Interview Professor Chris Hawkesworth
Deep time, deep thinking…
Interview Chris Hawkesworth
Deep time, deep thinking
Scientists are always under pressure to come up with exciting new ideas to improve our day-to-day lives, but sometimes trying to discover what we don't know or exploring “what we don't know we don't know” can lead to the greatest – and most unexpected – advances in science...
Professor Chris Hawkesworth confesses that he has a “low boredom threshold,” and this is perhaps what continues to drive his research. Hawkesworth is Deputy Principal and Professor of Earth Sciences at the University of St Andrews, after a research career at the Open University and the University of Bristol, but after all this time, he still describes the “joy of working with colleagues and students in isotope laboratories.”
Hawkesworth's passion for geology has taken him around the world in search of new discoveries, along the way developing his specialist interest in isotope geochemistry. More recently, he's turned his attention to “the nature of the geological record and the extent to which it is biased by sedimentary and tectonic processes and the development of supercontinents.” But the question he would most like to answer today is what the Earth was like during its first 600 million years – the period for which we have almost no geological record.
The biggest thing we know about this early period is probably “how much we don't know,” but what we learn may lead to major breakthroughs in our scientific knowledge and even the development of new applications – we simply don't know what we will discover. For Hawkesworth, it is just the kind of intellectual challenge which has always intrigued him, no matter what results from our future enquiries.
There are no rocks available which could provide the key to understanding this mysterious era – they've been recycled long ago. The planet is 4.55 billion years old but there are very few rock samples older than 3.9 billion years. One of the only clues we have to the previous 600 million years comes from looking at zircon, tiny crystals of zirconium silicate which are found in igneous, metamorphic and sedimentary rocks, and also in recent sands. When rocks are reworked during periods of major geological changes, the zircon they contain is often durable enough to survive processes such as erosion and even metamorphism, providing a record which pre-dates the rocks they now inhabit. Zircon yields high-precision ages of the zircon’s original crystallisation, and these ages have been the cornerstone of establishing the geological time scales, and the ages of events throughout the history of the Earth. “In many ways, zircon is therefore the workhorse of the geological record,” says Hawkesworth.
Traditionally, rocks and minerals for geochemical analysis were ground into homogeneous powders. However, they often contained material of different ages and different histories, and it is only with the development of ion microprobe and laser technologies that it is now possible to analyse tiny spots of individual minerals. The result, says Hawkesworth, is that we can now be much more certain of what we are analysing, and this approach is widely applied to zircons, since zircon crystals can often contain pockets of zircon of different ages.
“The isotopes involved in radioactive decay schemes, such as uranium decaying to lead, allow us to date rocks and minerals very precisely, because we know just how fast they decay,” says Hawkesworth. “But there are many isotopes not involved in radioactive decay, the so-called stable isotopes, and these are increasingly used to investigate metal–protein interactions, climate change, the effects of pollution and early signs of life in the geological record, to name but a few. Overall, isotopes are a fantastic tool – and we now have almost the whole periodic table to play with.”
According to Hawkesworth, zircon has already revealed a lot about the early days of the planet, with some samples dating as far back as 4.4 billion years. For example, the evidence from zircon suggests that there was water, that zircon crystallised from relatively low temperature magmas, and that new continental crust was melted to form granite time and time again over 1.5 billion years. But the big question is: was there life?
At this time, there were lots of meteorites flying around and colliding with Earth, causing huge geological changes – catastrophic enough to wipe out any primitive life forms that may have existed. For evidence of the damage, we only need to look at the Moon. But after this bombardment, we entered a relatively quiet time, and life as we know it evolved. Primitive, simple organisms started us on the road to life as we see it today.
Preservation vs Peaks
It is this mind-boggling time-scale that fascinates Hawkesworth. He may not yet have cracked the secrets of the earliest days of the planet, but in recent years, he and his colleagues have made a significant breakthrough in our understanding of how the continental crust was formed and how it evolved, over billions of years.
One of the striking features of the continental crust is that it contains peaks of ages of zircons, and hence of the granitic magmas from which they crystallised. That suggests that there were periods when more granite was generated and periods when much less granite was formed. Intriguingly, the peaks of ages also occur at the same times that different continents on the Earth came together to form supercontinents. Until quite recently, geologists believed that these peaks of ages reflected pulses of igneous activity, and that they were therefore caused by episodic “pulses” from deep in the Earth, resulting in the generation of unusually high volumes of magma at certain times.
Hawkesworth has another explanation.
