Farnham Geological Society

Although big advances are being made in waste minimisation, recycling and alternative means of treating waste, landfill is, and will remain, a major element in waste management for some time to come, and will continue to be necessary for some types of wastes. I will describe:

Brian Marker described the types of waste; the ways in which these decompose; the environmental issues surrounding the products of decomposition; the geological factors that influence their distribution; and the steps taken to reduce risks. He also reviewed the alternatives for landfill and implications for the future.

In science a new word is often needed for a new concept, leading to confusion if a word in common use is given a different scientific meaning to that available in the dictionary, as is unfortunately often done. The situation can become even worse in translation to a different language.

The word 'terrane' is therefore a deliberate corruption of the common word 'terrain', which refers only to a land surface. Its scientific meaning refers to a block of continental material from its surface right down to its base just above the mantle. An 'exotic terrane extends the concept to a block of material that has broken away from a continent, and moved until it collides and fuses somewhere else, a process sometimes referred to as 'docking', where its presence can give rise to an apparently confused geology over a fairly small area.

Not all terranes are small, of course. India is a large example, and whilst on passage from Gondwanaland near the South Pole to its present location docked on to southern Asia, it must have been isolated like present day Australia, which is probably an undocked terrane destined to join on to another continent in the far distant future. Greenland and Cyprus may also be examples, with the Seychelles as further candidates as they are made of continental materials, although they raise special problems due to their present location

The idea of terranes was first developed in the United States, where the concept made an immediate impact on the understanding of the geology of western coastal regions. Indeed even the inexplicable geology of Alaska has lately succumbed to investigation. Alaska has been found to be "a collage of terranes dismembered and repositioned over the past 160 million years by the wanderings and collisions of crustal plates, the flotsam of the ancient, vanished ocean that preceded the Pacific". (This is a direct quotation from the paper also titled "Terranes" by David G Howell)

Locally, too, the idea seems to be fruitful. There are several areas of this country where geology appears to change too suddenly for ordinary explanations, or where other anomalies exist such as traces of volcanic rock with no apparent volcanoes, or ophiolites where rocks not far away show any signs of abnormality. Suspicions are arising that this country, in common with other areas fringing the continental basement, contain many exotic terranes, some of which docked a long time ago and therefore share much of their geological history with the region on which they docked, but some more recently so that they share at most only a thin veneer of local sediments. Members of the Society may well have seen and puzzled over such features in Scotland, North Wales, Anglesey, and Cornwall, during field trips; the thought may now be entertained that adjacent areas may possibly have had a different geological history as part of a different territory or even a different continent.

Elegant, simple ideas like this deserve to be true, but there is still work to be done to explain how exotic terranes can cross oceans, if oceans always grow from mid-ocean ridges towards the continents under which they are subducted. Suggestions are being made that perhaps they move only along the edges of continents, in the way that, for example, Baja California is moving because of the San Andreas Fault, but this certainly does not fit for India, Australia, or Alaska.

It occurs to me, also, that there is still room for amateur speculation in such recent ideas. For example, the stability of small pieces of continent once they have developed an independent existence does not seem to have been investigated. Pieces of continent floating on the mantle will not be exempt from the general rules governing flotation and stability, of which some insight can be gained by floating odd- shaped pieces of wood on water. A thin flat slab will not float with its surface vertical, but will rollover until the surface is parallel with the water. A squarish block is unlikely to have any surface parallel to the water when it becomes stable, and might even finish with its diagonal vertical. Thus, a wide slab broken from a continent will obviously remain level as it floats on the mantle below, but a piece narrower than the continent is thick, typically about 35 kms, would be expected to rollover on to its side, no doubt revealing some very interesting rocks in the process, although no reliable example can be found. An intermediate slab up to perhaps twice as wide as it is thick would not remain level, raising one edge and lowering the other, causing a regional dip to stratified rocks that would probably be preserved after docking. The high side might also raise fragments of ocean floor -this would be the place to look for ophiolites, the presence of which would also suggest that neighbouring granites may well have been uplifted rather than intruded.

No doubt time will resolve these problems. New work like this shows once again that Geology is not a dry, static subject, as many suppose, but is dynamic and entertaining. Membership of a Society such as ours can be very rewarding, to keep us up to date with new developments.

