14 DAY TRIAL // In a paper published in the latest issue of Science, and entitled “A Change in the Geodynamics of Continental Growth 3 Billion Years Ago,” experts at the University of Bristol, in the United Kingdom, refine our understanding of how continents evolved over the ages.
The type of crust underlying Earth's continents is very different from the one that lies beneath the oceans, in the sense that it's both thicker and less dense, which is why it rises so high above the mantle.
What the new investigation suggests is that the continental crust experienced a significant growth rate during the first 1.5 billion years of our planet's history. This rate then subsided over the next 3 billion years, reaching current levels.
According to the researchers, this state of affairs suggests that significant changes occurred in the processes that both generated and preserved this type of crust. At this time, only 25 percent of the continental crust covering the planet is more than 1 billion years old.
Only 7 percent of its entire surface comprises rocks from the Archaean Period, which occurred before 2.5 billion years ago. Our planet's age is estimated to be around 4.5 billion years, meaning that it began forming only a few million years after the Sun itself.
The UB analysis also indicated that a large volume of continental crust was produced before 2.5 billion years ago, which may go a long way towards explaining the considerable amount of scientific debate currently revolving around the timing, rates and the geodynamical conditions of continental crust generation.
Scientists established that around 3 cubic kilometers of continental crust was being produced annually on Earth, during the planet's first 1.5 billion years. The growth rate then decreased over the ensuing 3 billion years to around 0.8 cubic kilometers, a level we see today as well.
“Such a sharp decrease in the crustal growth rate about 3 billion years ago indicates a dramatic change in the way the continental crust was generated and preserved,” explains Dr. Bruno Dhuime, who holds an appointment with the UB School of Earth Sciences.
“This change may in turn be linked to the onset of subduction-driven plate tectonics and discrete subduction zones as observed at the present day,” the investigator goes on to say.
“The next challenge is to determine which tectonic regime shaped the Earth's crust in the planet’s first 1.5 billion years – the Archean/Hadean eons – before this change,” he concludes.