Wednesday 19th April 2023
The last talk which was given by Chris Darmon was on the geology of the Canary Islands. Chris has led and will be leading further field trips to the Islands. More can be found about his field excursions at https://www.geosupplies.co.uk/geological-supplies-company.php
The Canary Islands are a group of seven main volcanic Islands located off the north-west coast of Africa trending in a south-west to north-east direction. They are located on ancient oceanic crust of the African plate and are a good example of intra-plate volcanism. The ages of the islands varies between the youngest El Hierro at 1.12Ma in the south west to Fuerteventura at 20.6Ma in the north east. It has been suggested that like the volcanism in Hawaii the islands have been produced by the presence of a volcanic plume over which the African plate moves in a direction from west to east although other ideas have been put forward and the topic is controversial.
Lava flow Lanzarote with columnar jointing by Chris Simpson MWGC
The canary Islands form what is known as the Canary Volcanic Province but this is not the only area on the African plate that is volcanic as the area around Madera is also volcanic and known as the Madeira Volcanic Province and volcanism is also around Cape Verde islands. ( Cape Verde Volcanic Province) But all of these are linked to a much larger area of seamounts and these cover an area that is approx 1300 km long and 350 km wide which is known as the Canary Islands Seamount Province. The sea mounts vary in age with the oldest in the region of 142Ma.
Aa Lava flow Lanzarote by Chris Simpson MWGC
The Canary Islands similar to the Hawaiian Islands, show different stages in their development over time. Firstly there is the submarine stage, followed by shield-building, declining, erosive and rejuvenation stages. The stages commence with the submarine seamount stage which contain submarine sediments and for example, alkali basalt pillow lavas and dyke swarms. This is followed by the emergent stage i.e. the stage at which the volcano is at the surface. It then begins to develop into a large shield volcano characterised by subaerial alkaline basalt and trachybasalt lava flows. As the volcano begins to decline then rocks like phonolite begin to appear. This stage is followed by erosion. Here there is little or no volcanic activity and there is vast erosional removal of previous lavas and ashes. Finally there is rejuvenation with minor volcanic activity and the formation of rocks such as nephelinites. Thus these structures need at least 10-15 million years to go through the various stages.
Multiple balsaltic dykes Fuerteventura by Chris Simpson MWGC
So what are the arguments for the formation of these islands. Firstly the idea that there is a large mantle plume over which the African plate moves but there are many arguments against this idea. Another suggestion is that they are related to a propagating fracture associated with the Atlas tectonic chain. Objection to this hypothesis is that there is no evidence of continuous faults connecting the two areas.
A third idea put forward by Paul van de Bogaard in 2013 is as follows: "shallow mantle upwelling beneath the Atlantic Ocean basin off the NW African continental lithosphere flanks produced recurrent melting anomalies and seamounts from the Late Jurassic to Recent, nominating the Canary Island Seamount Province as oldest hotspot track in the Atlantic Ocean and most long-lived preserved on earth.” (van den Bogaard, P. The origin of the Canary Island Seamount Province - New ages of old seamounts. Sci Rep 3, 2107 (2013). https://doi.org/10.1038/srep02107)
The geology of these islands appears to be complex with much more research necessary to understand their geology.
Saturday 11th March 2023
What triggered the Cambrian Explosion by Prof Rachel Wood University of Edinburgh
At the beginning of the Cambrian period there was a huge evolutionary burst of life markedly increasing the diversity of animals which includes many of the major groups of life we see today. The question has always been: “what triggered this event?”. The talk by Prof Rachel Wood” ( University of Edinburgh) on February 13th shows just how much work has gone into that question and that answers are finally being found. What is proposed is that the roots of the Cambrian explosion are not in the Cambrian but in the Ediacaran.
