The earth science SPLATs

What are SPLATs? They are explained here.

The principles of world structure and shape

• Our world is a large sphere, spinning once a day, and moving around the sun once a year. The moon is a smaller sphere which orbits the earth once a month.

• The study of the shape of the Earth is called geodesy. The shape was measured originally by taking the length of a degree of latitude in different places.

• The Earth's shape was no mystery to the ancient Greeks, who used observations of the horizon, and the planet's shadow on the Moon as proof that it was a sphere.

• Some time before 200 BC, the mathematician Eratosthenes, who lived in Alexandria, had estimated the circumference of the Earth with an error of about 4%.

• While most educated people and sailors all knew that the world was a globe, many of the less educated people clung to the old notion that the world was flat.

• In 1492, Christopher Columbus, like every other educated person of his day, was well aware that the earth was a sphere, and he also knew Eratosthenes' estimate.

• Because Eratosthenes gave his estimates in terms of a unit that had no standard, using the stadion, later scholars had troubling understanding his estimate.

• Christopher Columbus used a low estimate of the size of the stadion, and an overestimate of the land distance to China, and thought China was closer to Europe.

• In 1521, Ferdinand Magellan's crew completed the first circumnavigation of the Earth that he had commenced, establishing for all to see that it was a sphere.

• The circumference of the world is close to 40,000 kilometres, so that the distance from the North or South Pole to the equator is close to 10,000 kilometres.

• The Earth is not a perfect sphere, being slightly flattened at the Poles. This can be shown by measuring the length of one degree in low and high latitudes.

• Isaac Newton showed theoretically what we now know from measurements as a fact, that the planet is an oblate spheroid, a flattened sphere rather like a pumpkin.

• The extent of the flattening of the Earth is small, so the difference between the polar radius and the equatorial radius is about 22 kilometres, or about 0.33%.

• The rotation of the Foucault pendulum proves that the Earth rotates once in 24 hours. The rate of rotation is a function of the latitude of its location.

• A Foucault pendulum at the equator does not appear rotate at all when it is set going, while at the poles, it rotates once in twenty four hours, like the Earth.

• The period in hours of a Foucault pendulum's apparent rotation, when it is located at latitude lambda, is given by twenty four divided by sine lambda.

• The Earth has a magnetic field, but the magnetic poles are not in the same place as the geographical poles which lie on the planet's axis of rotation.

• We do not understand exactly how the Earth's magnetic field is generated, but it is assumed to be something to do with the iron core spinning around.

• Karl Gauss worked out where the geomagnetic poles are, placing the southern pole over the ocean and triggering a series of expeditions to locate the poles.

• In other words, it is possible for Antarctic navigators to sail to the south magnetic pole or even to the south of the south magnetic pole.

• At different places around the world, it is possible to measure the magnetic deviation, the difference between magnetic and geographical north or south.

• From time to time, about every 200,000 years, the Earth's magnetic field reverses, exchanging north and south in a period of perhaps a few thousand years.

• On average, there are four or five reversals every million years, but there has not been a reversal of the magnetic field in the past 800,000 years.

• The strength of the Earth's magnetic field has dropped by about 15% in the past 150 years, and some scientists think we may be heading for a reversal. 

The principles of plate tectonics

• Continents are made of lighter (less dense) crustal rock that floats on the more dense rock of the mantle, forming plates that can be pushed around.

• The continental crust makes up the continents, although small parts, like central America, are made up of material that has been uplifted by tectonic forces.

• Under the deep oceans, the crust is thinner, as there is much less mass of crust to support, and the mantle comes closer to the average surface of the planet.

• Areas of mountain lie above very thick crust, in just the same way that large icebergs extend further above and below sea level. This is called isostasy.

• In some cases, pieces of sea floor may be lifted up above the sea surface. This sort of formation is called an ophiolite and it reveals hidden processes.

• The surface of the planet is shaped by plate tectonics, because plates, as they collide, cause massive upheavals in the form of earthquakes and volcanoes.

• The surface of the earth is made up of plates that are in motion, driven by convection currents. Plate tectonics studies how the world changes over time.

• We can measure the actual movement of tectonic plates today by GPS stations located at fixed points. Most plates move about as fast as a fingernail grows.

• Good evidence of tectonic movement can be found at a mid-ocean ridge, where the sea floor can be shown to be spreading, seen in patterns of magnetic striping.

• In 1595, Abraham Ortelius suggested the Americas were "torn away from Europe and Africa by earthquakes and floods", an early suggestion of continental drift.

• In 1620 Francis Bacon pointed out the jigsaw fit of the opposite shores of the Atlantic Ocean, a first step to drifting continents and plate tectonics.

• In 1910, Alfred Wegener noticed the close 'fit' between the west coast of Africa and the east coast of South America and started thinking of continental drift.

• In 1912, Alfred Wegener developed his theory of continental drift based on fossil and glacial evidence, and first lectured about continental drift.

• In 1926, geologist Arthur Holmes saw that the Earth's internal heat had to go somewhere, and argued that there may be convection currents in the Earth's crust.

• In 1960, Harry Hess proposed that new sea floor might be created at mid-ocean rifts and destroyed at deep sea trenches, a key to plate tectonics.

• In 1963, Vine and Matthews explained the stripes of magnetized rocks as due to sea floor spreading and the periodic geomagnetic field reversals.

• The tectonic plates are thought to be moved by convection currents operating deep down in the Earth, bringing hot molten rock closer to the surface.

• Evidence for past movements of tectonic plates comes from the locations of fossils, geology and the observed distributions of plants and animals today.

