Short Wave - The Secrets Everyday Rocks Keep

Episode Date: November 4, 2025

Why does the New York City skyline look the way it does? In part, because of what happened there 500 million years ago, says geologist Anjana Khatwa, author of the new book Whispers of Rocks. In it, she traces how geology has had profound effects on human life, from magnetism of the ocean floor to voter trends in the Southern U.S.Interested in more geology episodes? Email us your question at [e-mail] to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. Hi, shortwavers, Sydney Luckin guest hosting today. If you were to look out over the New York City skyline, Angena Katwa says you might notice something interesting. What you see are clusters of skyscrapers all over Manhattan. Clusters. That's the interesting part. There are some in midtown, some are downtown, but not in between. As a geologist, Angina says,

part of the reason it looks like this has to do with what happened 500 million years ago when New York was sitting atop an ancient continent called Laurentia. Opposite side of an ancient ocean was another continent called Gondwana Land. Now over the course of about 150 million years, these two continents gradually moved towards each other, closing the ocean, and the ocean sediments beneath were squeezed and pushed upwards. And as these two continents collided, the landscape was transformed into towering mountains. Over time, these towering mountains were weathered down until just their bases were left, underground but near the surface.

And this, Angina says, is where New York skyscrapers are rooted. And those particular clusters of skyscrapers congregate around hard bedrock known as the Manhattan Shist. For Angina, rocks contain stories. And those stories explain our world, from the fossil fuel revolution to the taste of your coffee, which if your coffee beans grew near a volcano, could be different. Antony's new book, The Whispers of Rock, is rooted in the idea that we've always been searching for the deeper meeting in what's deep underground and sometimes hidden in plain sight. In each chapter, she mirrors the science with indigenous knowledge, showing how geology is literally

foundational to human life. Like the story from the Cherokee Nation about the great buzzard's quest. As the buzzard flew across the land, he grew very tired over these long distances, and as he flew, he flew closer to the ground. As his wings beat downwards, he actually created the valleys of the mountains, ranges.

And as his wings beat upwards, the land rose and created mountains. Today on the show, the hidden ways Earth's geological processes have shaped human life from our landscapes to our science to our culture. You're listening to Shortwave, the science podcast from NPR. So many parts of human life are made possible by rocks, which you think of as a finite resource, 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? 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 an experience them, they have a sense of solidity in our environment. They have a sense of longevity. So almost they become invisible in front of our very eyes. Now, for me, the rocks are ancient storykeepers 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 have had as we've interacted with them, they take on a whole new life and meaning. You're holding a rock right now. Tell me about the rock. In my hand is a beautiful, rounded pebble of Louisiana Nice, 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 felspar. Now, a nice 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. 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? 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 Louisiana nice 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. 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? 4.4 billion years ago, the Earth was a heaving mass of semi-multon 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 very, was very hard.

It was very unstable. And as these cratons began to develop, these rocks would essentially form part of a very barren landscape. And then how do we piece that together from just the rock? 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. 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? 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 crows. 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. Okay, so in 1957, Marie Thorpe and Bruce Heisen started mapping the Atlantic Ocean floor. Can you tell me what they found and how it supported this plate tectonic theory? 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 seafloor 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. 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? Chalk is such an incredible rock because not only is it such a beautiful dazzling white colour, 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, isn'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 seafloor 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. Wow. 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? Geology offers us incredible insights into how our earth has evolved and changed. Rocks are extraordinary storykeepers of this wisdom. And we 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. 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. Angela's book, The Whispers of Rocks, is out now. Thank you so much for joining us today.

I really appreciate it. Thank you so much, Sydney, for having me along. This episode was produced by Burley McCoy, edited by Brent Bachman, 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.