Professor David Dobson and guests talk about the geology of the UK in less than 2 minutes. These are typical things you might see when out walking and what they mean.
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Thank you. For years I have walked along a river bank banks wondered why I found rust deposits as you show in the video. The river in question cuts through Lower Devonian Sandstone, which is overlain by peaty soil. Now I understand what is going on
Yes, While Skolithos is a generic term for vertical burrows which covers a wide range of geological time, I guess yours are similar age in rocks deformed by the Appalacian Orogeny? Glad you like the vids.
These trace fossils aren’t great for dating rocks as they occur across a wide range of time. But in this case it is their very first appearance at the start of the Cambrian. Around 550 million years ago.
Thanks for the question.. as I understand it opal forms during diagenesis - as sediments consolidate to turn into rock. That occurs at really low pressures and temperatures 100s of m to a few km of burial depth. This milky quartz is metamorphic, requiring burial to at least 10 km depth - probably much more.
Does this means that nappe always formed from previous thrust folds? I mean does the existence of thrust folds is a requirements before another compressional stress is given to form nappe?
Hi, I'm not entirely sure I understand your question. But I think the answer is (probably) maybe not if I am interpreting it correctly. The model ends up forming these fold systems sequentially, but it is just a very simple model with totally different flow physics and boundary conditions from the Earth. In real world systems you can get several fold and fault systems developing concurrently (although strain weakening rheologies tends to cause one fault to dominate at any given time, this isn't so true for folding). Once you are into fully ductile regimes (say lower crust or even the mantle in somewhere like Ronda) there is a continuum between distributed ("fold-like") and locallised ("fault-like") deformation, but you still produce structures with a nappe-like geometry. Does that help?
Nice video Doctor. So (coming from an amateur) in a case of a feature like thiis, the depth of the dike must have been shallow enough for the country rock to have not been melted due to depth and pressure (and melted into magma) but deep enough so that the intruding magma would melt slowly and form crystals large enough to be classifed as granite? Two questions: In this case what is the country rock, and also would you expect to see contact metamorphism? Thanks as always.
Hi Mike, The Cornish granites were emplaced into the deep upper crust/shallow mid crust (perhaps 5-10 km depth) - the cuntry rock is Mylor Slate which shows extensive ductile folding but is only metamorphosed to greenschis facies (hence my estimate of 8+/- km). There is contact metamorphism - we commonly see quite extensive zonbes where the slate contains the low-P, high-T form of Al2SiO5 (Andalusite) and also cordierite, a magnesium-aluminosilicate mineral. I am actually writing this from Perranporth where there are great examples of cordierite 'spotted' slate - will record a video. The strange thing is granites are quite cool melts (around 700 C) compared to say basalt (1200 C) but the contact metamorphism is much more extensive around granites. This comes from 2 factors - granite bodies are large (so cool down slowly) and granitic melt is very wet and the water which is exolved as the granite crystallises enhances reaction rates in the surrounding rock.
Hi David, geologists borrowed the term from mining. Stoping is a way to follow a near-vertical seam. You drive a tunnel along the seam and then cut into the ore above and let it fall into the ore bucket - it is quite efficient as it saves having to lift the ore back up from the tunnel floor and was the preferred method in pre industrial mines in the UK. The method was called overhand stoping. Geologically it carries the idea that intrusions into the roof of a magma chamber can allow the country rock to fall into the magma, displacing an equal volume of magma upwards.
Andesite is intermediate between granite/dacite and basalt, so has lower silica than granite and dacite. Dacite has similar silica to granite, but lower potassium and higher calcium. So one way to produce dacite is to crystallise an andesite, preferentially removing mafic components as pyroxenes/ amphiboles and enriching the melt in silica.
It’s a Rab Borealis. Very light but good and wind proof and it stuffs down into the breast pocket, with a good stitched daisy chain for clipping when climbing.
@@ianhorsham7751 I'm expecting the actual terrain at that mine (and the flowers) is because of the mine, however the flowers will succeed better at any place where the mineral veins approach the surface and the drainage water 'contaminates' the soil. The association will quickly form with the miners observations then they will explore anywhere they see the flowers, especially if they can then use techniques such as 'hushing' to strip the soil to locate the valuable vein.
