Sydney Lufkin (2:43)

Today on the show, the hidden ways Earth's geological processes have shaped the human life, from our landscapes to our science to our culture. You're listening to Short Wave, the science podcast from npr.

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Sydney Lufkin (4:06)

Human life are made possible by rocks, which you think of as a finite resource.

NPR Announcer (4:12)

But.

Sydney Lufkin (4:12)

But a lot of us don't necessarily think about it that way. So what are the consequences of not thinking about it that way?

Anjana Katwa (4:20)

Well, rocks are strange things, aren't they? Because they surround us and sometimes they're invisible from view because they're underneath our feet, but where we can see and experience them, they have a sense of solidity in our environment. They have a sense of longevity. So almost they. They become invisible in front of our very eyes. Now, for me, the rocks are ancient sty story keepers of how our Earth evolved. But they also contain human experiences. And I think the minute we begin to think of them as these ancient story keepers of time, but also memory keepers of the experiences that we had as we've interacted with them, they take on a whole new life and meaning.

Sydney Lufkin (5:03)

You're holding a rock right now. Tell me about the rock.

Anjana Katwa (5:06)

In my hand is a beautiful rounded pebble of Louisian gneiss. And it comes from the Isle of Iona in the Western Isles of Scotland. It's a greyish rock and stretched across its surface are bands of pink and white crystals which are quartz and feldspar. Now, a gneiss is a metamorphic rock, which means it's formed from another source rock. Now, if we were to take this rock and ground it down into a fine powder, what we would be looking for is a crystal called zircon.

Sydney Lufkin (5:38)

Yeah, I wanted to ask you about this because when I think of geologists using rocks to decode history and understand how the Earth has changed, I think of them looking at layers of rock. But scientists can use this zircon you mentioned to understand the age of the rock. How do they do that?

Anjana Katwa (5:56)

Zircon is the superhero of minerals and they tend to be found in some of the oldest rocks on Earth. They reveal the age of the Rock because at the time of formation, and in my hand I'm holding a piece of Louisian gneiss that dates back to 2.7 billion years old. This rock formed when the Earth was in its infant stages. And as the magma cooled down and began to form a granitoid, the zircon became locked inside that rock. Now, the radioactive elements inside that core of zircon recorded that moment. And so what geologists can do is that they can extract the zircon and by calculating the half life decay of that radioactive element, they're able to establish the age of the rock.

Sydney Lufkin (6:41)

Okay, so you wrote that this zircon method has been used to date the oldest rock ever found at 4.4 billion with a B years old. The Earth is only 4.55 billion years old. So that rock was from a very young Earth, a baby Earth. What does scientists know about what the Earth looked like then?

Anjana Katwa (7:02)

4.4 billion years ago, the Earth was a heaving mass of semi molten magma. And it was a place that geologists have quite described as hell on Earth. This was a place where magma was beginning to settle out and form some of the earliest continents that are known. And they were actually called cratons, formed from cooling magma that originally formed a very primitive rock known as a granitoid. But this environment where this magma was cooling down, these rocks were devoid of any life forms, and water hadn't yet quite arrived by then. So we're thinking of a world that was, was very, was very hot, it was very unstable. And as these cratons began to develop, these rocks would essentially form part of a very barren landscape.

Sydney Lufkin (7:58)

And then how do we piece that together from just the rock?

Anjana Katwa (8:02)

Well, because we're looking at the crystalline structures of the rock, we're looking at the texture of the rock, and then the geochemistry of the zircon tells us how old it is. There aren't any fossils, there aren't any kind of bedding structures that we see in sedimentary rocks because water hasn't arrived yet on Earth. And so all of this information comes from the chemistry, the structures, and the texture of the rock.

Sydney Lufkin (8:25)

So as someone who grew up learning about plate tectonics in grade school, I was surprised to find that the theory for how they work was only really proven in the 60s. So can you first briefly just paint a picture of what the world's tectonic plates actually look like?

