12: Scratching the Surface

Last week, we learned how the idea of continental drift evolved from a scientific heresy into the cornerstone of modern geology, through the eyes of two scientists: Alfred Wegener and Marie Tharp. They thought that the Earth’s continents slowly drifted on top of the mantle like lilypads on a pond. Both were ridiculed by their colleagues, but Marie Tharp and others discovered that mid-ocean ridges were slowly spreading, pushing continents away from each other.

Before we go further, let’s take a step back. This is a show about Earth’s Earliest History, so why this detour to the modern day?

In Episode 10, we met the oldest thing on Earth, a small, dark purple zircon crystal from Western Australia, 4.4 billion years old and tough as nails. This zircon formed in the early crust, and to understand what the world looked like back then, it’s best to have a solid foundation in the modern crust.

Today, we’ll learn what the crust is made of, how it’s born, dies, and the changes in between. When this episode’s over, we’ll finally be able to compare our familiar modern world with the strange days of the Hadean.

Just like different types of pizza, there are two types of crust on Earth: thick continents and thin oceans. Listeners in Chicago will be delighted to hear that they live on thick crust. Listeners in New York will be horrified to hear that they too live on thick crust, but they can take comfort in knowing that thin crust is just a boat ride away, deep under the Atlantic Ocean. In fact, let’s start our journey in the Mid-Atlantic Ridge, where new crust is being born as we speak.

Part 1: The Seams of the Earth

You will often hear that the Mid-Atlantic Ridge is the longest mountain chain on Earth. Well, yes and no- the story is actually far more impressive. The Mid-Atlantic Ridge is just one part of a series of underwater volcanic chains stretching through every ocean. Together, they form the Mid-Ocean Ridge, which is truly the longest mountain range on Earth, 80,000 km. From above, these ridges look like stitches on a baseball, or train tracks on a global metro system. For the rest of the episode, I would recommend having a world map in your hands or in your brain, since we’re going to do some globe-trotting today. There are some maps on this episode’s web page if you want a more detailed view.

One end of the Mid-Ocean Ridge is around the North Pole, cutting south through Iceland to form the famous Mid-Atlantic Ridge. The mountains then curve around southern Africa and Australia through the Indian Ocean, with one side branch up the Red Sea. The main ridge tours the South Pacific before hooking north by Chile, and finally threading through the Gulf of California.

The length of the Mid-Ocean Ridge is impressive, but to geologists, it’s more important for another reason: nearly every new piece of oceanic crust is born at a mid-ocean ridge. To find out how, we need take a look under the hood, in the mantle just below.

Because the mantle is thousands of degrees hot, it’s easy to forget that it’s actually made of solid minerals. In the battle between high temperature and high pressure, pressure wins out deep inside the Earth, keeping the hot elements locked tight as crystals. But the closer to the surface, the lower the pressure gets. You can feel something similar in your ears when you swim up from the deep end of a pool or ride an airplane.

Eventually, the pressure is low enough that the hot mantle starts to melt into liquid magma. The new magma wants to keep rising and escape to the surface, where it can turn into lava. Like any good escape plan, the best exit is where the defenses are weakest, where the crust is thinnest- the rift cracking through the middle of Mid-Ocean Ridges. These were the valleys that Marie Tharp first saw in 1952, the features which helped clinch the idea of plate tectonics.

Ah, after millions of years the lava is free! Time to cover the Earth in crust!

Except, the new lava doesn’t get very far. The bottom of the ocean is nearly freezing. In a few seconds the cold, cold water turns the lava into a dark, lumpy rock.

This rock is called basalt- we briefly met it in Episode 2, when we were listing the common rocks on Earth. Let’s get to know it a little better. Basalts are usually black or dark gray, making them easy to identify in the field. That dark color comes from the mineral pyroxene, which we met on the moon in Episode 9. Other minerals in basalt include our old green friend olivine, and our new pale friend plagioclase. Don’t worry too much about the mineral names for now, we’ll come back to them at the end of the episode.

As we just saw, basalt is a volcanic rock- it forms when lava cools on Earth’s surface. That means when you find basalt, you’re looking at an ancient lava flow. Basalt can also form on dry land, but these spots just small dark freckles compared to ocean basalts, which cover most of the seafloor. This is because Mid-Ocean Ridges are constantly pumping out basalt into the ocean like a giant printer. The new basalt shoves its older siblings off to the side, before it is then shoved off by the next generation.

OK, so we know that oceanic crust is made of basalt, which is born at mid-ocean ridges. So where does all that crust go? To answer that question, we have to travel to the other side of the world, the Pacific Ocean.

2: Trench Warfare

Mid-ocean ridges divided the Earth into eight major tectonic plates and dozens of smaller ones, roughly one for every continent. Almost every plate has new crust added each year thanks to these mid-ocean ridges.

