Limestone is mostly made up of the mineral calcium carbonate ( CaCO ₃ )  this is not very soluble so the rocks don't dissolve very quickly. If you add an acid however you add Hydrogen  Ions ( H⁺ ) which will react with the carbonate to form hydrogen carbonate HCO₃^- ions which are very soluble in water, and the limestone will dissolve. Or if there is more acid about the two Hydrogen ions will react with a carbonate to form H₂CO₃ which will decompose to form carbon dioxide CO₂ and water H₂O.

The acid can come from a variety of sources sulphur and nitrogen oxides released by burning fuels will form sulphuric and nitric acids, can carbon-dioxide can dissolve in water to form carbonic acid.

If you imagine a plate with no ridge and there is some small piece of grit, food or dirt on the table the plate would rock and make it difficult to eat off it. However if you have a ridge it is very unlikely the grit is under the ridge itself and even if it is when you move the plate slightly the plate would fall off the grit becoming stable again.

It is also probably more difficult to manufacture a perfectly flat piece of pottery than a ridge that is the same height all the way around.

We asked our guest Marie Edmonds:

Marie- This is a very good question and it gives me an opportunity to dispel some of the myths about volcanoes and global warming and pollution. Volcanoes emit CO2 and SO2: carbon dioxide and sulphur dioxide are the main gases that might be construed to cause global warming or pollution. Volcanoes emit around 100,000,000 tonnes of CO2 a year. Compare that to man-made emissions of CO2 which comes to about 10,000,000,000 tonnes of CO2 per year. So volcanoes emit around 1/100^th of CO2 that we do and are therefore insignificant in terms of global warming. Sulphur dioxide on the other hand, volcanoes emit around a tenth of the anthropogenic emissions of SO2. That forms regional smog.

Chris- Isn’t there a benefit of sulphur dioxide in that it reflects heat back in to space? In fact it cools down the Earth so volcanoes are quite good in that respect because they keep us cooler than we otherwise would be.

Marie- Yes. There has been some research in the last few years that suggests very large eruptions such as Pinatubo in 1991. That eruption emitted about 20 megatonnes of SO2 into the stratosphere. Eruptions such as those which happen once every decade in fact have slowed down global warming a little bit. Without those eruptions we’d actually see the effects of global warming much more now than we do.

Marie -The quick answer to that is no. I think it would be foolhardy at best to try and interfere with the processes going on in a volcano. There have been instances of people trying to divert lava flows, for example, by spraying water on then and blowing them up with explosives –

Chris - And building canal systems for them to run into preferentially.

Marie - That’s right but actually trying to alter active processes at the vent wouldn’t be advised.

Our guest Marie Edmonds answered:

I think what we can do is harness geothermal energy. Usually we wait for volcanoes to be in periods of rest, not active eruption. This is certainly done in Iceland and New Zealand.

  That’s a very nice idea but I think in practise that wouldn’t work. Enormous pressures build up in conduits and magma chambers in volcanoes. I think you’d need a very large cork.

It certainly could be volcanic glass. There’s a glass called obsidian which is a solicit magma. The way to recognise it is if it’s got a fractured surface and it’s curved, Glass generally forms these curvy planar surfaces when you break it.

Harder materials scratch softer ones, hardness is measured on te Mohr scale, Diamond is 10, talc (of the powder) is 1. Window glass has a hardness of about 5.5 so there are a lot of materials which can scratch it, including hard steels, (6.5) silica in quartz or flint (7) etc. In fact flint is another possibility for your mineral as that can look very like a  glass so is another possibility  for your material.

We asked our guest Marie Edmonds

Marie - Yes. Certainly what we call refractory elements such as osmium, iron and nickel. Throughout history all of these heavy elements have been migrating towards the core and all the light elements have been migrating to for the crust so yes.

Chris - Aren’t there some companies being formed that want to exploit the minerals that are being brought from the surface in hydrothermal vent systems? Aren’t they enriched with minerals and things that are quite hard to get hold of but because the Earths doing the work geologically for us then that’s a useful way to do it?

Marie - Certainly hydrothermal systems around volcanoes: fluids bring a lot of these metals like gold, silver into the volcanic edifice and they get precipitated into the volcanic system. A very easy way of mining these is to dig into a volcanic edifice where they’re concentrated.

It depends on how long a time scale you are looking at. Over a few tens of years then probably not, the mantle below the crust behaves pretty much as a solid over periods of tens of years. But if you look over thousands or millions of years, then yes the areas where the ice has melted from will lift slightly and the areas the water has moved to will sink. During the last ice age Scotland was pushed down a few metres by the 2km of ice sitting on top of it, and since the ice melted, 10 000 years ago,  the crust has been floating back up again in a process called isostatic rebound.

This affect is very small though in comparison with the size of the planet though.

Yellowstone is a huge system and between eruptions which generally happen every six hundred thousand years or so it’s not a dead system. Instead the magma chamber is inflating. Indeed, we can see that now at Yellowstone – a very slow rate of deformation - but it’s certainly inflating, preparing for its next eruption which probably won’t happen in our lifetime.

We asked our guest Richard Davies

All rocks, all sedimentary rocks contain water. What we’re really looking for is rocks that have porosity and permeability. So you can look at outcrops to find out which are the right suitable rock types and you can drill holes and hopefully intersect those rock types. All sediments contain water and it’s actually when they’re buried that you’re actually squeezing the water out very slowly. It’s really a matter of looking for the suitable rock types. For example, chalk beneath London is a suitable aquifer. It has been historically, anyway.

We put this question to Marie Edmonds and Richard Daives:

Marie - To my knowledge this has never been observed but in theory yes. Because certainly distant very large earthquakes can set off volcanic eruptions so presumably this is possible.

Chris -  Richard, one would think they might have an impact on mud volcanoes?

Richard Davies - Absolutely. That’s proven that earthquakes can kick off mud volcanoes. If you go down to a beach and you jump up and down on the shoreline long enough you will create your own sand volcanoes through the same sort of process. You cause something called liquefaction when you turn the sediment into a liquid.

Chris -  Have we any evidence linking earthquake activity or bomb tests or anything like that to mud volcanoes really or is it just theory?

Richard -  Well, now I’ve investigated the Lusi volcano I’ve heard of stories where mud volcanoes erupted at the same time as seismic surveys etcetera. I don’t know if it’s proven yet but it’s certainly been suggested. If there’s already a mud volcano there, in other words the pipe works is there already, it could happen. It could increase its activity level.

Chris - The answer we trot out is the oil volume relative to the crust of the Earth is very minor. Situations like this highlight the fact that there can be consequences.

Richard - This is something that happens probably a few times a year around the world. What’s happened here has been unfortunate in that the area was ready for a mud volcano before. In other words geologically the conditions were suitable. Unfortunately this company drilled a well which had operational problems which provided the pipe work, the plumbing so that the mud volcano would form. This is something that happens a handful of time each year. This is the worst case in that unfortunately it happened in a populated area and it brought a lot of mud with it.

Chris - People are probably going to be quite surprised to learn that we do have one of these in Britain although, thankfully, not on the scale of the Indonesian example.

Richard - Yes. It is nowhere near on the scale of the Indonesian example and it is a natural occurrence. These are very small mud volcanoes outside Swindon but what I like about them is they provide an analogue for what’s happening in Java right now. It’s actually a limestone, a Corallian which is a Jurassic aged limestone: 150-160 million years old. It’s supplying fluid and the fluid is then passing through a clay called the ampthill clay and the clay is brought to the surface. It actually has with it a number of beautifully preserved ammonites.

Chris: That saves fossil hunters a bit of effort. Richard, thank you very much.