We will discuss much more about volcanoes soon. For now, note that Crater Lake sits atop one of a string of volcanic peaks: Lassen Volcanic National Park and Mt. Rainier National Park preserve other peaks in the Cascades range. Mt. St. Helens, Glacier Peak, and several others are protected federally. These peaks line up in a row, called a volcanic arc, parallel to the coast. A similar arc sits along much of Central America and forms the Andes of South America. And similar arcs also occur as island chains—the Aleutians, Japan, and others. In fact, the Pacific Ocean is almost entirely encircled by such volcanic arcs, forming the “Ring of Fire”.
Video: Ring of Fire (5:37)
Dr. Richard Alley: We saw in the last unit that there are spreading ridges out in the ocean that are making seafloor that moves away from them. And it might have bothered you at that time - the Earth is not being blown up, inflated like a giant balloon. So if we're making sea floor all the time, where does it go? And the answer is it goes back into the Earth to replace what is coming out at subduction zones.
And this is a diagram of a subduction zone that we have swiped from the United States Geological Survey. It has a number of details, maybe more than we need, but the down-going slab, the friction that it makes, tends to drag down the sea floor and make low spots that we call trenches. Those are often filled with very deep water -- the deepest spots in the world ocean. But sometimes they get filled with sediment and you see the accretionary wedge there. That's the scraped-off stuff. And sometimes this scraped-off stuff can make a little Coastal mountain range. We'll see that at the Olympic and the coast ranges of California. You also end up in such systems that the downgoing slab, it takes down some water, the water lowers the melting point of the deep rocks, and that allows magma to rise and then to feed volcanoes. So you have these volcanic arcs running along, parallel to the trenches. And you also find in such systems, that there's a lot of sticking and slipping and sticking and slipping that makes earthquakes as you go down. So these are features that you see in subduction zones.
Now, where do you find subduction zones? A lot of them are lined up around the Pacific Ocean in what we call the "Ring of Fire". And so you can see a diagram of the Ring of Fire here. These blue things are the trenches, including the deepest water in the world ocean, at the Challenger Deep. There is another trench over here, it just happens to be full of sediment so you don't need to worry about this, and later we'll sing about the Ring of Fire. So there's, there's the Ring of Fire for you.
Now we'll take a better look at this, a little more detail and more data. There's a lot of things in this figure. First of all, if you come over here on the right towards the Caribbean, you'll see this big red band. And there are several of such big red bands on this. These are the subduction zones that are close to the U.S. Holdings. So that's near Puerto Rico and the Virgin Islands and they're part of the U.S. and so these are near U.S. land and these are just a special concern of the United States Geological Survey which made this map. If we then look at other things on this map, there is some more red on here, and that's little tiny red dots. And you'll see a whole bunch of little tiny red dots for example, just below what I drew there at the top. All of those little red dots are volcanoes and you see volcanoes in the Aleutians where I was. You see them out in the Cascades, you see them in Central Mexico, you see them in Central America, you see them down the Andes. There's a whole lot of places around here that you can see volcanoes popping up. Also on this plot you can see earthquakes. So the black dots, such as the ones just inland from where I'm drawing down in the lower right there next to South America, those are all earthquakes. So this plot is showing you where the earthquakes are and where the volcanoes are, which ones are in the U.S. The very light red line out in the middle, these are the spreading ridges and then later we'll come back and we'll look at Hawaii and the hot spot chains. But so now you can get a pretty good view of what many of the features are that we have to worry about when we talk about the Ring of Fire.
We're gonna do one more view of things here. This is a figure that was generated by Gavin Hayes, who is a graduate of Penn State, and works for the United States Geological Survey. And what's shown here is not just the Pacific Ring of Fire, which is sort of over around this, right, so that's in the Pacific. Now you see all of the subduction zones on the planet. And the funny sort of coloring here, the yellow, the zeros, mean that the downgoing slab, the downgoing sea floor, has just started down. It's right at the sea floor. And you'll see it, say, over where I'm drawing now, it starts down under South America there. The deeper one, 600 kilometers, that's 400 miles almost, and it goes down and gets deep most of the way under South America towards the Atlantic. You can similarly see other places that the slabs are going down from shallow to deep and this one you'll also see the subduction zone that's continuing to Vesuvius and Aetna and the pushing up the Alps there. So we're going to look at subduction zones here.
Sitting offshore of the Ring of Fire, is a ring of trenches, which include the greatest depths of the ocean. The trenches parallel the volcanic arcs. Some trenches, which sit near continents, are nearly filled with sediments dumped off the continents, but other trenches are almost free of sediment and so have very deep water, to almost 7 miles (11 km) deep. (Figuring out depths is often complicated by sediment. The surface of Death Valley sits more than two miles lower than the adjacent peaks of the Sierra Nevada. But below the salt flats of Death Valley there are sediments as much as three miles thick, materials that were eroded off the tops of the peaks, so the valley has dropped by much more than three miles relative to the peaks.)
The trenches and volcanoes that ring the Pacific are a few of the many clues that tell us about subduction zones, and help solve a problem that might have been bothering you from Module 2. If sea floor is made at spreading ridges and then moves away, where does it go? The earth is not getting bigger. (Well, meteorites are adding a tiny, tiny, tiny bit, but not nearly enough to account for sea-floor spreading.) So, the sea floor must be disappearing somewhere, going back into the Earth.