A visit to Mupe Bay, Dorset, provides evidence for the opening of the mid-Atlantic, global sea-level rise and Africa’s collision with Europe.

The opening of the mid Atlantic in the Mesozoic caused crustal extension and thinning; huge areas subsided along a series of great listric (curved) faults to form basins such as the Weald and the Portland-Wight. Our journey around the back of Lulworth Cove, onto the lower slopes of Bindon Hill, and east to Bacon Hole and Mupe Bay, takes us along the major fault system which formed the north margin of the Portland-Wight Basin. Mesozoic sedimentary formations in the Portland-Wight basin are 21/2 times thicker than their equivalents on the platform to the north.

Mid Palaeocene (~61ma) collision of Africa with Europe began a long phase of crustal compression. In the mid Palaeocene there was regional uplift of NW Europe. In the Miocene a period of more intense compression caused "Basin Inversion"; the listric faults were "mended" and this led to elevation of the excess (compared with the platform sequence to the north) sediment in the basins into large asymmetrical anticlines with steep northern limbs - as in Lulworth Cove and the Isle of Wight. These steep, sometimes vertical, limbs above the basin forming faults are intensely deformed and fractured, clearly illustrated in the Chalk on the north side of Lulworth Cove. The southern limbs dip at a low angle and are comparatively undisturbed, for example, the Wealden beds of Worbarrow Bay.

The sequence consists of Cretaceous rocks, from the Purbeck beds to the Upper Chalk. There are two significant unconformities; regional uplift and tilting in the Aptian (Kimmerian) led to erosion of Lower Greensand and uppermost Wealden sediments; consequently Gault Clay rests unconformably upon Wealden sediments. Mid Palaeocene (~61 ma) uplift led to some 5my of erosion which removed early Tertiary Danian Chalk, and Maastrichtian and late Campanian Chalk. Consequently, late Palaeocene or early Eocene sediments (Reading Beds) rest directly upon eroded Campanian Chalk.

The Purbeck formation of Lulworth consists of Platform sediments (cf the Basin sequence at Durlston). There is a mixture of shallow marine, coastal, lagoonal and terrestrial sediments, which include limestone and evaporite beds (gypsum and anhydrite), and Dirt Beds. The Dirt Beds are fossil soils, and some of them have tree trunks and roots similar to those of modern Cypress or Juniper trees. The overall trend of sedimentation is a transition from the marine sediments of the underlying Portland Limestone up into terrestrial (fluvial and lacustrine) Wealden Beds. There are some remarkable deposits within the Purbeck, including:-

the Cinder Bed - a shelly limestone packed with Praeexogyra oysters; this is a widely correlateable bed, once used by British stratigraphers to indicate the Jurassic-Cretaceous boundary;

the Great Dirt Bed - a fossil soil with tree trunks and roots; the trees that grew in this thin Rendzina (carbonate-rich) soil were inundated by the sea which formed a very shallow lagoon; stromatolites grew on the dead trees thus preserving their form and shape before the wood rotted away; the Broken Beds - a sequence of thin limestone, gypsum and anhydrite beds deposited in a shallow lagoon subjected to a high level of evaporation; much of the gypsum and anhydrite was subsequently dissolved, and the remaining limestone beds collapsed into rubble; the "Purbeck Marble" - a limestone deposited in fresh water and packed with Viviparus snails; it has been used as a decorative stone in local churches.

The Wealden Group of terrestrial mottled red, purple and brown clay and sand beds often contain lignite. Although the basin continued to subside, sedimentation kept rate with subsidence, and the sea was unable to flood the basin; consequently, terrestrial sediments were deposited both on the Platform and in the Basin. The Lulworth "Platform" sequence is ~150m thick, whilst the Worbarrow Bay "Basin" sequence is over 400m. There are fluvial sandstone beds, some of which are remarkably coarse grained and contain tree logs eg the Coarse Quartz Grit.

There is an active oil seep near the slipway in Lulworth Cove. A conglomeritic oil sand at the foot of the steps leading down to Mupe Bay is believed to be a river into which oil seeped during the early Cretaceous.

The Lower Greensand may be represented by a few cm of impersistant ironstone which contains shallow marine molluscs - eg Preaeexogyra. There is an unconformity at the base of the overlying Gault Clay. The ironstone may be a remanie deposit formed during erosion of Lower Greensand and uppermost Wealden prior to Gault deposition.

