Salt of the Earth

In the past, salt was so valuable it could change the course of history: now it provides a window into the Earth's secrets.
23 May 2016

Interview with 

Professor Chris Jackson, Imperial College London


Salt might be something we associate with fish and chips, but once it was one of the world's most valuable resources, allowing armies to conquer and civilisations to thrive. To find out more about salt and how has it shaped our history Georgia Mills spoke to Imperial College London's Earth scientist, Chris Jackson. 

Chris - Salt actually spans quite a broad range of rock types. Essentially one of the common physical properties of these types of minerals is that they can be dissolved in water and then when the water is evaporated off, these minerals precipitate out. So, imagine taking a pan full of water and putting lots of salt in it and then boiling that pan for an hour or two hours, all of the steam will be rising off, so that's all the water turning into steam. And then at the end of that experiment you'll be left with a film of the material that can't be kept within the vapour and is actually left as a solid in the base of that pan. So that's what we mean by evaporite and precipitation.

Georgia - While salt can refer to a whole host of minerals, the one we're most acquainted with is sodium chloride, which has some quite useful properties.

Chris - Salt: it absorbs water and, therefore, it starves bacteria of water.  Therefore it can stop the food degrading and going rotten so salt is a fantastic preservative and this was recognised many, many years ago. For example, the Egyptians, for the mummification process the way that the pharaohs bodies, and the food, and the pets of the pharaohs were mummified was by basically embalming them with salt to draw out the moisture. Also salt, because of its preserving properties, it's also been used in the past to preserve food. So, before we had refrigeration, we'd have to actually put salt into them and, again, it's to draw the moisture to stop them becoming rotten. That allowed, for example, armies to transport food over large distances to keep the fighting force fed. It's had an illustrious history in the past and, probably we're a bit more dismissive and we don't really think about it too hard because it's so cheap, whereas in the past it was actually a huge valuable commodity.

Georgia - Indeed it was. As well as enabling armies to march these huge distances with food, Roman legions were even sometimes paid in salt and this is where we get the word "salary" from. Salt was once worth its weight in gold because it was so hard to find but nowadays we can pretty much all we need from the sea, so it's not seen as such a valuable commodity. However, the sea is not the only place we find salt, it's also buried within the earth and this is where it becomes valuable in a different way to researchers like Chris.

Chris - Salt is this unique mineral, this unique rock and when you actually start to bury it, it actually starts to flow like a fluid rather than breaking in a brittle fashion like most people envisage a rock does if you stress it too much. So, we're interested in how salt flows and then how it deforms the earth's crust as well, and then the landscapes that are generated or the seascapes if it's below the earth's surface, that are formed as a result of that. So there's an inherent scientific value for us as scientists to understand how the earth's crust deforms as a function of how salt moves but, from an applied point of view, it's hugely important for the oil industry as well.  This being because salt is an impermeable rock. That means that it's, actually, very hard to flow fluids or gases through that rock body, unlike sandstone or limestone, where we have small spaces in the rock where we can actually store hydrocarbons so we can store oil and gas. The salt is impermeable but that means if we put the sandstone against the salt, so we put permeable rock against impermeable rock and we can actually stop the migration of hydrocarbons and trap them. So a lot of the work we do is quite applied in terms of looking at how these salt structures form and how they can lead to the trapping of large volumes of hydrocarbons.

Georgia - And if that doesn't convince you it's important, I asked Chris to paint me a picture of a world without salt.

Chris - It's a very interesting question because some of the great offshore basins of the world contain salt so, for example, offshore Brazil where we work. If you were to just physically remove all the dissolvable salt material from there, the earth's seabed would collapse and you'd probably trigger giant landslides off the Brazilian coastline. I mean, in places like Cheshire, if you remove a lot of that salt from beneath the earth's surface, you'd have a lot of subsidence, you'd have presumably rivers being rerouted. You'd obviously have huge threats to infrastructure as well, like gas pipelines, telecommunication pipelines as well. But it sounds like a good - I'm not sure horror movie, but certainly an adventure movie of sorts.

Georgia - Well, I hear they're looking for ideas so...

Chris - Yeah, yeah, yeah... it would make good TV!


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