Why you weigh less in Cornwall

November 18th, 2013 | Posted in: Food and Drink, Science

Batholoth © Dominick Tyler 2013

The Cornubian Batholith sounds like it could have been the thirteenth labour of Hercules but is in fact a mass of granite formed around 300 million years ago from magma that pushed it’s way up through the earth’s crust. Rather than erupting as a volcano the magma collected, a bit like a blister, just under the surface, cooling and solidifying as it did so. Since then the surface layers have been worn away, revealing areas of underlying granite. The bulk of it is still under the surface, beneath layers of the surrounding rock. The full extent and shape of the batholith was only relatively recently mapped and this was made possible by technologies that allow very accurate measurement of the force of the earth’s gravity.

Granite is tough, one of the toughest rock types but it’s density is relatively low and as a result the Cornubian Batholith creates a localized gravity anomaly.  Simply put, you weigh very slightly less in Cornwall, and most of Devon, than you would somewhere the bedrock was denser, like the Isle of Skye. An compensatory effect is delivered by another force local to the West Country: clotted cream.

I’m a little sketchy on the maths and the physics but I’ve had a go at working out exactly how much clotted cream you need to consume to balance out the reduced gravitational pull:

Gravity above the Cornubian Batholith pulls at about -50mgals or 0.0005ms2 less than the continental average of 9.8ms2 (acceleration is a measure of the force of gravity). 1Kg at average gravity exerts a force of 9.8 newtons but 0.0005 newtons less in Cornwall, so a person of 70kg would exert (70 x 0.0005) newtons less (?):

70 x 0.0005 = 0.035

Then if we divide by 9.8 we should get the figure in Kg, right? (This seems right but I have a nagging feeling this might be where my understanding collapses)

0.035 ÷ 9.8 = 0.0036 (rounded up, let’s not get stingy with the cream)

0.0036Kg is 3.6g of clotted cream

I think that’s about 3/4 of a teaspoon.

I have to admit I was hoping for more but there you go.

Please do let me know if the maths is utterly wrong.