Global sea level rise resulted in inundation, and deposition of marine blue-grey Gault Clay with molluscs and ammonites. There is a basal pebble bed, composed largely of lydite and vein quartz, which represents a beach at the start of the marine transgression. Above is the upper Greensand, a sometimes muddy, bioturbated glauconitic sand; there were abundant molluscs, but the porous nature of the sand has allowed passage of fluids which dissolved the aragonitic shells to leave only moulds and casts. There are some Exogyra oysters; these were preserved because they have more resistant calcite shells.

As the sea rose and spread there was less and less land to weather, erode, and provide sediment; consequently, clastic material becomes finer grained and decreases up the sequence. The Lower Chalk consists of cyclical limestone and muddy limestone beds, bioturbated, with molluscs, ammonites and echinoids. The overlying Plenus Marl is a thin condensed muddy limestone unit with numerous disconformities. Above, the Middle Chalk is a clean nodular limestone, overlain by clean Upper Chalk limestone with flint layers. The Chalk is very hard, brittle and splintery, with numerous faults and fractures; this is a consequence of intense compression and deformation at the basin boundary fault during the Miocene.

On the far side of the Cove, Purbeck Beds, to the right, pass up into reddish Wealden, grey Gault and U Greensand, and pale grey Chalk.

The whole Cretaceous sequence can be seen within a very short distance, because the beds are nearly vertical. Upper Chalk is exposed high in the cliff at the back of Lulworth Cove; at the foot of the cliff is Middle Chalk; east and west of this point are Plenus Marl, ower. Chalk, Upper Greensand, Gault, Wealden, Purbeck and eventually Jurassic Portland Limestone beds. The intense faulting cuts out parts of the sequence; to the east the Upper Greensand is in faulted contact with the Lower Chalk and there is a distinctive fault breccia. At the back of the Cove is a Pleistocene raised beach, with some pebbles derived from the Budleigh Salterton Pebble Bed - their transport to this location is something of a conundrum.

There is a ledge and fossil cliff high in the cliffs south of Bindon Hill, cut during an Ice Age interglacial when the sea was higher than to-day. Along this ledge there is an outcrop of Lower Purbeck Beds which includes the Great Dirt Bed with the Fossil Forest of Juniper and Cypress like trees, and the overlying Broken Beds.

East of the fossil forest, Bacon Hole is a deep cove cut into the cliff line to expose a complete sequence of Purbeck Beds, overlain by Wealden Beds. The Cinder Bed is exposed towards the bottom of the cliff.

Beyond Bacon Hole there is a great bay cut through the line of Portland Limestone Cliffs. The eastern side is Worbarrow Bay, which exposes a large ("basinal") thickness of Wealden. On the western side is Mupe Bay which exposes a much thinner ("Platform") Wealden sequence. At the back of the bay on either side of Arish Mel, is a sequence of Gault, Upper Greensand and then the Chalk which is exposed in the great cliffs at Cockpit Head. The structural attitude of the sequence is similar to that of Lulworth, an antiform with steeply dipping intensely faulted and fractured beds dipping steeply to the north, passing southwards into gently dipping beds with minor parasitic folds.

The Island of Hawaii is built from five separate shield volcanoes that erupted somewhat sequentially, one overlapping the other. These are (from oldest to youngest): Kohala (extinct), Mauna Kea (dormant), Hualalai (dormant), Mauna Loa (active) and Kilauea (very active).

Kilauea is the youngest and south-eastern most volcano on the Big Island of Hawai`i. Topographically Kilauea appears as only a bulge on the southeastern flank of Mauna Loa, and so for many years Kilauea was thought to be a mere satellite of its giant neighbour, not a separate volcano. However, research over the past few decades shows clearly that Kilauea has its own magma-plumbing system, extending to the surface from more than 60 km deep in the earth.

Kilauea is the youngest and south-eastern most volcano on the Big Island of Hawai`i. Topographically Kilauea appears as only a bulge on the south-eastern flank of Mauna Loa, and so for many years Kilauea was thought to be a mere satellite of its giant neighbour, not a separate volcano. However, research over the past few decades shows clearly that Kilauea has its own magma-plumbing system, extending to the surface from more than 60 km deep in the earth.