Prof. Wood explained that for the vast majority of time the planet was dominated by unicellular life. Then between 575-540Ma organisms on the sea floor changed dramatically. These organisms - the Ediacara biota, were much more complex but at what point the first animal appears is actually still disputed. Although if one looks at various characteristics that suggest complexity eg. burrowing, surface locomotion or biomineralisation, it can be seen that they do start to appear in the Ediacaran. Then at about 540Ma a huge abundance and diversity of life appeared. This being the “Cambrian Explosion”.
Prof. Wood and her multidisiplinary team along with international collaborators have been working on this problem for many years. The main location for their field work is in Namibia. The reason this area is so important is due to the ability to a.) date the ash beds to a narrow time sequence of between 547 - 538 Ma a critical 10 million year period where they find these first fossils that are known to be bilaterians b)the area is divided into two basins, c) the necessary outcrops are present and vegetation free.
Research site in Namibia
One fossil find they have named Nomacalathus shows bilateral symmetry which suggests it is an animal and although from the Ediacaran biota is very similar to the Entoprocta found in China from about 520Ma. Therefore she suggests that they are now pulling the Cambrian Explosion down into older rocks.
One suggestion as to the cause of this increase in diversity is related to changes in oxygen levels. When oxygen levels increase it leads to an increase in cellular energy production. This could lead to a development of greater complexity with an increase in size, more complex body structures and enable more energy-intensive movements.
Part of the work in this area is in tracking anoxia. For this they use proxies one being the iron speciation paleoredox proxy which can distinguish three major oceanic redox states - oxygenated, ferruginous and euxinic. Anther proxy is the Cerium anomaly which can track manganous conditions. These two provide detail necessary to understand oxygen distribution in the basins under study.
From this work they discovered that the organisms only lived in well oxygenated areas of the basins. To begin these areas were very restricted but as oxygen levels increased so did habitable area which itself allowed spread into new areas and also an increase in diversity. Not only is change in habitat important but most probably nutrient levels will play a part but is difficult study. Individual animal physiology and adaptability will also play a part.
Prof. Wood then summed-up the talk with the following conclusion:
Late Ediacaran metazoans restricted to well-oxygenated, but dynamic, oases
There were differential turnovers and body size changes within the Cambrian
- may have been structured by intervals of dynamic shallow marine
Physiology may cause differing responses to these drivers
-created distinct record of early metazoans during the Cambrian Explosion, and also the form of its demise.
Oxygenation expanded habitats - dynamic chemocline - and governed ecological complexity
Cambrian Radiation was not linear, but cyclic, driven by oxic pulses
Wednesday 2nd November 2022
Evolution of the eye
At the last meeting 19th oct Tony Thorp gave the second of his very interesting talks on the Evolution of the Eye.
To begin he gave a summary of the previous talk which had taken the time-line of evolution back to the late Cambrian.
Eyesight began with light sensitive cells which could detect light but not direction, images or shapes. The next step was the development of pits covered in photoreceptors. These were able to detect direction of light but not images or shapes. This allowed development of phototropism. Further evolutionary change led to deepening of the pit and the cavity becoming fluid filled and eventually covered over. The narrowing of the aperture produced the “pinhole camera eyes” as in Nautilus. This increased directional detection and some shape detection but not colour or depth.
Nautilus by Archibald Tuttle Creative Commons https://creativecommons.org/licenses/by-sa/4.0/deed.en
Eventually the eye developed a lens so that now there was a fluid filled eye with lens and cornea which allowed image focusing which was followed by the development of the hard lens we know today which can focus light much more accurately. Tony concluded the summary of the first talk by talking about lamprey vision. The lamprey split off from the rest of the vertebrate group about 500MA. What is of interest is the rods and cones of the lampreys utilise the same mechanism of photo-transduction as other vertebrates therefore suggesting the mechanisms used in vision emerged very early on.
Western Brook lamprey bu US Fish and Wildlife Service Creative Commons
In the second half of his talk Tony showed that the development of vision could be shown to have evolved even further back than the Late Cambrian.
