Wednesday 4th February 2015

The AGM was held on 21^st January, a cold evening with frequent snow flurries. The AGM was followed by one of Dr. Geoff Steel's very interesting talks. On this occasion his talk was entitled, “The Tyrone Ophiolite: ancient ocean floor on land in Northern Ireland”.

Geoff commenced the talk with an explanation of ophiolites in general. They are interpreted as fragments of ocean floor that have been sheared off from subducting crust, caught between colliding continents and then elevated to a continental location. A typical ophiolite suite can be defined as consisting of the following (from base up): unlayered serpentinised peridotite, layered gabbro, massive gabbro, sheeted dyke complex, pillow basalts, deep sea sediments. Although not all ophiolites contain all the layers. Ophiolites are important as they allow access to oceanic crust and upper mantle that otherwise would be unobtainable. The age of oceanic crust is dated to about 200my. Therefore, an ophiolite that is dated as much older allows study and understanding of much earlier events and provides evidence for the existence of old oceans and oceanic basins. The Tyrone ophiolite was obducted onto the continental margin of Laurentia during closure of the Iapetus Ocean during the Ordovician Period.

It was Geoff's intention to observe as many elements of the ophiolite as possible. On his tour he visited Beltonanean Mountain and Black Rock where he observed the gabbro. The gabbro here consisted of coarse to very coarse-grained varieties. The gabbros at Black rock are also cut by younger balsaltic and doleritic intrusions which are in the form of 1 to 2m wide dykes. These sheeted-dykes are also part of the ophiolite. They were also viewed at Carrickmore where they cut through Hornblende gabbro. Originally these dykes would have acted as feeders for the overlying sequence of pillow lavas. The latter were viewed at Creggan rock. The final location on the tour was the Carraghinault gold mine. This gold was formed as part of the orogenic phase of the closure of the Iapetus Ocean. As this is a working quarry it could only be viewed from a distance.

The talk ended with some of the members now thinking of a visit to Ireland!

The next meeting will be on Wednesday 18th February. Club Secretary, Bill Bagley will be giving a talk entitled: “Quartz and Other Forms of Silica.”

Thursday 18th December 2014

23 members attended and had an enjoyable evening at the November meeting. Members were entertained by a DVD ( Picnic in Siluria) produced by the geological section of the Woolhope Naturalists Field Club. The film was a re-enactment of an 1850’s field visit by Sir Roderick Murchison to a site near Aymestrey in Herefordshire. The scene was set by a short talk given by Robert Williams who was part of the re-enactment in the guise of Sir Charles Lyell.

The next indoor meeting will be held on Wednesday 21st January.This will be the AGM followed by a talk by Dr Geoff Steel entitled: “ The Tyrone Ophiolite; ancient ocean floor on land in Northern Ireland.”

Tuesday 28th October 2014

The next event will be a member's social evening on the 19^th of November.

Sunday 19th October 2014

Mid Wales Geology Club’s evening meeting on 19 November will be more of a social event than usual. Billed as Mulled Wine, Mince Pies & Murchison, we will show the DVD produced several years ago by the Geology Section of the Woolhope Naturalists Field Club. The Woolhope arranged a re-enactment of the 1850s field visit by the eminent geologist Sir Roderick Murchison, to a site near Aymestrey in Herefordshire. Picnic in Siluria was complete with steam train and a large cast in Victorian costume. It concludes with a short modern footage of a geological visit to a nearby working quarry. Mid Wales Geology Club member Dr Geoff Steel is the current chairman of the Woolhope Geology Section. He says he played the part of a servant, and was not permitted to eat with the gentleman geologists!

Robert Williams, who attended the re-enactment in the guise of Sir Charles Lyell, will set the scene for us, and introduce the characters. Then we will enjoy mulled wine and mince pies (non-alcoholic for those who wish); and afterwards we will watch the DVD. Coffee and tea will be available before we close. We will be at the usual venue, Plas Dolerw, Milford Road, Newtown, commencing at 7.15 pm.

Wednesday 8th October 2014

At the last meeting we were treated to a super talk given by Chris Darmon. He commenced the talk by suggesting that the traditional view of how Iceland came to be where it is is now hotly debated. He would reveal more at the end of the talk! Iceland is an island that spans the mid-ocean ridge along which the Eurasian and North American plates are moving apart. It also sits atop a mantle plume where magma upwells from from the earth's mantle ( it is this that is now in dispute). And the combination of the two are responsible for Iceland. Geologically Iceland is very young with the oldest rocks exposed at the surface between 14 and 16 million years old. It's surface is constantly changing due to rapid erosion and replenishment from volcanic action.

It is one of the most volcanic places on earth with hundreds of volcanoes many of which commence as fissure eruptions. One of the most deadly volcanoes was the eruption of Laki in 1783/4 which led to huge volumes of Sulphur dioxide being emitted into the upper atmosphere which in turn caused thousands of deaths both in Iceland an other areas of Northern Europe due to starvation and respiratory illness.

The main rock type on Iceland is volcanic and you will find pillow lavas and basalt columns. 90% of the igneous rock is basalt but there is also rhyolite, dacite and andesite present. One anomaly here is the presence of larger amounts of rhyolite compared to similar areas eg. Hawaii. Sedimentary rocks are also present and vary in type from volcanic in origin to lake, river and lagoonal. Thus there are tuffs, shales, mudstones, sandstones and conglomerates. There are even layers containing lignite and coal beds. This suggests that the environment on Iceland has been changing over time. Fossil plants are frequent and contain Angiosperms, Gymnosperms, Horsetails and many others. This suggests that the Miocene Period on Iceland was one of a humid, warm temperate climate. The decline of these species at the end of the Miocene suggests a cooling of the environment.

Most of the population of Iceland live around the coast and inland is deserted. They also make use of all the geothermal energy present for example to heat their homes. Iceland is now a very desirable destination for tourists and Chris has led tours there for the past twenty years.

At the end of the talk Chris presented some evidence to suggest that a mantle plume may not exist under Iceland.

1. The spreading rate at the ridges is to fast. This suggests that an expanse of older crust underlies Iceland submerged beneath younger lavas.
2. The dating of zircon from the Hvitserker ignimbrite dated to 126-242MYA. That is the Mesozoic. Therefore it was suggested that a continental crust must exist beneath at least part of Iceland.
3. The crust is too thick.
4. The crust above the mantle plume is not hot.

He went on to say that Gillian Foulger found that her seismic data showed that instead of a long narrow plume of magma coming from deep below Iceland there is actually a broad reservoir of molten rock less than 400km down. This suggests that remnants of crust left behind in the mantle at the fault line have a lower melting point than the surrounding mantle. Where this ancient crustal rock crosses the ridge it should melt more than the rock around it creating a shallow reservoir of magma that feeds Iceland's volcanoes. Thus there is no need for a mantle plume.

This idea of no plume links in with much other research suggesting that mantle plumes have no need to exist to explain other areas like Hawaii. A very controversial area of geology!

The next talk will be on Wednesday 15th October to be given by Tony Thorpe and entitled “ A new look at metal extraction in the Bronze Age and the iron Age”

Also the members evening will be on Wednesday November 19th and will be a “Mulled wine, Mince pies and Murchison” evening.