Anjana Katwa (8:41)

Well, the world is made up of a jigsaw of all sorts of different types of tectonic plates. There are two main types There, there are continental plates that are quite buoyant, and they sit upon the mantle. Then there are the oceanic plates that are thinner and denser, and they sit beneath our oceans. Now, when a continental plate comes into a meeting with an oceanic plate, something extraordinary happens, because the oceanic plate is denser, but it's also weaker. And so it sinks beneath the more buoyant and thicker continental crust. And this leads to all sorts of activity, tectonic activity, such as the birth of volcanoes all the way through to earthquakes and tsunamis.

Sydney Lufkin (9:28)

Okay, so in 1957, Marie Thorpe and Bruce Heezen started mapping the Atlantic Ocean floor. Can you tell me what they found and how it supported this plate tectonic theory?

Anjana Katwa (9:40)

Their careful research of the profile of the seabed at the Mid Atlantic Ridge revealed all sorts of incredible ridges. And what they could do was they plotted an incredible line, a ridge at the bottom of the seafloor that basically indicated that the ocean floor was spreading apart. So there was almost like a zebra pattern of rocks being formed as the ridge was diverging, as it was spreading apart. And so lava and magma upwelling from the middle of the ridge would erupt onto the sea floor. And then later on, as the ships passed over the Mid Atlantic Ridge, the navigators saw something so strange happening. With their compass needles, they were flicking backwards and forwards, and what they were actually picking up was the changing polarity of the rocks that were deep beneath them.

Sydney Lufkin (10:31)

Oh, wow. So there's also a great story in the book about how geology, in some ways, shaped human history. The story was about chalk. Can you tell me about that?

Anjana Katwa (10:42)

Chalk is such an incredible rock, because not only is it such a beautiful, dazzling white color, but when I'm holding it, I feel like I'm holding death right in the palm of my hand. Now, that sounds really gruesome, doesn't it? But essentially, chalk is an organic rock. It's made up of the trillions of microorganisms that used to live in the oceans between 140 to 60 million years ago. And as they thrived in these tropical oceans, their calcium carbonate skeletons would collect on the bottom of the sea floor, so cement together to form this dazzling white rock. Now, in the southern United States, there is an arc of chalk that stretches across the southern states of Alabama, Georgia, and essentially, as the chalks weathered down, they formed a dark, rich soil that became known as the Black Belt. Now, the soils became a very prime area for growing crops like cotton by planters who brought in enslaved people from Africa to work on the plantations. When we look at the demographics today. It's heavily influenced by that history. In 2002, Stephen Dutch, professor emeritus of Natural and Applied Sciences from the University of Wisconsin, Green Bay, created a series of maps and we still see a reflection of that geological belt of chalk, reflected in the voting patterns and reflected in the demographics of the region.

NPR Announcer (12:14)

Wow.

Sydney Lufkin (12:15)

Okay, so this is my last question for you. What can studying the geology of the past tell us about the present and the future?

Anjana Katwa (12:25)

Geology offers us incredible insights into how our Earth has evolved and changed. Rocks are extraordinary storykeepers of this wisdom, and when we look at them, we can understand that life is actually a cycle of creation and destruction. Some of these rocks have taken tens of millions of years to form, and they've been brought up to the Earth's surface. So we are privileged enough to look at them, to access all of the incredible gifts they've given us. And using that wisdom is really critical as we move forward with all of the challenges that nature is posing to us, from climate change to mineral consumption to extraction and exploitation of the natural world. We live in such a delicate cycle where our impacts and actions have serious repercussions on the planet. And I think when we look at rocks, they are solid, they are stable, but they're also incredibly vulnerable because once we modify them, once we extract them from the Earth, their story is gone forever.

Sydney Lufkin (13:36)

Anjana's book, the Whispers of Rocks, is out now. Thank you so much for joining us today. I really appreciate it.

Anjana Katwa (13:43)

Thank you so much, Sydney, for having me along.

Sydney Lufkin (13:48)

This episode was Produced by Burleigh McCoy, edited by Brent Baughman, and fact checked by Tyler Jones. The audio engineer was Robert Rodriguez. Beth Donovan is our senior director of podcasting strategy. I'm Sidney Lupkin. Thanks for listening to shortwave from npr.

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