Welcome to the Fact File page. I hope you will find some of the information easy to read and informative. Factually, it is as accurate as I could make it with what I consider to be a very limited knowledge of science, geography and natural history. I have had to refer to reference books for some information and these are credited below at the bottom of the page. There are odd snippets however, where relevant, where my own belief system in respect to God and the creation of the Earth have crept in. These are my ethics, in these instances, you should make up your own mind!
It is impossible to have an interest in rocks, minerals and fossils without wondering how they came to be formed or where they come from. I have always struggled with the fact that not everyone finds them fascinating! That whilst some people like myself find them awe inspiring, breathtakingly beautiful, nature at its best and intriguing, other people just see them as little pieces of rock.
When I hold a particularly beautiful piece of rock or mineral, I am reminded that in my hand, is something of extreme beauty that nature formed quite literally in some cases, many millions of years ago. In comparison, the Human race could be viewed merely as young upstarts who live approximately three score years and ten and then die. In reality, in terms of the Earth's history and time line that is not even a quick blink of the eye!
Geology in terms of science could be considered to be relatively new. As recently as the Middle Ages, ammonite fossils from the north of the UK were sold to the religious as Christian relics who believed them to be snakes petrified by St Hilda. However,, it has provided support for many other sciences including history and natural science. It was geology that determined whether the Normandy landing beaches in World War 11 would be able to support the heavy tanks and machinery needed for the British offensive toward occupied France.
We can also tell from geology how different the coast line was in 1066 compared as at today. For instance, the Battle of Hastings took place at Battle in Hastings. If you saw the battle field today it is a far cry from the layout in 1066. Previously it was more hilly and marshy and a lot further from the coast line than it is in distance today.
A basic knowledge of minerals and their uses was held as far back as prehistoric times. As far back as 3500 BC copper and tin, gold and silver were used, as well as many gemstones, for jewellery, weapons and tools. Minerals systems were not considered until Aristotle (384 - 322 BC) wrote the first scientific publication complete with a mineral system.
During the middle-ages, mineralogy, as a science, remained dormant whilst the natural sciences focused around alchemy, astrology and the occult powers of stones. It wasn't until the 16th Century that a revival of natural sciences took place. During that time, Dr Georgius Agricola (1494 - 1555), who is known as the "father of mineralogy" wrote a book of observations, a scientific presentation of mining and minerals. His systematic subdivision of minerals remained valid into the early part of the 19th Century.
The industrial revolution gave mineral sciences another push start. The demand for raw materials saw a need for scientific foundations in mineralogy before ore extraction and the development of new deposits could be found. The Saxon mineralogist A.G. Werner (1749 - 1817) devised a new classification for minerals, the basis of which is still valid even today.
So how do rocks and minerals form then? What separates the classification of a rock from a mineral? How can we tell different minerals apart and how do they relate to each other in terms of chemical structure? How old is the Earth? What kind of structure does it have? How old are the fossils that were once living creatures? To answer these questions in part, we must start from the beginning and ask ourselves, how and when did Earth itself, the mother of these natural wonders, come into being?
Indulge me in the following section while I give a very brief potted history of the relevant bits!
In the Bible, the book of Genesis states that God created both the world, animal and human kind in six days and took a break on the seventh day. Darwin's Theory of Evolution, combined with the theory of Natural Selection and the fossil record has unequivocally proved the fixed species theory to be a myth, or if you like, a parable. (For an explanation of the theory of evolution & natural selection, click here.)
Science further suggests that the Earth and the universe as we know it, to be the result of a massive nuclear explosion, or a "Big Bang", the ripples of which are still occurring thereby expanding and widening the universe even as we sit reading this! Now in terms of history, this is a fairly new, albeit widely accepted theory. My understanding of the big bang is as follows.
Approximately 15 Billion years ago God was very bored. He went into his laboratory and he picked up an atomic nucleus he happened to have lying around ( known as a 'singularity') and tossed it around a bit as he thought what to do with it. At approximately 10 minus 43 seconds ( measured after time and space were created by the following events) he dropped the nucleus as it accidentally slipped out of his hands. After muttering the immortal words "oops that's buggered it!" and a few more choice words that the Anglo-Saxons' came to view as their own, there was a terrific explosion.
