Rombeporfyr

Feldspar forms light-colored crystals (mostly 1–3 cm in size) that are embedded in a very fine-grained, dark gray to light brown matrix, which is also feldspar. Strangely enough, the entire lava flow received the same type of large crystals, even though it is 30 km long, while the lava on top could have a completely different type of crystals. In this way, we find a great variety of rhomb porphyries throughout the series of lavas that were built up during more than 100 major eruptive periods.

How old is the rhomb porphyry?

In 1806, the German geologist Leopold von Buch discovered a peculiar rock at Tyvholmen in Oslo. He had never seen anything like it before and named it Rhomb Porphyry, because it was a porphyry (a volcanic rock with well-visible crystals in a fine-grained matrix), but the large crystals had the shape of skewed quadrilaterals (rhombs)126. Since then, geologists have found that the rhomb porphyries were lavas that flowed out nearly 300 million years ago and covered the area from Skien to Mjøsa (a large part of the Oslo Rift), and that this type of volcanism, with the vast extent it achieved in the Oslo Rift, is unique in the world.

Where do we find rhomb porphyry?

In the southern part of the Oslo Rift, volcanic eruptions built up more than 3 km of new crust of rhomb porphyry on top of the old one. There are areas with rhomb porphyry in Lardal, Siljan, and Skien. Andebu, Hof, and Holmestrand have preserved most of the thickest lava flows.

In Siljan, there is a lava field between Prestegårdseter and Vanebuvann. In Skien, rhomb porphyry can be found at the top of Skrehelle and along the southern side of Venstøptjern. Lardal has a significant sequence of layers in the Svarstad area. It is likely that there was a thick lava cover in this entire area, while only remnants are preserved. Rhomb porphyry-like layers are also found near Nevlunghavn.

If you walk up the forest road from Gjetrang in Lardal, you pass through a large series of rhomb porphyries that all look the same and must have been one of the longest-lasting eruptive series from the same source in the entire Oslo Rift. Perhaps these came from an area west of Breidvann and Svartangen in Lardal. There, we also find magmatic rock (larvikite) with the same appearance. (Glowing hot rock mass in the Earth's interior is called magma, but when it reaches the surface it is called lava).

Why rhomb porphyry?

This is one of the most puzzling things about the rhomb porphyries: We know they came up from great depth, below the solid crust (the mantle), and we know that larvikite and rhomb porphyry are two sides of the same coin: The magma crystallized into larvikite deep down, while what reached the surface through cracks became rhomb porphyry lava. We do not know why larvikite and rhomb porphyry formed only in the Oslo Rift, and not in other places in the world where magma has come from the mantle.