Where tectonic plates meet (quietly)

[cthulu2016]

Reading the post about creationists (hilariously) citing the Loch Ness monster as a disproof of evolution reminded me of why Loch Ness is so weird. (The lake, not the monster.) Ness is one of a line of long, narrow scottish lakes that are ridiculously deep.

When you look at a map of Scotland there is a line of lochs is part of an unusually straight line separating northwest Scotland from the rest. (The lochs look like perforations, to make NW Scotland easy to tear off.) You can see the line in the rift through the land where those lochs are, and in the configuration of the ocean inlets. That weirdly straight line is where two plates in the Earth's crust meet.



The two plates are not crashing together these days. It's a pretty quiet area, geologically. They met and then pulled a little apart, leaving a very deep rift between them. The deepest parts of the rift filled up with fresh water over time and became the line of lochs.

So that's why they are such unusual lakes... long, narrow and in places deeper than they are wide. Like cracks in the Earth.

The obvious line across Scotland is one of many instances where, on a map, the idea of the Earth's crust being in sections kind of jumps out at you. The most obvious is the jig-saw puzzle fit of east South America and west Africa. But the idea of plate tectonics was so startling that all such map oddities until quite recently (1960s) were written off to coincidence.

[muriel_volestrangler]

1. Strictly, I think it's a fault inside one plate

rather than the boundary between 2 plates - which roughly coincides with the Scottish-English border (or at least south of the Scottish Highlands). But the Great Glen was formed as a result of the 2 plates meeting. See eg

One of those bits is Scotland but our Laurentian credentials are only really convincing from the Hebrides and the northern Highlands down to the Highland Border. South of there, things are a wee bit less certain as the foundations of the country turn out to he the geological flotsam of the Iapetus Ocean, swept up and accreted onto the edge of Laurentia as the oceanic lithosphere was subducted. A big contribution was added about 470 million years ago when an oceanic volcanic arc collided with an early Midland Valley continental fragment and shunted it northwards to crush the oceanic rocks of the Highland Border Complex and deform the Dalradian. Part of the arc causing all the trouble ended up as an obducted ophiolite fragment on top of the Midland Valley basement block; we see a bit of it now at Ballantrae. Thereafter, with subduction established under the new and extended Laurentian margin, the end of the lapetus Ocean was inevitable. The Southern Uplands was sequentially scraped off the sea floor and built-up into a thrust belt advancing south. The sand grains in its rocks tell of other volcanic arcs, now lost for ever. Its northern, Ordovician belt still has convincing Laurentian faunal links but by the time the southern margin was added, in the middle Silurian, Laurentia and Avalonia had converged; Iapetus was no more. Scotland came out on top in this first meeting with England as the margin of Laurentia over-rode Avalonia. The Southern Uplands thrust front marched on southwards into Avalonia with a load-induced foreland basin ahead of it. Some of the rocks deposited therein are now preserved as part of the Windermere Supergroup in the south of the English Lake District.

http://www.edinburghgeolsoc.org/edingeologist/z_31_04.html