The temperature of the explosion reached trillions of degrees above any scale invented by man, What occurred was not only infinitely dense, creating subatomic particles and matter, but it also created time and space itself. The explosion caused this nucleus the size of an atom to expand to the size of a grapefruit in a tiny fraction of a second.
So God decided to run with what was happening. At 10 minus 32 seconds He took some nuclear energy, some gravity and some electromagnetic energy and turned it into a seething hot mass of electrons, quarks and other particles. He let it cool down for a while causing the quarks at 10 minus 6 seconds to combine into protons and neutrons, but all he ended up with was a very dense fog with no light. He decided to let it cool down some more and 300,000 years later it reached the temperature of 10,000 degrees C. He combined the electrons with the protons and neutrons which formed in turn, hydrogen and helium atoms.
In the blink of his eye, he had created light!
The first stars that had been created in this explosion started to die, spewing heavy elements into space. God took these elements, placed them in his preferred choice of position in relation to the sun and lo and behold, the planets and stars were created! The one he chose to create life upon, (Earth, at least in this Galaxy), was formed in approximately 4.5 billion BC.
The heavy element we call Earth, before it cooled, was a giant sphere of molten magma suspended in space. During the process of cooling, the heavier metallic minerals began to gravitate towards the centre of the earth due to the force of gravity. The lighter compounds and chemicals raised themselves to the surface as an oceanic Solicitor hot and fiery magmatic lava.
As the surface of the Earth cooled down, the minerals with the highest melting points started to form crystals. The very first crystal structures were formed several billion years ago! The structure of the rocks that formed depended upon the rate of how quickly it cooled and what chemicals the magma contained. If it cooled quickly, the result was a fine grained, or glassy textured rock with crystals not visible to the naked eye. If it cooled slowly, a courser structure occurred where the individual crystals could be seen by the naked eye.
The Earth's crust started to mature. It became thicker and cracked during the cooling period. Deep below however, volcanic intrusions of magma occurred. Molten lava along with gasses and vapours were forced into cracks within the older rocks and here, they cooled, hardened and formed crystals and minerals.
This process has not declined and indeed, still occurs today deep within the earth's core many miles below the surface. It occasionally evacuates through volcanoes and geysers.
Bishop Ussher (1581 - 1656) had used the time lines contained in the bible, (which at that time was considered to be accurate in it's portrayal of the fixed creation theory) to calculate that the Earth was 6006 years old. According to his calculations, it had therefore been created around 4004 BC. It wasn't until during the 19th Century that science began to realise through river erosion and fossil records in rocks that the Earth was indeed much older. The now generally accepted age of the Earth is around 4500 million years old.
The history of the Earth is quite literally written in the rocks. We have been able to divide the rocks into clear units by sequence. The fossils contained within them have given us knowledge of how life, and rocks and minerals developed in geological time.
Approximately in 3 Billion BC the first signs of primeval life, taking the form of bacteria and blue green algae, appeared in the seas and oceans. The first fossils can be traced back in existence to 600 Million BC. Life began to develop over the various era's below.
The Earth had a little bit of a hiccup during the Cretaceous period. No one knows for definite what actually happened but it is evident that God must have decided to go back to the drawing board again and do a little tweaking with his experimentation on life creation.
The life that had formed, most famously the dinosaurs, were suddenly wiped out. What happened? Here are a brief précis of the most popular theories.
In 1978 Walter Alvarez, a Nobel-prize-winning physicist, and his son Luis, first proposed the idea that the dinosaurs extinction was due to a meteorite strike. This was widely disputed in scientific circles. Such an impact would have been massive and left a distinguishable crater of great size.
In 1991 a crater that fitted the criteria was found, buried beneath the coastline of the Yucatan Peninsula, Mexico. The central crater, left by the impact 65 million years ago is at least 180 km wide and nearly 20 km deep, The outer impact rings may stretch to 300 km in diameter, making it the largest crater produced in the solar system in the last 4 billion years. However, the fossil record shows that the dinosaurs were already in gradual decline before the meteorite hit. The impact upon the Earth from the meteorite would have been catastrophic. Scientists produced graphics to show the impact that such a blast would have.
