Member Kit Moorhouse gave a talk on The Geology of Salt in the Cheshire Basin
Although on our doorstep, this topic has been sadly neglected by our club. Kit's talk was therefore particularly welcome.
The halite deposits which underlie the Cheshire Basin originated in the Triassic Period when the area was part of the vast supercontinent of Pangea, at a latitude of 20-30 degrees north. i.e. about that of the Sahara or the Persian Gulf.
Sea water comprises three and a half percent salt, together with other dissolved salts such as Carnallite (potassium magnesium chloride) Polyhalite hydrated potassium magnesium chloride, Sylvite (potassium Chloride), Sylvinite (sodium chloride with potassium chloride) , Gypsum, and carbonates.
The evaporation of about 100 m of seawater is required to produce just 5cm of deposits so simple evaporation of sea water is insufficient to produce such massive deposits so some replenishment mechanism is necessary.
In Cheshire, halite is dominant, with no calcium sulphate or potassium salts.
The preferred model for their formation is that of a "Barred Basin" in which access to sea water is limited and evaporation leads to a gradient of salinity in which carbonates, sulphates and chlorides are crystallised out in that order progressively at increasing distance from the "bar". This could occur in a situation like the Mediterranian, fed from Gibralter, or more interestingly, the Gulf of Garabogazköl in Turkmenistan. This is fed from the Caspian Sea and in 1980, it was felt that it was drying out the Caspian Sea so the inlet was blocked up. Unfortunately it triggered a problem with wind blown salt contaminating surrounding land and in 1990 the inlet was opened up again. To produce thick deposits, space must be created and a subsiding basin is required.
In the case of the Cheshire Basin, like the Zechstein Sea in the north of England, the flooding is associated with the opening of the North Atlantic and the formation of half grabens. 2000m of Jurassic and Cretaceous deposits followed before erosion brought the deposits to the near surface.
The halite beds form part of the Mercia Mudstones Group (Once called Keuper Marl) and comprise salt with associated muds and silts which give them their characteristic yellow, red and blue colouration.
In Cheshire salt was recovered from briney springs by the Romans by evaporation from lead pans, This developed in medieval times to shallow pumping from the "Wet rock head" (brecciated mudstone where meteoric water had dissolved salt) Round Winsford salt was recovered fron wells and open pans. Subsidence led to craters, meres and lakes.
In 1850-1919 the industry grew, based on wells and pans. Subsidence became a problem, often at a distance and in 1890 a levy was imposed on the industry to provide compensation. In addition in the 19th and 20th centuries, it was mined, leaving pillars to support the roof. This led to subsidence when mines were flooded including the "Great Subsidence" in 1881.
The largest workings are at Winsford where it is mined leaving pillars to support the roof.
The beds are not structureless with bedding planes showing up as laminae of muddy layers and interesting polygons are visible on a 4-14 metre scale with dish-like forms corresponding to their edges.
Winsford produces about a third of the total UK salt, used for treating roads in winter and as a beet fertiliser in East Anglia. Brines are also produced by controlled pumping for table salt and process salt for sodium hydroxide and chlorine production.
Spherical cavities produced by controlled brine extraction can be used as storage facilities for natural gas and have been applied for that purpose in East Yorkshire.
This overview of the geological and industrial aspects of our salt industry was much appreciated by members who resolved to try to include a field trip to the area in the future.
Our next meeting is: "A guide to Astronomy & Space Science" by guest speaker - Danny Craig.
Thursday 4th April 2013
“U.K. earthquakes and John Milne's contributions to the science” Dr Ian Stimpson, FGS
On Wednesday, the 20th March, Dr Ian Stimpson FGS, of the School of Physical & Geographical Sciences, Keele University. talked to an audience of 36 members and guests.
Ian last visited us two years ago, just after the disastrous earthquakes in Japan and New Zealand, when he gave us a splendid lecture on them in lieu of a talk on UK quakes. This talk is an updated version of the one we missed.
The first part of Ian’s talk focussed on John Milne, a geologist who could reasonably be described as “The Father of Seismology” because his legacy is so huge. In spite of this, he is comparatively unrecognised. This year is the centenary of his death.
John Milne was raised in Rochdale, read Natural Sciences at Kings College, London (Precursor of the Royal School of Mines) and then Mineralogy at the University of Freiberg in Germany, before surveying for coal in Newfoundland. He then took up a post as Professor of Geology and Mining at Imperial College, Tokyo. Typically, being of an adventurous nature, during 1875-76, he went there the difficult way, overland, making geological notes along the way! The earthquake which occurred on his first night in Japan triggered his lifelong interest in them. By 1889 he was a Fellow of the Royal Society, a considerable achievement for a geologist.
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During this period in the 1880s there was a transition from a belief in the supernatural origins of earthquakes to a scientific one and in 1893 at the Koto convention, it was mooted for the first time that faults caused earthquakes rather than being the results of them.
In the late 1800s there was no accurate recorder of seismic vibration before Milne and Gray developed the Milne-Gray seismograph which measured the three components (north-south, east-west and up-down). His invention was not confined to geology, he developed a vibration detector for steam engines which facilitated the improvement of the track.