In fact, the summit of Kilauea lies on a curving line of volcanoes that includes Mauna Kea and Kohala and excludes Mauna Loa. In other words, Kilauea is to Mauna Kea as Lo`ihi is to Mauna Loa. Hawaiians used the word Kilauea only for the summit caldera, but earth scientists and, over time, popular usage have extended the name to include the entire volcano. Kilauea is a very low, flat shield volcano — vastly different in profile from the high, sharply sloping peaks of stratovolcanoes like Mt. Fuji, Mount Hood, and Mount St. Helens.

Kilauea is considered to be the present home of Pele, the volcano goddess of ancient Hawai?ian legend. Several special lava formations are named after her, including Pele's Tears (small droplets of lava that cool in the air and retain their teardrop shapes) and Pele's Hair (thin, brittle strands of volcanic glass that often form during the explosions that accompany a lava flow as it enters the ocean).

Eruptions at Kilauea occur primarily either from the summit caldera or along either of the lengthy East and Southwest rift zones that extend from the caldera and run approximately parallel to the coastline. In recent decades, eruptions have been continuous, with many of the lava flows reaching to the Pacific Ocean shore. About 90% of the surface of Kilauea is lava flows less than 1,100 years old; 70% of the surface is younger than 600 years.

There were 45 eruptions of Kilauea in the twentieth century. The Mauna Ulu eruption of Kilauea began on May 24, 1969 and ended on July 22, 1974. The 1990 lava flow was notable for its destruction of property. The current Kilauea eruption began on January 3, 1983.. This eruption has covered over 117 km² of land on the southern flank of Kilauea and has built out into the sea 2 km² (230 hectares) of new land. Since 1983 more than 2.7 km³ of lava has been erupted, making the 1983-to-present eruption the largest historically known for Kilauea. 189 structures have been destroyed. In the early to middle 1980s Kilauea was known as "The Drive-By Volcano" because anyone could ride by and see the lava fountains — some as much as 1,000 feet (300 m) in the air — from their car.

Lava from Kilauea destroyed three abandoned houses in the week of February 25, 2008 in a nearly deserted neighborhood.^[3] On the night of March 5, 2008, lava from the flows again reached the ocean off the Puna coast, creating a spectacular show of light and color. Hawaii County Civil Defense officials have set up a viewing center nearby for the public to observe the phenomenon. In the early morning hours of March 19, 2008, Halema?uma?u experienced its first explosive event since 1924 and the first eruption in the Kilauea caldera since September 1982. A steam vent that had recently opened near the overlook area exploded, generating a magnitude 3.7 earthquake, and scattering rocks over a 75 acre area.

Peter Cotton, with the help of the USGS website: http://hvo.wr.usgs.gov/kilauea/

Nowadays, it is generally accepted by most people that the age of the Earth is around 4600 million years (4600 Ma). But this has not always been the accepted fact, for only ~360 years ago, the age of the Earth was decreed to be around 5650 years old, almost one millionth of today’s accepted value.

In fact it was James Ussher, Archbishop of Armagh in Ireland, who in 1650, stated that, by adding up the ages of all the important people in the Bible since Adam, he had calculated that the Earth had been created in 4004BC. Hereafter, this date was printed in the Bible as a note in Genesis, whereupon it became totally accepted as an indisputable fact. Indeed, my wife’s old family Bible, printed around the year 1850, clearly makes reference to the year 4004BC at the top of each of the 2 columns of notes appended to Chapter 1 of Genesis – see accompanying photographs.

This date of 4004BC remained largely unchallenged until the late 18^th century, by which time a number of eminent scientists were making significant inroads into understanding the fundamentals of geological processes. Even before this though, in 1715, Edmund Halley, after whom the famous comet is named, proposed that the age of the Earth could be calculated from the amount of salt in the oceans, assuming zero salt when the oceans were formed and hereafter a steady influx of salt; sadly he could not produce an actual age as he had no reliable figure for the rate at which salt was being added to the oceans.