The blast itself was a million times stronger than the world's combined nuclear arsenals. North America was hit and razed to the ground by a fire ball in 30 seconds. Within an hour, the whole world was in flames. In hitting sulphate - rich rocks, the effect was more catastrophic as the impact and heat vaporised them, causing a massive spew of sulphuric acid, measuring billions of tons into the Earth's atmosphere.
The blast caused a sulphurous, dusty atmosphere which left the world in darkness for 6 months. Global temperatures stayed near freezing for almost a year. Clouds of nitric acid that had been caused by the impact created rain showers of nitric acid. The top 100m of all the Earth's oceans became stagnant acid.
There is some thought that rather the impact being that of a meteorite, it may well have been a comet that crashed.
What would have been the effect upon life on Earth?
The effect upon life, animal, marine and plant would have been massive. The long periods of darkness would have almost certainly killed off plant life, which need sunlight to survive. This in turn would have killed off the herbivores (plant eating dinosaurs) who lost their life sustaining food. In turn, carnivorous (meat eating dinosaurs) would have died too.
Acid rain collecting in fresh water environments would have affected amphibians and fresh water fish very badly. Sea plankton would have been affected by the lack of light just as the plant life had been. Anything else would more than likely have been killed off by the sudden drop in arctic temperatures.
It has been suggested that such an event as this would have been the catalyst, sparking off volcano eruptions and changes in the sea levels which would bring all the other theories into play.
Another popular theory about the downfall of the dinosaurs is that a series of volcanic eruptions at the end of the Cretaceous period seriously affected the Earth. Theory suggests that the effects would have been similar, and just as deadly as the meteorite theory.
Long periods of darkness would have starved the plants of the required photosynthesis leading to the death of plant life, resulting in the death of herbivores, and in turn, carnivores. The fresh water fish would have been unable to have survive the highly acidic water. The darkness would have killed of the plankton, but deep water marine life may have had a chance of survival. Reptiles would also have been hit hard by the freezing temperatures.
The world, until approx. 100,million years ago was dominated by plant life, known as 'gymnosperms'. These plants did not have flowers and consisted of, amongst others, conifers and ferns. Toward the latter end of the Cretaceous period, flowering plants, known as 'angiosperms' appeared. They began to diversify and grow increasingly in number.
Although mammals existed alongside the dinosaurs, they were comparatively low in number. They started to develop and grow in number at the same time as the angiosperms began to form.
The new species theory argues that the dinosaurs may not have been able to have digested the new plants. The plant eating dinosaurs would have died off leading also to the death of the carnivores, who depended upon them for their food. In other words, the food chain was destroyed from the bottom.
It is worth bearing in mind however, that if we follow Darwin's theory of natural selection, this should surely have led to some life evolving the ability to survive on the new food.
The rise of the mammal species it is argued, could also have led to "egg eaters" who reduced the reptile population by eating their eggs.
Another common theory about the extinction of life, as shown in the fossil record, is that climatic changes occurred on Earth, with warm weather melting ice and raising the sea levels.
It has been suggested that sea levels had dropped prior to this in the Cretaceous period. The lakes and inland seas, common on the continents would have been cut off from the sea. The dense salt water contained in the stranded lakes would not be able to flow. Sense salty water is responsible for currents in the sea so the oceans had no current and would stand still. Currents form when denser water sinks to the bottom of the ocean or mixes with less dense water.
Eventually the seas would have started flowing again due to the melted ice at the poles (due to the heat) raising the water levels. The plankton life may also have started to recover.
The changes in the currents would have had a catastrophic affect upon marine life and plants. Normally, the nutrients provided from dead life would sink to the bottom of the ocean to be circulated by the currents and then rise to the surface. The life that depended upon this process for their food and nutrients would have died off.
On land, the retreating seas and rivers would have caused major changes to the land environment, causing huge coastal plains cut off from each other. Fragmentation of land can cause extinction to wild life. This being the main habitat of the dinosaurs, they would be heading towards extinction with the larger dinosaurs dying first.
Researchers at Leeds University have suggested that the climatic changes may have reduced the number of female dinosaurs being born. They suggest that like crocodiles, eggs depended upon certain temperatures to produce females. The cooling climate reduced the amount of females and therefore appropriate mates to propagate the dinosaur dynasty.
