The Oslo Graben or Oslo Rift is a

graben In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic conte ...

formed during a geologic

rifting In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben wi ...

event in

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...

time, the last phase of the

Variscan orogeny The Variscan or Hercynian orogeny was a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea. Nomenclature The name ''Variscan'', comes f ...

. The main graben forming period began in the late

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...

, which culminated with rift formation and

volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...

, with associated

rhomb porphyry Porphyry ( ) is a textural term for an igneous rock consisting of coarse-grained crystals such as feldspar or quartz dispersed in a fine-grained silicate-rich, generally aphanitic matrix or groundmass. The larger crystals are called phenoc ...

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or ...

flows. This activity was followed by

uplift Uplift may refer to: Science * Geologic uplift, a geological process ** Tectonic uplift, a geological process * Stellar uplift, the theoretical prospect of moving a stellar mass * Uplift mountains * Llano Uplift * Nemaha Uplift Business * Upli ...

ing, and ended with

intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

s about 65 million years after the onset of the formation. It is located in the area around the

Norwegian Norwegian, Norwayan, or Norsk may refer to: *Something of, from, or related to Norway, a country in northwestern Europe * Norwegians, both a nation and an ethnic group native to Norway * Demographics of Norway *The Norwegian language, including ...

capital

Oslo Oslo ( , , or ; sma, Oslove) is the capital and most populous city of Norway. It constitutes both a county and a municipality. The municipality of Oslo had a population of in 2022, while the city's greater urban area had a population of ...

. The lava production was high when the rhomb porphyry lavas were deposited. The lavas reflect a period of abundant earthquake-related movements, when

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...

forces tore the crust apart. In the

Vestfold Vestfold is a traditional region, a former county and a current electoral district in Eastern Norway. In 2020 the county became part of the much larger county of Vestfold og Telemark. Located on the western shore of the Oslofjord, it bordered ...

district, one lava flow was deposited on average every 250,000 years, resulting in a 3000-metre thick sequence of mainly volcanic material. In the

area, lavas were deposited on average every 800,000 years. Only a few plant remains have been found between these lavas. The bedrock in this area, roughly from

Skien Skien () is a city and municipality in Vestfold og Telemark county in Norway. In modern times it is regarded as part of the traditional region of Grenland, although historically it belonged to Grenmar/Skiensfjorden, while Grenland referred the ...

to Oslo and

Mjøsa Mjøsa is Norway's largest lake, as well as one of the deepest lakes in Norway and in Europe. It is the fourth-deepest lake in Norway. It is located in the southern part of Norway, about north of the city of Oslo. Its main tributary is the rive ...

, results in soil rich in nutrients important for plant growth. Since the Permian,

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...

has removed the volcanic peaks and indeed most of the lava layer and laid bare the

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it up ...

s and

volcanic pipe Volcanic pipes or volcanic conduits are subterranean geological structures formed by the violent, supersonic eruption of deep-origin volcanoes. They are considered to be a type of ''diatreme''. Volcanic pipes are composed of a deep, narrow cone o ...

s deep below, allowing scientist a rare view of what goes on beneath a

rift valley A rift valley is a linear shaped lowland between several highlands or mountain ranges created by the action of a geologic rift. Rifts are formed as a result of the pulling apart of the lithosphere due to extensional tectonics. The linear d ...

. Several of the old magma plumes are now quarried, the rich black

larvikite Larvikite is an igneous rock, specifically a variety of monzonite, notable for the presence of thumbnail-sized crystals of feldspar. These feldspars are known as ''ternary'' because they contain significant components of all three endmember f ...

(named from

Larvik Larvik () is a town and municipality in Vestfold in Vestfold og Telemark county, Norway. The administrative centre of the municipality is the city of Larvik. The municipality of Larvik has about 46,364 inhabitants. The municipality has a 110 ...

, a town south of Oslo) being one. The

Särna alkaline complex The Särna alkaline complex is a group of intrusive igneous rocks in Dalarna, Sweden. Emplacement and cooling of magma into rock occurred during the Carboniferous Period. The complex is aligned with the Oslo Rift The Oslo Graben or Oslo Rift is ...

in western Sweden, also of Late Carboniferous age, is thought to be related to the Oslo Graben as it is aligned to it.

References

External links

Mantle Plume Beneath The Oslo Graben?
Paleozoic rifts and grabens Carboniferous Norway Geology of Norway Permian Norway Aulacogens Volcanism of Norway Carboniferous volcanism Extinct volcanism Rift volcanism Valleys of Oslo {{geology-stub

See also

References

External links