In order to understand evolution of vision we first have to understand the biochemical pathway of photo-transduction. The pathway involves a cGMP (cyclic guanosine mono-phosphate) - gated mechanism which alters the flow of ions across the cell membrane. Each photo-pigment molecule consists of the trans-membrane G -receptor protein opsin and a chromophore 11 cis -retinal, the whole known as rhodopsin. When light is absorbed the rhodopsin becomes enzymatically active which causes a cascade of events which culminates in the decrease in cGMP concentration which causes closure of the cGMP - gated channels on the plasma membrane. Sodium and calcium channels in the plasma membrane of the outer segment are kept open in the dark by a high level of cGMP. A decrease in cGMP leads to a decrease in influx of the cations into the cell and therefore hyperpolarization of the cell membrane. This neuronal signal is then transmitted to the brain.
Research has shown that there are two classes of opsin. Type I opsin, common in bacteria, evolved separately from type II opsin that are common in animals. Opsin is a member of large family of detector proteins, called the 'G-protein coupled receptors' (GPCRs).
Further, all known visual pigments in Neuralia ( the clade containing Cnidaria,eg. Corals and jellyfish; Ctenophora- Comb Jelly; and Bilateria -animals with bilateral symmetry) are composed of an opsin and a light-sensitive chromophore, usually retinal. Also these opsins can be classified into the same three subfamilies into which the bilaterian opsins are classified: the ciliary, rhabdomeric, and go-coupled plus retinochrome, retinal G protein-coupled receptor (Go/RGR) opsins.
It has been found that opsins only evolved after sponges had diverged from other animals, but before the split between Bilateria and Cnidaria. Within this time there is one animal lineage i.e the Placozoans which are very simple organisms and do contain opsin although cannot detect light.
Therefore it is thought that opsin itself evolved at some point between 755-711MA with it’s ability to detect light evolving somewhere between 711 and 700MA
Monday 17th October 2022
Stonehenge by Colin Humphrey
General view. The figure on the right illustrates the scale
On 21st September 2022 club member Colin Humphrey summarised the present understanding of Stonehenge, a World Heritage site on Salisbury Plain. The monument is composed of two very different building stones: the huge sarsens of the lintelled circle and its horseshoe of trilithons; and the much smaller bluestone inner circle and horseshoe; although only about half of nearly 150 orthostats (standing stones) remain. Sarsen is English Paleogene sedimentary rock; and bluestone is much older Welsh Ordovician igneous rock. The monument is clearly designed to allow the summer solstice sunrise to shine through several apertures between stones, and onto an altar stone.
The sun must shine first through two heelstones (one is missing). Note root holes
The sarsens have long been assumed to come from the Marlborough Downs, 17 miles north of Stonehenge, the most abundant sarsen site in Britain. Recent X-ray fluoroscopy has definitely provenanced them to West Woods, just south of the Downs. Sarsen is silcrete, a silicified medium-to-coarse sandstone, very hard and erosion resistant. The sand was probably released by erosion of Lower Cretaceous Greensand (originally c100 Ma) exposed to the north, and conveyed by south flowing rivers to be deposited as the Reading Formation, sandy lenses on alluvial plains around 55 Ma ago in the early Cenozoic. Silcrete is found in many places across south-west Britain. The plains became vegetated and rootholes are frequent in the silcrete. Silicification probably occurred soon after, during the hot Paleocene-Eocene Thermal Maximum.
Silcrete boulders on Marlborough Downs
The bluestone comes from Mynydd Preseli, a small range of hills in north-east Pembrokeshire. Recent trace element analysis identifies various outcrops of 460 Ma intrusive dolerite, including some stones from Carn Goedog. There is a strong case that these and others were used to form a stone circle three miles west at Waen Mawn. The archaeologists now tentatively suggest, using several lines of evidence, that the Waun Mawn stone circle was then moved to Stonehenge. They also now conjecture that the local people carried their own stone circle to Stonehenge and remained there. It was a time of population movement; strontium analysis of cremation remains at Stonehenge include some people who were probably brought up on the rocks of west Wales. ‘Spotted’ dolerite (ie with phenocrysts) is not the only bluestone. There are others, including ‘unspotted’ dolerite and some extrusive rhyolite from the northern flank of Preseli at Rhosyfelin.