• The movement of different tectonic plates across the globe influences the distributions of animal and plant species and how they later evolve, when separated.

• Evidence that the plates are still moving comes from measurements with GPS equipment and from magnetic striping in the sea floor caused by polar reversals.

• On a large scale, geological structures such as mountain chains and island arcs relate to and are caused by the movements of the Earth's tectonic plates.

• Volcanoes are commonly found where plates are in contact, as there are planes of weakness there that plunge deep into the earth, and frictional effects.

• Mountains are most commonly formed when forces operate as the result of two tectonic plates coming in contact with each other and forcing material upwards.

• On the evidence, there was once a supercontinent that is now referred to as Pangaea. It later divided to parts now given the names Laurasia and Gondwana.

• Earthquakes are most severe when one plate moves under another, a process known as subduction. Japanese earthquakes involve subduction triggering deep quakes.

• The southern distribution of some groups of plants and animals reflects their origins in Gondwana. Laurasia gave rise to most of the northern continents. 

The principles of isostasy and orogeny

Various evidence tells us about the earth's inner structure, although the best evidence comes from the way earthquake waves are transmitted through the planet.

The Earth has a crust at the surface, a mantle below the crust, and a core at the very centre. The crust is less dense than the mantle, and floats on it.

The Earth's crust and the mantle are separated by a division called the Mohorovicic discontinuity, the point where the density of the planet changes.

There is a standard pattern at continental fringes: the continental shelf joins the continental slope that ends in the continental rise near the abyssal plain.

Under very high pressure, many solids such as rock and ice will flow as viscous fluids. They will also bend without breaking under some conditions.

The bending of rocks in the past can be seen in anticlines, synclines monoclines, each structure being formed by compression and other forces.

Isostatic effects in the earth include ice loading in Ice Ages and the later rebound, as the previously loaded rocks are able to rise back up from the mantle.

Mountains are a part of the less dense crust that floats on the more dense mantle. Mountain 'roots' sink deeper into the mantle, a principle called isostasy. 

The principles of glaciation

• Glaciers are made of solid ice flowing under pressure, flowing fastest at the top, midway between the sides, where it is furthest from any frictional effects.

• The earth has experienced a series of Ice Ages, detectable today because glacial erosion creates very distinctive landforms which indicate past history.

• The most obvious glacial landforms are moraines, cirques, drumlins, hanging valleys and 'erratics', which remain long after the glaciers are gone.

• One of the best indications of past glaciation comes from wide U-shaped valleys, quite unlike the V-shaped valleys made by flowing water in streams and rivers.

• As a Swiss, Louis Agassiz saw plenty of glaciers, and in 1839, he had discovered that a cabin, built on a glacier in 1827, had moved about 1.5 kilometres.

• Louis Agassiz drove a straight line of stakes into a glacier, and found they moved into a U shape as the ice flowed faster in the centre than on the edges.

• Glaciers transport sediment and also create it by grinding the base of the valley with the rocks they drag over the lower surface, producing 'rock flour'.

• Varved shales form when there is a regular seasonal variation in stream flow with summer flows being greater than winter flows, as in glacial outflows.

• At the moment, glaciers are melting, all over the world, which may mean summer droughts in areas reliant on the release of summer meltwater, like India. 

The principles of groundwater

• Water that falls as rain soaks into the ground, and fills the spaces between the particles in soil and rock. This groundwater flows slowly to the sea or lakes.

• Groundwater occurs wherever the geology allows it to exist, provided there are surface supplies available to top it up as it seeps to the sea, or is taken out.

• The ground below a certain point is saturated with groundwater. The surface of this zone is called the water table. Wells fill to the level of the water table.

• Groundwater moves through an aquifer with a rate of flow that depends on the aquifer's permeability and the gravitational gradient that it is flowing down.

• Groundwater supplies are a strictly limited resource, but they can be recharged in some areas by disposing sensibly of stormwater into the aquifer.

• Around the world, many areas are taking groundwater out faster than the aquifer is being recharged by rainwater. Much of this water is being wasted.

• A cone of depression forms in the surface of the water table when water is drawn from a well faster than it is replaced by inflow or local recharging.

• Wars in the first half of the 21st century will mostly be about supplies of agricultural and potable water, both from dams and rivers, and also groundwater.

• At a spring, the water table reaches the surface, at a river or a lake, the water table also reaches the surface, instead of being somewhere below it.

• Around the world, many wells deliver dangerous amounts of arsenic and other dissolved minerals which are capable of causing serious health problems.

• When too many trees are cut down, rainwater can reach the water table faster than it flows away, so that the water table rises, sometimes to the surface.

• When a water table reaches the surface, it brings dissolved salt with it. Sometimes the salt is concentrated enough to kill plants, a process called salination. 

Rocks and rock cycles

• The material that we call rock goes through cycles, being melted, weathered, eroded, buried and eventually heated and compressed until it melts again.

• Rocks are mostly made of minerals or their weathering products, which were, at one stage of their existence, crystalline, and may still be crystalline.

• Rocks erode and re-form in the rock cycle. The process involves chemical and mechanical weathering, erosion, transport, deposition and compaction

• Rocks and soil erode. Water transports sediments downstream, and the sediment particle size in a stream depends on the speed of the water flow.

• Sedimentary rocks which are buried under a sufficient load of more recent sediment may be compressed and heated so that over time they form metamorphic rocks.

• Metamorphic and sedimentary rocks may be melted to form magma that later becomes an igneous rock, but usually they are metamorphosed long before they melt.

• The properties of rocks are determined by the minerals in them, and the minerals in rocks often reveal their origins, even the depths at which they formed.

