Miniseries: The Oldest Rocks in South America
Hello, and welcome to Bedrock, a podcast on Earth’s earliest history.
I’m your host, Dylan Wilmeth.
We’re at the midway point in our mini-series on Earth’s Oldest Rocks. This series gives you a sneak peak into locations we’ll visit in Seasons 3-5, so think of these episodes like teaser trailers. If you want to follow along at home, we’ll have geologic maps of each continent on our website: bedrockpodcast.com
The rocks on this series only cover 5% of the Earth’s surface, but represent the first half of Earth history, a time period called The Archean. Some continents have a lot of ancient rocks, some very few. This week falls into the second category, a light appetizer compared to the multi-course meal of Africa. Break out your maps, it’s time to visit South America.
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South America is a giant triangle with three corners: one north, one east, and one south. The southern corner hangs just above Antarctica- they form two hands reaching out to each other. The south is full of rugged, beautiful scenery including the Chilean Andes, the Atacama Desert, and Patagonia. I’ve been very privileged to do fieldwork here, but that was on modern bacteria, not Archean rocks. If we keep trekking up the Andes to the northern corner, past Peru, Colombia, and Venezuela, these titanic mountains are still far too young for this episode. You’ll find Incan ruins here, but not Archean rocks.
Archean cratons connected across South America and Africa.
The rocks we’re looking for lie to the east, in the third corner of the triangle: Brazil. Brazil is the fifth-largest country in the world and takes up half of South America. So why are all the old rocks here and not in the Andes? To answer that question, we need to look across the Atlantic to Africa. If we move the two continents around like jigsaw pieces, both the coasts and the rocks on either side of the Atlantic form a perfect match. The pointy bit of Brazil fits snugly in the crook of western Africa. Rocks around the Congo River have long-lost siblings in the Brazilian city of Salvador, the land of samba and capoeira.
This Transatlantic jigsaw puzzle kicked off the greatest advance in geologic science: the idea of plate tectonics, that the continents break apart and crash into each other over millions of years. If you want to learn more about South America’s role in this discovery, check out Episode 11: Heretics and Heroes. When most people think about plate tectonics, one word comes to mind: Pangaea, the super-continent where dinosaurs roamed. But if we imagine plate tectonics like a long movie, Pangaea is just one recent freeze-frame. The continents have merged and split over and over again, just like an old rock band that can’t stay off the road.
Last week, we saw plate tectonics long before Pangaea, when a few large islands smashed together to form Africa. I didn’t lie to you back then, but I did leave out an important fact: some of those islands are now modern-day Brazil. For hundreds of millions of years, you could have walked from Rio to Cairo with no issue, except there would be no cities to visit, and there was almost no oxygen to breathe. Seasons came and went, other continents hooked up and broke apart, but Brazil and Africa were inseparable through October, November, and December of the Earth Calendar. This power couple eventually formed the heart of everyone’s favorite supercontinent, Pangaea.
Then came the volcanoes. Deep beneath Pangaea, a huge plume in the mantle rose to the surface as dinosaurs danced above. Like ice above a rising submarine, the brittle crust started to crack and push apart. The cracks grew wider, pushing the Americas away from Africa and giving birth to the Atlantic Ocean. South America is still plowing its way west an inch every year, mowing over the Pacific. This continental bulldozer forced up the Andes Mountains in mid December of the Earth Calendar. This is why South America’s oldest rocks are in Brazil and not Peru. So how old are these rocks, and what stories do they tell?
The ancient islands that built Brazil are called cratons, just like the oldest areas of Antarctica, Australia, and Africa. In this mini-series, our stories are shifting slightly younger with each continent: Antarctica’s were in February, 3.8 billion years ago, Australia’s in mid-March, 3.5 billion, South Africa’s in late March, 3.4 billion. Brazil also has rocks from late March, but 1: they cover less ground and 2: they’ve been highly altered by time, just like most of their African cousins.
