Published: 07.05.2019 | Author: Haakon Fossen
På de ytterste skjærene har havet vasket fjellet rent. Bergartene på Litla Kalsøy i Austevoll størknet på vei opp fra dypet i kambrosilurtiden. De tilhører de såkalte eksostiske bergartene i kakemodellen for hordalandsgeologien. (Haakon Fossen)
Much is hidden from us, but we know some of the main features in the history behind the different rock types and minerals that surround us. The Hordaland we experience today is the result of an exciting and sometimes dramatic geological history over many hundreds of millions of years - a result that is important for Hordaland: The bedrock influences the soil types and lays down the cultural foundation, by determining the possibilities for mining, quarrying slate, building stone and gravel for roads, and, not least, where we find mountains, valleys and fjords.
Hordaland's bedrock - like "a bottom-heavy layer cake"
Geologically, Hordaland is located near the margin of an ancient continent that is called Fennoscandia or the Baltic Shield." For the most part it is a large and stable shield area that has grown along its margins through collisions with other shields over geological time. We therefore find some of the world's oldest bedrock toward the centre of this shield (Finland and Russia), with younger bedrock out toward the margins (southern Norway). Most of the bedrock in Hordaland is nonetheless quite old, from over one and a half billion years to 155 million years old for the youngest.
The structure of Hordaland's bedrock reminds one of a "layer cake". The Precambrian Basement forms a very thick and solid base at the bottom. Above this is a layer of phyllite, somewhat resembling a layer of vanilla. Then comes another covering of Precambrian basement rocks followed by Cambro-Silurian bedrock that was formed in an ocean far to the west of modern-day Hordaland (so-called "exotic", meaning "from afar", rock types). These consist of among others greenstone, gabbro and granite together with phyllite. These bedrock types have at one time been overlain by Mesozoic layers and possible also deposits of Tertiary age.
Only crumbs left of the youngest layers of the ''cake''
The whole "cake" has now been sliced by faults and was penetrated by veins of magma during the Permian and Triassic Periods. The natural forces of nature have also "helped themselves" to the cake for a very long time, so that the different layers have been exposed at the surface and some have been partially eroded away. The layers from the Jurassic Period and any younger layers that had been deposited have now been totally "eaten away". Only a few crumbs are left under Vatlestraumen iin the Fjell municipality - otherwise one must travel westward beyond the coastal islands to find any remnants of these layers.
At the very top of Hordaland's bedrock "layer cake" lies the decoration: deposits of gravel, sand and clay from the Quaternary glacial melt and the subsequent interval of time. Most of this is around 10,000 years old, which in a geological time-frame is very young. So these layers have not yet been transformed into stone - they are still "just" sand and gravel.
The base of the layer cake
The Precambrian basement rocks, which comprise the foundation or "base of the layer cake" in Hordaland, were formed during the Earth's Precambrian Era. They arose when magma solidified as granite and gabbro in between somewhat older more superficial bedrocks, such as basalts, rhyolites and quartzites. In large parts of Hordaland these bedrocks were gradually pressed together and became transformed into different kinds of gneisses. This occurred both in the Precambrian Era (during the sveko-Norwegian mountain-building event) and in connection with the subsequent collision with Greenland (The Caledonian mountain-building event) In areas where there has not been gneiss metamorphism one can still find the original bedrock types. Deep intrusive igneous ¬rock types, and especially granitic plutons, are prominent in southeastern Hordaland (Hardangerfjord-Finse-Hardangervidda¬ area). Several of these rock types can be beautiful to look at, such as the Finse- and Eidfjord granites. The Eidfjord granite has been used both as a building stone and as a paving stone for a long time. (Quarry)
The surface bedrock is well preserved in the northeastern part of the Folgefonna Peninsula and further toward Sveio. The rock types present include quartzite, rhyolite and metamorphosed basaltic lavas squeezed in between large bodies of granite and gabbro. Ripple marks can be found some places and cross-bedding from when the quartzite was deposited as loose sand. There are also layers of conglomerate that once were gravel and stones in a Precambrian river system or coastal zone. These surface bedrocks recur several places southward toward Telemark, where they have been given the name "Telemark supracrustals".