• Most minerals in rocks are present as crystals: the size of the crystals in igneous rocks shows how quickly they cooled, with slow cooling giving big crystals.

• The hardness of minerals can be used to distinguish them by finding what they can and cannot scratch, and what will scratch them, using Mohs' scale of hardness.

• Rocks that cooled slowly at first may contain phenocrysts, because when the cooling speeds up, the phenocrysts may be surrounded by smaller crystals.

• Rocks weather chemically, with minerals changing chemically, often to soluble material which leaches away, resistant particles like quartz, and clay minerals. 

  The principles of fossils

• Fossils are traces of old life forms which need to be interpreted, allowing for changes in death, and the warping caused by the compression of sediments.

• Some fossils are too important to be left in private hands, which is why many fossil sites are off limits to amateurs who are likely to do more harm than good.

• The best fossils are formed from living things when the material of a live organism is replaced by other material that is fine-grained and slow to deposit.

• When conditions are just right, dead animals and plants may be preserved in sedimentary rocks. Over time, these fossil remains are chemically changed.

• When an animal dies, the long bones and the skull contain marrow and brain, and will be worked over by scavengers, so little remains of those bones.

• When an animal dies, some parts like the mandible (jaw), finger and toe bones, and the ends of the long bones, offer little nutrition and are usually left.

• Many fossils form by chemical replacement, where the material of the living organism is replaced by other, longer-lasting material, unrelated to the organism.

• Decayed plants may leave phytoliths as traces, and these can be recovered from deposits and used as hints on previous climates and crops by archaeologists.

• Some fossils form when bones are buried, and over time, the mineral material of the bones is replaced by other chemicals, even opal, dissolved silica.

• A stromatolite is a very ancient fossil form which is still around today. They date back to at least 3.5 billion years, possibly 3.8 billion years.

• The Burgess Shale of Canada contains very unusual fossils, as does the Ediacara formation of Australia, because they represent Precambrian forms.

• In 1947, geologist R. C. Sprigg discovered a rich deposit of Precambrian fossils in the Ediacara Hills of South Australia, the 'Ediacaran fauna'.

• Sometimes, a living fossil may be found: this is an animal or plant that was previously known only from fossils, and which is now found to still exist.

• Examples of living fossils include the ginkgo tree of China, the coelacanth of Africa (and more recently, Indonesia), and the Wollemi pine of Australia.

• There are many fossil types: some are formed when something rots away, leaving a mould that can be filled by minerals in groundwater, seeping slowly in.

• Very few things that die will ever be fossilized, as the dead animal or plant must be buried in oxygen-free conditions, and quickly covered by fine sediment.

• Fossils tell us what the past was like, the types of plants and animals that lived in an area at the time when the fossil-bearing sediments were laid down.

• Similar deposits in different areas may be linked by stratigraphic correlation, by looking for marker beds, identified by unusual minerals or fossils.

• Rocks which contain fossils of the same species are usually similar in age, and for this reason, fossils are often used to correlate strata over wide areas.

• Living things may perform the same function in different ways: lungs, gills, spiracles and diffusion are all used to supply oxygen in different groups.

• Legs in different animal groups are quite different in structure, even if the same homeotic genes are involved, and even if they serve much the same function.

• Arthropod legs have an exoskeleton with internal muscles, while vertebrate legs have an endoskeleton: presumably these two forms of leg evolved independently.

• In 1695 John Woodward published his 'Essay toward a Natural History of the Earth', saying that fossils formed when Noah's flood destroyed the Earth's surface.

• 1696 William Whiston published his New Theory of the Earth, which suggests that Noah's deluge might have been caused by a comet striking the Earth.

• In 1705 Robert Hooke's posthumous 'Discourse of earthquakes', completed 1668, speculated on the geological mechanisms responsible for the fossil distributions

• In 1787, Caspar Wistar described in Philadelphia a large bone, said to be a thigh-bone of a large animal, almost certainly duck-billed hadrosaur.

• Wistar's fossil has since been lost, but given the source, and Wistar's anatomical skill, it must have been a thigh-bone, and the first dinosaur bone known.

• In 1796, Georges Cuvier attributed the succession of fossil forms to a series of simultaneous extinctions caused by natural catastrophes, one of them Noah's flood.

• Georges Cuvier argued that the whole of an organism is to a pattern that is defined by its way of life in a predictable way, that every organism forms a whole.

• Cuvier argued that if the intestines of an animal are so organized as only to digest fresh meat, then jaws, claws, teeth, even legs and senses will match this.

• In 1823, William Buckland published his Reliquiae Diluvianae, in which he argued that fossils were formed when caves filled with mud during Noah's flood.

• Mary Anning of Lyme was a self-trained palaeontologist of the early 19th century, who found and prepared many famous fossils to sell, living on the proceeds.

• Mary Anning, fossil finder, features in the nursery rhyme "She sells sea shells by the sea shore, and the shells that she sells are sea shells, I'm sure."

• In 1913, Hans Reck discovered rich deposits of early mammalian fossils at Olduvai Gorge in East Africa, which would later take the Leakeys there.

• In 1917, Ferdinand Broili discovered the fossil remains of Seymouria, an organism showing both amphibian and reptilian characteristics, a missing link.

• Fish established the first tetrapod body form for vertebrates, the four-limbed body adopted with variations by amphibians, reptiles, birds and mammals.

• In 1932, A Danish scientific expedition found ichthyostegid fossils in Greenland. These were the oldest known fossils that can be classified as amphibians.

• In 1938, a live coelacanth was found off the coast of southern Africa. In 1998, another population of live specimens was found near Indonesia.

