Rocks and Time - A Beginner's Guide to Planet Earth PDF

2. Rocks and Time 2.1 Geology Rocks! When we look at the natural world around us today, in many places it is dominated by the influence of our human habitation. But bulldoze away the houses, roads and walls, scrape off the trees, grass and soil, and underneath it all you will find the rocks which...

2. Rocks and Time 2.1 Geology Rocks! When we look at the natural world around us today, in many places it is dominated by the influence of our human habitation. But bulldoze away the houses, roads and walls, scrape off the trees, grass and soil, and underneath it all you will find the rocks which make up the outer shell, or crust, of our planet. Quite simply, rocks underlie everything. Luckily, they poke through in a good many Rocks (or 'bedrock') underlie whatever loose soil, places, or peep out sediments (such as 'drift' from the last Ice Age), vegetation or buildings we see on the surface. from the edges of the soil and buildings at our coastlines. But, Soil, vegetation, buildings, roads & why should we care recent sediment about rocks? Rocks are formed ‘bedrock’ either inside the Earth or at its surface in a myriad of different ways. Ask the rocks the right questions and they are dying to tell you how, 13 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH where, when and why they formed, and this is a tiny piece of a great jigsaw puzzle which ultimately builds into a picture detailing the origin and evolution of our planet over billions of years. In fact, the basic building-blocks of rocks are the crystals of different minerals, typically only a millimetre or two in size. Many rocks are composed of a mixture of several different 'species' of mineral, either as a network of interlocking crystals, or alternatively, a mass of crystal grains stuck together. What type of mineral, or mixture of them is present in a rock such as a granite or a limestone, and whether they are crystalline or granular is actually determined by how and where the rock formed. So you will often see a geologist armed with a hammer to knock off a fresh bit and a hand lens, ready to read the story locked inside the rocks. Now, there is much more to geology than just looking Rock-forming minerals exist either as a network of interlocking crystals which have precipitated or at rocks. But as crystallized from a fluid, or as eroded and redeposited will become clear mineral grains (held together by a cement). over the coming Chapters, much of the narrative story of how our planet evolved and how we know, comes from nearly two and a half centuries of painstaking 14 INTRODUCTION TO GEOLOGY 2. ROCKS AND TIME study, of which reading and interpreting the rock record around us takes a central role. 2.2 Rock Classification We can broadly classify Molten basaltic magma pouring into the sea rocks into three main from lava tubes at Kilauea volcano, on the Big Island of Hawaii. categories. Igneous rocks are those which have formed hot from molten rock, called magma, which originates below the Earth's surface. If this molten magma cools and crystallizes below Granodiorite (from Jamaica) – an example of the surface then the an intrusive igneous rock, where molten rock it forms is magma has cooled slowly below the surface to referred to as an grow a network of large, interlocking crystals. intrusive igneous rock. Where magma reaches the surface, for instance erupting from a volcano, it forms an extrusive igneous rock. Sedimentary rocks, as the name suggests, 15 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH are made from sediment and are formed on the surface of the planet. One type of sediment, known as clastic sediment, is comprised of the loose fragments of a previous rock (lithic fragments), and/or Approximately 100,000 year old river grains derived from sediment from the East African Rift Valley (Lake Edward, Uganda); consisting of a soft individual crystals in a mixture of quartz and feldspar grains, with a previous rock, few rock fragments of schist, set in a soft clay. together both known as clasts. These are produced during weathering and erosion of rocks exposed at the surface, particularly in mountainous, or upland, areas. Transported clastic sediment will eventually Shallow water limestone from Tanzania, packed with shell debris weathering-out be dumped, or deposited, on its surface. to typically form distinct layers known as strata (singular; stratum), or beds. Alternatively, sedimentary rocks can be formed by directly precipitating the minerals from water, and this is usually seawater. Limestones are a good 16 INTRODUCTION TO GEOLOGY 2. ROCKS AND TIME example of this type of chemical precipitate, and because they typically form in warm, shallow tropical seas, they are often also packed with the debris of fossil shells, plates and spines of the organisms that would have been living there. Finally, metamorphic rocks can be formed from any previous rock type literally by Schistose gneiss (from Uganda) – a metamorphic metamorphosing rock that has been heated and squeezed so much, them into that the original minerals have melted and recrystallized in distinct bands, which themselves something different. have then been sheared. This process takes place below the surface of the planet and is achieved by applying heat and/or pressure to a rock to change its physical properties or mineralogy until it no longer really resembles the original. Metamorphic rocks will typically be formed on a regional scale during the intense heating and squeezing which takes place when building mountains, involving high temperature (T) and/or pressure (P). But it can also take place on a more localised scale, with high temperature contact metamorphism of cold rock sitting against a molten magma intrusion. 