So where should a discussion of volcanology begin? Well, the definition would be a good place to start. While no single all-encompassing definition exists, the most common one is this: a surficial deposit of magma and/or magmatically displaced material and it's corresponding plumbing sytem. It sounds complicated, but it's not too bad. The sentence states that a volcano is magma rising from the ground to influence the surface somehow. While there are a number of technical problems with this definition, it will work for our purposes here. There are several principle types of volcanoes which are as follows:
We'll begin with the most commonly seen form: the stratovolcano. Stratovolcanoes are well known for producing such pieces as Mount St. Helens. They typically produce explosive eruptions rather than fluid lava, although fluid lava is almost invariably extruded. Stratovolcanoes most often produce vulcanian, vesuvian, and plinian eruptions which will be covered shortly.
The most famous form of volcano, perhaps, is the shield volcano. These volcanoes are known for producing the Hawaiian Islands and are some of the largest in the world, in both height and width. Shield volcanoes are squatty, being typically lower to the ground, while having a huge base. They exist as such because they are built entirely of fluid lava flows which move from the vent and out toward the edge of the volcano before hardening. As a rule, shield volcanoes of higher viscosity have steeper slopes and higher summits. Viscosity is the measure of the resistance to flow or the "gooiness" of a lava. The lower the viscosity, the runnier the lava. Low viscosity lava spreads out further and thinner, while high viscosity lava stays closer to home and spreads out thicker.
Shield volcanoes that form in a chain as the Hawaiian Islands did, typically are the product of a magma plume. A plume is composed of a large portion of molten magma pushing it's way up through the mantle. As the overlying plate passes over the plume, a chain of shield volcanoes may be left in it's path.
Perhaps one of the coolest forms of volcanoes, in my opinion, is the fissure volcano. A fissure typically resides along a fault and may be one solid long volcano or a chain of volcanoes along the fault. These volcanoes may extend great distances and eject enormous amounts of magma (fissure volcanoes have historically wreaked havoc on Iceland).
Many people list Flood Basalts as a type of volcano, but I feel this is more of a subdivision of fissure volcanoes. Flood Basalts erupted the largest amounts of magma in Earth's history, but are only seperated from other kinds of basaltic volcanism by the sheer amount of material displaced (fyi: a flood basalt eruption has never been witnessed). As I don't personally consider Flood Basalts to be a principle form of volcanism, but rather an oddity among events, it will not be covered here (look for it elsewhere in the site, however).
The caldera is another cool form of volcano, but does not simply rise up from nothing. In fact it doesn't rise at all! A caldera forms when a volcano collapses in on itself. This occurs when the magma supply is exhausted and the chamber no longer has enough pressure to erupt; thus, the internal pressure that was supporting the volcano is gone and the surface collapses resulting in a caldera. A caldera usually has a rim (though it often quickly erodes), which is the remnant of the old volcano, and is often very active. You may ask why that is, since the magma has exhausted itself. Well, the fact is that there is still partially degassed magma down there and there may be fresh magma refilling the chamber. Features associated with calderas include cinder cones, fumaroles, and geysers. Note: Yellowstone National Park contains the world's largest caldera. Also the world's largest volcano, Yellowstone blew it's top and collapsed as a caldera. It remains one of the world's most active volcanoes to this day, despite its recent lack of eruptions.
An interesting, but fairly unknown type of volcano (as far as the public goes) is the
cinder cone. Cinder cone volcanoes often appear alone or in fields of other cinder cones.
They usually remain comparitively small and are active for only a short time. Named
cinder cones for the appearance of flying cinders, they "shoot" out lava in small,
explosive bursts. They appear as mounds formed of fine lava fragments and may form on other volcanoes as vents.
I will describe submarine volcanoes later, as they require a bit more background
information and are more technical.
Since I opened my mouth about geysers and fumaroles, I suppose I ought to explain them. It seems like everyone in the world knows what a geyser is: boiling water that shoots out of the ground. Very few people know why that is, though. Geysers are fairly unique to calderas where the underlying magma is very close to the surface. Now, you all hopefully know about ground water; the subsurface water in some places is close enough that it is heated to it's boiling point. As you know, when water boils, it expands. That boiling water forms a chamber of it's own and forces it's way to the surface. When it reaches the surface it explodes outward, releasing all of its pressure and energy. Basically it empties the chamber. The next eruption occurs when the water refills the chamber and is heated until it explodes upward again. This is how scientists are able to predict eruption times for such geysers as old faithful. The geyser will continue to erupt as long as the magma stays there and the water keeps coming. Earth quakes may disrupt or stop the flow of water in such regions. For example, an earthquake early in '99 occurred at Yellowstone which disrupted water flow in the region and caused Old Faithful to take seven extra minutes between eruptions and caused several extinct geysers to become active once again.
Now we come to fumaroles. If you have no idea what a fumarole is, don't feel bad- not many do. Basically it's a vent for escaping volcanic gasses. Magma contains a great deal of dissolved gas beneath the surface and as the pressure on a gas decreases, the gas expands. Well, as the magma rises, the pressure on it decreases and the gases within it expand and are exsolved. The gas rises toward the surface where it forms a vent and escapes. Fumarolic activity reflects the activity of the magma below. If the magma becomes inactive, then there will be no more pressure changes to release the gases and the fumarolic activity will slowly cease. Thus the fumarolic activity paints a descriptive picture of how magma is flowing in the volcano. An increase in such activity is an indicator of rising magma and may suggest that a volcano is nearing an eruption. This, coupled with various other data, is how scientists decide whether a volcano is about to erupt or not.