In 1895 when his home and laboratory burnt down he ended his 20 year stay in Japan, came back and set up home with his Japanese wife at Shide on the Isle of Wight. By 1908 the Milne-Shaw seismograph was the standard instrument for a worldwide network reporting back to him at his home, establishing the foundation of modern earthquake research, a remarkable achievement as he was in receipt of no government money.
His published log of recorded earthquakes clearly maps out what we came to understood as plate boundaries some 50 years later.
He died of Bright’s disease in 1913 at the young age of 62.
UK earthquakes
The scale of magnitude of earthquakes is based on the log of the size of the biggest displacement in the seismic trace, so from one step in magnitude to another is the equivalent of an increase of 32 times the amount of energy released. Shocks go from M 9.5 as the largest, recorded in Chile, to minus 5, when you break your pencil.
Most quakes are related to plate boundaries, but intraplate quakes occur, as in the UK. The risk is small here, but we are subject to magnitude 5 quakes and most of us remember one or two local “bumps”, including the Bishops Castle one in 1990, of magnitude 5.1. Strangely, we have more M 5 quakes than M 4 ones.
The UK is subject to compression leading to movement on ancient faults. Some small shallow quakes are related to coal extraction, when the stronger sandstones which separate the coal seams break; but larger quakes tend to be triggered at depth leading to only minor damage.
The distribution of earthquakes in the UK is problematic, being concentrated along the west coast, plus the Welsh borders, with the odd small patch round Dover. Very strangely, Ireland (North and South) has almost no quakes. We get on average one M 5-6 earthquake every 8 years. Fracking leads to shocks of about M -1 to M -2, of which we get about 23 every year.
Central within the UK there is a stable, roughly triangular, Midland Craton, within a complex patchwork of ancient terranes, all separated by faults along which movements can occur when intraplate pressure builds up. The overall distribution is however difficult to explain and some geologists invoke a “Hot Mantle Anomaly” to explain the larger deeper quakes which originate some 20 km down.
Colin Humphrey wound up the meeting by thanking Ian for a remarkably interesting talk and making the point that we should see our way to commemorating the life of John Milne, considering the huge legacy we have inherited from him.
April Meeting
At the indoor meeting, on the 17th April, Duncan Hawley will be giving a talk on “Early Geological Maps”.
Tuesday 12th March 2013
Next meeting will be on Wednesday 20th March. Dr Ian Stimpson will give a talk entitled “Uk Earthquakes and the Centenary of John Milne”.
This is expected to be an excellent talk so please come along.
Sunday 24th February 2013
At this well attended meeting Colin Humphrey gave a very interesting talk on the age of the earth.
In an Age of Ignorance, until mid 17th cent, western thinking about the age of the earth was polarised. At first, eternalist philosophers like Aristotle thought the earth had always been here. Later, scriptural literalists like archbishop Ussher took the opposite view. In 1650 he declared in Annals of the Old Testament that creation began on the evening preceding Sunday 23 October 4004 BC. Many authors at the time were priests, constrained to think of the earth as around 6000 years old.
In the Age of Reasoning, philosophers and priests were replaced by polymaths, whose empirical thinking quickly departed from scriptural literalism. The mistaken concept of a cooling molten earth dominated thinking during the 18th and 19th centuries. Newton and Leclerc calculated cooling at 50,000 to 100,000 years. From 1860 Lord Kelvin led with an age of 98 million years, though by 1899 he had reduced this to 24 million years. Other scientists, from Hutton to Lyell and Darwin assumed gradual geological processes like sedimentation and erosion to take several hundred million years. Some thought periodic natural catastrophe to be more important in shaping the earth. All believed the earth to be millions of years old, and all were wrong, but oh how they argued!
The Age of Science began in the 20th century with an explosion of knowledge. In 1896 Becquerel discovered radioactivity. Within eleven years the Curies, Rutherford and others had explained and calibrated radioactivity, and ages up to 2 billion years were being determined for some earth minerals. Another advance came in the early 1950s with much improved mass spectrometry, and with the realisation that earth was the same age as the meteorites from the asteroid belt – 4.55 billion years. Now it only remained for geologists and palaeontologists to support this age with the explanation that earth's history has been alternations of vastly long periods with very little change, interrupted by sudden catastrophe, repeated again and again.
The talk finished with recent geological evidence for Noah’s Flood, an event which puzzled for centuries. 7600 years ago, in an unusually warm and wet period the Mediterranean overflowed into the low-lying Black Sea basin, in just a few months inundating a region nearly half the area of Britain.
The next meeting is on 20th March at 7.15pm when Dr Ian Stimpson will give a talk entitled "UK Earthquakes and the Centenary of John Milne."
If his last talk is anything to go by then expect a riveting talk on earthquakes and seismology.
Monday 4th February 2013
The next evening event will be a talk by Colin Humphrey, on Wednesday 20th February - 'The Age of the Earth: a history'. Philosophers, scientists and religious men argued for thousands of years without getting near the truth. Then the last century saw an explosion in understanding, which has continued even to the present day. There are many aspects to this interesting story, from Aristotle to Ussher, radioactivity and recent investigation of Noah’s Flood, with a lot of nonsense and controversy in between.