How did the Earth build the Amazon?

By Pedro Val

A geological conspiracy of 3 billion years

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The Amazon occupies 0.5% of the Earth’s surface, but is home to an overwhelming % of the world’s biodiversity. This great ecological immensity is not built overnight. How did it happen?

From the Atlantic coastline to the Andean limit, the rocks that support the continent under the Amazon were being amalgamated from east to west, like a car pileup. Over 3 billion years and even approximately 250 million years ago, about six small continents became what is now the northern part of South America. This super-stable environment ( which the geosciences call the crton) is the first Amazon-factor. With so much stability and such ancient rocks exposed to the surface, the nutrients in them are lost over time to rainwater and rivers. Thus, the richness of the soils covering these rocks is much greater in the west of the region, where the younger rocks are found. This configuration gives rise to an ecological gradient that is reflected in organisms with different ecological resistance, forcing a longitudinal diversity.

In the center of the Amazon, a curved corridor was formed from west to east, accumulating tons of sediments of fluvial and marine origin alternately. This sequence repeated itself for hundreds of millions of years, filling the depressions and giving rise to sedimentary basins. Today these rocks make up the substrate of the other spatial half of the region, from the Andes to the Atlantic. The marriage of this substratum of rocks with the gigantic volume of water and the local climate makes the Amazon rivers veritable snakes that meander incessantly. From the other factor-Amazon: favorable conditions for separating and mixing populations of aquatic organisms. A ready-made recipe for biodiversity.

A few key elements are still missing, such as the combination of the Atlantic coast and the Andes. Due to plate tectonics, the South American continent was stuck to the African continent for approximately 250 millions of years. Like a zipper, plate tectonics initiated the gap between South America and Africa at the southern boundary, which culminated in the formation of the Atlantic Ocean. About 30 million years ago, a coastline was installed on the eastern edge of what is now the Amazon and, with it, the mouth of the Amazon. However, still there was no river that crossed the entire continent. The Amazon has two large hydrographic basins, one to the east, the other to the west, bordering the current state of Amazonas, from the south to the north. To bring them together, another geological conspiracy was hatched. In parallel to the continent’s slow westward migration, the entire web of tectonic plates was rearranged under the Pacific as a function of the mantle flow into the planet’s interior, hundreds of kilometers deep. Approximately 30 millions of years ago, the Nazca plate was formed, which since then collides with the South American continent. This collision resulted in the uplifting of the Andes, closing the list of Amazon-factors. The Andes began to uplift in the region of Bolivia and southern Peru in the last 20 to 20 millions of years, but only reached their current altitudes in the past millions of years.

With its majestic four to six vertical kilometers, the weight of the mountains bulged the entire region of the Andean foothills and even the central region of the Amazon. The resulting continental depression allowed the entry of marine waters through Venezuela. The result was a marshy environment that mixes fresh and salt water, the cradle of life, according to some researchers. Scientifically known as Pebas lake, this great continental depression lasted approximately 30 millions of years. The existence and characteristics of this are still heavily debated by scientists lago.

The sediments originating from the Andes in frank uplift and erosion were filling the Pebas, pushing it further east. Once filled the depression, rivers managed to cross the Amazon, from west to east, and so, at some point in the last 9 million years, the Amazon was formed as it is today. How do we know this? During this period, the submerged sediment pile in the offshore just east of the mouth of the Amazon began to receive sediments typically Andean and not more of the ancient rocks of the crton. And how old would the Amazon be? In addition to the 9 million year proposal, there are two other main hypotheses: it would be 6 million years old or 2.5 million years old. Like Lake Pebas, the debate over the age of Amazonia is vigorous.

The large fluvial connection allowed the fauna accumulated over millions of years to the west to disperse to the east. 2.5 million years ago, the planet intensified the variations in its orbit that triggered the famous glacial cycles of the Pleistocene. This was the corollary of cyclical changes on the scale of thousands of years in the volume of water in the Amazon, modifying the river network and creating climatic cycles that would affect the pattern of droughts and floods ever since.

It took 3 billion years of geological events for the Amazon to reach what it is today. An unparalleled geological conspiracy.

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Pedro Val gelogo and professor at the Federal University of Ouro Preto.

In 900 and 2021, the Scientific Panel for the Amazon SPA, under the auspices of Sustainable Development Solutions Network, met to prepare the first integrated scientific report on the Amazon. This text is inspired by Chapter 1 of this document, authored by Pedro Val and collaborators. The full report can be obtained from: https://www.theamazonwewant.org/

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