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There's a beach in Hawaii that shouldn't exist, geologically speaking. The sand is green. Regular sand is made of lots of different materials, but the most common ones are pieces of the minerals quartz and feldspar, plus shells and fragments of other stuff. Papakōlea Beach in Hawaii, however, is made of something else. Fragments of a mineral called olivine. Finding pieces of olivine that are big enough to see with the naked eye is somewhat uncommon, so you can imagine how rare it would be to find an entire beach made out of it. And the story of how so much of this mineral got in one spot and formed a giant green beach actually begins far below the beach itself.
[♪ INTRO] Like the other minerals that make up sand, olivine doesn't start off small and sand sized. Bigger pieces of it had to be broken down into those tiny pieces over time. It takes a very specific set of events for olivine to make its way to Earth's surface at all. And when it manages to do so, it's usually found inside a kind of volcanic rock called basalt. Globally, most basalts don't have large, visible olivine crystals. If there are any, they're usually really tiny … like microscopically tiny most of the time. And when bigger pieces of olivine do manage to make their way to the surface,
they don't usually last very long. This makes the odds of them forming an entire beach pretty low. To understand why big bits of olivine often don't make their way to the surface, we need to travel to where most olivine on the planet actually is, and that's in the Earth's upper mantle. In fact, over half of the upper mantle is olivine, where it's part of a rock called peridotite. The Earth's upper mantle ranges from just below the crust to as deep as about 400km. And this puts peridotite under a LOT of heat and pressure.
It's not pure liquid, though. It's still semi-solid but under certain conditions it will start to melt. When it melts, it forms magma, and that magma will start to make its way to the surface. This journey to the surface makes it start to cool again, and this causes minerals to slowly crystallize out of the magma. The first mineral to crystallize out of most magmas is olivine. But those olivines that crystallize are much denser than the magma they're crystallizing in, so as time goes on, they end up sinking to the bottom of the magma reservoir.
Now different minerals crystallize and separate from the liquid as the magma continues to cool, and this is a process that geologists call fractional crystallization. The longer the magma sits in the reservoir, the more time all of the olivine has to settle to the bottom. If a magma reservoir then ends up erupting after sitting there for a really long time, the lava that comes out probably won't have much olivine in it because it's coming from the top part of the reservoir first and all of the olivine is at the bottom by now. But, if the magma reaches the surface really fast, before the olivine has settled,
then more of it can reach the surface during an eruption, carried in a host rock of basalt. This rock from Iceland is a good example. It's a piece of something called an Olivine Picrite Hyaloclastite, and I was so excited when I found that I actually started crying, which also why I put a picture of it on my sweater vest. This rock is not for Rocks Box, though. This one is mine, so you can't have it, but you can look at it. The olivine picrite part of the name just means that it has a TON of olivine in it, and the hyaloclastite part means that it erupted
underneath a glacier and forms a kind of glassy rind on the surface. Not all magma can reach the surface fast enough to become an olivine picrite, though. Most magma sits around in the crust for a while before erupting. And even if a volcano does manage to erupt lava with a ton of olivine, the amount of olivine can change even in the middle of an eruption. There was an eruption in Iceland in 2021 where only the earliest lava that erupted contained any visible olivine crystals. So the early lavas formed from deeper, hotter magma that traveled
really quickly to the surface and allowed the olivine to make the journey with it. This is actually the eruption I have tattooed on my arm. I visited the site in 2021 and got this sample, which does have some olivine, from right about here. But the lava that came from the eruption later on looked completely different, and had almost no visible olivine. Like this one, which was collected right here. By the time the last lava reaches the surface, the olivine has settled out already.
Plus, the original magma had mixed with magma from a different reservoir with less olivine in it. While this makes it harder for olivine to get to the surface, it also makes it a really valuable tool for volcanologists. Olivine is so sensitive to the timing of an eruption that we can use it to help us understand how long the magma that carried it sat in the reservoir before it erupted. This is actually something I'm doing with my own rock samples for my PhD. I study rocks called PUBs or plagioclase ultraphyric basalts, which is basically just convoluted geology speak for having a lot of these big white plagioclase crystals in it.
However, they do also have a TINY bit of olivine in them, and by calculating how long the olivine sat in the magma before it erupted, we're able to figure out how long the lava as a whole was stored in its magma reservoir. We were also able to figure out that the magma that made this rock wasn't just stored in one place. It started out in a really deep magma reservoir, and then moved to a slightly shallower one, and then moved again to an even shallower one before finally erupting at the surface. And it was possible to learn all of that by looking at the chemistry of both the olivine and the plagioclase in the rocks. So olivine is essentially like a speedometer for the eruption.
But now that we've got these olivine crystals up on the surface, how do they become part of a green beach? Like everything else in geology, it's all about location. See, the olivine on that beach erupted from the Hawaiian volcano Mauna Loa, and Hawaii is well known for having lots of olivine rich volcanic eruptions. Basically, the magma beneath the Hawaiian islands usually erupts really quickly, too quickly for the olivine to settle to the bottom of the reservoir or mix with anything else.
This allows all that beautiful olivine to actually end up in the volcanic rock at the surface. Crystals of olivine are pretty susceptible to erosion, but the basalt they're found in is even more vulnerable to it. So when a volcano is located on a coast, the basalt that forms the volcano itself is going to get worn away. If you've heard of the famous black sand beach in Iceland, that black sand is made of eroded bits of volcanic rock and it does have some olivine crystals in the sand there, but it's nothing compared to Papakōlea Beach. The reason for this comes down to the shape of the bay the Hawaiian beach is located in. On other volcanic coasts, like Iceland, most of the olivine is going to get carried out to sea and sink,
just like it does in the magma reservoir. But at Papakōlea Beach, the half moon shape of the bay creates a circular wave pattern that keeps bringing the dense olivine back up onto the beach every time it sinks. This concentrates it in a way that isn't seen anywhere else in the world. This beach only exists thanks to a geographic quirk and a well-timed volcanic eruption. Pretty cool, right? And if you're a Rocks Box subscriber, you'll get your own sample of olivine in basalt this month! Now, they are not from Papakōlea Beach. That's a protected landmark, and taking rocks from there is illegal.
So instead, we found some ethically and legally collected samples you can have at home. You could even theoretically do the same chemical analysis that I did to find out how long yours took to erupt … if you can manage to get the grant money for that kind of thing. Every month, Rocks Box subscribers receive a piece of geologic history in the mail. Whether they're minerals, rocks, or fossils, each item teaches you about our planet or what has lived on it in some way. If you've been wanting to join the Rocks Box club, some new subscriptions just opened up and you can find them at Complexly.store/rocks.
Or if you prefer to pick out your own, we also have minerals for individual sale and plenty of other merch, like t-shirts, jeweler's loupes, and stickers. To learn more about the Rocks Box or just go rock shopping, visit Complexly.store/rocks. Thanks for watching! [♪ OUTRO]