The final theory that caused the death of the dinosaurs is that of the exploding star theory. supernovas, may produce cosmic radiation leading to cancer amongst Earth's animal life. This is supported by the fact that the fossil record shows mass extinctions occurring at regular intervals. However, there have been many reasoned explanations put forward about the cause of the gaps.
This theory is not as in depth as the others as its predictions are unspecific. Such an occurrence would mean that larger animals would be hit harder than plants and smaller animals. For instance, plankton should not have experienced much damage.
Whatever the cause, life began once again to develop. For want of a better word, the prototype of today's man was thought to have existed from around 4.4 Million years BC.
Homo habilis (Skilful Man) is evidently in existence as far back as 2.5 million years BC. He is believed to have used stone tools.
Homo Erectus (Upright Man) is thought to have emerged around 1.8 million years BC. By 70,000 BC we had the advanced
Neanderthal man who used fire and advanced tools.
It was Aristotle, the Greek philosopher who as far back as 340 BC put forward a coherent argument to support the fact that the Earth was round and not flat, but it took Christopher Columbus to prove this point. He shocked the (known) world by failing, as predicted by a lot of important people, to have sailed his ship off the supposedly flat Earth into oblivion. (Thereby making said important people to look very silly indeed)!
Aristotle further put forward the view that the Earth was stationary and that the sun, the moon and the planets and stars moved in circular orbits around the Earth. This was elaborated by Ptolemy in 2 AD but it was a Polish priest called Copernicus in 1514 who theorised that it is actually the Sun that is stationary and that the Earth and the planets move in circular orbits around the Sun.
As an interesting aside, Stephen Hawking, the author of "A Brief History of Time" (page 1) recalls how Bertrand Russell once gave a public lecture on Astronomy. He described how the Earth orbits around the sun and how the sun, in turn orbits around a vast collection of stars called our galaxy. A little old lady at the back of the hall reputedly stood up and said to him "What you have told us is rubbish. The world is really a flat plate supported on the back of a giant Tortoise". Russell was said to have smiled in a superior manner and asked, "What is the Tortoise standing on?". "Young man you're very clever, very clever", said the old lady. "But its turtles all the way down!". Anyone who has had the pleasure of reading the Disc world novels by Terry Pratchett will see a similarity in thought!
It was long known by man that gravity existed within the Earth. Even the ancient druids had drawn the conclusion that the sun, the moon, the tides and the seasons somehow worked together. Hence they built altars like Stonehenge c 2000 BC for Pagan rituals and worship of the moon and all things natural.
However, Issac Newton (1642 - 1727) gave us the Theory of Gravity following a famous incident when he watched an apple fall to the ground. Whilst it was commonly known that dropped items fell to the ground through the pull of gravity within the Earth, Newton worked out that it doesn't apply to the Earth alone, but also to objects in space. In other words, every particle of matter in the universe gravitates toward every other particle of matter with a force inversely proportional to the square of their distances. Newton also came forward with the theory of how prisms work, the secret of light and colour.
So why is gravity important when considering rocks and minerals? Because it is a major force shaping the Earth. It concentrates heavy materials like Iron into the core of the Earth whilst lighter properties dominate in the crust of the Earth.
Even assuming that the tunnel was protected from the intense heat and pressure at the centre of the Earth, you would not enjoy the experience. Gravitational pull gradually drops down to zero from the Earth's crust to the centre of the core. At first, you would be pulled down through the hole and past the core to the other side traveling at approximately 120 miles per hour. However, whilst you would go some way to reaching the other side, it would not be long before gravity pulled you back to the centre again. You would bound back and forth experiencing what you could call the bungee jump of your life. However, wind resistance would eventually stop you where you would float around in the core of the Earth in zero gravity for ever.
(Now be honest......who amongst us hasn't immediately thought of at least one anally retentive colleague or relative you would like to see attempt such a feat)!!
So we go back to the world's creation outlined in Genesis and how God, being lonely, created man in his own image thereby bringing Adam and Eve into existence. This fundamental statement of faith does not obviously fit the facts gleaned from archaeology, science and natural history. In fact, if it wasn't enough to set the cat amongst the pigeons that the world was found to be round and not flat, along came Charles Darwin who gave society a real fit of the vapours.
