Volcanoes occur where melted rock rises to the Earth’s surface. Almost all volcanoes are associated with one of three settings—pull-apart margins (spreading ridges), push-together subduction zones, and hot spots. We’ve already met the volcanoes that produce sea floor at spreading ridges, where low-silica basalt has erupted, and we have seen large, explosive volcanoes (called “stratovolcanoes”) such as Mt. St. Helens, made of higher-silica andesite at subduction zones. Next, we will look at wide, flat, basaltic shield volcanoes at hot spots. (Smaller volcanoes called cinder cones can form with stratovolcanoes or shield volcanoes, or in some other places; you can learn a little more about them in the Enrichment.) The short video below shows the shield volcano Mauna Kea in Hawaii, the stratovolcano Mt. Rainier, and the cinder cone Sunset Crater in Sunset Crater Volcano National Monument. We traced them for you, and put all the tracings on the same figure, to show you how different the shapes are.
Video: Volcano Shapes (2:47)
Dr. Richard Alley: We're going to look at various types of volcanoes and we're going to start with their shapes which are sort of telling us a lot about what's going on. So this is Mount Rainier. It is a strata volcano. It is steep, it is big, it is looming above Tacoma in this picture, in the state of Washington just south of Seattle. And if we trace it with this blue line then you can sort of see what the outline looks like. Now it turns out that Mount Rainier used to be even taller and steeper. The top slid off to make a massive Landslide about 5700 years ago. So here you see both the modern and the before that landslide. Next we'll look at Mauna Kea. This is Hawaii. It is a shield volcano, not so steep. It is even bigger. Mount Rainier rises about two miles above its base. Mauna Kea Hawaii starts at the sea floor and rises more than six miles above its base. But you can see rather clearly, it is not so steep up here at the top. So if we resize them so they fit in the same figure now you can see the comparison between Mount Rainier in blue and Mauna Kea in the redder one, much lower there. And if we put the top back on Mount Rainier you can now see that the comparison is very striking. These are the two most important types we're going to worry about. Just for fun, we'll add one more here. If we go back to Mauna Kea, there are little Cinder Cones up on the side of it that are pointed out with the green arrows. There are cinder cones at Mount Rainier, there are cinder cones at other places, such as Sunset Crater in Arizona. They are made of loose pieces, little bits of melted rock went through the air, they froze, they fell down, and they made a pile. If you try to walk up the side, it tends to roll out from under your feet and it's hard to walk up. And if we outline that with this green one here, we can put them all on the same picture here now and the cinder cone is sort of in between, with the shield volcano very flat and the stratovolcano very steep, especially if we put the top back on Mount Rainier. So, there are some volcano shapes and the shapes are telling us about processes about what goes on to make those volcanoes.
Hot Spots
Video: Hot Spots (3:36)
Dr. Richard Alley: I'm gonna go do hot spots now. This is a cutaway view of the earth and there is heat being generated way down in the core from radioactivity and other things that are going on. But the core is so dense that it can't rise by itself. So the heat comes up and it warms the base of the mantle, and when that gets hot enough, that starts to rise, and it will be coming up, and at some point, when it's coming up it might look something like this. Sort of having a mushroom head that we can see there. And that is headed up towards the surface. Later, if we come look at it, you'll have the heat down here. The thing will be rising up there, and the head will have made it up to the top and there'll be immense outpourings of basaltic lava going over the surface, in what we call a flood basalt. But eventually, that flood basalt will be carried away by the drifting plates and you will be left with something that looks more like this. It's coming up from below. It comes up here, and you have heat there that is trying to leak through. Now let's go to a different diagram that shows that. And we can do this. And so here is, coming up from below, the hot spot. It's trying to break through the lithosphere up here, and it does so in particular places, such as Hawaii, there at Mauna Loa and Kilauea. And, you might have two or three places that are active once, but eventually, as a Pacific Plate moves over, these will be carried away, and a new one will break through, and then a new one will be broken through as that comes along. And this has been going on for a very long time. This one out here is, for example, Kauai, it was sort of five million years ago, this was back here being active. And so this has been going on, and it's all a fascinating thing, and if you back up now, what do we see? The previous picture that you were looking at was right down here. That's Hawaii, and this is way out in the Pacific, and what we're going to do now is zoom in just a little bit on this so that you can see it more clearly. And so here we will go. And so the previous picture was down there in the corner in Hawaii. The flood basalt that came from the start of the Hawaiian hot spot is already gone. It has gone down the subduction zone and been turned into volcanoes or other things up here in the Aleutian Arc. This is sort of 81 million years since this was down here at the active hot spot. This corner here is about 43 million years old. So this has been going on for a long time. The fact that the angle changes is probably because the direction that the plate is moving changed, about 43 million years ago. Although it's possible there is a little bit of motion of the hot spot as well. And so there you see a hot spot.