• Reconstructing fossils, a form of forensic reconstruction and preparation which adds flesh back to bones in a plausible way, is a skilled task. 

The principles of igneous rocks

• Igneous rocks form when magma cools: granite cools slowly, deep down and forms large crystals, basalt cools faster near the earth's surface and has no crystals.

• There are many kinds of igneous intrusions: a sill is a horizontal intrusion between beds of rock, a dike is a vertical intrusion by way of a joint plane.

• In 1785, James Hutton predicted and discovered a number of pink veins of granite, pushing their way up into the dark schist above, the first record of dikes.

• When a basalt flow cools fast, this can produce columnar joints, resulting in columns with 6, 7 or 8 sides. These may be seen all over the world.

• Crystals form in igneous rocks in accordance with Bowen's reaction series, and this can lead to different types of rock forming from one batch of magma.

• Pumice forms when dissolved gas expands in molten rock, which then cools and solidifies before the gas has time to escape, leaving a rock that floats.

• When igneous rocks push their way through other rocks, or flow over them, they cause local changes in those other rocks and this is called contact metamorphism.

• If a sheet of basalt has traces of contact metamorphism both above and below, then it formed originally as a sill, pushing between two other layers of rock.

• If a sheet of basalt between two other rocks only has contact metamorphism below it, it was originally a flow that was later covered over by other material.

• Some igneous rocks undergo weathering faster than others, but fast or slow, igneous rocks usually form a soil which is rich in the minerals plants need. 

 The principles of sedimentary rocks

• Sediments may be compressed and heated to form sedimentary rocks, while sedimentary rocks are eroded in turn and weathered to form yet more sediments.

• When you look at sedimentary rocks, the ones on the bottom are the oldest, because the rocks are laid down in order as sediments that later harden into rock.

• Sedimentary rocks are formed when sediments are covered, compressed and heated to some extent, so the grains of sediment become cemented together.

• The oldest sediments contain the oldest fossils, so lower sedimentary rocks contain older fossils. This provides some of the evidence for evolution.

• In 1669 Steno, in his 'Prodromus', suggested that tilted strata of geology were originally laid down horizontally, and were later lifted up by some force.

• When sediment is carried to the front of an advancing bank of sediment and pushed over the edge, it forms a characteristic slope at the angle of repose or rest.

• Sediments are laid down in strata, comparatively horizontal layers, except in cross-bedding or current bedding, when they are laid at the angle of rest.

• When sediment forms a slope at the angle of repose, this angle is determined mainly by the shapes of the particles and the medium (air or water) of deposition.

• When sediment is pushed down a slope at the angle of repose, it forms a bed laid down at that angle, rather than horizontal. This is current bedding.

• Wind transports sediment, but generally over short distances only, unless the particles are fine. Dust particles may be carried from one continent to another.

• Fine sediments can be carried long distances by wind: from Australia across to New Zealand, from China to the US or from the Sahara to the US in proven cases.

• The sediment particle size depends on the speed of the wind or water flow, and coarser sediment settles as the speed drops away, producing graded deposits.

• Glaciers transport sediment, picking up rocks and grinding them across the countryside, producing 'rock flour' along the way, and washing it out in meltwater.

• Glaciers may move and create sediment throughout the year, but more sediment is released in summer when the ice melts more and the meltwater flows increase.

• The seasonal variation each year in the flow of meltwater from the head of a glacier produces varved deposits, which later may be compressed to varved shales. 

 The principles of metamorphic rocks

• Metamorphic rocks are formed by the action of heat and pressure on pre-existing rocks, mainly sedimentary rocks, which are changed by that action.

• Local or contact metamorphism, extending over a few metres or tens of metres, can be caused near the Earth's surface by a flow or sill. It happens quickly.

• Large-scale regional metamorphism happens only at great depths, and may extend over very large distances: it involves heat, pressure, and a long time scale.

• When sedimentary rocks are heated and compressed for long periods, limestone changes to marble, sandstone changes to quartzite and shale changes to slate.

• In some cases, fossils formed in a sedimentary rock may still be recognizable after the rock has undergone metamorphism, confirming its sedimentary origins. 

Earthquakes

• Earthquakes happen all the time, all over the Earth. Most are too small or too far away for us to feel them, but they can be measured with instruments.

• Earthquakes happen when there is movement along planes of weakness called faults in the Earth's crust, when built-up tension is released suddenly.

• Earthquakes happen as a result of tension or compression within or between plates, leading to slippage of large masses of rock along planes of weakness.

• Major earthquakes happen at faults where tectonic plates slip past each other and at subduction zones where one plate is slipping downwards under another.

• An earthquake is a shock wave that results from sudden movement when a build-up of tension is released because something gives way, releasing energy.

• Under some circumstances, rocks will move past each other along a joint plane. When two blocks of rock move, relative to each other, a fault is formed.

• The surface of the earth is made up of plates in motion, and earthquakes often happen at plate boundaries, where two plates are in relative motion.

• In 1760, John Michell suggested that an accurate timing of the arrival of the waves could help locate the center of an earthquake that had happened elsewhere.

• Earthquakes travel through the Earth as waves, following several different paths, and arriving at seismographs at different times, so the source can be located.

• Structures beneath the earth's surface are mapped either by using the information coming from earthquakes, or by looking at the reflections of small explosions.

• Earthquakes may be placed on a scale of intensity, either on the basis of the damage done at the epicentre, or in terms of the energy released.

• Points recording the same earthquake intensity are joined by an isoseismal line: in early times, these showed scientists the location of the epicentre.

• In 1935, Charles Richter invented a logarithmic scale to measure the strength of earthquakes, mainly based on the energy released in the quake.