Darwin argued in his thesis, "The Origin of the Species" that man was in fact, descended from Apes. That basically all forms of life have evolved from primitive means and adapted to the world around them accordingly. That their development over time was a process of evolution combined with the theory of natural selection. This had the effect of riling the "important" people even more (having not yet recovered from the embarrassment of finding out that the world was round). It went against everything written in the book of Genesis which was believed by the Christian faith in the literal sense. Darwin was considered to be a heretic.
As we know now, science has expounded on that theory and it is born out by the evidence of discovery of the remains of early man. Could it be that both explanations of how the Earth and life began can be pacified by taking the stance that the Book Of Genesis is a parable? The six days of the creation equalling millions of years as opposed to our understanding of the 24 hour day?
Really the answer lies in one's own individual belief system and how we reconcile our knowledge with our faith. The above is my own personal belief. Personally, whilst I have made some irreverent comments regarding God's omnipotence they are indeed tongue in cheek. In my opinion, this world is far too beautiful, thought out and magnificent to have been created in any other way other than by God.
The structure of the Earth can be classified into three different structures, the core, which is very dense, a mantle, also deep and a crust - the top, which is fairly thin. The core has a solid inner layer with a liquid outer layer. Gravity, discussed above, concentrates the heavier materials down nearer the centre of the Earth, whilst the lighter materials lay more towards the top within the Earth's crust.
The upper mantle of the Earth is made up of plates. New plates are formed at constructive plate margins, usually around ocean ridges and they move apart at a rate 2 -18 cm per year. This process is created by heat which produces large scale convective currents in the mantle. The heat helps to move the plates around the surface of the Earth. Whilst new plates form, the plates at the destructive plate margins move down to subduction zones where they melt to form magna, thereby balancing the whole process. Just imagine! This is constantly happening all the time, so the world we live on is in a constant state of change, with the majority of us not even aware it is happening! (that is, apart from the odd Earthquake or volcanic eruption)!
A mineral is something natural made from inorganic substances. Its make up will include elements or substances, usually two or more, that cannot be broken down into other substances by chemical means. Some minerals like gold only have one element, these are known as "native elements".
Rocks are made up of lots of mineral grains and vary in quantity. Some may have one dominant mineral quantity but will always have a percentage, albeit a small one, of other minerals present.
Fossil fuels are are elements like coal, oil and natural gas. They are known as fossil fuels as they are formed from organic chemicals, those that were once living matter.
It is hard to identify minerals as many of them look the same. Generally you have to have a lot of experience and a practiced eye. There is also a lot of what can be seen as baffling jargon in relation to the properties of minerals. The most common are identified here with brief explanations.
The bullet point headings are discussed in more depth here.
A very clever chap called Friedrich Mohs, an Austrian mineralogist devised a scale in 1812 to measure how strong minerals are. The scale allows us to determine hardness of minerals, or to be precise, the resistance of each mineral to scratching or abrasion. He defined a scale from 1 - 10 number 10 being the hardest. Now the clever thing about it is that each mineral can scratch a lower number, but it, in turn, can only be scratched by a higher number!
So, to find out how hard a mineral actually is, all you have to do is find one on Moth's' scale that will scratch your specimen and find another mineral that your specimen will scratch!
To experiment for yourselves you can use the following guide:-
Cleavage refers to the fact that a lot of minerals, when broken, tend to break along sets of well defined planes due to the internal arrangement of their atoms. For example, a basal cleavage occurs with some minerals where the atomic bonding is stronger within the sheets of the mineral as opposed to between them.
This defines the shape of the broken surface of the mineral. Not to be confused with cleavage! There are four common terms to describe fractures:-
Tenacity refers to how a mineral will react to shock, cutting, bending or crushing.
A sectile mineral can be cut with a knife and the slice broken by a hammer, for example graphite.
If a mineral is malleable, a slice from the mineral can be flattened when hammered, like copper!
If a mineral is flexible, thin sheets of the material will lean to being bent, like talc.
A mineral that is elastic will demonstrate the bent portion of a slice bouncing back into shape.
Brittle, most minerals come under this heading and will crumble when hit, like fluorspar.
The principle of this theory is that specific gravity equals the ratio of the weight of a body to that of an equal volume of water. Most minerals and rocks show a specific gravity within 2.5 - 3. However, generally, the darker the specimen, the higher the specific gravity - in principle! This is not very accurate though as there are exceptions! Graphite for example, dark grey graphite has a specific gravity of 2.23 and feels light! The theory is not accurate so should only be used as a useful indicator or guide.