Quartzite that might possibly be of the same type as in Telemark is also found in the bedrock farther north, in western Stølsheimen and Norhordland east of Fensfjorden. The Precambrian basement in this area is mostly dominated by gneisses and migmatites without the massive bodies of granite and other intrusive rock types that are so common in the Hardanger region. Smaller bodies of eclogite are found in the northwestern part of these gneisses, formed during the deep burial in Silurian times. A third area with Precambrian basement is found in Sund, Fjell, Fedje and the western part of the Bergen municipality. This area is also dominated by gneisses with occasional bodies of granite and gabbro. Reddish pegmatite veins are common both here and in other parts of the Precambrian basement, and they break up the other rock deposits from Precambrian times. Geological dating suggests that these Precambrian bedrocks were formed roughly 800-900 million years ago.
Vanilla layer of phyllite
A layer of phyllite is spread out over the Precambrian basement and is metamorphosed remnants of clay-rich sediments that were deposited on top of the basement in the beginning of Cambro-Silurian time. The layer once extended continuously from eastern Norway over the Hardanger plateau, out toward the west coast of Hordaland. Much of the phyllite layer is now gone after millions of years of erosion. The black flecks in the geological map of Hardanger plateau area are places where the phyllite is preserved, in some places with remnants of the so-called thrust sheet overlaying it. In the western parts (Voss and westward), where the temperature was highest during the Caledonian metamorphism, the bedrock has been transformed to mica schists with large flakes of mica and the mineral garnet.
At a few places conglomerate can be found at the base of the phyllite layer, in contact with the Precambrian basement (p. R463 ). This is petrified beach sand from the beginning of the Paleozoic Era (the Camrbian Period), when the basement rocks of southern Norway were flooded by an ancient sea. The phyllite is metamorphosed clay that had been deposited on the bottom of the sea. Layers of quartzite and limestone testify to the deposition of sand layers and marine life. In easternmost Hordaland fossils can be found in the phyllite belt. At Hårteigen, Finse, Holberget and Dvergmednuten on the Hardanger Plateau fossil remains of 500-600 million year old organisms have been found - signs that the Paleozoic seas of the Cambrian Period were teeming with life. The Cambro-Silurian calcium-rich phyllite disintegrates easily. This bedrock is often called "decaying stone" in many parts of Hordaland. But this "decaying stone" gives a good growing soil both in the high mountains and in the lowlands. The high calcium content prevents the mountain lakes from becoming acidic and losing their fish. It can therefore be a good idea to check the geological maps if you are planning on fishing on the Hardanger Plateau. Proximity to phyllite in the water's catchment area gives good hope for finding fish. An example of a settlement that has benefited from the lush phyllite belt is the Voss-Bulken area, where the phyllite layer is several hundreds of metres thick.
The top of the “cake” - pushed in from the side
Remains of a thick cover of Precambrian basement bedrock rests on the phyllite. During the Caledonian Mountain-building event enormous flakes were torn completely off from the bedrock and thrust up onto the Cambro-Silurian deposits. Such detached bedrock units are called thrust sheets. We find remnants of such thrust sheets a number of places in Hordaland, among other places on the Hardanger Plateau, around Bergen and around Oppheim. Some of the lowermost bedrock units (Bergsdal nappes) are surrounded by phyllite both above and below. This disruption of the otherwise systematic "cake structure" is the result of complex movements in the earth's crust during the Caledonian Mountain-building event. Some of the displaced thrust sheets consist of rock types that we recognise from the Precambrian basement underlying the phyllite, albeit a more flattened, or perhaps on their way to becoming gneisses.
Other thrust sheets consist of quite distinctive rock types. These include first and foremost the displaced Precembrian basement bedrock that stretches from the southern part of the Bergen municipality via the Osterøy, Lindås, Austrheim, Radøy and Meland municipalities (the so-called Lindås nappe), in addition to the area around Stallheim-Mjølfjell (the Jotun nappe). We find large volumes of anorthosite and anorthosite-gabbro in these two units. They are light, nearly white bedrocks and are relatively uncommon. The name "Mjølfjell" (="flour rock") comes from the light, flour-white colour.
