The Royal Society of Edinburgh Scotland Rocks: a Tartan Tour Of
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The Royal Society of Edinburgh Scotland Rocks: A Tartan Tour of Planet Earth Professor Iain Stewart Friday 26 February 2016, Greenwood Academy, Irvine Report by Kate Kennedy Many of the fundamental ideas which underpin our current understanding of how planet Earth works come from the work of Scottish scientists. This talk explored and celebrated this tartan contribution to modern Earth Science, and examined how many of the contemporary challenges facing society in the 21st Century also root into Scotland's industrious past. The current layout of the continents and countries of the Earth is instantly recognisable from the images provided in atlases; however, this well-known depiction is just a snapshot in time. For geologists, the image of Earth today is the product of millions of years of multiple, ever- changing worlds. Indeed, 250 million years ago, Earth looked very different and the countries we know today formed a ‘supercontinent’ known as Pangea. The break-up of Pangea, commencing around 200 million years ago, created the image of the Earth as we now know it. In 1844, the Rev. Thomas Dick, a Scottish Minister, teacher and writer, first suggested that the continents of Africa and South America once fitted together and were ‘rent asunder’ by physical processes. This suggestion was dismissed by early scientists and it was to be another fifty years before similar proposals were put forward by ‘modern’ Earth scientists. Evidence supporting Rev. Dick’s theory can be found in rocks near the Victoria Falls in Zimbabwe, created by a cataclysmic geological event that formed today’s Africa. Layers of volcanic basalt rock were laid down during a series of eruptions over thousands of years. As the rocks cooled, small crystals formed within the rocks, giving a speckled appearance. The small size of the crystals indicates that the rocks cooled rapidly. While the fragmentation of Pangea created iconic landscapes such as the Victoria Falls, Professor Stewart commented that Scotland’s landscape also contains similar evidence of geological activity, sometimes even older than that found in Africa. James Hutton FRSE, the ‘Father of Modern Geology’, was a farmer’s son from near Edinburgh. His 1788 paper, Theory of the Earth, clarified some of the fundamental principles of geology. Through careful observation of Scottish geology, he realised the importance of the Earth’s internal heat to the perpetual formation of the planet. For Hutton, the Earth should be considered as a physiological system and the heat inside the planet, as its engine, moving material around and keeping it alive. Hutton’s discoveries were extraordinary for the time; the popular explanation being that rocks were solely formed through sedimentary processes where layers are built up over time. His ideas focused on the notion that the heat in the Earth made some rocks molten, which lead to changes in their structure. One example of this is Glen Tilt in Perthshire, which Hutton visited in 1785. He noted the existence of outcrops of metamorphic schist that showed injections of pink granite, implying that the granite had been molten when violently intruded into the surrounding rock. Within Hutton’s theory is the concept of a very long geological timescale with “no vestige of a beginning, no prospect of an end”. At the time of his writing, the standard theological thinking was that the Earth was very young, about 7,000 years old. Hutton considered it to be pointless to discuss the age of the planet because, as a natural system that is constantly changing, there is no beginning and end, just a succession of different worlds. Hutton regarded Siccar Point in Berwickshire as conclusive proof of his geological theories; today, it is perhaps the most famous geological location on the planet. Professor Stewart described how at Siccar Point there are two obvious types of rocks; horizontal Old Red Sandstone overlies the older, grey, vertically-upstanding rocks (Silurian greywacke). The older grey rocks are formed from mud laid down on the ocean bed; marine organisms in them show that it is from the deep ocean – over 1000 metres deep. The Old Red Sandstone rocks are completely different and are derived from sand and gravel laid down in a desert environment. Between the two rock forms there is an irregular erosion surface. Hutton studied this environment and understood that it would take millions of years for this landscape to form; first for the sediments to form, then for these to raise up to the surface, plus the time needed to erode that landscape and then for the next desert landscape to form on top of this. Thus, the basic template of the theory of the planet was devised by Hutton in the late 18th Century and Scottish rocks underpin the understanding of modern geology. In the mid-1800s, geologists