Getting to know igneous rocks

Igneous rocks are formed when a descending crustal plate nears the base of the crust where very high temperatures prevail, and the plate begins to melt. Molten material, or ‘magma', ascends within the crust to a point where it is in equilibrium in terms of density, and it begins to crystallise to form solid rock.

If it crystallises at depth where the surrounding temperatures are warm, cooling is slow and crystals are large. These coarse grained rocks are called plutonic rocks. Magma which crystallises within fissures or fractures forms dykes or sills, and the rock cools fairly quickly to form a medium grained rock. Magma which flows on to the surface of the earth chills rapidly to form a fine grained rock called lava.

There are two main kinds of magma, and they have a different composition. One of these crystallises to form light coloured rocks which are also relatively light in weight. These rocks, which contain minerals made of silica, sodium, potassium, with a small proportion of other elements, form the granite family. The other kind of rock is dark in colour, and relatively heavy in weight. The constituent minerals contain iron, magnesium and calcium, with a small proportion of other elements. These are rocks of the gabbro family.

magma diagram

Schematic diagram of magmas rising from base of crust to form sill-like masses at about 5km and at about 2km.
Vertical bodies are dykes. Pink/orange represents granite family.
Gabbro family shown here mainly as dykes in green, with tiny sill near the surface.
Undersea volcano depicted at top.
Plutonic rocks (granite, gabbro) at 5km depth would normally be larger and more rounded than shown here.

THE GRANITE FAMILY

plate 5

PLATE 5 THE GRANITE FAMILY
A. Porphyritic granite, a coarse-grained rock with pink feldspar phenocrysts in a matrix of smaller white feldspar, shiny black mica and
glassy quartz. Note the white feldspar phenocryst on the top edge. B. Felsite, a medium-grained rock with small white feldspars,
glassy quartz and black mica  C. Felsite similar to B but with occasional large pink feldspar phenocrysts.
Note the piece of fine-grained rock plucked from the country rock as the molten magma was ascending
D. Rhyolite lava, always a fine grained rock but sometimes - as in this case - with fine needles of feldspar visible.

Sample 5A (Plate 5, sample A) shows a typical granite from this part of the highlands. The rock has large pink feldspar crystals (phenocrysts), and a nearly-square crystal of white feldspar is visible at the bottom edge of the sample. There are shiny black flakes (crystals) of black mica, which is known as biotite mica. The remainder of the rock consists of glassy and shapeless masses, and this is quartz. This granite is a plutonic rock which crystallised at considerable depth in the crust, but is now exposed on the surface because millions of years of erosion have removed kilometres of overlying rock.

Granite is one of the commonest rocks on the Earth's surface. In the past it was popular as a building stone, particularly for important buildings but was also used for ordinary dwellings where supply was plentiful, as in Aberdeen. It is still popular as an ornamental stone for gravestones. The colour varies according to impurities in the feldspar, but is commonly light grey, dark grey, or pink.

Ben Cruachan

Ben Cruachan, the "hollow mountain', is composed of granite.

Sample 5B is felsite, which crystallised at shallow depth below the Earth's surface. It is similar to felsite sills which occur to the southwest of Inveraray on the far side of Loch Fyne. The rock is medium grained, and although small, most minerals can be recognised. The pink regular shapes are feldspar, and both quartz and biotite mica can be recognised. This rock was once used as building stone, but is now more important as aggregate for road construction, and as ballast for railway lines. Aggregates, which are mechanically crushed granite, felsite or dolerite, are ‘the most important minerals produced onshore in Scotland' (Scottish Executive Factsheet, 2006), so even common rock types can be important to the economy. This sample of felsite was found on the Strachur shore, although it may have broken off some of the large rocks used to construct walls on the foreshore. There are many felsite sills to the southwest of Inveraray, and this particularly hard rock is quarried for a variety of purposes, such as building stone and gravel chips (aggregate). The stone wall bordering the Strachur Estate is made of felsite, probably from the Inveraray area.

wall Strachur Estate

Stone wall made of felsite blocks, Strachur Estate

Sample 5C is similar to Sample 5B, but has a few large feldspars as phenocrysts. The magma started to crystallise at depth where a few large crystals formed, then the magma ascended to a higher position in the crust and cooled more quickly, giving a fine grained groundmass. . Notice the piece of dark, fine-grained rock within this sample - it was plucked from the country rock by the ascending magma.

