Photos of the lake in Pridhamsleigh

[mrodoc]

Gus' post is way over my head. I think Chris Proctor considered it once.

 

[Roger W]

'Twas probably dug out by some Mendip guys on holiday...

 

[graham]

Gus

We have a hydrothermal cave here in Bristol, at Pen Park Hole. It is thought to have been active as a hydrothermal vent in the Rhaetic and is thus about 200 million years old.

 

[gus horsley]

Graham, this is turning into a very interesting post.  Do you know what evidence there is for a hydrothermal origin for Pen Park Hole?  The UK has many examples of post-mineralisation caverns, which could have formed at virtually any time in the past 200 million years, but we're talking about something different here aren't we, which are caverns forming at the same time as late-stage hydrothermal mineralisation due to uplift following the closure of the Rheic Ocean in late Carboniferous - early Permian times.

 

[graham]

Gus

Here's the paper.

Essentially, the cave has hydrothermal features additional to the mineralisation & the overlying Rhaetic clays contain disseminated lead that came out of a hydrothermal vent.

 

[gus horsley]

Thanks Graham, I'll have a good look at the article later but I can see from the survey that there's a resemblance to Prid.  Of course that doesn't mean they have the same type of origin.

 

[Roger W]

'Tis a very interesting bit of stuff, that, Graham.

How common are caves produced by the limestone being dissolved by hot volcanic water from below rather than by rainwater/streams from above?

I would think that some really hot and possibly acidic volcanic water could be quite agressive.

 

[Peter Burgess]

There are quite a few in Hungary.

 

[paul]

There are quite a few in Hungary.

Plus Carlsbad Caverns and Lechuguilla in the US which were largely formed from water containing amounts of sulphuric acid relating to petroleum deposits.

 

[graham]

Then there's the things with the gert big crystals. There are quite a few hydrothermal caves around.

But only one is a mile down the road from me.  8)

 

[whitelackington]

Gus

We have a hydrothermal cave here in Bristol, at Pen Park Hole. It is thought to have been active as a hydrothermal vent in the Rhaetic and is thus about 200 million years old.

Canada Combe Cave,
near Hutton, Mendip is also thought by some to be hydrothermal.

 

[gus horsley]

A possible scenario could be that caves such as Pen Park Hole could have originally formed at depth by ascending thermal waters charged with CO2; these theoretically would dissolve along lines of weakness but create only small cavities initially.  Indications of these would be calcite-filled vughs and evidence of altered rock.  At a later time uplift would have raised the caves where normal phreatic solution could take place, enlarging the existing voids.

How does that sound?

 

[graham]

A possible scenario could be that caves such as Pen Park Hole could have originally formed at depth by ascending thermal waters charged with CO2; these theoretically would dissolve along lines of weakness but create only small cavities initially.  Indications of these would be calcite-filled vughs and evidence of altered rock.  At a later time uplift would have raised the caves where normal phreatic solution could take place, enlarging the existing voids.

How does that sound?

In part it depends on whether the enlargement is pre- or post-mineralisation.

IIRC a number of Derbyshire lead mines are 'normal' caves formed by meteoric water which have undergone subsequent mineralisation at depth.

PPH not only has all the features of a hydrothermal cave, but is quite close to currently operating hydrothermal features, there are hot springs only a few miles away in the Avon Gorge, as well as, famously, in Bath, just down the road.