220 million years ago, glowing hot molten rock masses intruded into fractures in the earth's crust in the outer parts of Hordaland. Some of these are believed to have reached the surface and formed lava flows, which since have been eroded away by wind and weather. But, most of these flows solidified into diabase sills before they got to the surface.
At Flintaneset by Finnåsvika, in the centre of the municipality, we find the most beautiful and best preserved igneous rocks in western Norway. We must go to Hawaii or Island to find as fine structures as at Bømlo.
If we study the group of islands south of Selbjørns Fjord from the air or on a sea map, we will notice that many of the islands are elongated and lie systematically in rows. The islands are divided by long sounds, for example Trollosen, Nuleia and Hjelmosen, which are oriented in a south-southeast to north-northwesterly direction.
Håøy lies centrally in one of the main shipping lanes going into Bergen --- with Håyøsund on the south side facing Meland, and Hagelsundet facing Lindåslandet. Names like Nordfarskista and Nordfarsskorane explain things. The strategic position was important in Viking times and it has been important in our days too. The beacon on Håøy can have been built in the establishment of the coastal administration in Håkon the Good’s time, about 950 AD. The defence structure of which we say remains on Håøy Summit held a critical position during Norwegian neutrality during the First World War. The defence structure was taken down in 1957.
The highest mountainous area on Stord, including Kattnakken, Midtfjellet and Stovegolvet, has more in common with the mountainous terrain on the mainland than in the low coastal landscape of Sunnhordland. The volcanic bedrock together with the erosive powers of nature has resulted in a unique plateau landscape.