Rhosyfelin bluestone quarry
Around the outside of the present Stonehenge sarsen circle is a circle of now covered Aubrey Holes, named after its discoverer. We know from rock fragments and dating techniques that bluestones stood in these holes around 3000 BC. The much bigger sarsens were not installed until around 2500 BC, when the bluestones were then rearranged within the sarsens. Carbon dating of hearths at bluestone extraction sites in the Preseli hills suggest that a bluestone circle may have been constructed locally, at Waen Mawn for example, around 3400 BC. Stonehenge appears to have consisted of the smaller bluestones for hundreds of years before the more iconic sarsens arrived.
Rhyolite with fabric, a curious structure at Rhosyfelin
A dispute rages between a small number of people who believe the Welsh ice sheet transported the bluestones from Preseli at least to the Somerset Levels during the Anglian advance around 400.000 years ago. This has not been scientifically dismissed but the consensus now is that it is unlikely. Most glaciologists do not place the ice front far enough west and there is insufficient evidence of a train of large glacial erratics. Any argument that human transport is impossible is of course disproved by the movement of sarsens ten times the size, at least 15 miles.
Solitary remaining standing bluestone at Waen Mawn circle in Wales. Possibly the previous site of the Stonehenge bluestones
Sunday 4th September 2022
Study of Church Stretton Fault near Gladestry. by Dr. Geoff Steele
At the last meeting Dr. Geoff Steele gave a very insightful talk on events along the Church Stretton Fault and the Welsh Borderland Fault System (WBFS).
His study began with a question which was posed to him whilst on a geology field trip. The question asked “ why is Weythel Common a ridge when it consists of Raglan Marl whilst the other areas of Raglan Marl in the area form lowland?” This was in 2017 and led to Geoff studying the area in detail during his spare time. Weythel Common lies about 7km West of Kington,( grid reference OS235565.)
He presented his findings to us in this talk.
WBFS comprises the Tywi Lineament, the Pontesford Lineament and the Church Stretton Fault and forms the terrane boundary between the Cymru Terrane to the north west and the Wrekin Terrane to the south east. Various tectonic events have led to movements along the WBFS. One of these occurred during the late Ordovician in which the movement was dextral strike-slip. During the Silurian the movement was extensional as a normal fault with downthrow on the Welsh side. During the Devonian faults were reactivated as reverse faults and movement was sinestral strike-slip.
His fieldwork utilised three geology maps: one inch to 10 miles (1979); 1:50000 (2004); 1:250000 (1994). The section covering Weythel common on the latter map was scaled to the equivalent of the 1:50000 map from which differences in the mapped structure of the faults could be seen.
East side of Weythel Common
Line of east fault
Geoff went on to show that the Church Stretton Fault in the area under study has a break in it and showed that if a fault is not straight then a strike-slip movement producing transpression would lead to uplift of the strata in a “palm” structure whereas transtension would lead to basin formation “flower” structure. Therefore the structural hypothesis for this study was as follows: “if dextral strike-slip movements produce a transpressional high then sinestral strike -slip movements should produce a transtensional basin”
So Geoff underwent a detailed study of the area around Weythel Common. He looked at the stratigraphy, using easy to observe quarries and measured dip and strike of the area’s rocks. Using photography and field walking he could determine the line of the fault and also the pattern of some of the rocks just below the surface. The weather helped greatly in this latter endeavour, with snow melt indicating the trend of the fault and very dry and parched fields showing the underlying vertically orientated sandstones.
West side of Weythel Common
Line of west fault
From his findings Geoff was able to put together his own geological map of the area. His conclusions are as follows:
Weythel Common is a “tulip within a palm”
It is a hill because:
- Hard sandstone bands form a scaffold
- The surrounding Wenlock shale is more crumbly than the Raglan marl
It’s structure is an asymmetrical anticline and may be thrust faulted
Geoff's final map from the fieldwork