The anorthosite rock types represent magma that intruded into the earth's crust roughly one billion years ago and then solidified. Since then it has been altered and obtained its light colour. The light colour, chemical composition and other characteristics have made it interesting for a number of uses, from an abrasive in toothpaste to road gravel and garden stone. It contains a lot of aluminium and it is hoped that advances in technology will make it possible to make use of this sometime in the future. In addition, titanium iron and nickel deposits associated with the anorthosite in the Bergen area have been mined in earlier times. A variety of the anorthosite-gabbro known as Coronite is also found here. It consists of olivine surrounded by black pyroxene and a reddish garnet in a white matrix and is Hordaland's municipal stone. Anorthosite bedrock is quarried in considerable amounts both in the Bergen area (R334 ) and in Nærøydalen. In the Meland municipality some of the anorthosite includes some eclogite. This is important for understanding the Caledonian development of western Norway. Deposits like the titan-rich mineral rutile in the eclogite can potentially be of economic interest in the future.
Both the Jotun nappe and the Lindås nappe contain another type of unusual intrusive rock - the so-called mangerite-syenites. Mangerite is the best known of these and is in the intrusive rock type that lies between granite and gabbro in composition, formed deep below the earth's surface. This rock type got its name from Manger in the Radøy municipality.
Ancient sea floor - came from America?
Remnants of the so-called "exotic rock types" (meaning that they came from another place) of Cambro-Silurian age are only found near the coast and overlie both the phyllite belt and the detached sheets of Precambrian basement. The rock types are quite complicated and consist mainly of metamorphosed intrusives such as greenstone, rhyolite, serpentine, granite and gabbro, together with phyllite, quartzite, marble and conglomerates. These are exotic rock types since they were not originally formed in Hordaland, but were transported long distances by violent forces during the Caledonian mountain-building event. They were originally formed on ancient oceanic crust far to the west of modern day Hordaland, roughly 4-500 million years ago. There is much to suggest that they originated nearer to the North American continent than the Norwegian. However, some of the rock types have more in common with the more indigenous bedrock. Phyllite and mica schist are examples of this. The exotic phyllite is nonetheless different; it is more basic and in some places greener than the indigenous rocks in the phyllite belt. Together with the layer of marble they make very fertile soil and this is a sharp contrast to the granitic bedrock in the same unit. One can see this especially well in Sunnhordland where there is a visible contrast between the phyllite- and the mice-rich shale in the southerly part of Stord and Tysnes and the more barren granite-dominated area in the north toward Austevoll and Reksteren. Parts of the northwest side of Hardanger Fjord (Øystese-Nordheimsund-Tørvikbygd) are also enriched by these exotic Cambro-Silurian shales. Nearer Bergen there are elements of Cambro-Silurian phyllite and mica schist both in what are known as the "Bergen Arcs" (Os-Samnanger area) and in the "Little Bergen Arc" (from Grimstad via Lake Nordåsvatnet and centre to the east side of Askøy).
Phyllite, mica schist, greenstone and greenschists are rock types that disintegrate relatively easily. Therefore they commonly form lowlands or valleys in the terrain. In contrast, intrusive rock types like granites and gabbros tend to form hillsides and mountainsides. Therefore there is a sharp contrast between the lower phyllite- and greenstone area on the southern side of Stord and the mountainous central and northern parts of Stord where intrusive rocks dominate.
The limestone that is found in this exotic bedrock was long ago transformed into marble, and the phyllite is for the most part compressed and folded. Nonetheless, it is still possible some places to find fossils of organisms that lived in the ocean during Ordovician and Silurian times.
Conglomerate from the Devonian - a geological splash of colour
Devonian conglomerates are mainly found along the coast of Sogn and Fjordane, especially in the areas of Solund, Kvamshesten and Hornelen. They are also found in Hordaland at Holmengrå in the Fedje municipality (p. 403 This little island consists entirely of Devonian conglomerates. The conglomerates were formed when up to metre-sized stones including quartzite, granite, gneiss and shale were deposited on top of the exotic Cambro-Silurian bedrocks. The Devonian deposits are remnants of the Caledonian mountain chain that was eroded down about 390-360 million years ago. After the deposits we can see today were deposited, several kilometres of new sediments were laid down over these, so that the conglomerates were pressed together to form solid bedrock.
