as a building stone throughout Europe for over 900 years. The portal of St Kilian’s Church in Bad Lausick is an early example of this.