As humans we marvel at natural landscapes and how ‘Nature’ has carved and eroded them over eons of time. Our ancestors saw these natural landscapes and landforms as sacred and spiritual and embodied mountains, rivers, hills, waterfalls, trees and rocks with earth spirits, which they worshipped and appeased. They knew they were an integral part of the natural cycles of the seasons and were dependent on nature for food, water and survival.
To a geologist, every natural landform and landscape we see provides a window into the geological processes that have been at work over millions of years. These processes give insights into how a landscape was formed; how it has evolved over time; and the soils, plant life and crops it can support. Whilst scientists talk of geological processes occurring over millions of years, it is more accurate to think of them as slow, on-going processes which are happening all around us, all the time. For example, tectonic plates move at a speed similar to the rate of fingernail growth i.e., 2-5mm/year. Mountain ranges such as the Himalayas, the Andes and the Alps have been uplifted and eroded over millions of years; and the Himalayas continue to rise at the rate of a few millimetres a year as the Indian plate moves inexorably northwards. Beneath the Earth’s surface pressures build up slowly to be released dramatically in a matter of seconds in the form of earthquakes, which can cause massive changes on the surface. For example, the Indian Ocean tsunami of Boxing Day 2004 was caused by a ‘mega-thrust earthquake’, which ‘unzipped’ a section of the sea floor along many hundreds of miles and built huge underwater cliffs in a matter of seconds. On a different scale, pebble beaches along many British coastlines can change their shape overnight, when hundreds of tons of pebbles are moved by a combination of high tides and strong winds.
When it comes to understanding the landscapes we see and, the energy flows many people are now feeling, it seemed appropriate to include something about geology on this website. I have, however, tried to keep the content informative and light, on the basis there are few readers who want, or indeed need, a science lecture at this time. For those readers who would like more detailed information, there is a reading list at the end of this webpage.
The geologist observes and studies rock forms, landscapes, volcanoes and the sea bed, to try and understand the ongoing processes and events which have caused this planet Earth to be the way it is today. In addition, studying and understanding previous geological events, and patterns of events, provides insights into how the Earth might look in future, given current trends. For example, there have been periods in the Earth’s history when the planet was mostly covered by ice; and others when there has been no ice at all. And then of course, many rocks and minerals have a huge monetary value and play an important part in the material, highly connected world we currently live in. For myself, I am a ‘late bloomer’ and it wasn’t until I was in my 50’s that I began to study geology, part-time, with the Open University.
The following overview is based on ‘A Guide to Rocks’ by Chris and Helen Pellant, which is available as a Field Studies Centre guide (Pellant & Pellant, 2003).
Rocks are all around us everywhere; in our landscapes, exposed on seashores and in river valleys; or in our gardens, where they are used for decoration or paving; or on our road surfaces and even the slates on our roofs. In Cities, towns and villages our buildings are commonly made of local stone or brick, which is made from clay; and our office blocks and shops sometimes decorated with polished slabs of beautifully crystalline granite. Even the ubiquitous concrete is made from a mixture of sand and limestone.
Rocks are made of minerals, in fact a broad definition of rock is ‘an aggregate of mineral particles’. These minerals are combinations of elements arranged into compounds, and the commonest rock-forming elements in the Earth’s crust include silicon, oxygen, aluminium, iron, manganese, magnesium, calcium, sodium and potassium. A small variety of minerals go to make up most rocks; including silicates (minerals of silicon, and oxygen combined with various metals) such as feldspars, micas, amphiboles and pyroxenes; quartz (an oxide of silicon) and calcite (calcium carbonate).
There is, and always has been, a continuous cycle of rock formation on this planet; and there are three main groups of rocks: Igneous (or volcanic) rocks, which are formed by the cooling and crystallisation of molten material generated deep in the Earth’s crust or upper mantle. Sedimentary rocks, which form on the Earth’s surface from the accumulation of particles, from other eroded rocks; and are characterised by layering, with grains that can sometimes be easily rubbed off. And Metamorpic rocks, which are created when previously formed rocks are buried at high pressures and/or temperatures that cause permanent chemical changes. These rocks are crystalline and some types have a wavy structure reminiscent of layering.