• Seismology depends on the use of instruments to get intensity measures for earthquakes, using either the Richter scale or the modified Mercalli scale.

• Tsunamis are typically caused either by sudden underwater block movements in earthquakes, or when large blocks come off the side of undersea volcanoes.

• A tsunami is a water wave generated by sudden earth movements. Tsunamis may travel thousands of kilometres as barely visible waves before hitting a coast.

• In shallow waters, a tsunami builds up to a considerable height, and may flood a large coastal area, without any warning, far from any seismic activity.  

The principles of volcanoes

• Volcanoes bring molten rock to the surface, erosion and weathering convert these rocks to sediments. If they are compressed or heated, sedimentary rocks change.

• A volcano erupts when magma gets close enough to the surface of the Earth to force its way out: when it erupts onto the surface, the molten rock is called lava.

• Volcanoes are of different types, determined by the sort of magma that is working its way to the surface, as the geochemistry influences the type of eruption.

• Volcanoes happen where plates are in contact, and also over 'hot spots' which can cause a chain of volcanoes as a plate moves over the hot spot.

• Many of the volcanic island chains in the Pacific Ocean are caused by a plate moving over a hot spot. Hawaii is probably the best-known example.

• Volcanic areas often have geysers, where groundwater is heated under pressure until it boils, pushes out overlying water, and then boils explosively.

• While volcanoes cause a great deal of local damage in the short term, they are very useful in long term because they bring valuable new minerals to the surface.

• Volcanoes produce more than lava flows: they also produce large clouds of ash and dust which can travel long distances, and large amounts of noxious gases.

• The form and shape of a volcano depends on the chemical composition of the magma which determines how it erupts. 

 The principles of economic geology

• Minerals may be detected in many ways, and in the past, mainly involved surface prospecting to look for indications of what lay below the surface.

• Minerals may be identified by some of streak test, cleavage, lustre, hardness, fracture, specific gravity, fluorescence, radioactivity, or reaction to acid.

• Information about what lies below the surface comes from magnetic and gravitational anomalies, seismology data and careful geological mapping of the surface.

• Modern prospecting relies largely on gathering data about subsurface structures and then drilling test holes at the most likely sites to get samples.

• In 1625, gunpowder was first used in a mine in Chemnitz, Germany, as a way of breaking up rock and ore, so it could be hauled out of the mine and processed.

• When geologists work out how a particular geological system originally formed, then they are able to predict where valuable minerals might be found. 

  Principles about geological structures

• If a body of rock is subjected to enough force, it will break and slip along a plane of weakness, the process that we call geological faulting.

• Some faults have developed as a result of two tectonic plates moving past each other. These faults are earthquake zones: an example is the San Andreas fault.

• Most modern geological structures are the result of past tectonic activity which applies force to the world's rocks.

• Finding most types of mineral deposits depends on being able to envisage the structures which lie under the planet's surface.

• Geological structures can be economically important. Oil and gas are often found in anticlines or salt domes, and other structures can indicate mineralization.

• Some minerals are found near certain geological structures, usually indicating something of the way in which the deposits were formed at some time in the past.

• Given suitable pressure and force, apparently solid rocks can fold into complicated structures without breaking. This can happen on a small or large scale. 

The principles of geological history

• The earth changes as time goes by: mountains are uplifted and eroded away, continents move, the magnetic poles move, volcanoes erupt and are eroded.

• All of geology is consistent with standard processes applying over the standard geological time scale lasting some 4.6 billion years: there are no exceptions.

• One explanation of the earth is based on uniformitarianism. One explanation of the earth is based on catastrophism: neither is a perfect fit to the facts.

• Land forms have been shaped by factors such as weathering and erosion that we can see operating today: this is the principle called uniformitarianism.

• One aspect of geological history is geomorphology, which studies the way in which geological forces that we see today have shaped the major landforms.

• Uplift of rocks is followed by erosion, but equally, erosion is followed by isostatic uplift, since the crust of the planet floats on the mantle.

• Erosion changes the surface of the planet, by wearing down hills and mountains, and by cutting new valleys, both with ice and with water carrying sediment away.

• A gap in the geological record may be an unconformity, where one set of beds has been tilted, folded and eroded, before being overlain by later sediments.

• A gap in the geological record may be represented by a disconformity, where the beds above and below a gap are in alignment, bet deposition stopped for a while.

• Landforms may be determined by the underlying rocks, since more resistant beds will tend to remain, forming ridges that must be bypassed by rivers and glaciers.

• The formation history of a sedimentary rock aeolian or alluvial deposits as may often be found in the rock, either in structures, or in the fossils

• Wind erosion causes dust storms and sandhills, and given the right winds, has even been known to move fine sediments from one continent to another.

• Ice or glacial erosion creates unusual landforms such as moraines, which allow us later to recognize the influence of ice in shaping the landscape.

• Glacial valleys have a different shape from those cut by water erosion, because the grinding action of the valley-filling flowing ice makes a U-shaped valley.

• The ratios of stable isotopes in fossils provide good evidence of past climates because they generally give an indication of past temperatures.

• Tree rings provide good evidence of past climates, because the tree rings formed in good years are thicker. This is called dendroclimatology.

• We can obtain evidence of past climates from fossil data of many sorts, anything which varied with the conditions such as temperature at the time.

• We can gain evidence about past climates from palynology, the study of pollen grains, because the grains are distinctive to a particular species. 

The principles of dating methods

• Dating takes two forms: it can deliver an absolute age in years or an age relative to other events. Relative dating is sometimes all that is available to us.

