Rock type
A rock type is a natural combination of one or more minerals, for example granite, gneiss or limestone. Bedrock is the solid rock as a whole, and the rock type tells you what the rock consists of at a specific place. Among other things, the rock type affects how demanding it is to blast, how stable the rock is and whether there is a radon risk.
Rock types fall into three main groups based on how they formed. Igneous rocks are solidified molten material, such as granite, gabbro, syenite, larvikite and basalt. Sedimentary rocks are deposited in layers and hardened, such as sandstone, limestone, shale and alum shale. Metamorphic rocks are transformed under high pressure and high temperature, such as gneiss, mica schist, quartzite, marble and amphibolite. Gneiss and granite dominate the Norwegian basement rock in the bedrock.
In construction, the rock type determines rock quality and therefore costs. Granite and gneiss are hard and take longer to drill and blast, while schist and limestone are weaker. Fracture density says a lot about how stable an excavation pit, a rock cut or a tunnel will be. Mica schist and shale weather and give poorer rock quality, and swelling clay in fractures weakens the rock further. The rock type also decides whether blasted rock can be used as crushed stone or natural stone. Larvikite is Norway's most important natural stone export.
Some rock types carry their own risks. Granite, pegmatite and alum shale can have a high uranium content and give high radon susceptibility. Alum shale is also acid-forming and needs special handling of excavated material when you dig in it. Limestone and marble dissolve in water and can form cavities (karst), which matters for foundations in some areas. However, the rock type tells you nothing about the superficial deposits above the rock, and that is where quick clay is found.
Norges geologiske undersøkelse (NGU) (the Geological Survey of Norway) maps rock types in the bedrock maps N250 and N50. Each area is coded with main rock type, age of formation and main tectonic division, and the areas can be coloured by rock type or age. The line between two rock types is called a rock type boundary. N50 is more detailed, but does not cover the whole country as well as N250.
In Placepoint, rock types are in the map layer Bedrock under Geology. You can show the areas by rock type, age of formation or tectonic division as separate sub-layers, and a click on the map gives the Main rock type for that point. Use it in early site analysis together with superficial deposits and ground investigations from NADAG.
How the dataset appears on the map
The dataset is part of the map layer Geology as Bedrock:
From Placepoint's glossary: Rock type
More information: NGU: about bedrock, NGU: bedrock maps, Store norske leksikon: rock types
Norwegian: Bergart.
Frequently asked questions
What is the difference between rock type and bedrock?
Bedrock is the solid rock beneath the superficial deposits, seen as a whole. The rock type is what the rock consists of at a specific place, for example gneiss or limestone. A bedrock map therefore shows many rock types, separated by rock type boundaries.
Which rock types are most common in Norway?
Gneiss and granite dominate the Norwegian basement rock. There are also large areas of schist, limestone and other sedimentary and metamorphic rocks, especially in the mountain range and around the Oslo Region (Oslofeltet).
Why is alum shale a problem in development?
Alum shale can have a high uranium content and gives high radon susceptibility. It is also acid-forming when it comes into contact with air and water. Excavated material must therefore be handled and deposited separately, which increases project costs.
Where can I find which rock type lies under a site?
Norges geologiske undersøkelse (NGU) publishes bedrock maps in N250 and N50. In Placepoint, the map layer Bedrock shows the rock type areas, and a click on the map gives the main rock type for that point. Local conditions such as fractures and weathering still need on-site investigations.