Another geology question

[al78]

The difference between the east and west highlands.

The Grampian mountains and Cairngorms are rounded heather covered domes or big bulky lumps of granite, the latter having steep glens carved through by glaciers. The west highlands is home to mountains with sharp ridges, pinnacles, and, especially in winter, look like a scaled down version of the Alps in places. Why is there such a contrast in the shape of the highland hills/mountains from west to east. I'd have thought they would have formed around the same geological time?

A supplementary question: in Sutherland, the landscape takes a different form, it is characterised by a low rugged platform with hills rising up abruptly and in isolation in places (e.g. Suliven, Quiniag), and in the case of Suliven, the rock look stratified, as though it was laid down over time. Did this area used to be under the sea, and it has been uplifted, or is it the cumulative result of volcanic lava flows? Do the well known Sutherland hills represent localised regions of hard rock around which softer rock has eroded away?

[jmarkb]

The Grampian mountains and Cairngorms are rounded heather covered domes or big bulky lumps of granite, the latter having steep glens carved through by glaciers. The west highlands is home to mountains with sharp ridges, pinnacles, and, especially in winter, look like a scaled down version of the Alps in places. Why is there such a contrast in the shape of the highland hills/mountains from west to east. I'd have thought they would have formed around the same geological time?

This is mainly due to the effects of glaciation in the last Ice Age rather than the underlying geology. In the west, there was more snowfall (as there is more rainfall today), so the ice was thicker and also the glaciers were steeper and faster flowing (aided by a warmer climate in the west, which increased water lubrication at the base of the glaciers), and hence had a greater erosive effect. You can see the same pattern in Norway - the further east you go the more rounded and plateau-like are the mountains.

A supplementary question: in Sutherland, the landscape takes a different form, it is characterised by a low rugged platform with hills rising up abruptly and in isolation in places (e.g. Suliven, Quiniag), and in the case of Suliven, the rock look stratified, as though it was laid down over time. Did this area used to be under the sea, and it has been uplifted, or is it the cumulative result of volcanic lava flows? Do the well known Sutherland hills represent localised regions of hard rock around which softer rock has eroded away?

The geology north-west of the Moine thrust is quite different from the rest of Scotland - there are three main layers of rock - gneiss which forms the base and is exposed in the low-lying areas, sandstone in the middle (which is indeed a sedimentary rock laid down in layers on ocean floors around 1 billion years ago) and finally quartzite on the top. The steep sandstone mountains are likely locally harder or less fractured bits of rock, and/or have been protected from erosion by the harder overlying quarzite.

I highly recommend this book for more in depth reading: https://birlinn.co.uk/product/land-of-mountain-and-flood/

[ChrisButch]

Apart from glaciation etc already mentioned, it's useful to know that the Highlands are made up of three quite distinct slices of the earth's crust, which were at one time far apart, formed in quite different ways with different rocks, and were only subsequently sandwiched together. These chunks all run in a southwest to northeast direction: the far northwest already described; the rest of the northern highlands down to the Great Glen (known collectively as the Moinian); and the southern chunk, the 'Dalradian', which includes everything between the Great Glen and the Highland Boundary Fault.

[al78]

I highly recommend this book for more in depth reading: https://birlinn.co.uk/product/land-of-mountain-and-flood/

Thanks for the recommended reading, I have ordered it.

[nigheandonn]

It doesn't really seem to be the reason for the difference here, but through a lot of Britain, the further south and east you go, the newer the rocks at the surface, as the whole thing is tilted down to the east.

The smooth Pennines are just the rough Lake District hills with a band of newer rock on top - you can see that small hills along the western edge of the Pennine ridge have the pointy shapes, where the upper covering is worn away.

[al78]

Apart from glaciation etc already mentioned, it's useful to know that the Highlands are made up of three quite distinct slices of the earth's crust, which were at one time far apart, formed in quite different ways with different rocks, and were only subsequently sandwiched together. These chunks all run in a southwest to northeast direction: the far northwest already described; the rest of the northern highlands down to the Great Glen (known collectively as the Moinian); and the southern chunk, the 'Dalradian', which includes everything between the Great Glen and the Highland Boundary Fault.

This is another thing I don't get. If the UK's mountains are the result of bits of the Earth's crust coming together that were initially far apart, why is UK hundreds of miles away from a tectonic plate boundary? Normally when I think about volcanic activity and formation of mountain ranges, I think of the motion of big tectonic plates, like the Himilayas formed from India crashing into Asia, or the African plate crashing into the European plate forming the Alps. It is not just the UK, the Norwegian mountains are twice the height of the Scottish ones yet they are a long way from a tectonic plate boundary, although some of their height might be due to isostatic rebound after the ice age.

[nigheandonn]

Because some small older plates are now fused together as parts of larger plates, it seems. There's an almost endless list at https://en.wikipedia.org/wiki/List_of_tectonic_plates#Ancient_plates_and_cratons

[ChrisButch]

Yes, there's a continuous (very slow!) process of plates coming together to form supercontinents, then breaking apart again, but breaking along different lines than the previous join. Thus the far northwest of Scotland was originally part of what's now the North American + Greenland continent. When a previous wide ocean closed, it was welded to the rest of the Northern Highlands, which had been a separate microcontinent formed in a deep basin to the east. (The Northern Highlands were themselves in turn welded to other continental fragments to the south.) When everything moved apart again with the formation of the Atlantic Ocean, the far northwest was 'left behind' on this side. Some of the rocks in the area round Scourie are identical to formations in Eastern Greenland. You can think of it as four-dimensional jigsaw. Once the pieces have fitted together, the complete puzzle is recut in a different pattern.

[vuirich]

This is another thing I don't get. If the UK's mountains are the result of bits of the Earth's crust coming together that were initially far apart, why is UK hundreds of miles away from a tectonic plate boundary?

The other important thing to remember about geological processes is that of time. So even though geological processes driving plate tectonics can be very slow, geological time is very long. For example, northern Europe is currently moving apart from North America at about 5 cm per year, so, at this rate, in the 60 million years since the north Atlantic started to open along the Mid-Atlantic ridge, Europe and North America could have separated by 3000 km. So, although the UK (and northern continental Europe) is currently not at a major plate boundary, in the past this was not always the case due to the dynamic movement of tectonic plates creating new continental crust and reconfiguring and destroying earlier crustal fragments.

[DopeyLoser]

Here is a superb wee book, which I have a copy of but which I see is also downloadable as PDF:

https://www.nature.scot/landscape-fashioned-geology-scotland-creation-its-natural-landscape

I see a couple of links on that page for their regional booklets which are also excellent. There was a series of them, not sure if they have a single web page that lists them. I have this one on my bookshelf:

https://www.nature.scot/landscape-fashioned-geology-arran-and-clyde-islands

[DopeyLoser]

Update: if you put 'landscape fashioned by geology' into the SNH search box then the first twenty or so hits are for the regional booklets:

https://www.nature.scot/search?query=landscape%20fashioned%20by%20geology&page=0

All free as PDF but the printed booklets are even better for browsing and reading IMO.