The relative density of the mineral can be summed up as being the weight per unit volume weighed first in air, then water (which has a specific gravity of 1)
SG = Weight in air divided by the weight in air minus the weight in water.
Some minerals have distinctive properties that can be useful in identification. Here are a few!
Crystals form from minerals that are able to grow naturally and unrestricted. They are more or less symmetrical. The shape that the crystal takes, is dictated by the internal pattern of the atoms inside it. Some amazing crystals are formed from those allowed to develop slowly without interference. Others are more prone to developing quickly. There are a few minerals that only vary rarely form crystal structures and these are said to be amorphous, that is, they form masses with no structure.
The type of shape that the crystal develops in is known as its form. These can take many shapes and one mineral once converted to a crystal can take many individual forms on its own. A 'closed' crystal occurs when the form totally encloses a space, an open crystal is not closed at the ends and always combines with other forms.
The crystals can form in many habits, some of which are described below.
The symmetry of crystals fall into seven distinct groups. Crystals can show one or more planes and/or axes of symmetry. Larousse, (see credit notes) gives a good example of how this works. A matchbox can be cut three different ways to form two parts, one half being the mirror image of the other. This therefore shows three planes of symmetry.
The matchbox also shows three lines of axes of symmetry, each passing through the centres of two opposite faces so when it is rotated once about each axis, the same view appears twice. The matchbox is therefore said to have three axes of twofold rotation. Crystals can have three, four or six fold rotation.
Crystals can form twins. These can be identified by angles on the crystal that points inwards - known as a re-entrant angle. Twinning can occur when crystals meet across a flat surface or where halves seem to have grown into each other. These are known as interpenetrant twins. They can occur singularly or repeated (polysynthetic)
Minerals have individual chemical compositions which are expressed through chemical formula. Those minerals with only one element are known as native elements although most minerals are made up of two or more elements. The two charged parts are ions, the negatively charged anions often contain oxygen. Chemical tests can help in identifying minerals. Dilute hydrochloric acid placed on calcite for example, will fizz and give off a gas. Place it on quartz however, and there will be no reaction.
You can view the chemical formulas and the atomic numbers here.
Crystals usually only form in fissures cavities and joints. There are only a few minerals that contribute to the rocks in the Earth's crust. These are known as rock-forming minerals.
Mineral deposits form by special processes in several ways. When magma cools to form igneous rocks, minerals can become concentrated in particular areas. Pegmatites can be formed in the last stages of crystallisation of the magma. The late stages of the cooling of the magma can see heated gasses with volatile elements stream into the adjacent rock. The minerals contained eventually crystallise as pheumatolitic ore deposits. During the final stages of the magma cooling hot fluids called hydrothermal solutions can react with the host rock to form veins of ore or gangue.
Minerals can also drastically alter when exposed to surface weather. Deposits react with the air and water and it can seep down and react with the primary ores, forming oxidised ores above the water table and secondary sulphide enrichment below it. Metasomatism can occur with the host rock reacting with hot fluids creating new minerals.
Rocks can be placed in three groups. Igneous, metamorphic and sedimentary rocks.
Igneous rocks are formed from hot magma rising up from the lower crust or upper mantle which then cools and solidifies. Here we can classify Igneous rocks into two categories, intrusive and extrusive. Extrusive rocks forms from magna that is ejected from the crust, for example from volcanoes and is left to cool. Intrusive rocks form from magna which solidifies underground.
The cooling rate of the ejected magma is obviously quite quick and impurities do not have time to form. This results in fine grained or glassy rocks where there hasn't been time for crystals to grow. Obsidian is an example. However, igneous intrusions have a far slower cooling rate giving crystals more time to form.
The main textural feature of an igneous rock is its grain size. Fine grained rocks measure less than 0.1 mm across so cannot be seen by the naked eye. Medium grain has crystals that can be seen between 0.1 mm& 2 mm in size. Course grained rocks have a grain size of approx. 2 mm and above. Sometimes, large crystals that have a higher melting rate can appear on finer grained material which cools quicker. This is said to be situated in a 'matrix'.
