The fascinating world of granite!

I started finding out about granite after reading that it was naturally radioactive. I hadn’t really given it much consideration before and suddenly felt very ignorant. Then I ended up staying awake until 5am reading more and more about it! For the information about radiation you can skip through to the end, but there was also plenty of other interesting stuff I found out so I’ve popped that in too.

**What on earth is granite?**

So let’s start with the absolute basics. Granite is an intrusive igneous rock – that just means it is formed below the surface of the earth as magma or lava cools. Because the cooling happens underground it takes a very long time and that allows the rock to form large crystals which are visible to the naked eye. That’s why granite can end up looking like a rocky fruit cake.

Granite is composed of between 20-60 percent quartz (silicon dioxide), lots of feldspar (a family of silicate minerals) and then a selection of other minerals which give it different darknesses and textures. To work out if something is a true granite or just a granitoid you need to know the relative proportions of quartz and feldspars. There’s even a helpful triangle to help with that – the bits in green are the true granites:

How it works is you find out the percentage quartz in the rock and plot that as a horizontal line, then you look at the feldspar content and work out the what percentage of that is plagioclase feldspar as opposed to alkali feldspar. That goes along the base of the triangle. Then you draw a line from that point on the base to the apex of the triangle and the zone where it intersects the horizontal line will tell you what kind of rock you have. Neat!

So what’s so interesting about granite? Well, it turns out there’s a lot.

**Granite is a magnetic encyclopaedia**

The crystals of feldspar and quartz which make up the majority of granite are sensitive to the Earth’s magnetic field (the magnetosphere) as they cool. Once they have solidified they contain a snapshot of the polarity and intensity of the magnetosphere which is called a paleomagnetic record. Provided the rocks aren’t contaminated by heat at a later date you can use them to read the strength of the Earth’s magnetic field billions of years ago. Smaller magnetic inclusions (the name given to the crystals) give more accurate results as they are less vulnerable to temperature changes.

John Tarduno of the University of Rochester used this property of granite to look at how the Earth’s magnetic field worked 3.2 billion years ago and found that the strength of the ancient field was at least 50 percent of the current one. That’s important because the magnetosphere is what protects the ozone layer from being destroyed by solar winds and the ozone layer protects the earth from the Sun’s harmful ultraviolet radiation. Without the magnetosphere life on earth wouldn’t be possible.

Another neat touch was that because the magnetic properties recorded by the crystals include the polarity of the Earth’s magnetosphere so Tarduno could check whether the rock had been contaminated at a later date by comparing the polarity with other samples of a similar age.

**Granite dominates the land, basalt owns the sea**

Both granite and basalt are igneous rocks, created when magma cools and solidifies but in terms of distribution, the continents are dominated by granite while the ocean floor belongs to basalt;. One of the differences between basalt and granite is that granite crystals are visible to the naked eye while basalt has much finer grains. This happens because on land the magma cools at a far slower rate giving the minerals time to grow larger while magma released underwater cools a lot faster making for teeny tiny crystals and that’s a massive part of why you will find granite on the continents but basalt in the ocean beds.

**Granite shows an ancient supercontinent linked the US and Antarctica**

A lump of granite found on the Nimrod Glacier near the Ross Ice Shelf in Antarctica had such a specific mineral composition that geologists were able to identify it as being related to a unique belt of rock which runs through parts of North America including California and New Mexico. The belt actually stops abruptly on the West Coast of the US hinting that a chunk of rock which used to be part of the supercontinent Rodinia — a precursor of Pangaea — had rifted away, eventually coming to rest in what is now Antarctica.

**Granite countertops can cause radiation exposure**

Most stone has the potential to emit radiation so the fact that granite can do so shouldn’t necessarily set alarm bells ringing, however granite has developed a bit of a radioactive reputation because some of its forms contain elevated concentrations of naturally-occurring radioactive materials.

Uranium 238 and thorium 232 are two such materials and can both cause radioactive gases like radon to be released into the surrounding atmosphere as they decay. In living areas — especially those with bad ventilation — the radon gas can accumulate and be inhaled potentially causing lung cancer.

Apparently there aren’t consumer guidelines for radiation safety when the emissions come from naturally occurring materials but using the safety standards which apply to controllable radiation sources a study by Daniel Steck of Saint John’s University in Collegeville, Minnesota (yeah, I had to look that up and check it wasn’t a spoof place name) discovered that putting lots of granite in small living spaces with poor ventilation can generate levels of radon which would be considered excessive.

The thing is, it depends completely on individual types of granite as to the level of risk. Most of the hoo-hah seems to focus around granite countertops and other home renovation materials but it also means that cellars carved into granite rock or towns built on granite foundations could experience radiation-related problems too.

**Granite radiation at Grand Central Station**

While writing this I found something which sounds suspiciously like an urban legend stating that the granite in Grand Central Station in New York emits more radiation than would be permissible at a nuclear power plant. After a bit of investigating it looks like this isn’t an urban legend but neither is it the cause for concern that it’s phrased as.

According to MIT the average annual background radiation dose for humans is 300 mrem and there’s a safe limit of 500 mrem for children and pregnant women. Grand Central exposes employees to 120 mrem per year (I don’t know how accurate that last figure is but it’s the only one I could find). That means that working at Grand Central will give you an annual radiation dose of around 420 mrem – still very much within the safe exposure zone for humans.

Basically, this all says more about the strict regulations governing nuclear power plants than it does about granite in Grand Central!


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