Igneous rocks: are the first rocks to form in the rock cycle. When molten material is underground it is called magma; and once erupted onto the surface, it is known as lava. Magma may rise slowly towards the surface under pressure, to form intrusions near the surface, called sills (horizontal) or dykes (vertical). Depending on its chemical composition and gas content, lava may flow quietly out onto the surface as a sheet of molten rock e.g. shield-type volcano, as in Hawaii; or it may erupt violently from a steep-sided volcano, with clouds of gas, ash and rock segments. Lavas tend to have very small crystals, because they cool rapidly. Examples of igneous (volcanic) rocks produced by cooling lavas include basalts, andesites and rhyolites; and all reflect the magnetism of the earth at the time they were formed. On other occasions, rising magma may cool and solidify at relatively shallow depths (e.g. 6km below the surface), as a large mass of igneous rock called a pluton or a batholith. Such igneous intrusions can be tens of kilometres across and are commonly found in the roots of mountain chains. Because these vast masses of rock retain their heat and chemical activity for what can be many millions of years, large crystals can form as they slowly cool. These igneous, or plutonic rocks as they are sometimes called, occur as granites, diorites, and gabbros and have large crystal textures. They also reflect the magnetism of the Earth at the time they were formed. Granites can also be highly radioactive, dependent on the elements and minerals present when they are forming. For example, granite from the Rubislaw Quarry in Aberdeen has been used in buildings all round the world, including the Sydney Harbour Bridge. What many people don’t know though, is the granite is radioactive; although, I hasten to add, not at dangerous levels for passers-by. But it is a standing joke amongst local Aberdeen geologists, to take visitors through the main streets of Aberdeen with a Geiger counter and watch their faces as the readings go off the scale.
Sedimentary rocks: are the second to form in the rock cycle; and they occur as three main types:
- Detrital sedimentary rocks: are composed of fragments produced by the weathering and erosion of pre-existing rocks, including newly formed mountain chains. Weathering is the breakdown of rock in situ ; by either the freezing of water in joints and cracks in the rock; or chemical weathering by rainwater; or biological activity. Erosion involves rock breakdown through movement; and can occur in river systems, during the movement of glaciers, or by wind action. Along a coastline, for example, the sea is very effective in eroding cliffs and wave-cut platforms. The rock fragments broken off by these processes range from very fine clay particles to huge boulders. When these have been transported and naturally sorted (often by size), for example in a river; they are then deposited and sedimentary layers begin to form. The vast majority of sedimentary rocks are made in the sea, as layers of sand and mud on the sea bed. Others are formed in desert regions, in lakes and rivers and, by glacial deposition. Geologists will use the content of layers, bedding features and grain size/shape to determine which processes were involved. For example, congolmerates and breccias are coarse-grained, often with large-sized pebbles and small boulders; whilst sandstones are medium grained; and shales and clays fine grained. Grain shape will indicate how the grains have been transported. For example, wind-blown grains from a desert sand are rounded, whilst grains in a marine sand are angular; and pebbles in a water-deposited congolmerate are rounded, whilst those from a glacial environment are angular.
- The most common mineral in sedimentary rocks is quartz, mainly because it is durable, both mechanically and chemically and, resitant to erosion and weathering processes. Sandstones have a very high quartz content and come in a variety of colours dependent on the minerals they are coated with. Red sandstones have a coating of iron oxide (hematite); yellow sandstones are coated with iron hydroxide (limonite); and green sandstones are coated with a greenish mineral called glauconite.
- Organic sedimentary rocks: contain mainly organic material. Limestones, for example contain fossil shells with a high percentage of calcite and calcium carbonate; whilst Chalk is an organic rock composed of the remains of microscopic marine creatures. Then there is coal, which is formed from decaying, dense, vegetal matter and has a high carbon content. Peat is the earliest stage of coal formation and is a brown ‘rock’ with roots and plant fibres still visible in it. The transformation from peat to coal involves burial beneath a great thickness of sediment, with the consequent heat removing water and other impurities.