17 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH 2.3 Minerals and Crystals As we have mentioned Simple summary of rock classification. already, the basic component building blocks of rocks are minerals. Minerals exist in a crystalline state. Therefore, they can be found in rocks as a network of interlocking crystals, such as in igneous rocks, metamorphic rocks and chemically precipitated sedimentary rocks. Or, alternatively, they can exist as a mix of eroded and re-deposited crystal grains, such as in clastic sediments or volcanically-derived fragments and grains, known as volcaniclastics. Of all the different types of mineral which exist on Earth, an amazing 90% of the crust that we live on is composed of those made from silica. In fact, all these silicate minerals build their crystals by simply sticking together a basic building block, made of an individual silicate tetrahedron, in five different ways. Silicate tetrahedra are made from a lone, positively charged silicon ion (carrying a charge of +4), surrounded in a four-sided, triangle-faced tetrahedron by four negatively charged oxygen ions (each carrying a charge of -2). This gives the SiO4 tetrahedron an overall negative charge of -4, so in order to form a stable compound with a neutral charge it must do one of two things. Firstly, either stay as a single tetrahedron but attract 18 INTRODUCTION TO GEOLOGY 2. ROCKS AND TIME in some positively charged ions to join the party, such as iron or magnesium (both ions of these elements carry a positive charge of +2). Or, secondly, get The Silica tetrahedron and its representation as the basic building block in silicate friendly with a minerals. neighbouring silica tetrahedron and share oxygen ions. In reality, of course, they can do a bit of both between these two end members, and hence the five different crystal lattice structures that give rise to five main families of silicate minerals. The first family are known as island or independent, and pack positive ions around each of their individual tetrahedra in order to neutralise the charge. A good example of this group is the mineral olivine, which uses both Fe2+ and Mg2+ to do this. The next group joins tetrahedra together into a long chain sharing an oxygen ion at each link in the chain. This still gives an overall negative charge, so again some helpful positive ions are also needed in the structure. Pyroxene is a good example of this type of mineral. Double chain silicates take two single chains in parallel and then share oxygen ions where the chains touch. Sharing more of the oxygen means double chains need fewer positive ions in the lattice. Amphiboles are a common double chain silicate. The mineral mica arranges its tetrahedra into hexagons which join together to make flat sheets, arranged 19 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH one on top of the other, with cations in-between. Finally, the fifth family is known as the framework silicates. These go for maximum sharing of oxygen ions and can form tightly packed crystal lattices with no need for positive ions in the structure. Two of the most common minerals in Earth's crust, quartz and feldspar are such framework silicates. The five main families of silicate minerals as a function of the way they stack or join their silica tetrahedra. Ultimately, crystal lattice structure determines the physical and chemical properties of a mineral, such as hardness, shape (known as habit), density, colour, and how it fractures. We can often identify particular minerals in a rock by hitting it with a hammer and seeing how the minerals within it split open; micas 20 INTRODUCTION TO GEOLOGY 2. ROCKS AND TIME fracture between their tetrahedra sheets, whilst amphiboles will split between Characteristic fracture planes (cleavage) in pyroxenes and amphiboles as a consequence of whether they their double contain single or double chain silica tetrahedra; chains. single chain pyroxenes have two fractures at 90°, but with a double chain in amphiboles, the fractures With silicates occur at a wider 120° angle to each other. accounting for 90% of all rock- forming minerals, there is not much room for non-silicates. By far the most important of these are the carbonates and sulphates. Carbonates, typically composed of the mineral calcite, take a calcium ion and bond it to carbon and oxygen to form CaCO3; calcium Gypsum deposit, Tanzania; this mush of carbonate. Calcite is crystals is soft enough to be scratched by your fingernail. the mineral component of the rock, limestone. In sulphates, such as the mineral gypsum, the calcium ion is this time bonded to sulphur and oxygen to form calcium sulphate. Gypsum is naturally precipitated from 21 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH seawater when it evaporates, for instance in a tropical lagoon, so it occurs as a hydrated form of calcium sulphate, CaSO4·2H2O. If it is buried the water can be driven off to form a dehydrated, slightly denser mineral, known as anhydrite. 2.4 Geological Time We discussed the emergence of Geology as a Science in the last Session. The Neptunist – Plutonist debate saw the discovery of ‘deep time’ and Earth’s internal heat by the Edinburgh Doctor, James Hutton. There was no way in 1795 that James Hutton could have imagined just quite how old the Earth actually is, although he could appreciate the immensity of geological time. We now know from measuring the slow decay of radioactive isotopes, such as uranium, locked into some minerals, that the Earth is 4,550,000,000 years old. In geology, we use the American definition of a billion as a thousand million, so that makes the Earth just over 4.5 Baron ‘Georges’ Cuvier (1769- billion years old. But the use of 1839). From a portrait by an radioactive decay in isotopes of unknown artist. Cuvier is depicted here about the time of his work on an element to date rocks was ‘catastrophies’. only discovered by Rutherford in 1911. Interestingly, one of the first applications he made of his discovery was to try and find the age of the Earth. So, for the whole of the nineteenth century nobody knew the actual age of the Earth. What the 22 INTRODUCTION TO GEOLOGY 2. ROCKS AND TIME Victorian geologists did instead was to acknowledge the huge amount of time that Earth had existed, and erect a geological timescale which was relative. Relative to what you may ask, and the answer is fossils. An Englishman, William Smith (1769-1839) and a Frenchman, Baron Leopold Chrétien Fréderic Dagobert Cuvier (known to his friends as Georges) (1769-1832), were the first