Now let's discuss eruption styles. These are even more complex than the types of volcanoes. These are:
Strombolian eruptions, named after the Italian volcano, Stromboli, eject small to medium sized lava fragments to form incandescent arcs which can be traced in photographs. Cinder cones often use this eruptive style.
Vulcanian eruptions, named for the Italian volcano, Vulcano, produce short blast-like eruptions of gas and ash. These and all such eruptions occur due to the gas content of the magma being expelled. When a volcano's magma contains too much gas, it explodes. As the magma approaches the surface, the gas expands at a wild rate. When the magma reaches the surface, the gas contained within the magma literally explodes, blowing the lava into lapilli (ash and small fragments). The force involved causes a column of debris to rise high above the volcano. As the tiny pieces of magma and mountain rise, they cool and are carried by the wind to be deposited meters to miles away, and in some cases have been know to circle the globe. Aiding the explosiveness of the eruption is a magma plug that blocks the vent leading to the overpressurization of the magma.
Plinian eruptions, named after the guy who reported Vesuvius to the Romans (Pliny the Younger), are the most violent there are. The most famous plinian eruption for Americans is probably the eruption of Mt. St. Helens. Plinian eruptions are long duration explosions which send ash and debris billowing kilometers into the atmosphere. They typically last hours and can expel truly massive quantities of debris.
Vesuvian eruptions are similar to plinian, but are not so enormous. They release large amounts of ash and gas while the cloud typically forms a sort of cauliflower shape. Vesuvian eruptions are named after the Italian Mt. Vesuvius, which destroyed the cities of Pompeii and Herculaneum.
I'm saving phreatic and pelean eruptions 'till last; so, that brings us to Hawaiian. If you can't figure out what hawaiian eruptions were named after, then you should probably begin brushing up on your geography. Hawaiian eruptions occur in two places: on shield volcanoes from a central vent and along fissures. Fissure eruptions spurt out lava along a fissure and feed lava streams down slope. Central vent eruptions may shoot that lava several hundred feet in the air, although not necessarily. Hawaiian eruptions differ from strombolian in that strombolian eruptions are marked by gentle effusion punctuated by short violent blasts. Hawaiian eruptions generally consist of sustained activity; anywhere from fountaining to gentle oozing.
Now we come to pelean eruptions. Pelean eruptions occur when the vent is held at an angle so that the eruption shoots out laterally, not up. The result of a pelean eruption is a Nuee Ardente (it's french. pronounce Nway Ardentay). Also known as a pyroclastic flow, these incandescent burning clouds of ash and gas plow over and incinerate anything in their paths. Sometimes heated to as much as 1500 degrees and travelling up to 150mph, these pyroclastic flows are among the most dangerous styles of eruption.
Now we arrive at the phreatic eruption. Phreatic eruptions require two things: magma and water. They can occur in a variety of places: Shallow lakes and the ocean, volcanic crater lakes, sub-glacial volcanoes, and underground. Extremely explosive, phreatic eruptions occur when magma meets water. The magma, at over 1600 degrees F, when in contact with water, flash boils any thing near it. Simply put, the magma turns water into vapor-a gas.The rapid change in volume comes in the form of a sizeable explosion. The blasts shatter the magma into the air, creating quite an impressive spectacle. In addition to the explosions taking place, as the magma flash boils the water, the magma is rapidly cooled. As the lava is supercooled, it will frequently shatter, adding to the explosivity. For this reason, phreatic eruptions are pound for pound more explosive than plinian eruptions.
Now let's delve into submarine volcanoes. Submarine volcanoes
are by far the most common form of volcano on earth. This makes sense, seeing as how
two thirds of the earth is covered by water. You say, I already know about this. Water
and magma=phreatic explosion. It's not that simple, though. When you add water to magma, it explodes,
but, if the pressure is too great...no explosion! Why? Because the pressure on the gas
within the magma is too great to let it escape and the pressure on the water touching the
magma is too great to let it flash boil. The result? Pillow lava! As the lava is extruded,
it is quickly cooled. As it cools, the exterior of the flow hardens. Meanwhile, the lava
inside is insulated and continues to flow. The end of the flow hardens
with the rest of the exterior, but the inards are still flowing. Consequently it punches a hole
into the end through which the lava flows and the exterior hardens, repeating the process.
This forms pillow lava which basically looks like squeezed tooth paste. Submarine eruptions
don't necessarily have to take place in the ocean. They can take place in deep lakes
and in subglacial volcanoes. Once the volcano builds itself up enough to overcome the pressure,
though, it starts producing phreatic eruptions. Phreatic eruptions can occur underground
when sub-surface magma reaches ground water. The magma, of course, flash boils the water
and causes a massive explosion. The explosion literally blows out, without warning,
any land above it in a massive explosion. A crater, known as a maar, forms as a result and often develops a
lake.
You have just finished part one of the two part series. You now know the very basics
of volcanology. Do you want to know more?
