Magma intruding into sedimentary layers at Arthur’s Seat, Edinburgh

At the end of May, during the Edinburgh Marathon Festival weekend, I ran the 10K, and it got me thinking about time. Runners are obsessive about time, of course, always trying to improve their best time over various distances, but so are geologists – for entirely different reasons.

Runners and geologists think about time on very different scales. If you’re in a 200 m or 800 m race – or a 10K – you’re thinking in minutes and seconds. But for a geologist, even a million years is the blink of an eye. The earth’s age is measured in billions of years. The 10K race in Edinburgh, strangely enough, brought both these scales into focus. Why was that? First there was the run itself: although I’ve been a runner all my life I hadn’t taken part in an organized running event for many years, and when I filled out the entry form they wanted to know what my anticipated finishing time would be. I had no idea, but I made an educated guess and wrote down 60 minutes. A nice round number, and it wasn’t too far off; my actual time was 52:28. Minutes and seconds. Then there was the map showing the course of the race: it circled around Arthur’s Seat, a prominent hill and an Edinburgh landmark. But to a geologist Arthur’s Seat is more than just a hill – it’s the remnant of a very ancient (almost 350 million years old) extinct volcano, and perhaps more important it’s where a famous eighteenth century scholar, a man named James Hutton, made observations that helped him develop his ideas about the immensity of geological time. Hutton gave us one of the most memorable phrases ever written about time when he said that geological time has “ … no vestige of a beginning, no prospect of an end.”

Hutton did what good geologists still do: he observed the natural world and used reasoning to explain the features and phenomena he saw. He wasn’t swayed by preconceived ideas or the conventional wisdom (which in his day still included the idea that God created earth in a week about 6000 years ago). When he observed a cliff with hundreds of meters of sedimentary layers piled up one upon the other and pondered how they got there, he realized that huge expanses of time must have been involved. Every small particle in those layers had been eroded from the land, transported to the sea or to a lake by streams and rivers, and settled slowly to the bottom, gradually building up the sedimentary layers. It was, he understood, an incredibly slow process.

At Arthur’s Seat Hutton saw evidence that molten magma had forced its way into and through older sedimentary layers near the earth’s surface, and he concluded that in some places rocks must get deeply buried and eventually melt before making their way back up to the surface as magma (he didn’t actually use the word ‘magma,’ however – although the English word was known in Hutton’s day, it wasn’t used in a geological sense until much later). Hutton developed the idea of geological cycles – erosion, buildup and deep burial of sedimentary layers, melting, and, finally, uplift. Each cycle starts with erosion and ends with uplift, and then it begins all over again. You can imagine such cycles going on forever, hence Hutton’s phrase ‘no vestige of a beginning, no prospect of an end.’

See the photo above for an illustration of what Hutton saw in Edinburgh near Arthur’s Seat, where the 10K run took place. The young people are standing on tilted layers of ancient sedimentary rock; the darker rocks above are the now-frozen magma. Can you discern the way the magma intruded into and lifted up a block of older sedimentary rock?