How old is a mountain range?
PhD geology student, Eleni Wood, explains how the science of ‘geochronology’ can be used to effectively measure the life-expectancy of a mountain range.
When we think about mountains we might think about them being constant, permanent features of our landscapes. However, through geology, we know this isn’t the case. The surface of our planet is forever changing and evolving, due to the movements of tectonic plates that make up our rocky crust. Mountains grow in the collision zones between plates over millions of years, but their presence is only transient as they are slowly eroded away again.
The science of ‘geochronology’
How do we know the timescales, and effectively the ‘life-expectancy’, of a mountain range? One way we work out how a mountain range has grown through time is via a science called geochronology. Geo- relating to the Earth, and -chronology relating to the timing and duration of events. Geochronology is the science of dating geological events and processes and in particular, we can use geochronology to calculate when a mineral first grew.
I am applying this science to rocks that I collected from the Himalayan mountain range during fieldwork in Bhutan. The rocks that I study are metamorphic rocks; the minerals they contain have undergone chemical and physical changes, due to being buried 10s of kilometres beneath the surface. At great depths beneath the mountain range, extremely high pressures and temperatures can cause new minerals to crystallise and grow. We can establish when some of these minerals grew using geochronology.
When any minerals of zircon and monazite, within the metamorphic rocks, incorporate radioactive elements such as uranium into their crystalline structure when they grow. Uranium is unstable and decays over time to stable lead. When the minerals grew they started a radiogenic stopwatch; they incorporated very little lead when they grew and therefore most of the lead we can measure in the mineral today formed from the breakdown of uranium.
Geochronology uses the principle that uranium decays exponentially at a known rate and measurements of the relative proportions of uranium and lead isotopes in the minerals, to calculate when they grew. This can be imagined as if we were looking at a sand timer where we know the rate at which sand falls through the timer. If we turn over the sand timer, letting the sand fall from the top to the bottom, and want to know how long it had been turned over for, we could compare how much sand is in the top versus the bottom. Similarly, we can compare how much uranium and how much lead there is in a mineral to work out when it grew.
Why is this technique important for understanding the life-expectancy of a mountain range? By dating lots of different minerals in lots of different rocks from a mountain range, we can find out when different packages of rock were buried deep beneath the mountain range and also how quickly they returned to the surface. This is important for understanding how the mountain range evolved as it was buckled and folded between two colliding continents. Through geochronology, we know that the history of the Himalayan mountain range spans the past 50 million years.
Dr Eleni Wood is a PhD geology student at The Open University. This article was previously published on OpenLearn in May 2018. Subscribe to our newsletter for more free courses, articles, games and videos.