This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. The Earth was formed just over four and a half billion years ago. What happened during its early history? What sort of climate existed then? What covered the Earth's surface, and was it liquid or solid? And what was it composed of? You might think that it would be impossible for us to answer questions about events that took place so very long ago.

John Valley is Professor in the Department of Geoscience at the University of Wisconsin. He's an expert on the early Earth. His research has shown that we can indeed get some answers to questions about the very early Earth. John Valley, welcome to Geology Bites.

John Valley: Thank you. Those are fascinating questions. I'm looking forward to discussing them with you today.

Oliver Strimpel: When we talk about the early Earth in a geological context, what do we mean?

John Valley: Well, the age of the Earth is a little bit more than four and a half billion years, and that's 4,500 million years, such a large number that it's sort of hard to wrap your brain around what it means. Most of what we know about the Earth is based on the last 500 million years. We have a lot of samples, and we know a lot about what's going on. This was the period when animals evolved on Earth, starting with the first hard-shelled trilobites, up until the modern day. By contrast, the first 500 million years of Earth history are often called the geological Dark Ages. We know comparatively little, in large part because we have no rock samples from the Earth of this age. But yet there are many important events that happened in the first 500 million years. After the Earth formed, the Moon formed, and the Earth was totally transformed. The Earth's metallic core separated from the silicate mantle, and the crust started to separate from the mantle and to thicken, eventually forming continents and oceans and setting the stage for life.

Oliver Strimpel: What's the very earliest we should, at least in theory, be able to see back to?

John Valley: Well, the Earth didn't actually form at a single instant. It accreted from large and small bodies in the solar system, and in fact, in detail, it's still accreting today, every time a meteorite lands on the Earth. But the growth rate is so slow. The majority, 90 to 99 percent, of the Earth formed within the first few million years to 100 million years. And so when we talk about the age of the Earth, it's when most of it formed. But there may have been important events later. We think that there may have been a late period of heavy bombardment about 3.9 billion years ago, which would have added material and perhaps changed the composition of the crust of the Earth. But it was largely formed at 4.55 billion years.

And then, shortly thereafter, the prevailing hypothesis is that the Moon was formed by the impact of a giant planet-sized body, which we now call Theia, perhaps the size of Mars, which struck such an intense blow on the Earth that it was almost destroyed. We almost never became a planet. A large amount of the Earth was vaporized, and from that vapor and dust circling the Earth, the Moon coalesced, and then the Earth was re-formed from what was left. So that's younger than the age of the Earth, but perhaps by only 50 million years.

Oliver Strimpel: So does that mean that the Moon-forming event is the farthest back we could ever see, because everything solid was essentially wiped out at that point?

John Valley: There's really no hope of finding something earlier than the Moon-forming event. And the oldest materials that have been dated on both the Earth and the Moon support that, returning ages of about 4.45 billion years.

Oliver Strimpel: OK, we'll come back to what we know now about the early Earth. But first I'd like to talk about how it is that we can find out about conditions and processes that occurred so long ago.

John Valley: Well, geologists are historians, and we read rocks like a book. So to study the earliest Earth, we need solid samples, preferably rocks that we can study, and there are no known rocks on the Earth older than 4 billion years. So there's this roughly 500-million-year period of time with no complete books. We do find older rocks on the Moon, and meteorites mostly date to the age of the solar system, at 4.55 billion years. So there are older materials in the solar system, but we don't find them on Earth, because the earliest rocks have been destroyed. The processes of destruction are plate tectonics, but also simpler things: exposure to rain and weathering, where rocks can become altered on the surface of the Earth. So the oldest rocks have been destroyed by a range of processes on the Earth.

The one clue that we find from the earliest period is minerals that are very resistant to destruction, in particular zircon. Once a zircon forms, it's very hard to destroy it. These are very tiny crystals that mostly form in igneous rocks. They solidify, or crystallize, from melts, from magmas. Once they form, the rock itself can be weathered and disaggregated. The parent rock can even be melted, and the zircon will not be destroyed. It will be preserved and then incorporated into younger rocks. And so we sometimes find very, very old zircons contained in much younger rocks, and this allows us to study those earlier events.

Oliver Strimpel: So, as you said, zircons form when silicate magmas cool and crystallize, which could be at depth, such as where granites form, or on the surface, when the magma is erupted as lava. And such zircons didn't just form in the early Earth; indeed, they're forming to this day in crystallizing magmas, and are fairly ubiquitous in igneous rocks. The largest ones are even used as inexpensive gemstones, as they look a little like diamonds. So what is it about zircons that makes them so incredibly useful for dating purposes?