• All geological dating methods come with a small amount of uncertainty, because they rely on probabilities and inference, based on the best available data.

• Some dating methods can be interfered with by contamination of the sample, but combining several methods can help avoid the risk of error from this source.

• The oldest fossil traces we know of go back to about 3800 million years, but as most rocks of that age have been since destroyed, life may be a little older.

• In 1920, Andrew Douglass suggested dendrochronology, using tree rings to build a sequence of years, and using other timber with overlaps to extend the scale.

• Dendrochronology can be used to date artefacts very accurately for thousands of years, relying on unique patterns that can be traced from one tree to another.

• In 1947, Willard Libby introduced the idea of carbon-14 dating. By 1949, he could present carbon dating as a fully developed technique, ready to use.

• Material less than 50,000 years old can be dated by carbon dating, provided it has organic material which has not been contaminated since it was formed.

• Thermoluminescence can identify how long some things have been buried. The thermoluminescence clock is 'reset' when the objects are exposed to direct sunlight.

• Ice cores provide good evidence of past climates and temperatures. The cores preserve stable isotope ratios in water and gases, and solids like volcanic ash.

• Isotope dating works with many igneous rocks, and this can be used to determine absolute limits to the age range of fossils lying between two igneous layers. 

The principles of the age of the Earth

• Around 1640, Bishop James Ussher, using traditional ages and dates found in the Old Testament, calculated that the world began at noon on October 23, 4004 BC.

• In calculating that the world was created in 4004 BC, James Ussher was merely repeating a view widely held before his time, but with rather greater precision.

• William Shakespeare, who died in 1616, reflected this view when he wrote in 'As You Like It', the line: "The poor world is almost six thousand years old . . ."

• Jean-Baptiste Fourier argued that the earth's central heat was clearly revealed in higher temperatures observed deep in mines and by volcanic activity.

• Fourier explained the earth's observed central heat by assuming the whole earth was once hot, and that the temperature of the earth was now falling.

• In 1830, Charles Lyell began to publish his Principles of geology. In this, he proposed the revolutionary argument that the Earth is several million years old.

• In 1846, William Thomson (Lord Kelvin ) wrongly estimated the Earth to be 100 million years old, based on heat calculations, assuming no internal heat source.

• In 1862, Lord Kelvin estimated the age of the Earth, from its cooling time to be between 20 and 400 million years. Again, he assumed no internal heat sources.

• In 1892, Sir Robert Ball gave the world about four or five million years more to go before it ended when the Sun used up all its energy, after 18 million years.

• In 1903, George Darwin and John Joly suggested that radioactivity might warm the Earth, making the earth potentially much older than previously thought.

• In 1904, Ernest Rutherford suggested the age of Earth might be longer than previously assumed on cooling estimates, due to internal heating by radioactivity.

• In 1907, Bertram Borden Boltwood first proposed the use of radioactivity to date minerals, and offered dates for some rocks of 410 - 2200 million years.

• By 1931, on the basis of assorted radioactivity and geological data, the age of the earth was now considered to be at least two billion years.

• In 1954, a revised estimate, based on the best information, put the earth at 5 to 6 billion years, while estimates these days are more like 4.5 billion years. 

The principles of weathering and erosion

• In 1802, John Playfair wrote about geomorphology, giving us 'Playfair's Law', that rivers cut their own valleys, rather than following pre-existing routes.

• Rock minerals undergo both physical and chemical changes as they weather to form soil, with some of the soluble products being leached away by groundwater.

• When water with dissolved limestone evaporates, a stalactite or stalagmite may form, as soluble calcium hydrogen carbonate forms calcium carbonate.

• The loss of rocks from the surface of the earth is balanced by the uplift caused by tectonic forces and also volcanic eruptions.

• Rocks weather due to chemical effects caused mainly by the atmosphere and water. Erosion often exposes fresh surfaces to weathering.

• Erosion can be caused by wind, water or ice. Plants and animals can have more minor influences on weathering and erosion. 

  The principles relating to soil

• Soil forms slowly from basement rock when the rock minerals undergo changes as they weather to soil, which includes minerals as inorganic parts.

• When volcanoes cover an area with new rock in the form of lava, or when glaciers scrape it clean, new fresh soil, rich in mineral nutrients, can form.

• Igneous rocks are eroded and weathered to form sediments and soil, but over time, the more valuable minerals are leached away, leaving a deficient soil.

• Soil contains humus, a complex mixture of partly-decayed organic matter that supports a broad range of Fungi, bacteria and microscopic life forms.

• The soil is commonly regarded as dead, but it includes organic parts as well as the more obvious minerals, and is of little use to plants if it is sterile.

• Leaf litter is a key part of the soil and forest floor, and supports its own rich culture of living things, before it enters the soil as humus.

• Many things live in the soil, and the animals in soil can be extracted and studied. The richer the soil, the more animals there will be living in it.

• The quality of soil can be assessed by taking a soil profile, which means either digging a trench, or using an auger to bore out a sample to study the layers.

• Hydroponics is a popular method of growing plants without soil, using water to supply the essential mineral nutrients that are needed by the plants. 

  The principles of the weather

• Meteorology is the scientific study of weather effects and patterns, and it includes a great deal more than simple daily weather forecasting.

• All parts of the world are inter-connected by weather patterns, by cycles of energy and matter, air and ocean currents and migrations of animals.

• Most weather systems are metastable: they will continue while conditions remain the same, but small changes may switch them to a new metastable pattern.

• Weather is metastable: world weather patterns can flip when a single wind or current pattern is disrupted, and the new pattern can be hard to switch back again.