@@ianhorsham7751 yes, the hummocky ground follows along the mineral vein for several hundred meters. It was caused by early mining where (in this location) miners would sink shafts and then dump the spoil immediately around the shaft. Many of the depressions around here might sit above shafts with relatively thin infill blocking them - a greater risk to life and limb than lead poisoning!
Glad you like it! I use Camtasia for putting together the viedo clips, audio track and any visuals. The red and yellow shapes were created in Powerpoint, just tracing over a frame from the video. Then I paste them into Camtasia and use the object attributes options there to make them transparent and synch their position with the moving image. Quite a slow process, but easy to do.
So Doctor? Is this uncomformity, and missing time, coincident with the "Great Uncomformity" that to my undestanding is virtually world wide? Or is this a different event? I believe that the great uncomformity is of a longer duration. Nice video!
Good question Mike. It is quite similar to the Grand Canyon Great unconformity, which juxtaposes 550 Ma rocks on top of 770 Ma rocks (here we have 400 Ma on 750 Ma or so). Hutton's Great unconformity has Devonian (350 Ma) over graptolitic Silurian (435 Ma), so not quite so 'great'! The transcontinetal unconformities are either related to global sea-level changes, or the large-scale "Wilson" cycles of continent growth. This one is related to continent growth and the Caledonian Orogeny. The term 'great unconformity' was coined early on in geology for one with clear changes in deformation style either side which implied massive geological events in the missing time.
I love seeing smoking gun examples of how bedding and cleavage run at varying degrees of perpendicularity! I'm amazed by how tight and how local those folds are!! Think about how far we had to walk to find the other sides of that nappe fold up in Glencoe! :0)
@@jncolligan1 all the highly folded rocks are east of the Moine thrust, on the overriding plate. It is very common for the upper block in thrust systems to become highly deformed while the lower block stays relatively unaffected. At root this is because the upper surface of the overriding block is free air where the lower surface of the lower block is the rest of the Earth. Torridon sandstone is below the Moine. That is the main reason. There are details to do with where in the continental suture each part is as well, but that is the simple story.
No matter how many tight isoclinal folds I see... nor how much I know about the mechanisms of formation (confining pressures, long periods of time, etc...). There is still a small thrill when marveling at how these 'impossible' (at earth surface temps and pressures... and hence our intuitive starting points) have come to be!
Yes! and these are even wierder - they are isoclinal periclines. Periclines are folds whose amplitudes decay along their lengths (leading to opposite sense of axial plunge at either end) so that the strain in one fold 'switches' to the adjecent fold as it decays out. It is quite a difficult concept but you can see it in the fold to the feft of the main one in the video. It gives the folds here quite a surreal aspect - almost like legs with twisted knee joints. My photography just can't do it justice.
@@OneMinuteGeology That’s really cool! A three dimensional expression of compressive accommodation. Also a reminder of the importance of not rushing to judgement when first assessing outcrops. :)
Great explanation, and cool illustration :0) Evaporite.. another cool word for a cool aspect of geology! Starting to get my head around Felsic vs Mafic aspects of rocks now.. that's making a lot of things you shared on the last walk make more sense!
Cheers Joe. There is another one from the same place about to drop, looking at some of the fine detail. It is great to see what can be quite complex taxtires in high-grade rocks (due to all the subsequent history) in these shallow gypsums. I’m there with students right now.
@@OneMinuteGeology I think there's a really interesting story (that i don't fully understand yet) when it comes to 'crust' Ocean and Continental, then there seems like another relevant story when it comes to the lifecycle of Magma... and how the melts work and how the different temps cause the different elements to melt at different times as temperature change has the component of time in it, and since melts can rise, and the different types of crust (ocean or continental) they rise through impact the rise, the order of the rise and what actual does rise then the different sequence of melting seems to reveal a story of where it is at that time, almost like stratigraphy, but melting lifecycles?.. and all these stories seem to weave into each other.. but i've still not made conceptual sense of them all. :0) Hope that makes sense :0)
Feldspar couldin clay is used to naturally bong limestone to granulated granite for making Rose granite quartz geopolymer they used woodgrain and self drying hydraulic limes that self dry when compressed
