Prästberget 1

Masses of molten rock became red granite and dark gabbro

Two melts pushed their way up here from deep within the earth at the same time but separately. The magmas were of completely contrasting compositions and so behaved in different ways.

When in due course the magmas cooled and became solid they formed rocks. One of the magmas was dark, almost black and became gabbro. The other one was red in colour and became granite.  

Pulses of magma filled the chamber

Pulses of warm magma were continuously adding to the magma chamber from below. Magma that had already solidified could then heat up and become liquid again. The magmas interacted in batches, forming different patterns depending on whether they were liquid at the same time or not. Gabbro and granite melt at different temperatures. Gabbro is several hundred degrees warmer when melted compared to granite. The difference in temperatures between the two magmas meant that they would not fully blend everywhere.

Magma mingling – "pillows" of gabbro in granite

In many places you can see patterns of the gabbro in the granite, looking like pillows. This is called magma mingling and occurred at the time when both magma types were partially liquid at the same time.

The magmas may have been cold, hot or tepid at the time of the encounters. Around the gabbro you can sometimes spot a darker outer edge; this appeared when the warmer gabbro cooled off quicker in contact with the colder granite.

Rapakivi granite – a rare granite that crumbles easily

The rapakivi granite is odd in appearance. Scientists have different theories for this but there is no consensus. The rock is most often meaty red with large crystals and a yellow-white shell. The colour of the crystals and the shell stems from two kinds of the mineral feldspar. The semi-transparent, grey grains are the mineral quartz.

Rapakivi granite is quite rare in Sweden and nowhere else are you able to study the rock as well as in Ragunda.

Cavities filled with large crystals

In the hill there are cavities filled with large crystals. The cavities were formed when hot magma rich in gases moved from deep within the earth up towards the earth´s surface. On the way the gases were released and formed bubbles, a bit like when you open a bottle of sparkling water. The bubbles contained various solutions of minerals depending on the composition of elements in the rock. As the magma cooled the bubbles were stabilised in the form of cavities. In the openings of the cavities the minerals grew at their own pace. Once they had hardened they formed large crystals. The more time they had to grow the larger the crystals became.

Weathering worns down rocks

Rock is continuously worn down by, for example, the sun, wind, and water, meaning that rock surfaces are in constant stage of change.

Here you can see traces of the weathering rock. In the border area where gabbro encounters granite you can see, in some places that the gabbro has abraded more than the granite. One reason for this difference is that the rocks contain minerals that weather more or less easily. Quartz is in general a mineral that does not weather easily.