John Valley: Well, zircon is an unusual mineral. It contains just three elements as major elements: zirconium, silicon and oxygen. But they arrange themselves in a crystal structure that has certain sites that allow the substitution of trace amounts of uranium. So uranium fits very nicely into this structure. But at the same time, the daughter element of the uranium-to-lead radioactive decay, which is lead, is much bigger, and it doesn't fit into the zircon structure. And so when the zircon forms, it will have no lead but a lot of uranium. As the zircon then becomes older, the uranium is spontaneously decaying to lead, and the lead-to-uranium ratio increases with time. Because we know the rate of decay, and we have very accurate and precise instruments for measuring these ratios, we can determine the age.

It's actually even better than that, because there are two isotopes of uranium that we can date. Isotopes are different forms of the same element that have different numbers of neutrons. So uranium-235 and uranium-238 are different because 238 contains three additional neutrons. And those two uranium parent isotopes decay to distinct lead isotopes, lead-206 and lead-207, at different rates. The half-life of the uranium-238 decay is about four and a half billion years, and the half-life of uranium-235, which is the isotope that one can make bombs from, is about 700 million years. So when you measure the uranium-lead ratios of the two different isotope pairs, you actually get two independent ages, and this allows you to check the accuracy, what we call concordance of the two ages. It's very important for establishing the reliability of this technique.

Oliver Strimpel: And so with these two clocks in one, we know they were both reset, if you like, at the same time, because for both of them, it's the crystallization age of that zircon crystal, which had only uranium and no lead when it originally crystallized.

John Valley: Exactly. And the crystallization of the zircon is the same as the crystallization, or the solidification, of the rock, and that's what we call the age of a rock. For instance, when a volcano erupts, the rocks then crystallize, and the zircons will record that age. So that's very important. Zircon is the gold standard for determining the ages of ancient events on the Earth. And in fact, there's even a third clock. There's a thorium-lead pair that also gives us an age that you can use with zircons, but we mostly talk about the uranium-lead ages.

Oliver Strimpel: So you've actually been to the place where these very old zircons were first really discovered.

John Valley: Yes, it's an interesting story. There's one place on the Earth where we find a very high concentration of zircons that date from what we call the Hadean Eon, which is the first 500 million years of the Earth. That place is in Western Australia, in a somewhat arid expanse of sheep stations that are about a million acres each, and the old zircons that we've studied come from a place called the Jack Hills. The zircons are found in what were formerly sandstones and pebble conglomerates in the Jack Hills. These were sedimentary rocks, laid down on beaches and as debris-flow deposits about 3 billion years ago. So a long time ago, but still quite a bit younger than the ages of the zircons themselves, which go back as far as 4.4 billion years. So the zircons were actually just grains of sand on the beach, and slowly the rocks hardened into a quartzite, which is what we bring back today.

The first time I went there, we brought back 1,000 pounds of quartzite on the airplane, 14 suitcases' worth, and we crush these rocks up. Zircons are a trace component, less than a part per thousand of the rock, which is mostly quartz. In fact, on almost any beach, a small number of the grains of sand will be zircons that are older than the beach itself, and those are the materials we date. They're totally disaggregated from their original rocks. We have no rock evidence to tell us their genesis, just what we can learn by studying the zircons themselves.

And the zircons are small. A typical zircon in this situation would be 100 micrometers, which is a little bit more than the diameter of a human hair. Human hair is about 80 microns in diameter, so it's about the diameter of a human hair and twice as long. If you had it on the palm of your hand, you couldn't see it without a lens. So we have to be very careful when we separate these zircons. But if you crush a kilogram of rock from the Jack Hills, from the right localities, you'll get a pencil point's worth of zircons, which is thousands of these little crystals.

Oliver Strimpel: So once you've got these tiny zircons, how do you measure the isotope ratios inside them?

John Valley: Well, there are a couple of different ways. In the past, this was done by taking hundreds or even thousands of zircons, dissolving them and analyzing them in bulk. But there are now microanalytical techniques using a special type of mass spectrometer called an ion microprobe. A mass spectrometer is an instrument that measures isotopes. It actually weighs ions one at a time. We charge the lead ions, and then we project them into a magnetic field, and they are deflected according to their mass. The lighter-mass lead atoms will be bent more and the heavier ones will be bent less, and so we can separate them, and then we can count them. We are basically weighing the lead isotopes and the uranium isotopes using this machine.

In order to analyze things with an ion microprobe, we take the zircons one at a time rather than en masse, and this is a huge advantage. We pick them up with a needle. They're so small that if you touch a needle to the side of a zircon, it adheres, and you can then place it down on sticky tape in rows. Then we cast them in epoxy, and once that's hardened, we polish and grind it down until we expose the center of each zircon. The zircons have grown concentrically. They look sort of like a hailstone, or tree rings, where the center is older and the outer rings are younger. With this instrument called an ion microprobe, we can focus a beam of either oxygen or cesium atoms to a spot that's about 10 or 20 microns in diameter, much, much smaller than the zircon itself. We then sputter material from a tiny pit in the surface, and that's the material that we then accelerate into our magnet to weigh the isotopes. So we're analyzing samples that are nanograms, little tiny, tiny domains of material. And we can determine the uranium and the lead isotopes, and we can actually measure many, many other isotopes and trace elements as well.