In 1785, James Hutton, frequently called the “Father of modern geology”, argued that as the land of today had been fashioned by the seas and rivers of yesterday, and that in his view there had been at least three major cycles of land formation in Earth’s history (Hutton’s unconformities), geological time was just too long for humans to imagine. At around the same time, Charles Buffon, an eminent French naturalist, suggested that it might have taken the Earth some 75,000 years to cool from a molten ball of rock to its present temperature, basing his calculations on experiments he undertook using hot balls of iron. Also at this time, Georges Cuvier, a French scientist mainly interested in evolution, proposed that complete major groups of plants and animals, not just individual species, had been repeatedly swept from the face of the Earth in great catastrophes, and that many thousands of years must have been needed for this depopulation and repopulation to occur.

In the 1790s, William Smith, a surveyor working on building canals, recognised that there was a regular and systematic order within rocks of Southern England. He observed that not only were the rocks ordered, but also the fossils contained within the rocks always succeeded one another in the same order; thus emerged the crucial understanding that fossils allowed rocks to be ordered one above the other in a chronological sequence. In 1830, Charles Lyell, whose interest in geology was fuelled only after attending lectures given by William Buckland whilst studying law at Oxford University, published the first volume of his great work “Principles of Geology”. Interestingly, he had already come to the conclusion through his studies of lava flows on Mount Etna that the age of the Earth must be immense!

All these emerging ideas that the actual age of the Earth must be way in excess of the ~5800 years put forward in the Bible continued to be dismissed out of hand by the Christian Church, who for example, argued that the occurrence of fossils on the tops of mountains resulted from a redistribution of landscapes and seascapes during the time of Noah’s flood!

In his first edition of “On the origin of species”, Charles Darwin estimated the minimum age of the Earth by attempting to calculate the time it had taken to erode the dome of the Weald. Using rather sketchy figures for both the amount of rock originally present, and its subsequent removal rate, he came up with a figure of just over 300Ma; the inference from this was that the age of the Earth must be greatly in excess of this figure. Sadly, after a great deal of criticism from both fellow scientists and the Church, the calculation disappeared completely from the third edition of his book onwards.

In 1862 Lord Kelvin (formerly William Thomson), the Professor of Natural Philosophy at Glasgow University and the world’s expert on thermodynamics, led a blistering attack on geologists for overlooking thermodynamic principles in attempting to calculate the age of the Earth. Using best available data to hand on the melting points of rock and heat flow properties etc, he calculated that the age of the Earth must lie in the range 20Ma to 400Ma. Geologists at the time were very happy with the figure of 400Ma, but were much less impressed when a few years later, after acquiring what he thought were more reliable rock property data, revised his calculation down to a more definite 100Ma. Finally, in 1893, following publication of some experiments undertaken in America, Lord Kelvin was able to refine his calculations even further, concluding that the age of the Earth lay between 20Ma and 40Ma. Lord Kelvin was king of the physicists at the time, and his immense scientific stature made it very difficult indeed for geologists to put their case that the Earth’s age had to be reckoned in hundreds of millions, not just tens of millions years.

At the very end of the 19^th Century, two final attempts were made to estimate the age of the Earth by observational methods. In 1897, John Joly, Professor of Geology at Trinity College, Dublin, picked up on the idea of Halley some 150 years earlier, that the Earth’s age could be calculated from the salinity of the oceans, or in Joly’s case, the amount of sodium in the oceans. He arrived at a figure of 89Ma, which happened to agree quite nicely with Lord Kelvin’s latest prediction. A few years earlier, Samuel Houghton, an Irish geologist, estimated the total thickness of the rocks on the Earth’s surface, and after ascribing a rate of denudation of just over one foot per thousand years, came up with an age within the range 200 to 2000Ma; in fact, such a method could produce almost any age one desired, depending on the input parameters selected.

So it was that at the very end of the 18^th Century, Lord Kelvin and his followers estimated the age of the Earth as being in the region of 20Ma to 40Ma, whereas the geologists and naturalists like Darwin postulated that the Earth required hundreds if not thousands of millions of years for it to have achieved it present state. The dates from both sides of the argument were all estimates based on assumptions backed up by little or no actual indisputable scientific measurement. Luckily for science, the key to solving this conundrum was just emerging from physics laboratories around Europe with the discovery and understanding of radioactivity.

In 1896, a year after the German physicist William Röntgen had discovered X-rays, Henri, a French physicist, discovered that Uranium not only glowed in the dark after being exposed to sunlight but also gave out mysterious rays capable of fogging a photographic plate wrapped in black paper. Soon afterwards Marie Curie discovered that other elements, for example: thorium, polonium and radium, gave out rays similar to those emitted by uranium, although she could not give a rational scientific explanation of what she had discovered.

