By Mark Jefferd
When I meet someone, regardless of them being a family member, an old friend, a scientist or even a stranger, and say that I earn a living studying and researching rocks, I am usually greeted by one of three responses. The first starts with a nervous laugh, followed swiftly by awkwardness when they realise I’m not joking. The second, the classic motherly response, is to try and feign interest just to be nice. The third response is the one I’m going to attempt to answer in this blog: “Why?”
Many aspects of our lives are directly linked to the how rocks respond to the stresses and forces within the Earth. These range from catastrophic natural disasters such as earthquakes and volcanoes, to our on-going reliance on hydrocarbons like oil, for energy generation as well as the production of many common items such as plastics and tyres.
Now that we’ve agreed that rocks are important to study, what are the ways in which this can be done? The first method is to use fieldwork to study the real world. But this method has its problems, such as having to reach inaccessible areas of interest, and the large costs involved.
Another option is to study the natural world in the laboratory, allowing scientists to create the conditions they want to study in much more accessible locations. The third method is to use computer models, which allows us to simulate scenarios that are too big and complex to ever do in a lab, at a fraction of the cost. However, a model is only ever as good as the parameters and variables put into it. So fieldwork and laboratory work are still the bread and butter of geology and are essential to measure the inputs and validate the outputs of any computer model.
Working in the field is vital for geologists to be able to study different landscapes and geological features. However, it can be nearly impossible for them to observe all of the processes that led to their formation, due to the huge time scales involved. Which is part of the reason why many geologists have a very warped sense of time, seeing little difference between yesterday and 10,000 years ago.
If we compress the Earth’s history into one year, then the dinosaurs died out at the end of October, and modern humans have only been around since noon on the 31st December. When you deal with such vast expanses of time, 10,000 years starts to seem like pocket change.
Some geological events, such as earthquakes or landslides, do occur at observable rates, but the chances of being in the right place at the right time to collect meaningful data, and actually surviving to publish the paper, are very low.
Other fascinating geological sites simply cannot be accessed; most earthquakes occur too deep under the crust to be observed directly. The deepest man made holes are typically mines used for mineral extraction. The Mponeng gold mine in South Africa is the deepest mine in the World at 4km deep. At these depths the rocks are at a scorching 70°C, and the air has to be cooled to a survivable temperature of 35°C by pumping ice slurry down the shafts!
Humans are capable of drilling deeper than this; the deepest hole ever drilled was the impressive Kola borehole in Russia, which reached 12.2km, where the temperature is 180°C. But even this falls tens or hundreds of kilometres short of where most earthquakes happen. The devastating earthquake of 2011 that caused the Japanese tsunami occurred 30km below the surface. Even if it was possible for us to be at those depths, would you want to be down a 30km hole when a 9.1 magnitude earthquake was occurring? I know I wouldn’t.
Clearly, geologists need to carry out experiments to replicate the conditions found in inaccessible regions of the Earth, and to allow them to observe geological processes on a sensible time frame. For example, the area of geology I am currently most interested in is rock mechanics, or ‘what happens to this lump of rock when I smash, squeeze, pull, spin or heat it?’
Rock mechanic experiments can range in complexity; simple experiments may involve crushing a sample of rock in one direction (uniaxialy) to see where it fractures. This can tell us the basic mechanical properties of the rock, and information about its strength, which allows for more complex experiments to be carried out. Knowing at what pressures a rock will break or deform under is also important for other fields such as engineering and construction, where many buildings or structures use the bedrock as a stable foundation, or rock pillars to provide support.
More complex experiments involve squeezing a sample of rock in three dimensions (triaxially) at the same time. These tests are more realistic for the conditions found deep within the Earth, and can help us to understand where rock fractures form during earthquakes. They can also tell us about how permeability changes through a rock as it deforms – an important characteristic for predicting the location of hydrocarbon reservoirs and aquifers, which may yield oil or natural gases.
Other experiments involve squeezing two rocks together and spinning them in opposite directions. This can represent an earthquake, where two pieces of rock slide past each other, at speeds up to 8km/s. At these speeds, the friction creates intense heat, which can melt the rock. Melted rock then solidifies along the large crack, known as the fault plane, which can effect earthquake reoccurrence rates and the movement of underground fluid in the local area. If liquid rich in mineral is prevented from freely flowing it can easily lead to the deposition of metal ores and other important mineable products, hence why many mines are located near faults.
So while rocks may appear dull and inanimate, many things we take for granted in daily life can be located and understood from the geology of our planet. Likewise, through studying geology, disasters can be understood and predicated a little bit better, potentially saving millions of lives. So why do I study rocks? The real question should be why don’t more people study rocks?