What he and his colleagues have questioned is whether there's a link between the ages and volumes of the igneous rocks and the formation of the supercontinents. And they suggest that the larger volume of rocks associated with particular ages can be better explained by “biases in preservation” – in other words, the conditions which existed at particular times helped to preserve larger volumes of rock, rather than that there were times when dramatic pulses of magmatic activity resulted in the generation of larger volumes of igneous rock.
Hawkesworth expands on this theory by suggesting that the continental crust was formed as part of a continuum rather than a series of sudden events, much the same as the continuous process observed by scientists today as supercontinents build and separate, producing magma
to generate new crust, building mountains and melting and recycling the old crust as one tectonic plate sinks beneath another. And this would also help to explain the “unevenness” of the rock record available for sampling – older rocks and younger rocks mixed up together.
What Hawkesworth concludes is that when tectonic plates or supercontinents move, they have a greater potential to preserve rocks at some stages during the process than at others – for example, “subduction-related magmatic rocks are better preserved in extensional basins that lie inboard from the subduction zone.” In simple terms, most samples of zircon, those peaks of ages, are preserved from the times when continents collide to form supercontinents, rather than from the periods when the continents are moving around or when the supercontinents start to break up. As we interrogate the geological record, says Hawkesworth, it seems important to consider not only the volumes of magma generated in different tectonic settings, but also the extent to which they are likely to be preserved.
Thus, the geological record is not always quite what it seems – it is biased by sedimentary and tectonic processes and the development of supercontinents.
Why does geology matter?
This is the kind of research that has fascinated Hawkesworth throughout his career, since the early days at Trinity College in Dublin and at the University of Oxford, where he was a member of a group that developed one of the first thermal models for an orogenic belt (a mountain belt) in the Eastern Alps in Austria. Apart from how and when the continental crust was formed, his other major interests are the generation of large igneous provinces and their role in the break-up of supercontinents, and the formation of associated – and economically important – base metal sulphides. He is also intrigued by how meteorite bombardment and biological processes have shaped the evolution of the Earth.
Hawkesworth is also passionate about education – particularly the teaching of geology. In an age when more and more attention is being paid to climate change, as well as to volcanic hazards, oil and gas exploration and the search for precious minerals and metals, he is concerned that Scotland may be falling behind in international league tables for the number of school students enrolling in earth and environmental sciences. Not only does Scotland have a noble tradition in geology, including some of the biggest names in the history of the science, but we also have a practical need to produce more geoscientists and improve the knowledge of geology among the general public. “We are the custodians of the Earth for future generations and we need to know more about it,” says Hawkesworth.
The teaching of geology is coming under threat, however, with Higher Geology due to be phased out in 2015 and only basic elementary geology being subsumed into parts of other courses. So even now Scotland is falling behind other countries, says Hawkesworth. In Norway, for example, and in England and Wales, the ratio of students taking the equivalent of a geology Higher or A level compared to those taking physics over the last five years is ~1:15, while in Scotland it is currently ~1:150. Looked at another way, about 9,000 students in Scotland have recently been taking Higher Physics but only about 60 have been taking geology. If the ratio was similar to Norway, or for those taking A levels, we should expect 10 times more geology students taking Higher Geology – roughly 600. According to Hawkesworth, the reason Scotland isn’t submitting more pupils to Higher Geology qualifications is insufficient provision of teachers and support for the subject.
Another big concern is that non-scientists are missing out on a basic education in earth sciences – including politicians responsible for drawing up our policies on climate change.
Hawkesworth clearly feels very strongly that the study of science has practical value as well as helping to develop analytical skills, and a deeper knowledge of environmental issues. “It is important that as many people as possible should have the opportunity to study geology,” Hawkesworth explains, “not least to have a reasonable understanding of the history and workings of our planet as we plan for the future, in addition to the more tangible links to industries such as oil and gas.”
The public debate about the value of pure science versus applied science will be never ending, but Hawkesworth is relaxed about his personal role: “Both are important, they feed off each other, and often they operate on different time scales. I have greatly enjoyed working with industry, but the delight is to follow up on ideas and to see where they lead.”
Climate change is one area where there's a lot of research focus and excitement today. Geologists are interrogating the recent records of climate change and helping construct new models, thus making possible a more informed debate, says Hawkesworth. “In detail, there is much to be debated – for example, the extent to which past records are helpful in predicting future change, or whether our present climate system has already been modified too much for the past still to remain a useful key. More widely, it remains important that 'blue sky' research continues to be valued and supported. We don’t know where the next breakthrough may happen, and it is a high priority that people of the calibre to make such breakthroughs are encouraged to undertake their research in Scotland.”
There are lots of things in geology we know nothing about, he continues – especially the first 600 million years of Earth. “But when I interview students and research staff, the key is surely to look for those who are intrigued by these big questions – not frightened by them.”