Sample 5D is rhyolite, the lava of the granite family. It was erupted from an ancient volcano. Rhyolite is typically a very fine grained rock, although this one has needle-like phenocrysts of feldspar throughout. Rhyolite is not known to occur in the Strachur area, and although this sample was found on the shore near town it may have travelled a long way from its source.

THE GABBRO FAMILY

plate 6

PLATE 6 THE GABBRO FAMILY
A. Coarse-grained gabbro, with dark-coloured hornblende crystals and white feldspar crystals B. Medium-grained dolerite
C. Drill core of fine-grained dolerite D. Fine-grained basalt E. Vesicular basalt with the steam-holes preserved

Sample 6A (Plate 6, sample A) shows a coarse gabbro from the Tyndrum area. The main mineral is hornblende (possibly altered from pyroxene), a dark-coloured slightly greenish mineral which here occurs in large crystals which glint in the light. In between the hornblende crystals is feldspar, a pale-coloured material which seems to fill the spaces between the hornblende. Quartz is scarce or absent in these rocks.

Gabbro is a plutonic rock which crystallised at considerable depth in the crust. It is exposed on the surface because millions of years of erosion have removed kilometres of overlying rock. The Cuillins in Skye are composed of gabbro, and they have a rugged appearance. In contrast, the more rounded Red Hills in Skye are made of granite.

Cuillin, Skye

Mountains of gabbro - this view looks north-west into the heart of the Cuillins, towards Loch Coruisk.

When this dark coloured magma crystallises nearer the Earth's surface to form dykes or sills it is medium grained in texture and called dolerite (Sample 6B). A sill is horizontal (like a window sill) because the rising magma was intruded between sedimentary layers when it reached the limit of its buoyancy. Salisbury Crags is an example of a dolerite sill, and with the overlying sediments now eroded, the resistant dolerite has formed a prominent scarp dominating the Edinburgh landscape.

Salesbury Crags

Salisbury Crags, Edinburgh. This is a thick dolerite sill which was intruded horizontally
between beds in a thick sedimentary sequence. The sediments above the sill have now been eroded away.

Dykes are so called because they sometimes erode more slowly than the surrounding rocks and can be left standing above the general terrain just like a stone dyke. A spectacular example of this can be seen on the west coast just south of Ardfern, about 20km south of Oban (photo on page 7). Sample 6C is an example of a fine-grained dolerite which occurred as a narrow dyke in the Tyndrum area and which was intersected during drilling for gold. This type of dolerite is black and attractive when polished and is used for some gravestones under the trade name ‘Black Granite', although it does not belong to the granite family at all. In very thin dykes and sills the rock may be so fine grained that it is more appropriate to call it basalt, as in some of the thin dykes which cut through the schists on the Loch Fyne shore just south of town.

Dolerite

Dolerite dyke cutting through schist, shore of Loch Fyne at Strachur

If this dark magma reaches the surface of the Earth and is erupted from a volcano, the material cools quickly to a fine-grained lava called basalt (Sample 6D). Basalt is a very common rock in the west of Scotland. The spectacular columns at Fingals Cave on the isle of Staffa were formed by rapid cooling of a basalt flow. Sometimes a lava cools so quickly that the steam holes are ‘frozen' in the rock (Sample 6E). These holes are called vesicles (pronounced VEE-sicles), and can be quite large. Sometimes they become filled with other minerals such as quartz, or the semi-precious stone called agate (for example along the beach at Wormit in Fife). The circles on a cut agate are a cross section of the coatings of silica which were progressively deposited within the vesicle until the hole was completely filled, with each successive layer being slightly different in composition from the one before. The middle of some agates is filled with quartz crystals. Agates can be seen in many local shops, but note that bright colours such as red, blue and purple are usually from dyes.

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