Volcanoes during the Dinosaur Age?
Along the coast of Hordaland there are hundreds of places where one finds intrusive rock types that are younger than both the Cambro-Silurian and Devonian bedrock. These igneous bedrocks occur in a North - South to North/Northwest -South/Southwest oriented depositional break. They were formed when magma from the mantle flowed up toward the surface of the continental crust along these breakages. The veins have a basaltic composition and are seldom more than a half of a metre wide. They can, however, be up to several hundred metres long. They appear very dark and cut through all the other bedrocks and bedrock structures. Using modern dating methods it has been possible to determine that these were intruded during the Permian and Triassic Periods. It appears that there were a series of such intrusions around 260 million years ago and also a newer pulse roughly 220 million years ago. This is very far back in time, but the bedrocks are still quite a bit younger than the Devonian and older bedrocks of Hordaland.
There is much to suggest that these relatively young igneous bedrocks have come up from great depths. It is not impossible that some of them reached the surface at that time and caused local volcanic eruptions in the area around Sunnhordland and Sotra. If the lava flows did reach the surface, they were in any case protected from wind and weather since.
Jurassic Rock under the Vatle Waterfall
Until just a short time ago, it was believed that the Permian and Triassic veins were the youngest bedrocks in Hordaland. That was until sedimentary rocks of Jurassic age, roughly 150-160 million years old, were discovered in connection with building a tunnel under the Vatlestraumen passage between Alvæen and Bjorøy near Bergen (page 364). We know these layers well from the North Sea Basin, where they can be followed eastwards toward land. They disappear roughly 15 km from land, but now it is known that they are preserved locally under the Vatlestraumen passage. Previously they covered parts of the Hordaland coast, but were since eroded away. The little remains we know of from the tunnel under Vatlestraumen passage were protected from erosion where they lie preserved in a fault zone. Similar bedrocks are only known for certain in Beitstadfjorden (in the Trøndelag municipality) and on Andøya, and probably also in the Karmsundet Sound at Haugesund.
This is our view of how Hordaland's bedrock was formed, with its history stretching from 1700 to 150 million years back in time. Hordaland has undergone changing landscapes and climate throughout this enormous interval of time. There were mountains as high as the Himalaya here, but also landscapes consisting of low plateaus. Hordaland was flooded by the ocean during some periods, and shaken by earthquakes or covered by volcanos in others. The landscape has been liften up, and eroded down, several times. Again the bedrocks lie like a thick-bottomed cake of Precambrian basement, phyllite and thrust sheets. We find them everywhere, like silent witnesses to Hordaland's long and dramatic history: Gneiss, granite, gabbro, limestone, phyllite, greenstone and anorthosite - and many others. Quiet and lifeless, but testimony to a continuously changing earth.
The geological timeframe
In order to understand Hordaland's geological history it is necessary to reflect on the time aspect involved in geological processes. We are used to thinking in terms of years, generations or perhaps up to a few thousand years back in time. It gets more challenging when Quaternary geologists operate with tens of thousands of years up to a couple of million years. When we learn that Hordaland's bedrock was formed from several hundreds of millions of years to over a billion and a half years ago, most of us have difficulty imagining this.
It helps to make a picture scale. The oldest parts of Hordaland's bedrock can be as old as 1.7 billion years old, if not more. Let us use as a starting point the railway line between Finse and Bergen, which is about 165 km long. Let us consider a trip from the oldest bedrock (Finse station) to today (Bergen station). A generation (75 years) would compare to 7.5 millimetres along the railway line, one metre would represent ten thousand years (time since the Ice Age), while a trip 16 kilometres long would represent 160 million years, which is the age of the youngest rock types in Hordaland (Bjorøylagene, see page 364). If one stands at the station in Bergen, one would see many details near the starting point, but fewer and fewer as one travelled back in time along the tracks. Even though one stood high up and could see all the way to Finse station, one would only be able to catch a glimpse of some of the mountaintops or main features in the landscape. This is so with the geology, also; the farther back in time we go, the fewer details we are able to see.
