Geologists travel deep underground!

Yr 14 Geology trip to Carrickfergus Salt Mines

At the beginning of December 2013 the yr 14 Geology class experienced a tour of the Salt Mines at Carrickfergus which are owned by the Irish Salt Mining Enterprise Co Ltd.  As there was a limit of 7 visitors per tour, the class visited on two separate days, the first with Ms Sloane and the second with Mr Lynn. The trip was organised for A2 students to learn about different mining techniques, the economic, environmental and social impact of mining.


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On arrival each group had a safety talk and an opportunity to ask questions to Jason, the Mining Manager.  He pointed out to us that the company only ‘lease’ the minerals from the government and that they don’t buy anyone’s land!  They currently have leased enough of the mineral to keep them in production at the current rate of extraction for another 30yrs! 

After the safety talk the groups had to kit up in fetching bright orange coveralls, hardhat and lights and were ushered onto a 4x4 jeep.  As we arrived at the entrance of the mine shaft/tunnel we were surprised at how narrow it was?  Jason told us that it was built in the 1960’s and even though it was only the width of 2 * 4x4’s vehicles all the equipment we would see had to be taken down through this tunnel. Some of the equipment was so big it had to be dismantled and reassembled at the mine face.

At the bottom of the steep shaft, the mine opened out into a series of roadways/tunnels where the salt had already been mined out using a Pillar and Stall technique. This is where tunnels are excavated and pillars of the rock are left to support the weight of the rock above.  For every 15m of salt that had been mined out the company have to leave pillars 50m wide to support the roof of the mine and the 400m of rock above!!!  The mine itself was pitch dark and was only lit up by the machinery!!!  The temperate was 17°C and the groups couldn’t help but think of the Chilean miners trapped underground for 66 days in much higher temperatures!!

We were down the mine for 2hrs, although time passed quickly as we were driven through the network of tunnels that lay 400m below Carrickfergus. We were able to see the different stages in the mining operation from the undercutting of the salt face to where the crushing of the salt takes place.  The vast network of tunnels extend in excess of 30 miles underneath Co Antrim.

How is the salt mined?

Every day a cutting machine with a 3m saw on the end of an extendable arm undercuts the rock face that is currently being worked.  Holes are then drilled through the rock salt to the top of the mine face and explosives set in them.

At night the explosives are discharged and the 3m thick 15m high wall of salt is dislodged, ready for collection by the day shift starting the following morning.  Three seismometers above ground monitor ground movements. The explosions are set off at night as salt crystals are very fine and it would make it very uncomfortable and hazardous for the men to work in if they were blasting during the day. 

The solid blocks of salt are then taken to the crushing machine where they are reduced into 6mm pieces of salt.  These fragments then put on a conveyor belt and sent to the surface where they are mixed with chemical which prevent it from solidifying. The chemical mixture is then storedin large storage sheds to await distribution. Local supplies are distributed by lorries and overseas orders by ship from the company's own quay adjoining the mine. 

The salt extracted at Carrickfergus is used principally for de-icing our roads and last year 750,000 tonnes of salt were extracted and sold.

What about the machinery?

All the machinery is maintained down the mine, should it be the lorries and diggers, the machine that tests the strength of the wall and floor of the mine, the crushing machine or the conveyors.  The mechanics workshop was full of trucks and the mechanics themselves looked dwarfed against the large trucks they had to fix!


There was a good ventilation system down the mine.  As all the machines operate on diesel it is very important to extract this polluted air.  A system of vents and pumps circulated clean air from the surface and extracted the diesel charged air to the surface.  The photo below shows workers erecting a screen to divert the clean air around the mine.


There were 3 safety chambers in the mine.  These were stocked with a generous supply of drinking water and a portable loo (no privacy though)!  In former days the army used to be employed by the MOD to ensure that all the explosives that were bought by the company were used down the mine and none got into the wrong hands during the troubles!!  They are no longer employed in this capacity although there are strict checks on mine safety and explosives.

Who buys the salt?

Much of the salt produced in Carrickfergus is for the domestic market – local councils will order the salt based on the weather conditions in the winter.  Scotlandand Donegal are also big markets for the salt as they do not have any salt deposits as the rocks in both Scotlandand Donegal are all igneous or metamorphic rocks, formed by either magma or recrystallisation of other rocks by heat and pressure.


How did the salt get there?

This halite (rock salt) formed during the Triassic period when Northern Irelandwas on the edge of the ‘ZechsteinSea’.  This was a warm shallow lagoon that stretched from Northern Irelandto Eastern Poland.  During the Permian age, Britainand Irelandwere located 20°N of the equator where the arid conditions allowed for intense evaporation of the water from the lagoon, leaving the salt behind on its floor.  The salt deposits are reported to be around 400m thick at Carrickfergus as the lagoon was continuously replenished with salt water from the open ocean.  The salt was discovered at Carrickfergus when exploring for coal deposits.


Both groups had an enjoyable and highly informative day experiencing the conditions of those working in the mine, the health and safety measures involved, the significant difficulties regarding the introduction and servicing of mining equipment as well as the geological factors for its existence.

The mining manager Jason deserves special recognition for facilitating us for 2 days and giving us a unique insight into mining operations deep below our feet!