The drifting tectonic plates of the lithosphere are thin—roughly 100 miles (160 km), although with notable variation—compared to the 4000 miles (6000 km) radius of the Earth. Hot spots come from deeper, with the big ones from about halfway down near the base of the mantle just above where it meets the outer core. There, a rising tower of hot rock sometimes forms and then lasts for quite a while, powered by heat coming from the core. (To see something that looks vaguely like the formation of such a hot spot, go back and view the “lava lamp” film of Dr. Anandakrishnan in the introductory material to this module.)
As the lithosphere drifts overhead, the hot spot may “punch through” to make a volcano. Then as the lithosphere carries that volcano away, the hot spot punches through a new place to make a new volcano, a little like a quilter shoving a needle through at discrete places, although a quilter never sticks a needle through two holes at the same time, whereas the hot spot may be wide enough to feed two or more volcanoes at the same time. Hot spots bring melt from the mantle, and so normally make basaltic volcanoes that are only a little different from sea-floor basalt at spreading ridges. However, where a hot spot pokes through a continent rather than through the sea floor, silica from the continental rocks may mix with the melt to increase its silica content, as at Yellowstone.
When a new hot spot first rises from below, the top must push through the mantle and crust, and the resistance of the stuff in the way of the rising column causes its top to spread out like the head of a thunderhead rain cloud, or of a mushroom cloud from an atomic bomb, or of a blob in a lava lamp, and for the same reasons. When that wide head reaches the surface, it can produce immense lava flows that spread across state-sized areas and bury them hundreds of feet deep. Much of central and eastern Washington and Oregon is buried by the “flood basalts” from the head of the Yellowstone hotspot. After the head of the mushroom cloud has made a flood basalt, the “stem” may continue for millions of years or more, supplying melt to the surface. At Yellowstone, the continent has moved across the hot spot, which has fed a string of volcanoes including those at Craters of the Moon National Monument in Idaho. The hot spot now fuels Yellowstone (which is why it is called the Yellowstone hot spot…a lot of this stuff isn’t that difficult!). The flood basalts from the Hawaiian hot spot have been subducted beneath the Aleutians, but the string of hot-spot volcanoes that have not yet been subducted can be traced from the Aleutians south all the way to the volcanically active big island of Hawaii. A new volcano, Loihi Seamount, is growing undersea just off Hawaii as the big island slowly drifts away, and Loihi will someday (not in our lifetimes!) be the new, volcanically active “big island” while the volcanoes of the current big island die and the island slowly erodes away. The upward flow of the hot spot raises the crust above it, and volcanoes slide slowly down from this raised area as the drifting plate carries them away, which along with erosion causes them to disappear beneath the sea surface.
The flood basalts from the Hawaiian hot spot have been subducted beneath the Aleutians, but the string of hot-spot volcanoes that have not yet been subducted can be traced from the Aleutians south all the way to the volcanically active big island of Hawaii. A new volcano, Loihi Seamount, is growing undersea just off Hawaii as the big island slowly drifts away, and Loihi will someday (not in our lifetimes!) be the new, volcanically active “big island” while the volcanoes of the current big island die and the island slowly erodes away. The upward flow of hot rock that feeds the hot spot also raises the crust above it, and andanoes slide slowly down from this raised area as the drifting plate carries them away, which along with erosion causes them to disappear beneath the sea surface.
What controls the type of volcano?
We have now seen that melted rock can leak up from below to feed volcanoes at spreading ridges, at hot spots, and above subduction zones. But, very different volcanoes develop in these different settings: sea floor forms from spreading ridges; flood basalts and then wide, not-very-steep Hawaii-shaped volcanoes form from hot spots; and, steep Mt. St. Helens-type volcanoes form above subduction zones. The type of volcano that develops at a place depends on a host of factors, including the temperature, composition, and rate of supply of the melt, how long the melt is supplied, and several others. We will focus on two of these here, which are probably the most important: composition of the melt (primarily how much silica versus other chemicals) and volatile content (mostly how much water, although carbon dioxide, hydrogen sulfide, and other compounds that are gases under conditions at the Earth’s surface may be present and classified with volatiles).