You might ask “Wait, if Earth keeps printing out crust at ridges, why doesn’t the planet get bigger every year, like an inflating balloon?” If so, you’re not the first person. Charles Darwin came up with a similar idea on his famous Beagle Voyage in the 1830s, though he eventually changed his mind. Remember Marie Tharp’s supervisor, Bruce Heezen? He also thought that the Earth was expanding before Marie finally convinced him otherwise. It’s hard to believe, but only sixty years ago the Expanding Earth hypothesis was a serious topic in geology.

The reason it’s wrong can be found just north of New Zealand. The islands sit on top of a death struggle between two tectonic plates.

In blue corner is the Pacific Plate, the largest plate on Earth. It is also the only major plate to have almost no continental crust at all, with the notable exceptions of southern New Zealand, Baja California, and Los Angeles. Some pretty good slices of Earth there.

In the red corner is the Australian Plate, home to Australia, southern New Guinea, and the rest of New Zealand. Also, not a bad slice of the planet. A lot more continental crust here, folks- but for tonight’s tussle, it’s oceanic vs. oceanic, basalt on basalt.

These two plates were printed out at mid-ocean ridges hundreds of kilometers away just to meet here, and there can only be one survivor. Before placing final bets, one last note: around New Zealand, the Pacific crust is 100 million years old, while the Australian Plate facing it isn’t a day over 40 million.

So, who will win? Will size and experience make the Pacific Plate the champ? Or will the Australian Plate float like a butterfly and sting like a bee?

Floating like a butterfly is actually a pretty good analogy. Around New Zealand, the young Australian plate is lighter than the old heavy Pacific and rises to the top. The losing Pacific, meanwhile, gets pulled deeper down, melting into hot magma before getting recycled into the mantle. For a less dramatic re-enactment, take two carpets in your home and push them against each other. Eventually, one should slip beneath the other- usually the heavier one.

Scientists call this sinking process subduction, from the Latin for “to lead underneath”.

As the old Pacific Plate subducts down, the ocean forms a deep linear crack- the Kermadec Trench. Every trench under the sea, including the famous Mariana Trench, is where a tectonic plate is slowly being dragged back into Earth’s underworld. If mid-ocean ridges are printers, trenches are the shredders of the crust.

That might sound bleak, but on Earth destruction and creation often come hand in hand. As the old oceanic plates start to melt, a few pieces escape and transform into a completely different type of crust.

Part 3: From Surf to Turf

On a world map, try to find some trenches in the ocean- the Pacific is your best bet. Some examples are the Aleutian Trench, the Japan Trench, and the Tonga Trench. As you spot more trenches, you may begin to see a pattern. All these places have curved strings of volcanoes popping out of the ocean- scientists call these arches volcanic arcs. This pattern is not a coincidence- trenches always form volcanic arcs but how?

Let’s return to the Kermadec Trench north of New Zealand. As the old Pacific Plate sinks deeper down, it starts to melt back into magma. As we saw in the Mid-Atlantic Ridge, that hot magma wants to rise back up to the surface, it wants to break free. And eventually, it does- into a volcanic arc. But as with most sequels, this second rising isn’t quite the same as the first time.

Remember, a mid-ocean ridge pulls the mantle up from the depths of the Earth. But a trench pulls surface material from above- old basalt with a coating of deep-sea mud, soaked through with water. The ingredients are different, so the end-result is also going to be different.

There’s one more major difference- as they say in the real-estate business: location, location, location. At a mid-ocean ridge, magma has an easy path to the surface- up through a crack into cold ocean water. Back in our prize-fight at the Kermadec Trench, the sinking, melting Pacific Plate now has to claw its’ way up through the overlying Australian Plate before it can see the surface. This slow crawl upwards takes time, and as any cook knows, changing the baking time will drastically change your recipe. By the time the molten rock erupts on the surface in a volcanic arc, it is a very different material than before.

As the magma slowly cools, certain crystals form first- our friends olivine, plagioclase and pyroxene. These early crystals gobble up the tastiest elements from the magma: iron, magnesium, and calcium. Any minerals that form after have to use other elements. It’s like coming late to a party and everyone has already taken the best snacks or the best drinks- and you just have to make do with what’s left.

Eventually, a completely new series of minerals crystallizes from the magma. These minerals create rocks on volcanic islands that look nothing like basalt. I’m not going to cover all these other minerals here, but I will introduce you to the final stage, the last crystal to cool down from magma. You might not have heard of the first stages before this show: olivine and pyroxene. But I’m very sure you know this one: its’ name is quartz.

Quartz has a very simple chemistry: silicon and oxygen- the only ingredients left as the magma cools. Quartz is the second most common mineral on Earth’s surface, after plagioclase. But there’s a reason almost everyone knows quartz and not plag- quartz is far more common in continental rocks, ones we see every day. Quartz comes in a variety of colors, from smoky to rose to amethyst. But for now, to keep things simple, I want you to think of quartz as white.