Rocks can also be classified by structure. For example, a layered or banded structure can be evident vesicular structure can occur where bubbles of gas escape from the magma leaving behind cavities known as vesicles in the rock. Or a zenolith can occur. Zenoliths are pieces of other rock embedded in an igneous rock.
An igneous rock can also vary in colour.
Igneous rocks contain essential minerals as part of their identifiable make up, they usually come from a small group of rock-forming silicates. Most of them carry as few as two to four essential minerals.
Minor mineral deposits in the rock are called accessory minerals. Rocks with a colour of less than 90% are identified using a QAPF scale. A diagram which allows plotting and the naming of a mineral dependant upon its felsic minerals. Rocks can also be classified by the amount of silica contained. The content can be classed as acid, intermediate basic or ultrabasic.
This type of rock is produced by alteration of all types of rock in the solid sate under high pressures and/or temperatures. This involves the recrystallization of existing minerals or formation of new ones. It usually changes in texture from the original rock type. Rocks can be subjected to elevated pressures and temperatures during mountain forming episodes, usually referred to as regional metamorphism.
Mountain building is a serious and time consuming task! The forces involved cause the rocks to be broken to form faults and bent to form folds. This can affect several hundred kilometres at a time.
Dynamic metamorphism concerns the alteration of rocks under high pressure but low temperature.
Contact or thermal metamorphism can occur around igneous intrusions where the heat bakes the surrounding rock to form a contact metamorphic aureole.
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The degree of metamorphism is referred to as grade and increases from low to medium to high grade with increasing temperature or pressures. Higher grade rocks have a more crystalline texture.
Texture of metamorphic rocks
Grain size varies in metamorphic rocks. It becomes coarser as the temperature increases.
Colour & Mineralogy of Metamorphic Rocks
Metamorphic rocks can sometimes but not always, be identified by their colour. They are generally light to dark grey but with a few exceptions. For instance, Quartzite and marble are recognisable by a sparkling white colour. and Serpentines have distinctive red and green streaks with a waxy appearance.
The term 'assemblage' is used to denote the total number of minerals in a metamorphic rock.. There are generally not very many. They will depend upon the composition of the original rock and whether recrystallization was completed.
Basically, sedimentary rocks are formed from either the result of organic remains, or through physical or chemical processes. A process called lithification takes place where the sediments are buried, squashed and over time, converted into rock. The most common type are detrital or clastic sediments. These occur where weathered rock fragments get transported by wind or water and accumulate in one place to form sediment. The sediment accumulates and compact. Material builds up on the top acting as a filter to water the result of which turns the sediment to rock.
The other common way for sedimentary rock to form is from organic matter, namely plants or animals. Most of the calcium carbonate content comes from the skeletons of marine life which form limestones. Coal and oil are important sediments formed from organic matter.
Another form of sedimentary rock comes from a high percentage of salt in high temperature of hot dry regions from shallow seas or lakes. These type of sedimentary rocks are known as 'evaporites'.
Texture & Structure of Sedimentary Rocks
The main textural feature of sedimentary rocks are made up of grain sizes. The range is huge from boulders down to microscopic clay particles. Fine grained rocks are known as argillaceous, medium grained arenaxeous, and course grained, rudaceous. Where grains travel a lot through water a rounded grain is produced. These are reworked by tidal waters, for instance at the beach and are roughly sorted out to the same size! A sediment with well sorted and rounded grains is said to be 'mature', whilst angular grains are said to be 'immature'. Immature grains are unlikely to have traveled far from their area of source.
rock's history can basically be seen by looking at its structure, rather
like counting the rings on a tree stump to tell its age! Each stage leaves
Larousse, Field Guide to the Rocks & Minerals of the World. Wendy Kirk & David Cook. ISBN 0-7523-0051-2 an excellent reference book written in terminology for the beginner but also enough information for the more knowledgeable.
An Introduction to Geology - Brian Lee ISBN 0 946284 53 9 (1987)
www.bbc.co.uk history and natural science web site.
The Colour Treasury of Gemstones - Dr Eduard Gubelin Gemologist C.G. F.G.A (1969) ISBN 0 7290 0041 9
Collins PhotoGuide Rocks, Minerals & Gemstones W Schuman - ISBN 0 - 00 - 219909-2
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