largely subscribed to the traditional theory that rocks formed in sedimentary layers with the oldest at the bottom and the youngest on top. Some amateur geologists, including Charles Lapworth, suggested that this wasn’t always the case and quoted the example of Knockan Crag in Assynt, where it appeared that the rocks at the bottom were the younger specimens. Discussions on the subject continued for decades until Archibald Geikie decided to put an end to these claims and arranged for a team of experts to visit the area and map the geology with the intention of reinstating the Survey’s reputation as the ‘expert’ geologists. Two experts, Ben Peach and John Horne, visited the outcrop and did indeed discover that, as the amateurs had suggested, the older rock was on top; a lack of fossils in rock indicates an older age. However, they also noticed a narrow band of crushed rock lying between the other layers of rocks; described as a “vast rolling and crushing mill of irresistible power”. Their discoveries led to the assessment that the order of the layers of rock was being disturbed by a slab of rock being pushed up and over another one through tectonic action; the old rock from lower down was being pushed up and put on top of younger rock. Until this time, theories relating to ‘building’ mountains focused on the notion that they were pushed up from the base; this notion of a sideways thrust creating mountains was therefore a revolutionary thought in geology. Peach, Horne and their team continued to map the area for ten years, in incredible detail. A new global theory of the Earth developed in the 1960s, including an understanding of plate tectonics developed by Arthur Holmes at Edinburgh University. He first presented his theories in the 1920s, but his ideas were not widely accepted until the 1960s. Today, his model is the one used in the modern understanding of plate tectonics; the planet exudes heat, causing the surfaces to crack, which creates mid- ocean ridges and spreads a ‘conveyor belt’ of rock across the ocean floor. This new rock cools, becomes denser and, as it gets older, is so cold and dense that it becomes unstable and sinks back down into the mantle – resembling an elegant planetary recycling system. In geological terms, it is difficult to define the age of a country – the rocks under our feet are all part of history and have had an extraordinary journey through a whole series of past ‘Scotlands’. Indeed, in reality, Scotland is made up of fragments of other places and has a complex history. Professor Stewart cited the example of rocks found in a stone wall in Rhynie in Aberdeenshire, which are around 407 million years old. At the time they were formed, the world looked very different; places we now know as America, Mexico, etc. were all located in different parts of the globe and Scotland and England were nowhere near each other. The rocks at Rhynie are important because, when they are cut and polished, exquisite fossil evidence of stems of plants, including some at cellular level, is clearly evident in the layers of volcanic sinter. These rocks are the earliest evidence of ecosystems and, as such, Rhynie is a site of international importance. Such rocks, therefore, are not just rocks, but portals to the past; environments of life preserved in rock. Scotland is one of the best places in the world to study geological formations; geologists have been studying and recording here for centuries. There are many varieties of rocks and the country has been exposed to periods of glaciation in the past. Carbon dioxide (CO2) levels from 400 to 500 million years ago can be ascertained from the chemistry of plants left in vegetation traces. Land plants first developed about 450 million years ago in an extremely warm time period, when there was a lot of CO2 in the atmosphere. Plants photosynthesising immediately started to draw down the CO2 and, at the same time, oxygen was being pumped into the atmosphere; CO2 levels went into freefall over the next 50 to 100 million years. Normally, when vegetation dies it decays and the CO2 is released back to the atmosphere; however, this was not happening and the CO2 remained trapped in the root systems, leaves and stems. As such, the atmosphere was becoming less CO2 rich and, consequently, the air temperature was dropping. Evidence of the Carboniferous period, about 300 million years ago, can be seen in the fossilised tree trunks found on the Fife coast, with carbon still locked in. The changes in atmosphere towards the end of the Carboniferous period led into the Ice Age, with ice sheets nearly reaching tropical areas. In the Carboniferous period, the oxygen-rich atmosphere meant that giant beasts, including dragonflies up to 1.5 metres wide, were common and, although the Carboniferous period was short-lived, importantly, Scotland’s coal deposits were formed in this period from the preserved vegetation.