The most well-studied area is the Sao Francisco Craton in southeastern Brazil. If you’re listening from Salvador on the coast, or munching on cheese bread in Belo Horizonte, today is your lucky day. The oldest rocks you can find are granites and their deformed siblings: gneisses. Granites are formed in old magma chambers below Earth’s surface, the backbones of the first continents. Gneisses tell us what happened afterwards, when minerals are cooked and squeezed so hard that they shape-shift. To learn more, check out Episodes 2 and 13.
Some of Brazil’s oldest rocks, gneisses 3.4 billion years old from the Sao Francisco Craton, southwest of Salvador (from Lopes et al 2021, Precambrian Research)
Sprinkled between these pale blobs of granite are smaller valleys of dark green rocks. These are called greenstone belts, and usually draw a lot more scientific interest. For example, in my interview with Nadja Drabon, she was working in the Barberton Greenstone Belt in South Africa. Greenstone belts tell tales of Earth’s surface: volcanoes, oceans, and if you’re very lucky, fossils. Many fossils in the Australia and Africa episodes were found in greenstone belts 3.5 billion years old. While South America’s oldest rocks are the same age, we won’t see fossils until much later 2.7 billion years ago, Season 5 in this podcast. Some are in Brazil, but the prize samples are actually in Uruguay to the south- fossil bacteria near Montevideo. We can’t let the Brazilians have all the fun here. I have a gut feeling that there are other Archean fossil sites just waiting to be discovered in the next few years, and I would love to help describe them one day.
While South America is short on Archean fossils, ancient life left other traces that are more economically relevant. Between April and June of the Earth Calendar (3-2.5 billion years ago), some bacteria stumbled across a new form of solar power. Life had been harvesting sunlight for billions of years, a process we call photosynthesis. These new bacteria found a way to combine two different solar harvesters, creating a lot more energy to live their lives. Why do we care? Because the waste-product of these new biological engines was oxygen. The bacteria weren’t looking to make a breathable world- in fact, oxygen is toxic to many microbes. But like it or not, these small cells were slowly changing their world.
The Carajas iron mine, the largest in the world, founded by the smallest organisms billions of years ago.
How does this tie in with Brazil’s economy? Iron. In the years before oxygen, invisible iron floated around in the oceans. But iron plus oxygen equals heavy rust, which sinks to the bottom of the ocean. The more oxygen the bacteria farted out, the more rust fell on the seafloor. Fast forward to today, and these rusty rocks form the largest iron mines on Earth. In Brazil, the Carajas Mine on the Amazon’s edge holds more than 7 billion tons of iron ore. An entire Brazilian state is named after the industry: Minas Gerais, the General Mines. From small things, big things grow.
In the Australian episode, we met iron deposits around 2.5 billion years old, halfway back in Earth history. The iron ore in the Carajas Mine formed around the same time, and if you held an Australian sample next to a Brazilian sample, both would look very similar. Each have bands of blood-red and shiny gray layers. Because these rocks are banded and iron-rich, geologists creatively call them banded iron formations, or BIFs for short. BIFs also exist across Africa, and we’ll run into them many times going forward.
Speaking of which, our brief time with South America is at an end. While the rocks are not as abundant as our previous destinations, I have a feeling that the continent still has some aces up its’ sleeve. Next episode, we’ll head to the northern hemisphere and tackle Earth’s largest continent: Asia.
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Thank you for listening to Bedrock, a part of Be Giants Media.
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Images:
South America craton map: https://upload.wikimedia.org/wikipedia/commons/thumb/0/0f/Cratons_West_Gondwana.svg/640px-Cratons_West_Gondwana.svg.png
Gneisses, modified from Lopes et al 2021: https://www.sciencedirect.com/science/article/abs/pii/S0301926821002618
Carajas Mine: https://commons.wikimedia.org/wiki/File:Isa_Carajas.jpg