Alston earthquake

[Pitlamp]

Small earthquake at Alston a couple of days ago: http://www.earthquakes.bgs.ac.uk/earthquakes/recent_uk_events.html

Take care when in those rickety old mine workings!

[RobinGriffiths]

Ooh nice. That'll be the granite adjusting its isostatic equilibrium then!

That reminds me of an OU program on a geology course I did back in the Stone Age. I think conventional wisdom was that the granite under the Alston block was intruded after the carboniferous sedimentation, but they had a look at the Rookhope, or maybe Stanhope borehole, and I think if I remember rightly that the carboniferous beds were deposited on top of the pre-existing granite. So the mineralisation would not be due to hot granite being intruded, but maybe due to radioactive thorium, potassium and uranium heating the country rock and mobilising metal containing fluids. Lesley Judd presented it, so if I'm wrong with the above, it's her fault not mine.

[ChrisJC]

It sounds like an interesting theory, but I find it hard to believe that enough heat is generated by radioactive decay to reach the sort of temperatures required for mineralisation to occur... (happy to hear facts to put me right here).

Chris.

[legendrider]

Yes, the Rookhope Borehole found that the upper surface of the Weardale Granite was weathered, the implication being that the batholith was in place prior to Carboniferous sedimentation.  The WG has been dated to 400 million years (Devonian), and is probably the eroded root of a Himalaya-esque mountain range.  The mineralisation events have been postulated as emanating from re-activated fractures in the Granite, rather than radiogenic heating.

MARK

[andrewmc]

It sounds like an interesting theory, but I find it hard to believe that enough heat is generated by radioactive decay to reach the sort of temperatures required for mineralisation to occur... (happy to hear facts to put me right here).

Chris.

Heat from radioactive decay is the source 90% of the Earth's internal heat flow - which why the centre of the Earth is still hot, volcanos happen, plate tectonics etc...

So while I don't know the specifics of heat produced in granite batholiths and the temperatures required for mineralization, I have no problem believing radioactive decay is a plausible explanation.

[RobinGriffiths]

Be interesting to know where the mineralisation came from in such a scenario. Was the mineralisation already present in the granite, or has it been mobilised from the subequently deposited sediments, or maybe the deposits into which the granite was originally intruded? In the Welsh Basin, I beleive that positive metal ions were captured by negative clay particles which formed the Silurian shales, and these then got mobilised and depsoited as ore bodies.

[Fishes]

I'm not sure about the Alston area but in some other areas its thought that granite releases mineral rich water during cooling. Granite can also heat and mobilise water in surrounding rocks which dissolves minerals from them and then deposits them in a more concentrated form when the pressure and temperature reach the right point.

[Andy Farrant]

The mineralising fluids can also come from adjacent sedimentary basins, where compaction of sediments can lead to the expulsion of pore-waters (often saline brines), which then migrate into adjacent carbonates.

see http://earthwise.bgs.ac.uk/index.php/Northern_Pennine_Orefield  - an extract is given below.

By analogy with the zonation of minerals around the granites of south-west England, and with support from early geophysical studies, a concealed Late Carboniferous (‘Variscan’) granite beneath the northern Pennines was postulated to account for the mineralisation. However, the proving of an Early Devonian age for the Weardale Granite in the Rookhope Borehole, drilled in 1960–61, clearly demonstrated that the mineralisation could not be the direct result of granite intrusion. Instead, as a ‘high-heat-production’ granite, the Weardale Granite is now considered to have driven a convective circulation of mineralising fluids long after its intrusion. From fluid inclusion and isotopic evidence, mineralising brines were derived by dewatering of adjoining sedimentary basins before scavenging metals from the rock through which they passed. Sources of the required chemical elements probably included Lower Palaeozoic basement rocks, the Weardale Granite itself, Carboniferous sedimentary rocks and the Whin Sills. Minerals were deposited in fissure veins by crystallisation from solutions which cooled as they flowed outwards from emanative centres.

[Cantclimbtom]

Reading with great interest especially the comments about the magnitude of the heating effect from decay. It seems to be an area where opinions differ. Reading this https://www.scientificamerican.com/article/why-is-the-earths-core-so/ the author seems to play down the significance of decay, but then doesn't offer much explanation for heat production other than tectonic stresses etc which I'd have thought to be a method of energy loss to the surface to be radiated away also the author made no mention of insulating effects of our atmosphere or the heating of the upper atmosphere.

Very odd that there's so little consensus. But maybe since in the early 70s the expanding earth theory was still holding ground, I shouldn't be so surprised that there's still scope for further discovery and clarity. If decay was such a significant driver, why doesn't mars have obvious activate volcanic geology? It's all puzzling

It is startling how different the earth is from the other planets in the solar system, being a giant magnet is perhaps the most significant (allowing us to stop the atmosphere being blown away, like mars' one) and the mechanism for that is still speculative. The inner core being a mass of aligned crystals gets my vote unless something better shows up.