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Rocks for coast protection

Rocks are often used as the outermost layer in protection of the coast at our latitude because they are easy to obtain. Sea boulders have been used for many years, transported to Denmark during the Ice Age, so these boulders are smooth. Rocks have a specific gravity of about 2,600 kilos per cubic metre.

The pale rock on display is a so-called quartzite or quartz arenite which consists mainly of the mineral quartz, the same material in, for example, rock crystal and flint. The almost pure quartz sand is like sand from the west coast. It was turned into stone by mechanical/chemical processes deep within the earth. Only a thin section can reveal the precise mode of formation.
The small holes on the surface were probably left by less stable minerals which have disappeared from the surface of the rock after its formation, which presumably occurred in the sea, which is also responsible for its smooth shape. The rock could have come from Sweden or Bornholm and be a so-called Balka sandstone.

Natural rocks are also used today, but only from quarries, that is, they were blasted out of cliffs and transported to where they were laid down.
There are two obvious advantages in this type of rock. The first is that it is possible to choose the type of rock to be used. Norwegian norite is therefore often used as it has a very high specific gravity (3,100 kilos per cubic metre) and it is black.

The second advantage of quarry rocks is that they are rough, which makes them more stable, unlike the smooth sea rocks. Smaller and lighter rocks can therefore be used, helping to reduce the price.
The dark rock is a norite, which is an ultramafic rock. This means that it is a magma rock with more than 90% dark minerals (olivine, pyroxene and hornblende) by volume.

Magma rocks arise via the partial melting of rocks in the top layer of the earth’s mantle or the bottom layer of the crust. This process is triggered by a rise in temperature, a fall In pressure or the addition of water and other volatile compounds. This particular rock is from a quarry in Norway.