Indeed, almost all the sea floor is younger than 160 million years old, but the continents contain rocks as old as almost 4 billion years, showing that the sea floor is being consumed before it gets very old. (Remember, before the class ends, we’ll discuss how geologists date rocks.) (And remember that when a geologist dates a rock, it involves physics or chemistry but not dinner or a movie.)
Credit right: Swanson, Don A., USGS Volcano Hazards Program, Public Domain
More clues to what happens at subduction zones
The sea floor is made of basalt. This is just the kind of rock that would be made if you melted a little bit of the deep, convecting rocks of the Earth’s mantle, and let that melt float up to the surface and “freeze” (cool until it solidifies). If you take basalt, plus a little ocean sediment and some ocean water, and heat them enough to cause a little melting, and then let that melt come to the surface and freeze, you obtain a rock called andesite with a little more silica and a little less iron and magnesium than basalt, lighter in color and lower in density than basalt. Interestingly, the dominant rock in the walls of Crater Lake, and in the other Cascades and Ring-of-Fire volcanoes, is andesite (named after the Andes, which are part of the Ring of Fire), giving you a clue to how these peaks were formed. (Some of the melted rock freezes below ground, making granite or similar rocks.)
If the sea floor were plunging under the continents and melting to make andesite, you might expect that occasionally the downgoing rocks would get stuck and then break free, making earthquakes. Indeed, a three-dimensional map of earthquakes shows that shallow ones occur near the trenches, and the quakes are progressively deeper inland beneath the volcanic arcs, along the descending slab of old sea floor. The great 1964 Alaska earthquake was such an earthquake, which happened where rocks of the Pacific Ocean floor plunged to the north under coastal Alaska and the Aleutian chain. The more southerly of the earthquakes there occur at shallow depths, with the earthquakes getting deeper to the north, occurring along the downgoing rocks. The disastrous 2011 Tohoku earthquake in Japan was of the same type.
Earthquakes make waves that travel through the Earth, at speeds that depend on the characteristics of the rocks, including their temperature. Careful analysis of the speed of the waves, which can be learned from the time it takes for a wave to get from an earthquake to a listening device (a seismometer), shows the higher speeds of the cold slabs going down into the hotter mantle. As these initially-cold downgoing slabs of rock are heated, with their water and sediment, a little melted rock (magma) is produced. (Interestingly, wet rocks melt at a lower temperature than dry rocks, just as adding a little water to flour and yeast speeds cooking of bread in the oven.) When the melt rises to the surface and cools, andesite forms, such as is seen around Crater Lake, in the Andes, or in the Aleutian volcanoes.
So, the sea floor is made at the spreading ridges. It is hot and low-density initially, but cools and contracts as it gets older and loses heat to the colder ocean water. When the sea floor becomes cold and dense enough, it can sink back into the mantle, and we call the place where it sinks a “subduction zone”. The sinking sea-floor slab drags along a little sediment and water. The slab warms because of friction with the surrounding rocks and heat flowing from those surrounding rocks into the colder slab. This causes the sediment and a little of the sinking slab to melt, and the melt rises to feed the volcanic arcs. Old sea floor is going down around much of the Pacific Ocean, and in a few other places such as beneath the Caribbean, and beneath portions of the Alps. Wherever this happens, andesitic volcanic arcs form as shown in the video and figure below. The subduction beneath the Alps created the volcanoes Vesuvius that buried Pompeii, and Santorini that may have destroyed Minoan civilization.
Video: Subduction Zones (2:46)
Dr. Richard Alley: So let's do a little review on what subduction zones are and what they mean for us. Recall that in the deep earth, there is hot, soft rock which is undergoing convection and that some of that convection leads to mid-ocean ridges. The hot soft rock comes up and it makes ridges and the ridges make seafloor and the sea floor moves away from those ridges. The Earth is not inflating like a balloon so if you're making seafloor somewhere it has to be disappearing somewhere else and that goes down at subduction zones. Subduction zones cause a number of features. If they are not filled with sediment, they drag down the rocks where they start down and they make deep trenches and the deepest water in the world oceans happens to be at such trenches. Very often though, there is sediment being eroded from the neighboring land or volcanic ark and so the sediment comes down in rivers and it fills up the trenches as it happens in the west coast of the U.S. off of Washington and Oregon. When that sentiment is dragged down the subduction zone and any sediment from out under the ocean, a lot of it is scraped off and it may make a pile that you would see as the coast ranges or the Olympic out in the western U.S. A little of that sediment is dragged down along the subduction zone together with a little of water and the water and the sediment lower the melting point down there and that allows the rocks to melt, which makes magma, which rises and that eventually feeds to Mount St Helens and Crater Lakes and other Crater Lake and other volcanoes that line up in an arc along there. You also end up with a lot of earthquakes in the system, so this can get stuck and then break, and stuck, and then break. There is a possibility too, as the Rock is taken way down, that it changes into other forms, other minerals, and then occasionally that may cause a sort of implosion that shakes the ground when it happens and makes a really big, really deep earthquake. And so you see, a whole lot of different things can happen at subduction zones. We see volcanic hazards, landsides from steep slopes, we see giant waves made by earthquakes that we call tsunamis. So subduction zones are really, really important.
The following diagram shows the same process as described in the video above. Take a look and see if you would be able to describe it to a friend.
