When most people think about the Amazon, they think of vast green forests alive with the sounds of monkeys, colorful birds, and millions of insect species. I think of those things too. But I also think about what lies beneath the surface. 

I’ve spent the past 4 years studying Geology, learning what rocks can tell us about the long (4.6 billion years long) story of our planet. So I wanted to write about something not many people think about: why do these massive and beautiful forests exist? How did they form and where does its story begin? 

The story of the Amazon, its rich biodiversity, ecological and cultural diversity, and role in global climatic and political systems is is driven by geology. The changes and movements in the earth itself over millions of years have created the modern conditions for the modern Amazon to exist.

This complex, dramatic, and fascinating story begins (as the very best stories do), with rocks. 

At one point in time, earth’s land masses were fused together as giant supercontinents. About 184 million years ago, one of the most intense and massive periods of volcanic activity began. This volcanism was so powerful that it broke up the Gondwana supercontinent, which at one point contained present-day Africa, South America, Australia, Antarctica, and parts of Asia (6).

After Pangea and Gondwana broke apart, two of earth’s tectonic plates drifted into each other. This collision would eventually cause the rise of the Andes Mountains, which began around 25 million years ago (5).

15 million years ago, a massive freshwater lake covered the Amazon basin. But during the several ice ages that followed, water started to flow eastward from the Andes, sea level fell, the lake started to drain out to the ocean, and the Amazon River was born (2).

From rock to rainforest

The water flowing from the Andes Mountains eroded the landscape and brought sediment from the mountains to the Amazon basin. This movement of sediment created the soils necessary for the rainforest to grow (1, 7).

How Ice Ages created the Biodiversity of the Amazon

There is still a lot of scientific debate over how such incredible biodiversity emerged in the Amazon. Here are some theories: 

During periodic ice ages, earth’s colder and drier climate reduced the Amazon into isolated patches of rainforest instead of the vast and continuous forest it is today (excepting deforestation). This separated and isolated animals, which increased competition between them. Competition often leads to the emergence of new species (3).

Another theory is that biodiversity rose because the geology of the region was so dynamic– sea level rose and fell, the climate cooled and warmed, and the rising Andes caused deposition, erosion, faulting, and earthquakes. These stresses can also quicken the pace of speciation. More stress, more species, more biodiversity. (3). 

Lastly, some argue that the tropics were left uncovered by ice during the last ice age. When glaciers creep over landscapes, many species die and new ones emerge when the ice melts. But if the tropics weren’t covered by ice, then the species that lived there survived. This means that there was just more time for new species to form and for all the species to evolve (4, 6).

How Rainforest Partnership Uses This Science

As part of the Rainforest Partnership team, we use all of this important and incredible information to help map the current biodiversity and resources of the Amazon. This helps us better understand how things like climate change and deforestation are affecting rainforests and rainforest communities. We ground both our advocacy and our work with our community partners in science.

The Amazon Rainforest, teeming with life and biodiversity, home to millions of species of plants, animals, and insects, was made possible by volcanoes, colliding tectonic plates, the uplift of the Andes, and Ice Ages. The Amazon, then, is hundreds of millions of years in the making. Earth history is long and slow and punctuated by climate changes, moving continents, volcanic eruptions, and mass extinctions. Humans are a tiny piece of that timeline, but as we know well, have incredible power to both destroy and protect this ancient and fascinating place.