The geological timescale is divided into four main parts. The Precambrian Era (until around 550 million years ago) is the Era when the basement rocks of Norway were formed. During the Paleozoic Era the Caledonian Mountain Chain was formed and eroded down. Fish and plants appeared for the first time (ca. 550-250 million years ago). West Norway's most fertile areas are situated on bedrocks from Cambro-Silurian time, which is the first part of the Paleozoic. During the Mesozoic Era (250-60 million years ago), the oil-bearing layers in the North Sea were laid down. At the beginning of the Tertiary the dinosaurs went extinct. Norway then gets lifted up to a nearly 2000 metre-high plateau before the Ice Age set in roughly two million years ago. We have come to the Quaternary Period, which lasted until about 10,000 years ago.
Three main rock types
The bedrock around us consists of a large variety of different rock types. Some are soft, many are hard, some er red or green, others nearly black or white. Each of them has been through a unique history. Just as no two snowflakes are alike, neither are two rocks exactly alike. Rock types can nonetheless be divided into three main groups, according to their composition and the processes that formed them.
Igneous rocks are rocks that have melted deep down at the bottom of the earth's crust or even deeper, in the mantle, and thereafter have been brought up to the surface. Some of these solidify on their way up and form deep intrusive rocks such as gabbro and granite. These are common rock types in Hordaland; the intrusive rocks of Bømlo, Stord and Tysnes are especially well known. Others melt when they reach the earth's surface and are called extrusive igneous rocks. These flow out at the surface as lava or are ejected during a volcanic eruption as volcanic ash. When such layers form bedrock they are called rhyolite and basalt. There are many lava rock types in Hordaland, but they are all quite altered.
When bedrock at the earth's surface gets worn down, mineral grains and rock fragments are transported by rivers, ice or wind until they accumulate as sediments. If new layers of sediments get deposited over a long period of time, the burial and pressure from the accumulated weight will form them into sedimentary rocks. Layers of sand will form sandstone, gravel will form conglomerates, and clay will form shale. Limestone and marble can be formed from the accumulation of large and small shell fragments of organisms that lived in the sea or from coral reefs. Extrusive igneous rocks and sedimentary rocks are often called "surface bedrocks". There are not many sedimentary rocks in Hordaland that have not been metamorphosed. The least altered are found in Holmengrå in Fedje, in the area around Ulven in Os and in the southwestern part of Stord.
All rock types can be exposed to increased pressure and /or temperature. This can occur when they get buried or pressed down into the earth's crust where the temperature is higher, or when hot magma flows in and causes higher temperatures locally. The original mineral grains will then be altered to new mineral grains or minerals and a metamorphic rock type is formed. For example, an extrusive igneous rock like basalt might be altered into a greenstone or amphibolite, a shale into phyllite or mica schist, and a quartz sandstone into a quartzite. In greenstones one will get the metamorphic minerals chlorite and epidote from the plagioclase feldspar, amphibole and pyroxene. In phyllite, especially when it gets further metamorphosed to mica schist, the clay minerals will be altered into larger sheets of mica, while quartz will be recrystallised into new and larger grains of quartz. In mica schist, where the temperatures have been higher than in phyllite, one might also find the minerals garnet and amphibole.
If there is strong compression and high temperature, many rock types will be altered to gneiss - the most common rock type in Hordaland. If the temperatures get very high, the original rock type can begin to melt and form a migmatite - a mixture of melted and unmelted bedrock. This might occur when temperatures exceed 6-700 °C.
Examples of more unusual metamorphic rock types are soapstone and serpentine. These are extremely basic rock types that consist of serpentine minerals and are found as relatively small inclusions, spread around Hordaland's bedrock. The soft serpentine minerals make the bedrock so malleable that it can easily be sawn and cut into. Soapstone is therefore a gratifying building stone.
Perhaps the most special metamorphic rock type in Hordaland is eclogite. Eclogite is a greenish and quite heavy rock type formed of basic bedrock that has been buried very deep and exposed to unusually high pressure. The minerals red garnet and green amphibole are characteristic for eclogite. These are mainly found in the area around Meland.
Hordaland consists mainly of more or less metamorphic rock types as a consequence of earthquake movements during the Sveko-Norwegian mountain-building event (ca. 1 billion years ago) and the Caledonian mountain-building event (400-450 million years ago).
