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Workshop Summaries

 

27th March 2018 - Why does Cumbria have so many different rock types?

Led by Audrey Brown

Audrey Brown began this workshop by giving us each a simplified version of a BGS map of Cumbria. She then described how hot, radioactive rocks solidify as they cool, & other rocks rise to the surface. Tectonic plates can be moving apart which enables new material to move to the surface & form ridges, or moving together, in which case one plate is forced down beneath the other - a process known as subduction. Subduction is associated with mountain building, earthquakes & volcanism. The oldest rocks in Cumbria are from the Ordovician period (480 million years ago), when what is now England was situated in the southern hemisphere on the edge of the Iapetus Ocean. Material washing over us from the Gondwana land mass formed the Skiddaw slates & mudstone. As we moved north, there was volcanic activity creating the Borrowdale Volcanic Group & Eycott Hill, with differing mineral combinations depending on the eruption, & whether ash fell on land or in water, & with melting due to hydration from the subducting slab. Eventually the Iapetus Ocean disappeared (was subducted), & erosion was followed by sea water coming in again, & new sedimentary deposits. Audrey took us through the Silurian, Devonian, Carboniferous, & Permian & Triassic periods, describing processes & location (which we then coloured in on our maps). Her final slide, a prediction of how the land masses might look in the future, showed the continents forming a single land mass, with the British Isles to the north of it in (blissful?) isolation.  - Chris Anderson & Brian Kettle

27th February 2018 - Rocks of the lake District

Led by Clive Boulter

The second workshop of 2018 on ‘Rocks of the Lake District’, started with Clive Boulter explaining the differences between hand lenses (& how to use one), & why he always carries a pen-knife to check rock hardness (which we all had a go at doing). We then worked in small groups on some Shap granite, discussing what we observed & trying to identify what was in it. Clive told us how to estimate percentages of the different constituents of the rock, & when we were happy with this, we had a go at plotting it on an identification ‘pyramid’. We then examined various other rocks in our sample boxes, e.g. pumice which Clive had collected from the Cumbrian coast & probably came from the Caribbean. At the end, Clive let us look at part of his personal collection, which included some wonderful specimens.  - Chris Anderson

23rd January 2018 - Igneous Rocks

Led by Audrey Brown

Audrey brought along different rock samples & after our first attempt at identifying the igneous examples, she helped us sort them into igneous, metamorphic & sedimentary. We then looked at the range of igneous rock samples & tried to describe their characteristics – crystals, mineral characteristics, colour. The Igneous rocks had no layers. The proportions of different minerals could be roughly gauged. Audrey then gave an overview of the different mineral compositions & Mafic & Felsic minerals/rocks, also an overview on extrusive & intrusive. We completed the volume:elements chart for granite to start with, comparing Shap Granite to a Granite from Peterhead & then wrote in the names of the other minerals & rocks, while looking at the rock samples. At the end we looked at some Peridotite.  - Anne Thompson

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