Igneous raw materials

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Igneous rocks have solidified or crystallized from magma. Some igneous rocks have solidified at great depths in the earth’s crust in large masses and are called intrusive rocks. Other intrusive rocks are formed when magma penetrates into cracks and solidifies in these. During volcanic eruption the magma is forced to the surface and flows as lava over land and sea. The rocks which originate from the deep, but solidifies at the surface are called extrusive igneous rocks.


Igneous rocks consist, like all rocks, of minerals. For most of the intrusive rocks the mineral grains are visible, and these are described as middle to coarse grained. Granite is an example of an intrusive igneous rock which consists of the minerals quartz, feltspar, plagioclase and biotite. Intrusive igneous rocks solidified in cracks and extrusive igneous rocks are often fine grained because they have crystallized more rapidly. Diabase and basalt are examples of respectively a intrusive igneous rocks solidified in a crack and an extrusive igneous rock (Sigmond 1996:7-12).


In Norway, igneous rocks are most abundant in the geologically famous “Oslofeltet” which streches through the counties Oslo, Akershus, Vestfold and Buskerud.

The so-called crust of the Earth is about 35 km thick under the continents but averages only some 7 km beneath the oceans. It is formed mainly of rocks of relatively low density. Beneath the crust there is a layer of denser rock called the mantle which extends down to a depth of nearly 3.000 km. Much of the molten rock material which goes to make up the igneous rocks is generated within the upper parts of the mantle. magma This material, which is called magma, migrates upwards into the Earth's crust and forms rock masses which are known as igneous intrusions. If magma reaches the Earth's surface and flows out over it, it is called lava. Within some lavas, fragments of dense, green-coloured rocks are sometimes found which consists principally of olivine and pyroxene. These fragments are thought to represent pieces of the mantle, carried upwards by the migrating magma.

The overwhelming majority of lavas consist of the black, rather dense rock called basalt, and most eptrologists consider that the primary molten rock material which comes from the mantle has a composition which is near to that of basalt. Although basalt is the most abundant of the lavas, granite is by far the commonest of the intrusive igneous rocks. Granite is mineralogically and chemically different from basalt and for many years geologists have wrestled with the problem of how the two rock types are related. If basalt is assumed to derive from the mantle, is it likely that granite, which is of a quite different composition, could also come from the mantle? Nowadays it is considered that granite may be produced in two ways; either from basalt, or from crustal rocks. When basalt magma starts to crystallize in the upper mantle, or the lower part of the crust, the overall composition of the crystals is not the same as the overall composition of the magma. This means that the liquid part will have a composition different from that of the original magma, and the further the crystallization process goes the greater will be the difference in composition between the liquid and the crystals. If the crystals and the liquid should now be separated by some mechanism, then rocks of two types will result, and each will have a composition different from the original basalt. This process, called differentiation, is capable of producing a great range of rock types, one of which is granite.

The second and perhaps more important way of producing granite is thought to operate within the crust itself. When mountain chains are formed, considerable thickness of crustal rocks are squeezed and thickened, and probably the base of the crust bulges down into the mantle. At the same time large volumes of magma move up into the crust. The effect is to heat the base of the crust to temperatures high enough to melt the rocks, so producing more magma. This new magma, which has the composition of granite, is mobile and moves up into the higher levels of the crust where it cools and solidifies as large granite intrusions, which are found in most mountain belts. These two processes account for the majority of igneous rocks.

The recognition and naming of igneous rocks involves an assessment of grain size and the recognition and estimation of the relative amounts of the constituent minerals. Additional information is obtained from colour index, texture, structure, and sometimes from field relations."

(Hamilton et al 1976, 146-147)

Diabase

Diabase, or dolerite, is an igneous intrusive rock.


  • Appearance: When the diabase is fresh it is black, dark-grey or green. It can also be black and white-spotted. Exposed to erosion diabase can have a dull brownish color.
  • Fracture: Conchoidal fracture
  • Grain size: Fine-Medium grained
  • Properties: Homogenous, robust
  • Tools: Work well for axe production, but also for macro tools (large knives/scrapers)
  • Source: In Oslofeltet: Oslo, Akershus, Vestfold and Buskerud

 

diabase (dolerite)

Colour: When fresh it is black, dark-grey or green; may be mottled black and white.

Grain size: Medium.

Texture: Occasionally ophitic texture can be distinguished in hand specimen. May be porphyritic.

Structure: Vesicles and amygdales occur. Sometimes has segregations of coarser rock enriched in feldspar. Mineralogy: Phenocrysts comprise olivine (olivine diabase) and/or pyroxene or plagioclase. The groundmass comprises the same minerals with iron oxide, and sometimes with some quartz, hornblende or biotite.

Field relations: Dykes and sills. These may form swarms of hundreds or perhaps thousands of individual dykes or sills which often radiate from a single volcanic centre." (Hamilton et al 1976, 170)

Basalt

Basalt is an igneous extrusive rock.

  • Appearance: Fresh basalt is very dark (black/dark-grey), but erosion can make it brown/red.
  • Fracture: Conchoidal fracture
  • Grain size: Ordinarily the basalt is very fine grained and compact, but sometimes they can be found with bigger crystals in a finer matrix.
  • Properties: Relatively homogenous, though, robust
  • Tools: Mainly axes
  • Source: In Oslofeltet: Oslo, Akershus, Vestfold and Buskerud

Colour: When fresh it is black or greyish black; often weathers to a reddish or greenish crust.