• Weather is driven by the flow of air around the world: that is to say, weather is driven by the wind patterns, as these also carry moisture and warmth.

• Multi-year weather patterns include El Niño which runs on about a four-year scale and the Pacific decadal oscillation which is on about a ten-year scale.

• El Niño is either a symptom or a cause of large-scale weather patterns over periods of several years, or quite possibly both, depending on how you look at it.

• The Coriolis effect makes weather patterns move in roughly circular paths: in the south, winds go clockwise around a low, but counter-clockwise in the north.

• Monsoon systems drive weather in Asia and northern Australia, bringing wet weather around June in Asia, and December on the southern side of the equator.

• The monsoons of Asia and Australia are probably a relatively new phenomenon, driven by air circulation around the Himalayas, once they were uplifted.

• Weather maps help in weather prediction, which is the art of extrapolating from the best available present data to an expected future outcome.

• Ocean currents are interlinked and interactive, and form a delicate metastable pattern that pumps nutrients, warmth and weather patterns around the world.

• The Conveyor is a worldwide ocean current that includes the Gulf Stream that keeps Europe warm. It is driven by the formation of sea ice in Arctic waters.

• The main ocean currents of the world are driven by the formation of sea ice, which leaves cold, dense, salty water near the surface, from where it sinks.

• Seasons are caused by the tilt of the earth on its axis, relative to its orbit, which leads to the solar radiation falling more on one hemisphere or the other.

• In 1920, Milutin Milankovich suggested long term climatic cycles may be due to changes in the eccentricity of the Earth's orbit and in the Earth's obliquity.

• Weather patterns travel from west to east because the earth rotates once a day. The weather is driven by ocean currents and their warmth or coolness.

• Weather patterns are shaped by the so-called Coriolis force, the effect that makes wind leaving the equator move towards the east, generating eddies.

• Alexander von Humboldt gave us isothermals, lines joining points of equal temperature. Plotting these globally showed the influence of geography on climate.

• Air pressure changes from place to place, and in a single place, it varies with time. Isobars are lines on a map that join places with the same air pressure.

• Air pollution may be trapped by an inversion layer, where dense air is trapped, especially in valleys, allowing an increase in local pollutant levels.

• Weather shows extremes: floods occur when rivers spill over their banks, drought is usually driven by global effects, cyclones and tornadoes form over the sea. 

Climate change

Around the world, sea levels are rising, in part because the Earth is getting warmer, and the seas are expanding slightly, in part because glaciers are melting.

A number of gases in the atmosphere are causing global warming, or at least part of it. Some interest groups deny this to preserve their profit margins.

There is no absolute proof that burning fossil fuels causes global warming, but it makes sense, and there is absolutely no proof that it is not the culprit.

The greenhouse effect involves many gases, but water vapour and carbon dioxide are major ones which retain heat, and so assist global warming.

Small temperature changes in the oceans can have large effects on ocean life forms, because many life forms are operating close to some limit or another.

Coral bleaching is caused when water temperatures rise, and the coral animals are provoked to eject their symbiotic algae, which can also kill the corals.

Coral bleaching is not fully understood, but scientists think it may be caused by pollution, eutrophication, global warming , or maybe more than one of these.

The Earth has experienced a series of Ice Ages, some appearing to be more severe than anything that humans have experienced, but all causing great disruption.

The earth is getting warmer: most people believe that the major cause is human effects, a few people say that it is just part of a natural upswing.

Various methods may be used to determine past climates, including looking at fossil evidence and the ratios of stable isotopes in different sediments.

Global warming not only raises the temperature: it can also change climate patterns such as rainfall and storms, and may even cause some places to get colder.

Except for a few glaciers on or near volcanoes, every glacier in the world is retreating, melting faster at the lower end than new ice is added at the top.

Global warming is believed to be responsible for the increase in disastrous floods and storms in the past decade, because weather patterns have changed.

Weather patterns are metastable, which means that if they flip to a new mode, they probably will not flip back again unless something major changes.

All of the world's ocean currents are linked and influence each other, and all of them are driven by regular seasonal changes in world-wide weather patterns. 

The principles of catastrophes

• Catastrophism was an attempt to give a place to Noah's flood in the shaping of a world believed to be just 6000 years old, but the model is badly flawed.

• Occasionally, catastrophic events do shape the Earth, and these include asteroid strikes, volcanic eruptions, polar reversals and sudden changes in climate.

• In 1980, Luis and Walter Alvarez, Frank Asaro, and Helen Michel proposed that a giant comet or asteroid may have struck the Earth about 65 million years ago.

• This comet, they argued, caused massive extinctions and enriched the iridium in the K-T (Cretaceous-Tertiary) layer, and in passing, killing all the dinosaurs.

• In 1986, a hole in the ozone layer was first reported. In Europe and America, it was dismissed as 'only a threat to a few penguins who sunbake'. 

The principles of water

• Life evolved in the water, then came onto land: probably plants came first, followed by arthropods and amphibians, although this remains open to conjecture.

• Water is a polar molecule, a function of its shape with both hydrogen atoms being on one side of the oxygen atom, giving net charges across the molecule.

• The average particle in liquid water is more like H8O4 or H6O3 than H2O, as a result of water molecules being linked together by hydrogen bonding.

• The key to understanding water and its sometime strange physical properties is the hydrogen bond, which in turn depends on water being a polar molecule.

• A porous rock is one with spaces where water can fit, a permeable rock is one where the spaces are linked together, so water can pass through the rock.

• Air holds water, and when we measure this atmospheric water, we call it humidity. The whole water cycle relies on variations in the humidity of the atmosphere.

