A restless earth
THE EARTH’S CRUST
This rock collection relates to the Earth's crust, which is the surface layer of rocks measuring from 7km to 40km (4 miles to 24 miles) thick and containing the highest mountains and the deepest oceans. Importantly, the surface rocks have weathered to form soil, a thin, nutrient-rich layer which supports all land plants from forests to agricultural crops, which in turn provide a habitat and food source for all land animals, including mankind. The rocks also provide the stone, sand, and metals for everyday living. The science of rocks or the study of rocks is called geology.
Although we see little change from day to day or year to year, parts of the crust are continuously on the move. We are familiar with the sudden movements called earthquakes, but most geological movements are very slow. The surface of the Earth consists of huge crustal plates which move at a few centimetres per year. At 3 centimetres per year, this amounts to 30 kilometres over a million years, or 300 kilometres over 10 million years. Over time these giant plates meet in slow collision, and one plate (usually a thin oceanic plate, 7 to 10km thick) sinks below another (usually a thicker continental plate, 35 to 40km thick), wrinkling the edge of the continental plate as it passes underneath (hence the chain of mountains down the west side of South America). Less commonly, two thick continental plates collide, causing a buckling of the crust - hence the high plateau of the Himalayas.
The plates that make up the crust – note that a plate is not the same as a continent
The tip of a descending oceanic plate starts to melt as it nears the base of the crust, where the temperatures are very high. This molten rock or ‘magma' starts to ascend, until it reaches a position of equilibrium according to its density, anywhere between 5 kilometres depth and the surface of the Earth, including eruption on to the Earth's surface. Continental volcanoes are a familiar concept but lavas can also erupt from volcanoes under the sea, with the lava pouring on to the bottom of the ocean.
Schematic diagram of descending oceanic plate melting at depth.
NATURE OF THE CRUST
The crust consists of three major groups of rock, classified according to their origin. These are igneous rocks, which form from molten rock, sedimentary rocks which are consolidated mud, sand, and pebble deposits derived from erosion, and metamorphic rocks which are rocks of any origin which have been reconstituted due to ‘cooking' at depth.
Rocks which derive from melts or ‘magmas' are called igneous rocks. If the magma crystallises several kilometres below the Earth's surface where the surrounding rocks are still quite warm, the liquid cools very slowly allowing large crystals to form and the rock eventually becomes solid. These coarse grained igneous rocks are called plutonic rocks. The commonest plutonic rocks are granite or gabbro, depending on the chemical composition of the melt (discussed later). The Cuillins on Skye are mostly made of gabbro, as is the Fairy Knowe, that prominent knob in the hills which stretch from Strachur to Strathlachlan. In contrast, Ben Cruachan and its neighbouring peaks are made of granite.
Prominent on the skyline above Strachur is Fairy Knowe, a plug of gabbro
A magma's buoyancy may take it to just short of the Earth's surface, so that it spreads horizontally, wedging between layers of near-surface sediments to form horizontal intrusions called sills; cooling is quite rapid and the resulting crystals are medium sized. These medium-grained rocks are called felsite or dolerite, depending on the composition of the melt. Salisbury Crags at Edinburgh is a good example of a dolerite sill.
If magma fills a vertical fissure the intrusion is called a dyke. The cooling is quite rapid so the rock is medium grained in texture, and may be fine-grained at the edges where the liquid was in contact with cold country rock (chilled margin).
The Purcell Sill, Mount Gould, in Glacier National Park, Montana.
The dolerite magma was intruded horizontally into sediments and the sequence has now been exposed by erosion.
Upstanding dolerite dyke, south of Ardfern, Argyll.
If the magma pours out on to the Earth's surface through a volcano, cooling is very rapid indeed and the result is a fine-grained
lava, classified either as basalt or rhyolite (pronounced RYE-o-lite), depending on the composition of the original melt. Basalt dykes occur on the shore of Loch Fyne southwards from Strachur Post Office. They tend to occur in swarms, with each dyke roughly parallel to the others, and swarms can extend for many tens of kilometres.
Sometimes a magma starts to crystallise at depth, with some large crystals being formed, then the magma moves nearer to the surface and finishes its crystallisation in cooler conditions. This gives a medium-grained rock with large crystals scattered throughout a finer-grained rock. Geologists call this a porphyritic rock and the large crystals are called phenocrysts.
Commonly, magma reaches the surface of the Earth it is erupted either on to dry land or on to the sea floor, and the result is lava. Lava can be fluid (such as basalt, the compositional equivalent of gabbro) or thick and viscous (such as rhyolite, the compositional equivalent of granite).
Hot basalt lava on the Big Island of Hawaii.
This will cool to form a dark fine-grained rock as in Plate 6, sample D.
All rocks which form high ground are subject to erosion. The weather, including wind, rain, ice formation, rapid cooling and heating, all weaken the fabric of the rock to produce fragments and grains which are washed toward the sea or into lakes. The debris forms piles or layers of sediment which consolidate to form sedimentary rocks. Different rates of flow of the streams and rivers bring changes to amount and type of sediment carried and deposited, so that the sedimentary pile consists of layers of different colour and different composition. A slow-moving stream carries mud, whereas a faster flowing stream carries coarse sand and small pebbles. Erosion makes little visible difference on an annual basis, but it is impressive over time. It is believed that the highlands of Scotland were once as high as the Alps, and the "missing" material is now represented by the huge thicknesses of sedimentary rocks of younger geological age. It is this gradual erosion of kilometres of rock that eventually exposes on surface the granite and gabbro intrusions which crystallised at considerable depth.
As rocks become buried they become subject to heat and pressure in a process called metamorphism. The heat recrystallises the rocks to form new minerals, and these new rocks are called metamorphic rocks. The most common metamorphic rocks are schist and gneiss (pronounced ‘shist' and ‘nice'). Schists contain lots of mica flakes which lie flat within layers, making the schist an easy rock to split. Schist is the commonest rock type in the hills of the southwest highlands. It is well exposed in a small cliff face near Creggans Inn and in outcrop on the loch shore. The most common metamorphic minerals in the southwest highlands are white mica, black mica (biotite, pronounced BYE-otite), hornblende and garnet, which are described later in the text. In contrast, gneisses are very hard, tough rocks and although they are banded there is no tendency to split along these bands. A typical boulder of gneiss from Strachur beach is illustrated below, and another appears on the front cover of the book.
Banded gneiss from Strachur beach
Gneiss is not common in the southwest highlands. North eastwards towards Aberdeen and northwards towards the northern highlands the degree of metamorphism increases (the rocks were subjected to greater heat), forming gneisses and schists containing a variety of other metamorphic minerals, but these are not described here.
At a few kilometres depth where the rocks are quite hot (but not near their melting point) they can become deformed onto folds due to movements within the crust. Later earth movements - sometimes called tectonic forces - often bring these folded rocks to the surface, or the folded rocks can be exposed on surface by gradual erosion of the rocks above them.
Near-surface rocks are cooler and more brittle and they tend to fracture when subjected to stresses, forming fissures and faults. In other words they break, rather than bend.
Bending and breaking of rocks (folding and fracturing) is called deformation.