Minerals are the building blocks of bedrock. Some types of bedrock, such as quartzite, may consist of just one mineral. Most, however, consist of several minerals. The most common minerals in the so-called acidic rock types are quartz, potassium feldspar and mica minerals.
Quartz is a hard mineral that can be translucent in crystal form, but most often is white. Veins or lenses in bedrock often consist of milky white quartz and can be easily confused with marble, but quartz is much harder. Potassium feldspar is reddish to white, and breaks up along fine, shiny surfaces. Potassium feldspar is used as a glaze in ceramic production. There are several quarries in Hordaland. Mica minerals are silver-coloured (muscovite) or almost black (biotite). These minerals break into quite thin flakes. Thin, large flakes of muscovite have been used as windows in wood stoves.
In more basic rock types the quartz and potassium feldspar is replaced by darker minerals such as amphibole and pyroxene, in addition to biotite and chlorite. These are iron- and magnesium-rich minerals that give the bedrock a dark colour. They include plagioclase feldspar instead of potassium feldspar. The chlorite minerals resemble mica, but are green and characteristic for the greenstones of Hordaland.
Minerals crystallise out and grow when flowing masses of magma solidify within the crust or on the surface, or when sediments and bedrocks are exposed to high pressures and temperatures (metamorphosis). In addition, minerals can precipitate out of so-called hydrothermal solutions - warm, mineral-rich fluids that circulate in fractures and fault-zones in the earth's crust. Such hydrothermal mineral growth can give fine crystal faces if the minerals grow into cavity spaces in the bedrock. The vast majority of minerals, however, do not have well developed crystal faces and are identified on the basis of colour, cleavage (the way they fracture when broken), optical qualities and hardness.
The most common rock types in Hordaland
Granite and gabbro are among our most common rock types. These igneous rock types can make up large bodies many kilometres in diameter. They can also penetrate in and solidify along narrow cracks and form intrusive veins. Granites are massive and hard bedrock comprised of quartz, potassium feldspar and often containing dark mica (biotite). If they are of good quality they can be used as building- or decorative stone. For example, the Eidfjord granite and granites in the area around Austevoll. Gabbro is a darker bedrock lacking quartz. It is often found as a dark rock containing white grains of plagioclase feldspar. Sometimes the dark pyroxene - or amphibole minerals might be absent. Then the rock type is called anorthosite and can be almost entirely white, as at Mjølfjell and in the Bergen Arcs. Some granite and gabbro rock types can be altered to gneiss, which is usually banded ("banded gneiss") or can have small inclusions ("augen gneiss"). The inclusions are cm-sized grains of white or pink alkali feldspar.
Amphibolite is a metamorphic bedrock that is dominated by amphibole. It might be black or green, depending on the type of amphibole and composition of the green epidote and chlorite minerals present. Amphibolite is related to the greenstones and greenschists, which contain large amounts of epidote and chlorite. Mica schist is, as its name suggests, rich in the silver-grey flaky mineral mica and can also contain mm-sized crystals of the red-brown mineral garnet. Among several white bedrock types, marble is the only one made of calcium carbonate (usually calcite) and is a "soft" rock type that is suited both for making lime from and as a decorative stone. Marble is found in limited amounts near Os, at Fullbotn, on Varaldsøy and in the southern parts of Tysnes, Stord and Bømlo. Quartzite is another much more common grey-white bedrock type in Hordaland, and is harder than marble.
Facts about the earth's crust
Our earth consists of a series of different layers of solid and fluid rock masses where the heaviest elements are concentrated in the core. This division of layers is the result of physical processes that have been acting since the earth was first created. We humans wander about on a relatively thin continental crust made of cold and hard rock types. The earth's crust can be divided into oceanic and continental crust. All of the continents consist of continental crust, which is thicker, lighter and older than the oceanic crust.
The crustal rock types essentially "float" on the warmer and heavier masses down in the mantel. In this "syrup" there are weak currents that help to move or divide the continents - a process known as continental drift. These currents change occasionally, so continents that have been moving apart for a period of time might begin instead to move closer together again. In other cases the forces are not enough to move or divide continents, but might stretch the crust so that it becomes thinner than normal. Forces of this type have been active in the North Sea between Great Britain and West Norway. The crust here was made thinner during Permian-Cretaceous times when Scandinavia and Great Britain were drawn away from each other. Earlier (during the Silurian Period) these two areas moved closer together.