• Humidity is measured with a hygrometer and expressed as absolute humidity the amount carried, or as relative humidity, compared with the possible maximum.

• Water vapour is a gas, and it is invisible to us. Clouds are masses of condensed water vapour, as is the 'steam' that we see rising from boiling water.

• Water droplets form around condensation nuclei: this means that rain water, while it may be fairly pure, necessarily contains a measurable level of impurities.

• Water reaches the land in a variety of forms, known collectively as precipitation. Mist and fog are clouds at ground level, and can add to local precipitation.

• Precipitation in the fullest sense includes rain, hail, snow, sleet, frost and dew, where 'dew' also includes any condensation from mist and fog.

• Precipitation may be measured in either a rain gauge or a precipitation gauge, depending on the climate and precipitation forms at the location of the gauge.

• The available water at a particular place depends in part on the rainfall and its regularity, but also on the level of evaporation experienced there.

• The water cycle carries salts from the land to the seas, but over time, some ocean salt is lost in subduction zones and some is lost to halite deposits.

• Plants need to get water to their highest point, which in some cases, is up to 100 metres above ground, too high for water to get there by simple suction.

• Hard water is water with dissolved calcium or magnesium in it: the main effect of hard water may be seen in boiler scale and problems in getting soap to lather. 

Irrigation and water supply

• The aqueduct was an early means of transporting water with no energy cost, but it required complex masonry and the ability to make waterproof channels.

• Groundwater occurs wherever the geology allows it. The minimum requirement is for porous and permeable rocks to be in contact with a water source.

• The qanat was an early means of transporting water with no energy cost, once the tunnel was made. This Iranian technology spread as far as Morocco and China.

• A river bank may be raised to make a levee, and as the river gathers silt, the levee is also raised, giving an unstable river higher than the surrounding plain.

• In 1674 Pierre Perrault measured rainfall in the Paris basin, finding that it accounted for the flow in the Seine, ruling out a hypothetical underground source.

• In 1752 Phillippe Bruache uses the idea of river basins to divide the world into natural regions, and we still use this today when we speak of the Amazon basin.

• Water can be raised by an Archimedean screw, which uses the rotation of a crank to produce a continual gentle flow of water while the crank is turned.

• The standard pump relies on one or more valves, each of which is a cleverly arranged flap or ball that allows flow in a pipe or opening in one direction only.

• Water can be raised by different pump types, and has been done since ancient times: water management has been a major unifying influence in many civilizations. 

The principles of the atmosphere

• Air is a substance that can be measured weighed, dissolved, condensed, frozen, and even seen when it is trapped below water or when it is liquefied.

• Air has weight, and as a result, it exerts a pressure on everything around it, operating in all directions. Air pressure is measured with a barometer.

• The Earth's atmosphere protects the planet from radiation, and also from the surface impact of most cosmic of the material that reaches the planet.

• Global systems drive the weather on the whole planet, both in the short term of a few days, and also by seasons and several multi-year patterns.

• There are a number of recognized levels in the atmosphere, and these levels include the troposphere, the stratosphere the mesosphere and the ionosphere.

• Air is made of individual gases, each of them exerting a partial pressure on everything around them. Oxygen, nitrogen and noble gases make up most of the air.

• Air is largely transparent, but it can be seen on very hot days, when you look along a hot surface, and see a heat shimmer caused by air of varying density.

• At sea, wind speed is measured on the descriptive Beaufort wind scale, and at other times, it is measured in knots, one of the few universal non-SI units.

• Chlorofluorocarbons (CFCs) are effective refrigerants, but when they escape, they cause ozone depletion in the upper atmosphere by reacting with the ozone. 

The principles of the oceans

• The sea contains large amounts of dissolved material, including minerals and dissolved gases. It also contains very large numbers of organisms of all sizes.

• We know a great deal about the general shape of the ocean bottoms from echo sounding, where reflected sonar signals allow the bottom to be mapped accurately.

• We know about the ocean bottoms from drilling programs, from remotely controlled submarines and from instruments lowered to gather samples and data.

• Cold water contains more dissolved gases than warm water, and in particular, it contains more dissolved oxygen, making it easier for aquatic animals to breathe.

• The world's ocean currents are all interlinked, so that the blockage of any channel, anywhere in the world, could have major effects elsewhere in the world.

• Cold ocean currents that are forced to the surface carry large amounts of mineral nutrients, and these upwellings support high levels of productivity.

• Storm systems spin around a low pressure zone in accordance with the Coriolis effect, so they go clockwise in the south, counter-clockwise in the north.

• Tides and waves bring about smaller scale water movements than those created by ocean currents, but even these are able to move sediment on and along beaches.

• Winds and ocean currents spread life forms around the world, in large part as rafts washed down flooded rivers, which act as temporary refuges for vertebrates.

• In the past, it is likely that volcanic activity near Indonesia has changed local, and thus world ocean current patterns by opening and closing various gaps.

• The number of icebergs seen outside the Arctic and Antarctic Circles has increased with global warming, because this causes glaciers to lose ice faster.

• In the oceans, iron is usually the limiting factor: it has been suggested that algal blooms could be generated to act as a carbon sink by adding iron salts. 

There are other SPLATS to be found, but you will need to go back to the main SPLATS page to find the links.

© The author of this work is Peter Macinnis, who asserts his sole right to the product as it is packaged here, recognising that many of the ideas are common. You are free to use this as a model to do your own version. Copies of this whole file or site may be made and stored or printed for personal or educational use. You can contact me at macinnis44@gmail.com, but only if you add my first name to the front of that email address — this is a low-tech way of making it harder to harvest the e-mail address I actually read.