geologists, around 1811, to independently make the observation that certain layers of sedimentary rock appeared to contain a characteristic and recognisable assemblage of fossils. Fossils are any signs of life that once lived on our planet, preserved in the rocks. More commonly these will be shells or bones, but they can be anything as diverse as leaves, burrows or footprints. Moreover, when Smith and Cuvier found layers with different fossil assemblages in them, one overlying the other, they were always found in the same order. Assuming that the layer on top must be the younger one, using the law of superposition, then Smith and Cuvier hypothesised that the fossils in the overlying strata must have lived at a different time, younger than in the strata below. Therefore, following these observations, characteristic fossil assemblages could be used to work out the ages of the strata relative to each other; which is older and which is younger. This may seem obvious if you are looking at a cliff face with them in, but as we said right at the beginning of this Chapter, quite often rocks are poorly exposed at the surface. Using this method, Smith and Cuvier were able to produce the first ever geological maps, of England and Wales, and the Paris Basin respectively. These maps used different colours to 23 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH indicate rocks with different fossil assemblages in them and consequently a different relative age. The Smith / Cuvier method of producing the first geological maps and a composite relative geological timescale: Two cliffs, A and B, contain distinctive fossil assemblages. The top of cliff A contains the same assemblage as the base of cliff B, and therefore the two must be the same age. Consequently, the middle and top of cliff B must belong to younger Periods than anything exposed at A (N.B.: The actual Geological Period names were coined later). Soon the Victorian geologists were giving these different map colours names to signify that they were of a different geological time period, so producing a relative geological time scale. Many of these early workers were British, and in an age of romantic poets, they used ancient British celtic tribal names in many of these, or the geographic area where they were best found exposed. So, the Cambrian Period takes its name from the Cambrian Mountains in central Wales and the Ordovician and Silurian Periods are both from the British Celtic tribes of the 24 INTRODUCTION TO GEOLOGY 2. ROCKS AND TIME Ordovices and Silures, and so on. These names for periods of geological time persist today, and all that has happened since the discovery of radioactivity and radioactive dating techniques is to place absolute numbers on the age of each period to produce an absolute geological time scale. Simplified Geological Time Scale (see also p. 433). 2.5 Catastrophism or Uniformitarianism? Baron 'Georges' Cuvier had another role to play in the early development of geology as a science. To explain the sudden change seen from one characteristic fossil assemblage to another 25 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH in the strata, Cuvier hypothesised that the animals and plants were wiped-out by terrible floods unlike anything we see happening at the present day. New and more complex organisms would appear after each flood event to take the place of those that had gone extinct. This idea found much support from the geologists of the day as a still commonly held view was that the Earth was gradually cooling over time. Shifting climatic belts as the ice from the poles crept towards the equator might therefore be expected to trigger great catastrophic floods. In fact, this theory soon became dubbed Catastrophism. However, after 15 years of travelling widely across Europe on geological research, a young Scottish landowner, Charles Lyell (1797-1875) finally came up with a compelling challenge to catastrophism. Lyell had been trained in his youth as a lawyer, and he felt strongly that to invoke a hypothesis which we had no means of testing was unscientific. A young Charles Lyell (1797- 1875) in 1836, only a couple of Cuvier's ideas may have been years after all three volumes of correct, but as Lyell argued, if his ‘Principles of Geology’ had these catastrophic floods were been published. From a portrait by J. M. Wright. unlike anything that we see on Earth today, how could his hypothesis be supported by evidence? Lyell realised that the only way to be scientific about the geological history of planet Earth, was to interpret it in terms of the geological principles and processes that we can see in operation in the 26 INTRODUCTION TO GEOLOGY 2. ROCKS AND TIME world around us today. We must observe what geological processes are forming particular rocks now, see the way in which they operate and the timescales involved, and then apply this evidence to interpret how similar rocks must have formed millions of years ago. In a letter to his friend Roderick Murchison in 1829, Lyell set out this principle of Uniformitarianism. His forthcoming book, he wrote: '...will endeavour to establish the principles of reasoning in the science [geology] which are that no causes whatever have from the earliest time to which we can look back, to the present, ever acted but those now acting; and that they never acted with different A single, basalt lava flow on the Cape Verde Islands, showing columnar jointing degrees of energy from (top half of cliff). Applying that which they now uniformitarianism, we must assume that it exert'. would have formed in the same way, and over the same period of time, that we can observe other basalt lava flows moving, Lyell’s uniformitarian cooling and crystallizing today. principles are often today 27 C. J. NICHOLAS A BEGINNER’S GUIDE TO PLANET EARTH reduced to a single paraphrase of, ‘the present is the key to the past’. However, his original book called 'Principles of Geology', ran to 3 volumes and was so influential upon publication (between 1830-1833) that a young Charles Darwin set off on his voyage in the Beagle with a copy of Volume I tucked under his arm, determined to make his name as a geologist. *** 28