Fluid volcanic eruptions; this is by far the subject that everyone is most familiar with... at least, they think they are. It's the lava fountains you see in Hawaii or the strombolian spurts you can find at Etna. In reality, however, many volcanologists consider fluid eruptions to be relatively unimportant compared to other styles of eruption for several reason: the impact is more locally contained (it's not a global problem), danger to local populations is usually minimal or at least predictable and gives ample time to act, basaltic volcanism around population centers is uncommon compared to silicic volcanics.
So what is a fluid eruption, how is it defined? Basalt, andesite, dacite, and rhyolite all can and do form lava flows, but basalt stands apart from the others because it is extremely fluid. This allows for extensive lava flows along the volcano flanks, as opposed to the more silicic magmas whose flows appear "stocky" and do not travel far (the rare lava known as carbonatite also fits this description, but will not be discussed separately due to its scarcity). Thus, for the purposes of this article, fluid eruptions are all basaltic in composition. Viscous eruptions are discussed in a separate article.
Background information for this subject can be found throughout the MIVO website. For information on volcano physical characteristics, see the Basic Eruption Styles and Characteristics article; for background on lava flow characteristics, see the Basic Lava Formation and Composition Characteristics, and Lava Flows articles. To learn about the occurrences of these basaltic volcanoes, see the article entitled Tectonic Effects on Volcanism. To learn about the formation and ascent of the magma, see the Process of Magma Ascent article.
This section will look at the physical aspects of a fluid eruption; that is, exactly what forms an eruption may take and why. In the field of Geology, it is important to understand how the Earth works and this is an integral part: the formation of new surficial earth.
There are two types of fluid eruptions that meet the requirements outlined in the introduction; these are hawaiian and strombolian. The first style discussed will be hawaiian.
These types of eruptions generally begin with an earthquake swarm and the opening of fissures. The eruption itself may take place in a central crater or along a fissure to produce a "curtain of fire" effect which is absolutely amazing to see (these "curtains of fire" may extend for kilometers). As a rift eruption transpires, ramparts are built and some sections begin to cool (you can think of these sections as essentially becoming clogged). As these sections are sealed off, the eruption is contained to one or several smaller vents along the original fissure. The beginning of a hawaiian eruption is usually gradual, building to a crescendo, while the end is generally quicker and more dramatic (about an order or magnitude in minutes). Eruptions may last for hours and up to or over a day!
Hawaiian style eruptions can be truly massive with extrusion rates as high as 1000 m3/s; fountaining has been observed to heights of 1600m at Izu-Oshima during its '86-'87 eruption (that's 5200ft, or almost a mile). These eruptions are generally continuous, but exhibit sizeable pulses every 1-5 seconds.
Fluid eruptions, like all eruptions, are gas-driven. That is, the eruption is instigated and continued by the rapid ascension of tiny gas bubbles that are trapped in the lava. The lava is thus carried up by the bubbles (which are under high pressure) and rocketed out at the vent, resulting in what we call the hawaiian eruption. The magma chamber beneath the vent may cause gas to accumulate at the surface before rising, which results in the pulses described above. Moreover, it is interesting to note that most of the ejected lava falls back into the crater and is either recycled (carried back up into the fountain) or pushed over the rim of the crater like an overflowing cup of water; this makes estimations of gas content difficult to determine. The eruption ends when the underlying magma has been degassed to the point at which it can no longer rise to the surface (this degassing is why the end of an eruption is never instantaneous, but winds down over minutes). When the eruption is finished, lava in the crater drains back into the conduit to deep within the earth; if another eruption is imminent (meaning the underlying lava is being replenished and regassed), the lava then gradually rises within the conduit, pulsating as gas pockets push their way up to the surface.
This style of eruption is named after Stromboli volcano in Italy and is markedly different from the hawaiian style. These eruptions begin with a hawaiian fissure eruption (rapid degassing) or a vulcanian eruption (over-pressurized gas explosively clears out the blocked vent). Strombolian eruptions are distinct in that they don't form fountains at all, but are rather sizeable explosion-like bursts of magma from a vent (often containing a small lava lake at the surface). Strombolian eruptions are generally more subdued, with extrusion rates no higher than 100m3/explosion and ejecta heights of less than 100m (though that's still impressive by human standards). The eruptions occur at somewhat regular intervals, but can accelerate in frequency by one or two orders of magnitude during periods of high activity.
Like hawaiian eruptions, gas plays the integral role, but viscosity is what distinguishes between the two types. Hawaiian style eruptions are low viscosity while strombolian eruptions are the result of higher viscosity magmas. The higher viscosity causes the gas within to travel less freely and accumulate longer in the magma chamber. The eruption is actually caused by a large gas bubble rising up through the conduit that pushes the overlying magma up with it (picture a large bubble in a narrow glass with fluid overtop of it). The higher viscosity of the magma also causes the overlying magma to be pushed up with the bubble, rather than flowing to the sides of the conduit and allowing it to pass.
If you'd like to learn about the various characteristics of magmatic gas, how it forms and how it behaves, there is another article to deal with that. In this section we'll simply discuss, in greater detail, how it behaves during a fluid eruption.
In hawaiian style eruptions, gas is typically more diffuse (better dissolved) than strombolian due to the low viscosity. This causes the magma to become more buoyant and rise through the conduit more rapidly. The lower viscosity also allows for less friction between it and the conduit walls for more rapid movement. Upon reaching the magma chamber (note: magma chambers are rather poorly understood), the magma quickly loses momentum (like water flowing from a stream to a lake). Thermal energy and probably thermally driven convection within the chamber, draws the magma toward the top. During this process, however, the magma is partially degassed as bubbles rise up and begin to congregate at the chamber ceiling (which may be dominated by a crystalline mush zone). As these bubbles accumulate, they will build until they are suddenly released into the exit conduit. When these bubbles reach the surface, the result is a strombolian or hawaiian eruption.
What distinguishes between the two types is what happens at the chamber ceiling and in the conduit. In a hawaiian eruption, due to the lower viscosity, the bubbles can move easily through the magma and so don't have much time to build up before they are released through the conduits (remember that pulses occur at 1-5 second intervals). When the gas is released, it is not as a single bubble, but more as a gas-rich wave of magma between gas-poor sections. The gas remains dissolved within the magma and rockets out at the surface for the same reasons stated earlier (think of it as a shaken bottle of soda). Strombolian eruptions, on the other hand, are more viscous; this gives the gas time to accumulate at the chamber ceiling (on the order of minutes), as it is more difficult to move the thick magma away from the vent. As the bubble reaches the edge of the exit conduit, it is suddenly released upward. The gas in this instance is referred to as a "slug" as it is roughly the width of the conduit (several meters) and is literally a bubble (does not contain entrained magma, just pure gas). Because of it's size and the viscosity of the magma, overlying molten material is pushed up with it to the surface. Upon reaching the surface, the slug explodes for 1-10 seconds resulting in an often impressive fireworks display. You might think of it as a highly-pressurized bubble making it's way through a syrup bottle, or a slow-moving bullet through the barrel of a gun.
Hopefully you now have a better understanding of how fluid eruptions occur and why they do so. Fluid eruptions of this type are important for a number of reasons, mostly because they are occurring all over the globe and their mechanisms are still poorly understood. We have looked at what happens during these eruptions at the surface, how these eruptions occur, and why. There are a number of other great articles on this website that correlate well with this one and I encourage you to indulge your curiosity and continue reading them. Have fun.