Fortunately at around this time, a series of scientific findings began to unravel the mystery. First of these was J.J.Thompson’s discovery of the electron, thereby dispelling the previously held view that an atom was a fundamental particle that could not be subdivided. Then, in 1903, Ernest Rutherford and Frederick Soddy, working at McGill University in Montreal, Canada, discovered that in the process of emitting radiation, one element would change into another; they also noticed that helium gas was often found to be present in radioactive rocks, and suggested that its presence might be associated with radioactive decay. It was soon postulated that an unstable atom of uranium radioactively decayed to give a “Daughter” atom of radium together with some helium; this unstable radium then decayed to its “Daughter” atom radon, again with the liberation of some helium; this process continued until after many stages of decay, one eventually finished up with a stable “Daughter” atom and the liberated helium. Therefore, it was realised that, knowing the rate of decay of uranium, measuring the amounts of both “Daughter” and residual parent elements present in a rock containing uranium would provide a means of determining the precise age of the rock being studied, and as older and older rocks were discovered, the age of the Earth itself. Rutherford was the first to attempt to date a rock by this method, arriving at a date of 500Ma after measuring the amounts of radium and helium in a rock.

There followed a surge of interest by scientists all around the world at determining the age of rocks based on radioactivity and a quantitative measurement of their “Daughter” products. Professor Robert Strutt, Cambridge University, pointed out that there was a major flaw in Rutherford’s technique, namely that helium was a gas and would be escaping into the atmosphere during the chemical analysis procedure, and that the stated age could only be the minimum age of the rock, not the actual age. A more accurate dating method that did not involve the measurement of residual helium gas was therefore required.

In 1907 Bertram Boltwood, an American chemist, noticed that along with helium, unusually large amounts of lead were present in radioactive rocks, and postulated that lead was in fact the stable end product in the decay chain starting with uranium. Picking up on this finding, Arthur Holmes (author of “The age of the Earth” and “Principles of physical geology”), working under Strutt at Cambridge, began a painstaking series of experiments aimed at doing just this, and after many setbacks, finally arrived at a result in 1911. His major finding was that the age of his Norwegian sample of Devonian rock was at least 370Ma. In addition to his own age determination, he also revised, in line with more recent procedures, some age data published by Boltwood, and assigned a geological period to these data, something Boltwood had failed to do.

Unfortunately, the lead ratio method (like the helium one before it) for dating rocks had a major drawback – what if there was some lead present when the uranium started to decay? If this was the case, then ages would be over estimated. Other problems also began to surface. As the structure of the atom began to be unravelled, it was recognised that the nucleus of an atom was made up of two types of particle, protons and neutrons; the number of protons (atomic number) dictated the chemical properties of an element, whereas the combined total of protons and neutrons dictated its atomic weight. Hence, an element could have a range of atomic weights depending on the number of neutrons associated with the set number of protons. For example, an atom of lead derived from the nuclear decay of uranium²³⁸ is called lead²⁰⁶ because it contains 82 protons and 124 neutrons; similarly, lead derived from the decay of thorium²³² has 82 protons and 126 neutrons, hence is known as lead ²⁰⁸. The various forms of the same element are known as isotopes.

Hence, lead in a rock could arise from the decay of not only any uranium originally present but also any thorium present, these new quantities of lead adding to any lead present at the start of the decay period. With this in mind, in 1914, Holmes, in conjunction with Lawson and workers at the Vienna Radium Institute who had gained world expertise in atomic weight determination, reassessed earlier lead based calculations and came up with a figure for the age of the Earth as >1600Ma. [Sadly, unknown to these workers at the time was that natural uranium itself consisted of more than one isotope; although 99% of natural uranium is made up of the isotope uranium²³⁸, the small amount of uranium²³⁵ present is very significant as it decays to another stable lead isotope but at a much faster rate. Holmes’ corrected dates were therefore still inaccurate as he was overestimating what he thought was “original” lead, as Holmes’ “original” lead consisted of any “original” lead plus the lead derived from the decay of uranium²³⁵.]