Grain size: Fine.

Texture: Usually dense with no minerals identifiable in hand specimen; a freshly broken surface is dull in appearance. May be porphyrithic. Structure: Often vesicular and/or amygdaloidal. Xenoliths are relatively common and usually consists of olivine and pyroxene; they have a green colour. Columnar jointing is common and often spectacular.

Mineralogy: Phenocrysts are usually olivine (green, glassy), pyroxene (black, shiny) or plagioclase (white-grey, tabular). If olivine is present the rock is called olivine basalt. Microscopic examination show the groundmass to consist of plagioclase (usually labradorite), pyroxene, olivine and magnetite, with a wide range of accessory minerals. Amygdales may be filled, or partly filled with zeolites, carbonates or silica, usually in the form of chalcedony or agate.

Field relations: Lava flows and narrow dykes and sills. The edges of dykes or sills are often finer grained than the centers or even glassy, due to rapid cooling on intrusion. Most basalts occur as lava flows either in volcanoes or as extensive sheets building up a lava plateau, which may cover hundreds of thousands of square kilometres, and may be fed by numerous fissures. The surface forms of lavas are of two principal types; smooth or ropy (the surface looks like a rope) which is known by the Hawaiian term of pahoehoe, and scoriaceous which is rough and clinkery and has the Hawaiian name aa. Another common form is pillow lava which consists of pillows or balloon-like masses of basalt - usually with a very fine-grained or glassy outer layer. They are formed by the eruption of lava into water." (Hamilton et al 1976, 170)

Rhyolite 

  • Appearance: The rhyolite often has a light color that vary between white, grey, greenish, reddish or brown. The color can be even or bonded with different shadows.
  • Fracture: Conchoidal fracture
  • Grain size: Fine grained
  • Properties: Homogenous, brittle, elastic, sharp edges, color
  • Tools:
  • Blades, projectiles, small tools like scrapers, axes and knives

Colour: Usually light coloured; white, grey, greenish, reddish or brownish. The colour may be even, or in bands of differing shades.

Grain size: Fine to very fine.

Texture: Frequently shows altering layers that differ slightly in granularity or colour. Phenocrysts not uncommon (porphyritic rhyolite). Flow banding is sometimes evident, defined by swirling layers of differing colour or granularity, and by aligned phenocrysts.

Structure: Vesicles or amygdales may be present. (Pumice is a highly vesicular variety of rhyolite.) May contain spherulites which are spherical bodies, often coalescing, comprising radial aggregates of needles, usually of quartz or feldspar. Spherulites are generally less than 0,5 cm in diameter, but they may reach a meter or more across. They form by very rapid growth in quickly cooling magma, and the crystallization of glass. Mineralogy: As for granite, but rapid cooling results in minute crystals. Phenocrysts of quartz, feldspar, hornblende or mica occur.

Field relations: Flows, dykes and plugs. Rhyolite (or granite) magma is highly viscous and so flows only very slowly, so that if it is extruded it forms very short, thick flows or is confined as a plug in the throat of a volcano." (Hamilton et al 1976, 164)

Pumice

Pumice is a type of rhyolite. Due to its porousity and lightness it can float in water and is often found in coastal areas far away from its origin.

  • Appearance: Light to brown with large pores
  • Fracture: Undetermined fracture
  • Grain size: Coarse grained
  • Properties: No properties that make it useful for tool production
  • Tools: Used for grinding

Diorite

Colour: Speckled black and white in hand specimen; occasionally shades of dark green or pink. The dark minerals are more noticeable than in gabbro. Colour index: 40 to 90, but very variable, often over short distances.

Grain size: Coarse; may be pegmatitic.

Texture: Equigranular or porphyrithic. In porphyritic varieties the feldspar or hornblende may form phenocrysts. Diorites often vary rapidly in texture; an equigranular variety may grade into a porphyritic one within a few centimeters. They are sometimes foliated due to the roughly parallel arrangement of the minerals.

Structure: Xenoliths are common.
Mineralogy: Essentially plagioclase (oligoclase or andesine) and hornblende; biotite and/or pyroxene may occur. Alkali feldspar and quartz (quartz diorites) may be present, when diorite grades into granodiorite. Common accessory minerals are apatite, sphene and iron oxides.

Field relations: Forms independent stocks, bosses and dykes, but also comprises local variants of masses of granite, and sometimes gabbro, into which they merge imperceptibly." (Hamilton et al 1976, 158)

Obsidian

"Obsidian and pitchstone
Colour: Shiny black, also brown or grey. Pitchstones have a dull rather than a shiny lustre.


Grain size: None; the rock is glassy. Texture: Glassy, but obsidian may contain numerous phenocrysts.

Structure: May be spotted or flow banded and spherulites (see rhyolite) are common. Being a siliceous glass it breaks with a conchoidal fracture and may be fashioned to a sharp cutting edge. It was used for cutting tools by primitive peoples.

Mineralogy: Essentially a glass. Rare phenocrysts (abundant in pitchstones) of quartz and feldspar.
Field relations: Dykes and flows. Commonly associated with rhyolites to which they are chemically equivalent."

(Hamilton et al 1976, 164)