Oliver Strimpel: What sort of age accuracy can we obtain?

John Valley: The accuracy and the precision of ages are actually, unintuitively, better as you go further back in time, because of the way the ratios work. For ages earlier than about two billion years, we use the ratio of the two lead isotopes, which uniquely determines an age, and the analytical precision can be as good as 5 million years when you get back to the four-billion-year time scale. Now, accuracy is another question, because it's all based on how well we know the half-lives of the parent-to-daughter decay for the two uranium-to-lead pairs. Those were recently revised by less than a percent, but there have been recent revisions. Most people think that the accuracy is within millions of years. We actually are determining these ages exceedingly accurately.

Oliver Strimpel: You know, a million years doesn't seem like a lot, but when you compare it to four and a half billion, it's extraordinary how accurate that is, percentage-wise.

John Valley: Yeah, well, certainly a million years is easy for a geologist to say, as if it's not much, but you and I aren't expecting to live a million years. So on a human scale it's quite long, but relative to the Earth, that's a small percentage. And if your age is plus or minus five million years at four billion, that's quite good.

Oliver Strimpel: In addition to finding the crystallization age of these zircon crystals, can these instruments, or other measurements, tell us anything about the conditions under which these crystals formed?

John Valley: Well, there are actually several things that can be measured, and one that I've been very excited about is the ratio of oxygen isotopes. There are three isotopes of oxygen on the surface of the Earth that are what we call stable isotopes. They don't undergo radioactive decay, and so the ratio doesn't change with time, but they're very sensitive to the temperatures of processes. The two isotopes that we most commonly study are oxygen-16 and oxygen-18. Oxygen of mass 17 is less commonly studied on Earth. But the oxygen-18 to oxygen-16 ratio varies according to the temperature of processes, and so we can distinguish high-temperature events from low-temperature events using oxygen isotopes. This has also been very important with these ancient zircons.

Oliver Strimpel: What temperature-dependent process affects the ratio of oxygen isotopes in a zircon crystal?

John Valley: We talk about partitioning of isotopes, or of elements, or the preferential uptake of one isotope versus the other. Oxygen-18 and oxygen-16, because they're the same element, are almost exactly the same size as ions. The main difference is the two extra neutrons, which make oxygen-18 about 15 percent heavier, more massive. So the way the oxygen vibrates within the crystal structure, the vibrational frequency, will depend on which isotope you have, and this creates a preference for one isotope versus the other. The vibrational frequencies are quite temperature-dependent, and so this is the basis of what we call oxygen isotope thermometry. It's used in a wide range of geologic studies, including studies of past climate and Arctic ice records: the temperature of the climate changes the oxygen isotope ratio of the frozen water. We see it in marine shells, as seawater has changed temperature through time. And we see it in zircons from the early Earth. We see evidence of the temperatures of processes.

When my students and I first started studying oxygen isotopes in zircons some 30 years ago, the prediction was that we would get some old zircons, but we knew they were going to look just like the mantle, like the deep-Earth value. We didn't expect to see a difference, but we wanted to measure it, just to really nail that down, so we had a datum point to compare younger zircons to, where we were seeing evidence of interesting things. So it was a great surprise when we analyzed the very first Hadean zircon, zircons older than four billion years, to find that the oxygen isotope ratio is in fact a little bit enriched in oxygen-18, what we would say heavier, because it has more of the heavy isotope; a little bit heavier than the mantle of the Earth.

The only way that's been proposed that you can form these heavier oxygen isotope ratios is if you have a low-temperature process. At low temperatures, you get this preferential partitioning between, say, a mineral and liquid water. So we very commonly have high 18/16 ratios near the surface of the Earth because of these low-temperature processes, whereas in things that happen in magmas, because it's so hot, there's very little discrimination between the two different isotopes. And so the only way you can get these high 18/16 ratios is if there is a history on the surface of the Earth.

Oliver Strimpel: So how does that relate to zircons, which are forming in high-temperature magmas?