Composition of the melt
Silicon and oxygen are the two commonest elements in the crust of the Earth and get together in melt to form the material we call silica. Left to itself, each silicon atom will be surrounded by four oxygen atoms, which form a tetrahedron (a little pyramid). But, give them a little time, and the tetrahedra will start sticking together, or polymerizing, into chains and sheets and bigger clumps, with some oxygens being shared between more than one tetrahedron. If these lumps get big enough, we call them minerals, and the melt has solidified.
When the lumps are present but not too big, the melt is like lumpy oatmeal—it doesn’t flow very well. There are three ways to get rid of the lumps: make the melt really hot, which makes the tetrahedra vibrate so rapidly that they pull away from other tetrahedra and so stop polymerization; fill the melt with iron, magnesium, and other elements that interfere with the tetrahedra polymerizing; or, fill the melt with volatiles that interfere with the tetrahedra polymerizing. When polymerization is low, the melt flows easily. Lava comes out of the volcano quietly, without making big explosions, and flows easily and far from the mouth of the volcano. In extreme cases, flows may be nearly horizontal and cover much of a state, as in the flood basalts. If the melt spreads almost as easily as flood basalts, the lava will have very slight slopes of only a few degrees, forming shield volcanoes (they look like a warrior’s shield lying on its side) such as in Hawaii. Hawaiian lavas and flood basalts flow easily because they are hot and are high in iron and magnesium.
Volatile content
When water and other volatiles remove the lumps, a different situation develops. This is because the volatiles will stay in the melt only if the pressure on them is high. Just as a bottling company can force CO2 into the water of a soft drink under high pressure, but the CO2 escapes as the pressure falls when you open the bottle or can, the water and CO2 and other volatiles stay in the melt under high pressure down in the Earth but escape as the pressure drops as the melt gets close to the surface.
Silica-rich melts usually form with many volatiles. Remember that in subduction zones, wet sediment dragged down the trench releases water (and carbon dioxide and other volatiles) that promote melting. When the melt (called magma when it is in the Earth and lava when it reaches the surface) nears the surface, the lower pressure allows the volatiles to bubble off and escape into hot springs, geysers, etc. (Note that most of the fluids that come out of such hot springs are rainwater that has circulated down into the earth, but some of the fluids may be “juvenile” waters from the magma below.) Silica-rich, relatively cool lava that has lost its volatiles flows only with great difficulty. It may emerge from the volcano and flow a short distance as a very thick, slow-moving, steep flow. It may not even flow, but simply form a dome directly over the volcanic vent. The videos you saw on the last page for Mt. St. Helens show such a “plug dome” forming in the crater left by the great 1980 eruption. And, such “thick” lava may “plug the system” when it solidifies, preventing the escape of more lava and volatiles coming from below. Then the stage may be set for a big explosion.
The next melt that rises in the volcano cannot follow the same path, because the hardened lava above prevents escape. The gases are trapped, and pressure builds up. The volcano is like a hot pop bottle being shaken by little earthquakes. If the top is removed, either by a “bottle opener” (such as the landslide that released the explosion at Mt. St. Helens, or a crack opened by an earthquake) or just because the pressure becomes great enough to blow the top off, the sudden release allows the soda or the magma to come foaming out. A good champagne may fountain to many times the bottle’s height, and blast the cork across the room. A powerful volcano may blast ash higher than jet flight paths. The melt really does get foamy, and that foam hardens into little glass shards. The ash layer deposited by Mt. St. Helens, which stopped drivers hundreds of miles away, was mostly composed of such little glass shards, although torn-up bits of the former volcano were also included. The picture below shows volcanic ash that is composed of lots of broken bubble walls.
The andesitic volcanoes of the Ring of Fire are typically stratovolcanoes, formed of alternating layers of thick lava flows and of pyroclastics—the things thrown through the air by the volcano. The steepness comes from the flows, which cannot go too far from the vent. Some of the andesitic volcanoes, including the rebuilding of Mt. St. Helens, include plug-dome elements, the oozing lava staying right above the vent.
So, the major volcanoes for our purposes are the quiet, basaltic shield volcanoes of hot spots, the quiet basaltic rift volcanoes of spreading ridges, and the steep, scenic, explosive, andesitic volcanoes of the Ring of Fire. Other types exist, notably, cinder cones thrown up by typically minor eruptions tossing pyroclastics short distances (see the Enrichment on cinder cones). Also, hot spots or rifts trying to poke through continental rather than oceanic crust may pick up silica and water, and then produce explosive silica-rich volcanoes. But if you understand shields and stratovolcanoes, you will be well on your way toward understanding volcanism.