Sten – Dalarna

I'm a block of porphyry that comes from Älvdalen in Dalarna, and that's why people usually call me Älvdal porphyry or Dala porphyry (geologists usually think it should be called Dala porphyry, but common people probably recognize the name Älvdal porphyry more, myself I don't really care ). Porphyry is a rock type that consists of white (sometimes light green or light pink) millimeter-sized grains of quartz and feldspar in a dark dense matrix, where no mineral grains can be discerned at all, at least not to the naked eye. The matrix can vary in color from red or purple – porphyry comes from the same word as purple– to chocolate brown or almost black.

Like the granites here in the stone circle, I was formed from an acidic - silica-rich - magma, i.e. a red-hot mass of molten rock. But while the granite magma could not reach the earth's surface and solidified deep in the earth's crust, my magma penetrated all the way to the earth's surface and spewed out as lava during a volcanic eruption. And what volcanic eruptions there were! Really explosive eruptions on land with hot gas and ash clouds that spread for kilometers in all directions. It was lucky that these volcanic eruptions in Älvdalen occurred 1700 million years ago, when there were no plants and animals on land that could be affected; if it had happened now it would have been a huge disaster.

The bright scattered grains of quartz and feldspar are such grains that had already begun to crystallize from the magma deep down in the magma chamber before the eruption itself, and then followed the magma to the earth's surface. The dense matrix is lava, or perhaps rather volcanic ash (which is fine-grained powdered lava) that solidified at the surface just after the eruption. It happened so quickly that no real mineral grains had time to form, only volcanic glass (or very tiny mineral grains) that was welded together into a hard mass of rock, with the previously formed larger mineral grains embedded in it.

Seen like this, I may not look very remarkable, but if you polish me, I can become truly magnificent, with the bright specks like stars in a dark sky. Not that I want to brag, but in ancient times porphyry was said to be a rock for emperors and kings. Only Egyptian pharaohs and Roman emperors could afford objects made of hard porphyry. Here in Sweden, at the end of the 18th century, King Gustav III initiated a porphyry factory in Älvdalen, where magnificent urns and vases were made from blocks of porphyry for the court or wealthy noblemen. A little later, the porphyry works was bought by Karl XIV Johan. Across the water here, outside Rosendal castle which he had built, stands a huge vase of Garberg granite, a porphyry-related granite from Älvdalen, which was manufactured in Älvdalen. Over time, the first porphyry works burned down, but in 1897 a new factory was built, called the New Porphyry Works. It is still there and was used until 1985, although then mostly to make smaller objects such as boxes, jewelry or butter knives. Now it is a museum that is shown during the summer (see https://www.rockdale.se/#/). You can also take the opportunity to visit the Porphyry Museum inside the Älvdalen itself (Rockdale - Nature and cultural experiences in Älvdalen.). https://www.rockdale.se/#/).
You are welcome to visit the museum in Älvdalen.Rockdale – Natur- och kulturupplevelser i Älvdalen.).

Geologist Åke Johansson
Swedish Museum of Natural History