John Valley: The process is, in fact, quite common in modern rocks, where sediments will be weathered and will interact with rainwater and groundwater at low temperatures. Then, if they're buried, they can be heated again at depth within the Earth and melt, and they pass on their high oxygen isotope ratio to the melt. And when the melt crystallizes zircons, the zircons inherit that high oxygen isotope ratio as well. We see this all the time in younger rocks, and this is what we've proposed was happening all the way back to at least 4.3 billion years ago. We see evidence of these low-temperature processes, and what that means is that the Earth had cooled off by 4.3, and maybe by 4.4, billion years ago. It cooled off to the point where the steam atmosphere condensed and formed liquid water on the surface of the Earth, probably forming oceans, and these were below the boiling temperature of water and would have been clement, relatively habitable conditions that possibly could have fostered life.

Oliver Strimpel: But different from the fiery hell that Hades conjures up in the mind.

John Valley: Yes, we call the first 500 million years of Earth history the Hadean, which means hell-like. And when the Earth and the Moon formed, conditions really were hell-like. It was a very violent, fiery planet, covered with red-hot molten rock to great depths, and there would have been a much higher rate of meteorite impact. All of the Earth's water would have either been dissolved in these 1,500-degree-Celsius melts, or it would have been in the steam atmosphere. There was no liquid water, and certainly nothing would have been alive on the planet. So we call that the Hadean. But the question is, how long did the Hadean conditions really last, and how quickly did the Earth cool off after the Moon-forming event? What the zircons are telling us now is that the Hadean did not last 500 million years, as many had thought previously. It may have lasted just a few million years, or 50 million years, probably at most, before the surface of the Earth crusted over and cooled off to the point where you can have liquid water.

Oliver Strimpel: So do we know anything about the chemical composition, then, of the rocks, and whether some of them would not have been primitive mantle material but had already undergone some change through geological processes?

John Valley: This is a hot topic amongst geologists, and the question is, when were the first continents formed? We've just talked about when the first oceans formed. Some people think that continental land masses that rose above the oceans would have formed very early in Earth history, whereas others think that this process was delayed until the second half of Earth history. The continents are underlain by rocks that are chemically different from the rocks under the oceans. The oceans are low-lying because their rocks are dense. Continents are high relative to the ocean floor because they're made of rocks that are very rich in certain elements, principally silicon, and sodium and potassium, which make less dense granite, or granitic-composition rocks. And so we'd love to know if these zircons came from true granites, or even intermediate compositions that we might call granitoids, because that would be evidence that continents were starting to form as early as 4.3 billion years ago.

One of the hallmarks of a silica-rich, granitic, continental-crust kind of rock is the mineral quartz. Quartz is quite rare on the Moon and Mars, because those are undifferentiated planets; they're planets that did not separate a continental crust. And quartz is quite rare in the ocean crust, but the continents are full of quartz. In fact, your average beach sand is 90% quartz or more. And when we look at those zircons, even though they are these little tiny things that we can't see without a microscope, if we look at them with an electron microscope in the polished surface, we realize that included inside these tiny little zircons are other minerals. It's not just a zircon crystal; it's a time capsule. And one of the most common mineral inclusions that we find inside these zircons is crystals of quartz, and sometimes feldspar, which is another common mineral in granites. This has led us to suspect that these zircons are actually forming from, well, I would say granitic-composition rocks, maybe not true granites, but rocks that are close to granites in composition. And this means that the Earth was beginning to undergo this chemical differentiation. It was beginning the process whereby continents are separated from the mantle as early as 4.3 or even 4.4 billion years ago. We find quartz inclusions all the way back to the oldest, 4.4-billion-year-old zircons we've seen from the Jack Hills.

Oliver Strimpel: Does this chemical differentiation also have any bearing on when the Earth might first have become habitable?

John Valley: Well, there are several questions there. When we say habitable, we're talking about microbial life, single-celled organisms, probably what are called archaea, that like heat. First of all, you would have to have liquid water to form organisms like that, so the Earth has to have crusted over to become insulated from its hot interior. Elements like phosphorus tend to be concentrated by the same processes that separated the continents, and phosphorus is an essential ingredient for life. And so there are a number of steps here, but probably the most important step is the formation of these bodies of liquid water on the surface of the Earth.

Oliver Strimpel: Which actually brings me to my last question. There are clearly quite a few unanswered questions about this very early period of the Earth's history. So if I could actually teleport you right back to the Hadean to have a look, what would be the first thing you'd try to find out?

John Valley: Well, the first thing I'd do would be to put on my scuba mask, because the Earth's atmosphere was very poisonous for the first two billion years, so I would have to take my own oxygen. But right away, I would seek out bodies of liquid water, and just like the hot springs at Yellowstone, I would look for thermophilic organisms, like archaea, that might be living in those bodies of water. That would be a hugely exciting thing to know about.

Oliver Strimpel: John Valley, thank you very much.

John Valley: Thank you, Oliver. It's been fun.

Oliver Strimpel: For more about Geology Bites, as well as pictures and diagrams that illustrate this podcast, you can go to geologybites.com.