By Pedro Val
Landscapes have a chronological memory
When we talk about Earth’s geological past, fossils, especially dinosaurs, come to mind. Perhaps some think of rocks, others of minerals, but rarely will anyone evoke a landscape, the shape of a relief, a waterfall, a ravine. Landscapes like fossils, rocks and minerals now have an age, or better: a temporal memory.
Understanding how long it has been since natural beauty came about is the part of earth sciences that excites me the most. Since when has the Amazon been crossing the continent from the Andes to the Atlantic? How long has Pão de Açúcar been there? How long did it take to build the Grand Canyon? And the Andes to rise?
Insights into solving such questions can come from various sciences. One of them, geomorphology, works with the view that landscapes are shaped by physical processes that take a quantifiable time to complete. For example, many waterfalls are mobile. For thousands to millions of years they migrate in the opposite direction to the river of water that flows over them because the erosion of their wall is causing a setback. And when a waterfall crosses other rivers during its migration, other waterfalls of similar size form and spread throughout the hydrographic basin as the falls continue to migrate until they reach slopes and water catchment areas. In my research, I identify which waterfalls have formed this way and estimate the time it will take to get them to where they are today. With this I calculate when the geological process that led to the first waterfall took place, and this can be important if this assumption is consistent with the appearance of new species of fish, for example.
But nobody has to be an expert to guess the possible age of a landscape. A quick trick is to take the difference in elevation from the highest to the lowest point of the landscape of interest and divide that difference by the speed at which erosive processes are removing material from the surface of that landscape.
For example, suppose the ancient floor of all of Guanabara Bay was level with the highest point of the Sugar Loaf. Absurd? I don’t know, but let’s see. If this plateau was dug to sea level every 1 million years at a slow speed of 10 meters (reasonable speed for this region), the iconic postcard would reach 400 meters in height after 40 million years. It’s a rough calculation, but it makes us think that what we see when we stand in Pedra da Gávea took shape at least ten million years ago. When we are already overwhelmed by natural beauty, the dimension of time makes us even smaller. It is clear that there are scientifically sound ways to make these estimates – this trick only gives us an idea.
The age of a landscape can do more than just add to its natural beauty and answer great scientific questions. For example, the Amazon did not flow from the Andes to the Atlantic about 10 million years ago. Some processes since that date have caused it to take its current form. Which would be? If the change had happened about 10 million years ago, some researchers would say that the uplifting of the Andes would have been the main trigger. If the change has happened in the past 6 million years, the answer could even come from the Earth’s mantle, which is hundreds of kilometers deep! Other researchers would blame geological processes, whether or not associated with the Andes, in connection with climate change when the change is only 3 million years old. At the same time, the biodiversity of the Amazon also emerged during this period. What if one of these periods coincides with the appearance of new species? Well, there we would find out what geological mechanism triggered the formation of this extremely diverse biome.
The temporal memory of landscapes contains answers with the potential to transform basic knowledge about the evolution of the relief of a continent and its species.
Pedro Val is a geologist and professor at the Federal University of Ouro Preto
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