Why? Well, let’s return to an island volcano. In fact, let’s head to the island named Vulcano, in the blue Mediterranean just off Sicily. This is the original volcano, the island which has lent its’ name to every fiery mountain across this planet.

As the episode winds down, let’s relax and imagine ourselves on a nice beach on the island of Vulcano. As we sip some Marsala wine and watch the sunset over Sicily, we have two different coasters: mine’s made of basalt from the old oceanic crust. Yours is a rock from this new volcanic island. This rock is called rhyolite, but that won’t be on the test for now. The important thing is that it is definitely not basalt. Let’s count the ways.

First, you’ll notice a color difference: the basalt from the ocean floor is black, while the rhyolite from the island is light gray. If you hold both in your hands, the basalt is heavier than the rhyolite. Both these simple, observable facts are related to the amount of quartz in the two rocks. Quartz and its friends are lighter in color and less dense than olivine or pyroxene. And that density difference is important. If we threw both our coasters into lava, your quartz-rich island rhyolite would easily float like a cork, while my dark oceanic basalt would just barely stay at the surface.

The same thing happens on top of Earth’s mantle: dark, dense oceanic crust is a low-rider, forming huge pits for the oceans to sit in. In contrast, light quartz-rich rocks float higher, forming volcanic islands which stick above the water. You know what else sticks above water and is quartz-rich? Continents.

So how do we make continents from islands? Did this happen all at once, or slowly over time?

Now we’re asking some very interesting questions, questions that bring us back to where we started in Episode 10, looking at the oldest stuff on Earth. Because if we take this new, pale island rock and look at it with a microscope, we would see a familiar face. A small, dark purple crystal, tough as nails- a zircon, just like the Jack Hills zircons, 4.4 billion years old.

Summary:

Earth has two types of crust: the thick continents and the thin oceans. Oceanic crust is born at mid-ocean ridges, printing out dark, dense basalt. Eventually, this basalt is pulled back down into the mantle at trenches. As these dying rocks melt into magma, some escape their fate in the underworld, rising up to form new volcanic islands and eventually continents. These islands and continents are made of different rocks, lighter, whiter, and quartz-rich.

Next episode, we return to our main story, 4.4 billion years ago. Now that we know how the modern crust works, it’s time to see what the first crust looked like. Was it oceanic? Continental? Or something completely different? The answers to these questions are trapped inside the Jack Hills zircons, like a message in a bottle. Join us next time to start deciphering these ancient clues.

Thank you for listening to Bedrock, a part of Be Giants Media.

If you like what you’ve heard today, please take a second to rate our show wherever you tune in- just a simple click of the stars, no words needed unless you feel like it. If just one person rates the show every week or tells a friend, that makes us more visible to other curious folks. It always makes my day, and that one person could be you. You can drop me a line at bedrock.mailbox@gmail.com. See you next time!

Images:

Ocean Floor Map: https://commons.wikimedia.org/wiki/File:(Manuscript_painting_of_Heezen-Tharp_World_ocean_floor_map_by_Berann).jpg

Mid-Ocean Ridge Diagram: https://commons.wikimedia.org/wiki/File:Mid-ocean_ridge_cut_away_view.png

Kermadec Trench: https://commons.wikimedia.org/wiki/File:Map_of_the_Kermadec_and_Tonga_subduction_trench.jpg

Ridge and Trench Diagram: https://commons.wikimedia.org/wiki/File:Subduction.svg

Vulcano Crater: https://commons.wikimedia.org/wiki/File:Vulcankrater,_Vulcano-.JPG

Rhyolite: https://commons.wikimedia.org/wiki/File:PinkRhyolite.tif

Basalt Outcrop: https://commons.wikimedia.org/wiki/File:Pillow_lava_at_Oamaru.jpg

Basalt Sample 2: https://commons.wikimedia.org/wiki/File:Ice_Springs_basalt_is1.jpg

Music:

Pizzicato Dance by Haim Mazar

24 Caprices for Solo Violin by Niccolò Paganini, performed by Elias Goldstein and Christina Lalog

https://commons.wikimedia.org/wiki/File:Paganini_Caprice-24.ogg

This New Life by Big Score Audio

Hebrides Overture, Fingal’s Cave: https://commons.wikimedia.org/wiki/File:Mendelssohn_-_Hebrides_Overture_Fingal%27s_Cave.ogg

Retro Wave by Remember the Future

Elevator by Gangsterdave: https://commons.wikimedia.org/wiki/File:Elevator.ogg

Elfentanz by David Popper, performed by Hans Goldstein: https://commons.wikimedia.org/wiki/File:David_Popper_-_Elfentanz_(Hans_Goldstein,_cello).ogg

Catacombs by Big Score Audio

It Was All Italian by Brother Roy

Seven Days of Flying by Remember the Future

11: Heretics and Heroes

13: Message in a Bottle