The continental crust consists mainly of granitic rock types. The fluid masses in the mantle beneath the crust are more dense and richer in iron and magnesium. Serpentinite, found among other places in Raudeberget i Stølsheimen, at Lygra and several places in Samnanger, are bits of the mantle that were transported up to the surface by large movements in the crust and erosion.
The continental crust is less than 10 km thick under the oceans and ca. 30-45 km thick under the continents. The crust thereby comprises roughly 5 parts per thousand of the earth's radius and is therefore, relatively speaking, about as thick as an eggshell. The temperature increases rapidly with depth under the surface, usually about 25-30 °C for every kilometre. In deep mines and boreholes the temperature can be measured, and if the mine shaft is deep enough, it will be noticed!
Facts about the earth's crust #2
Scientists send sound waves deep down into the crust in order to learn more about the structure of the earth. The sound is reflected by the many layers and is recorded by microphones at the surface. The technique is called (deep) seismic analysis, and it gives information among other things about the thickness of the earth's crust. The data is analysed by powerful computers before it can be interpreted by geologists and geophysicists.
Two deep seismic profiles across the North Sea have been studied in detail by Norwegian scientists. The illustration shown here is based on this work. Under Hordaland the earth's crust has a relatively normal thickness of about 35 km, but it is only about half as thick under the middle of the North Sea. The reason for this is that the stretching of the earth's crust was strongest under the middle of the North Sea.
Trygve Hereid, eieren av granittbruddet i Eidfjord, på inspeksjon i steinhoggeriet tidlig på 1900-tallet. Granitten tilhører grunnfjellet og er nesten en milliard år gammel. Den høye alderen gjør den ikke mindre egnet til gatestein – i nyere tid har den som en av flere steinsorter blitt brukt i gategolvet rundt den blå steinen i Bergen sentrum.
Den kalkholdige og lett oppsmuldrende fyllitten gir godt jordsmonn og grønne bakker. Selv på de neddemte breddene av Hamlagrøvatnet kan gresset vokse opp før magasinet fylles. På fyllittblokken som stikker opp i forgrunnen, kan vi ta kilden for grøderikdommen nærmere i øyesyn. Geitafjellet i bakgrunnen. (Svein Nord)
Perspektiv over Hordaland mot nordøst, fargelagt med hovedenhetene i berggrunnen. Bergartsfordelingen slik den fremstår i dag, er resultatet av et komplekst samspill av jordskorpebevegelser, som foldninger, skråstillinger og forkastninger, og senere erosjon. Det er først og fremst i østlige deler av fylket at lagene i kaken ligger relativt flatt og uforstyrret oppå hverandre. (Haakon Fossen)
Anortositten i Rjoanddalen i Mjølfjellområdet er en lys, nesten melhvit bergart (derav navnet Mjølfjell) og tilhører et gigantisk skyveflak, Jotundekket, som ble presset inn fra nordvest under den kaledonske fjellkjededannelsen. De store steinurene i dalsidene er frosten og vannet sitt verk. Anortosittbergartene har mange bruksområder, fra slipemiddel i tannkrem til veipukk og hagestein. (Svein Nord)
- Fossen, H. 1998. Jordskorpen vår. Årbok for Bergen Museum 1997: 48–50.UiB.
- Ragnhildstveit, J.; Helliksen, D. 1997. Geologisk kart over Norge, berggrunnskart Bergen – M 1:250.000. Norges geologiske undersøkelse.
- Ragnhildstveit, J.; Naterstad, J.; Jorde, K.;Egeland,B.1998. Geologisk kart over Norge, berggrunnskart Haugesund – M 1:250.000. Norges geologiske undersøkelse.
- Sigmond, E. M. O. 1975. Geologisk kart over Norge, berggrunnskart Sauda – M 1:250.000. Norges geologiske undersøkelse.
- Sigmond, E. M. O. 1998. Geologisk kart over Norge, berggrunnskart Odda – M 1:250.000. Norges geologiske undersøkelse.