For many years to come eminent scientists continued to debate, sometimes somewhat heatedly, the age of the Earth, many pointing out likely inaccuracies in the uranium/lead methods and hence doubting Holmes’ revised age of 1600Ma. It was not until 1929 that Aston, in conjunction with Rutherford, using a mass spectrometer for isotope analyses, finally identified the Uranium²³⁵ isotope. Rutherford then, by making the assumption that originally there were equal amounts of both uranium isotopes present in a rock, came up with a revised age of the Earth as being ~3400Ma. As one can imagine, bearing in mind all the previous controversies, most scientists at the time were reluctant to accept that the Earth could possibly be as old as this!

Lead in a rock sample could be made up therefore of lead derived from U²³⁸, lead derived from U²³⁵, lead derived from thorium together with any “original” lead. In 1936 Alfred Nier, a young physicist at Harvard University who had access to the very latest in mass spectrometers, identified the “original” lead isotope as lead²⁰⁴. He and others then realised that a more accurate way of determining the age of a rock would be to determine the ratio of lead derived from U²³⁸ to that derived from U²³⁵ and by 1940 Nier’s oldest determined age for a rock was 2400Ma!. This lead-lead method as it was called, was a major step forward and is still one of the fundamental age dating techniques used in modern geochronology.

But, and again there is always a but, what if “original” lead, that is, the lead present when the Earth was formed, contained not just lead²⁰⁴ but all 3 isotopes of the element as this would again give erroneous results. It was eventually argued by a number of scientists that the best approximation to the lead isotope ratios present in “original” lead would be given by those present in material present at the time the Earth evolved, that is, meteorites. Although there is only a very minute amount of lead present in meteorites, advances in analytical techniques eventually made these difficult determinations possible, and by 1953, Claire Patterson and Harrison Brown, working at California Institute of Technology, and the German Fritz Houtermans, independently came up with a revised figure for the age of the Earth of ~4560Ma.

This is of course still only the latest estimate in a long line of attempts to determine the age of the Earth, and over the centuries scientists and others have gradually arrived at older and older values as geological, chemical and physical understanding of all the phenomena involved have been advanced. Is 4560Ma the final definitive answer for the age of the Earth? or are there new scientific discoveries and ideas lurking around the corner that will again lead to either an upward or downward revision of this figure? Only time will tell!

Date	By whom	Estimated Age of Earth	Methodology
1650	Archbishop Ussher	6012 years	Back calculation from the Bible
1778	Georges Buffon	75000 years	Rate of cooling of Earth since in molten state
1785	James Hutton	Many thousands	Realisation of time required for continuing geological processes to “build” the Earth (Uniformitarianism)
~1800	Georges Cuvier	>100000 years	Deduction to go alongside his acceptance of “Catastrophism”
~1829	Charles Lyell	Immense!	Study of Mount Etna
1859	Charles Darwin	>>300Ma	Rate of erosion of the Weald dome
~1862	Lord Kelvin	100Ma	Rate of cooling of Earth since in molten state
~1890	Samuel Houghton	200 to 2000 Ma	Rate of deposition of sediments on ocean floor
1893	Lord Kelvin	20 to 40 Ma	Cooling of the Earth from molten state – revised estimate
1897	Prof. John Joly	89Ma	Ocean salinity
1903	Pierre Curie		Radioactive decay produces heat
~1905	Prof. Ernest Rutherhood	>500Ma	Radioactive decay of radium
1911	Arthur Holmes	370Ma	Date of a Devonian rock using Uranium/Lead method
1914	Arthur Holmes et al	1600Ma	Improved Uranium/Lead method taking into account lead derived from decay of thorium
1929	Rutherford	3400Ma	Uranium/lead method taking into account both isotopes of uranium – U²³⁸ and U²³⁵
1940	Alfred Nier	2570Ma	Lead-lead ratio method
1953	Patterson, Brown and Houtermans	4560Ma	Lead-lead ratio method but using meteorite composition to define isotope composition of “original” lead

Darwin – a life in science, John Cribbin & Michael White, Simon & Schuster UK Ltd (1995)

The self-interpreting family Bible, Rev. John Brown, published by J G Murdoch (~1850)


Two tree boles, and a fallen tree trunk	The Broken Beds; Jack is sniffing the Great Dirt Bed !


Looking East, with the Chalk cliffs of Arish Mel beyond	Looking West, Portland Limestone at cliff base, Wealden top right