The name eclogite comes from Greek, and suggests something like "first choice". This is related to its beautiful appearance. The nutrient-depleted rock type is slow to weather, such that eclogite bedrock is left standing as peaks in the terrain. This is why Mt. Eldsfjellet has become a landmark on the coast.
The world famous geologists Carl Fredrik Kolderup and Pentti Eskola mentioned the eclogites on Holsnøy early in the 1900s. At that time it was not known how the rock type was formed. Now we know that it is created in areas where the rock gets pressed down to quite deep depths in connection with collisions that force one continental plate under another.
The material forming the eclogites was anorthositic and mangeritic rock types originating in the lower part of the earth's crust. They were formed at about 30 kilometres depth and at temperatures of 800-900°C and were therefore well prepared for an even deeper geological dive: During the collision between Greenland and Norway about 420 million years ago, they were pressed down to more than 50 kilometres under the earth's surface. This submersion caused the older minerals to be replaced by new. The molecular structure also became denser. Therefore, the rock type shrank in size by about 10-15 percent, and this made it heavier than before. Geologists believe that this shrinking caused movements in the bedrock, and that the shrinking thereby triggered some of the deepest earthquakes that have ever occurred on earth.
Geologists also believe that the warm fluids that seeped into the bedrock were important for the creation of the eclogite. Only where fluids streamed into the bedrock, namely, was eclogite the result. The fluids seeped in most easily where the bedrock because of shrinking got fractured and moved. Where there is most fracturing, thick zones of nearly pure eclogite are found. A good example of this can be seen on the southeast side of Sætrevika.
If the homesteads there did not get in the way, there could perhaps have been industrial mineral extraction on Holsnøy. The mineral rutile, which is found in eclogite, can namely be used instead of ilmenite as a source of titanium.
Titanium is used for the white colouring in paint and for light-metal alloys in bicycle frames, planes and other products. The production of titanium from ilmenite, such as is carried out i.e. in Tyssedal in Hardanger, is more polluting than the use of rutile. During the formation of eclogite on Holsnøy, an environmentally dangerous manufacturing process occurred 420 million years ago. Then, ilmenite and feldspar were unstable under the high pressures, so that they were altered to the minerals rutile and garnet. Rutile, which can only be seen under a microscope, is a titanium oxide that both forms small needles in other minerals and free standing grains of rutile. Rutile is found i.e. in a mica-rich bedrock by Landsvik, and in the ecologites north of the shop in Sætrevika.