- Chemical sedimentary rocks: include some limestones, some ironstones and evaporite rocks like rock gypsum and rock salt. All are the result of various chemical processes that occur after burial; or, are the result of fluctuating sea levels.
Metamorphic rocks: are the third in the rock cycle and are produced when previously formed rocks are buried under high pressures and temperatures. After metamorphism (meaning: a change of form) takes place, some features of the original igneous, sedimentary or earlier metamorphic rock may be visible, but chemical changes will create completely new rocks. No melting takes place during metamorphism and if it does, then igneous rocks (e.g., granites) will form. The chemical changes that occur are completely dependent on the chemical composition of the original rock prior to metamorphism. During metamorphism however, if the right elements are present, precious and semi-precious stones can be formed. Examples of the changes that can occur include:
- Clays and mudstones → slate (with very closely spaced, almost perfectly flat foliation surfaces – which is what makes slate so great for roofing)
- Mudstones with abundant mica → schist or mica schist (moderately spaced, undulating foliation surfaces with abundant mica – great for street cobbles and pathways)
- Igneous rocks, including volcanic rocks and granites → gneiss – pronounced ‘nice’ (widely spaced undulating foliation surfaces, often with bands of different chemical composition)
- Limestones → marble
- Sandstones → quartzite (extremely hard and looks like snow from a distance)
Diamagnetic vs Paramagnetic rocks: in the mid 1980’s a Dr. Philip Callahan (Callahan, 1984), suggested that certain types of geology emit subtle influences in a landscape and rocks have inherent magnetic qualities. All substances are either diamagnetic or repelled by a magnetic field e.g. organic material (peat, coal), water, chalk and pure limestone; or paramagnetic i.e. they attract magnetic fields, e.g. sandstones, granite and metals, like ironstone and slate. These paramagnetic qualities are mainly due to the quartz silica content of the rocks, which adopt the magnetism of the Earth at the time they are formed. He goes on to argue that organic soil with a high concentration of iron-rich clay is highly fertile, because of its balanced consistency of both diamagnetic and paramagnetic ingredients; a quality akin to the chi of the Chinese having a correct balance of yin and yang energies.
Callahan, D. P. (1984). Ancient Mysteries, Modern Visions. Acres, Kansas City, USA.
Cally Hall, S. O. (1995). Earth Facts. New York and London: Dorling Kindersley.
Hall, C. (1994, 2000, 2012). DK Handbooks Gemstones. London, SW11 7BW, UK: Dorling Kindersley, Penguin Random House.
Pellant, C., & Pellant, H. (2003). A Guide to Rocks. Shrewsbury, Shropshire, England: FSC (Field Studies Council) Publications.
Other useful geology sources for ‘beginners’
Edmonds, E. (Crown Copyright 1993). The Geological Map: an anatomy of the landscape. Institute of Geological Sciences, London. ISBN: 011 880721 8
Atkinson, Richard and Francis (1979, 1996). The Observer’s Book of Rocks and Minerals. Claremont Books, London. ISBN: 978185471044
Roberts, John L. (2001). A photographic Guide to Minerals, Rocks and Fossils. New Holland Publishers, London. ISBN: 9781859749395
Fejer, E., Frampton, S. and Fitzsimons, C. (1991,1993). Lett’s Pocket Guide to Rocks and Minerals. Charles Letts & Co Ltd. ISBN: 978185238117
Hall, Cally (1994,2000, 2012), DK Handbooks Gemstones. Dorling Kindersley Limited and Penguin Random House, London, SW11 7BW. ISBN: 9781405357975
There are also a number of very useful websites with geology based information, that can be found with a simple online search. For example: https://www.volcanodiscovery.com/volcanoes.html is excellent for finding out about the volcanoes of the world, where they are located and the different types, etc.