Sand and shingle – stars of the Jurassic Coast
Science feature by Professor Glenn Patrick
WHAT comes into your mind when you think of Lyme Regis and the surrounding coast? Images with sand and pebbles probably feature quite prominently.
I am afraid that I am no geologist and I have trouble identifying one rock from another. Nonetheless, I often pause to wonder about the true origin of all the materials that make up our beautiful Jurassic Coast. Where did it all really come from and what is it all about?
Luckily, there is a wonderful website called ‘Geology of the Wessex Coast of Southern England’ by Dr Ian West of Southampton University, which has 150 pages of geological information on this region.
Here, I learned that the pebbles on the 18-mile Chesil Beach – and there are 180 billion of them – are mainly composed of pieces of flint and chert. These are forms of the mineral quartz or chalcedony – a microcrystalline variety of quartz.
Most quartz is formed naturally in igneous rocks and crystallises when molten magma from inside the Earth cools and solidifies. We now know that quartz is a chemical compound known as silicon dioxide consisting of the elements silicon (one part) and oxygen (two parts).
The Earth’s crust is made up of just eight elements with oxygen at 47 per cent and silicon at 28 per cent the most common by weight – a whopping 75 per cent in total.
During the coastal protection works completed in Lyme in 2007, the town beach in front of the Cart Road was strengthened against the sea using 70,000 tonnes of shingle from the Isle of Wight – again rich in silica.
The 30,000 tonnes of sand used to replenish the sandy beach at the same time made the news because it came from Caen in northern France – apparently because of its grain size.
Sand is, of course, formed from the decomposition of rocks and minerals over many thousands of years. This means that there are different types and grades of sand depending on the local rocks and the precise ageing and erosion processes.
However, whatever its texture, the main constituent of sand in most parts of the world is again silicon dioxide! So, an essential constituent of all the sand and most of the pebbles that we see around us is silicon. In fact, the clue is already in the name as it is derived from the Latin word “silex”, meaning flint.
The problem with silicon is that it never occurs free on Earth because it likes to combine with oxygen. It was eventually isolated as a pure element by the Swedish chemist Jons Jacob Berzelius in 1824.
Today, silicon is one of the most useful elements to mankind. We make concrete out of it in the form of sand, it is used to make glass and when it is doped with other elements it is used extensively in the electronics industry as a semiconductor.
It is so ubiquitous that we talk of ‘Silicon Valley’, the ‘Silicon Age’ and even ponder whether there are silicon based aliens in the same way that we humans are based on carbon.
Silicon is the eighth most common element in the visible Universe by mass. This might sound impressive until we compare with hydrogen and helium, which respectively make up 73 per cent and 25 per cent of the Universe.
All of the rest, including silicon, is a puny two per cent! It is of course an important two per cent because most of the atoms in our bodies and in the Earth consist of these heavier elements.
To answer the question of why this happened we have to go back in time. In 1948, a famous paper established that the very lightest elements were of primordial origin made very shortly after the birth of the Universe.
Just after the Big Bang, the Universe consisted of a hot soup of protons, neutrons and electrons. After three minutes, this soup had cooled sufficiently – to a billion degrees centigrade – that the nuclei of light nuclei could start to form.
After only 20 minutes, the protons and neutrons had combined to produce the nuclei of hydrogen and helium along with small amounts of lithium and beryllium – this is why we see so much hydrogen and helium today.
At this point, the Universe became too sparse and too cool for further nuclei to form. It took a lot longer – 380,000 years – for the electrons to get trapped by these light nuclei and to form atoms of the lightest elements.
In 1957, the British astronomers Sir Fred Hoyle and the husband-wife partnership of Geoffrey and Margaret Burbidge teamed up with the American nuclear physicist William Fowler to explain the origin of the heavier elements – that two per cent including silicon and oxygen.
They presented strong evidence that the 11 elements necessary for life were cooked up by thermonuclear reactions deep inside massive stars, which eventually blasted them into space when they exploded as supernovae at the ends of their lives.
We think that this process of manufacturing the heavy elements up to iron was kick-started around 100 million years after the Big Bang, when the very first massive stars formed and illuminated the Universe.
These initial stars would only have been able to burn hydrogen and helium, but they would have died relatively quickly spewing the first heavy elements out into space and seeding further stars. We owe our very existence to them.
As Carl Sagan famously said, “All of the rocky and metallic material we stand on, the iron in our blood, the calcium in our teeth and the carbon in our genes were produced billions of years ago in the interior of a red giant star. We are made of star-stuff”.
In the last few years, planets orbiting stars outside of our own solar system have been found so we may not be alone. Over 4,000 of these exoplanets have now been detected – an amazing feat in itself – but, so far, the only planet we know that has both the right elements and the right conditions to support human life is Earth.
It is always exhilarating to walk along the sand and pebbles along our beautiful coast – but it seems even more remarkable when you realise the cosmic journey that has been undertaken through space-time to provide this precise mix of elements in this particular spot.
We live in a special place not just in Dorset, not just in the solar system, but in the entire Universe.
Glenn Patrick is a particle physicist and science communicator who explores the quantum world of sub-atomic particles (including at the Large Hadron Collider) and now lives in Lyme Regis