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Volcanoes. What determines whether a volcano extrudes magma violently or gently?
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What determines whether a volcano extrudes magma violently or gently? The primary factors include the magma’s composition, its temperature, and the amount of dissolvedgases it contains. To varying degrees these factors affect the magma’s viscosity. The more viscous (thicker) the material, the greater its resistance to flow. For example, syrup is more viscous than water.
The effect of temperature on viscosity is easily visualized. Just as heating syrup makes it more fluid (less viscous), the mobility of lava is strongly influenced by temperature changes. As a lava flow cools and begins to congeal, its mobility decreases and eventually the flow halts.
But more significant to volcanic behavior is the chemical composition of magmas. A major difference among various igneous rocks is their silica (SiO2) content. The same is true of the magmas from which rocks form. Magmas that produce basaltic rocks contain about 50 percent silica, whereas magmas that produce granitic rocks contain over 70 percent silica.
Note that a magma’s viscosity is directly related to its silica content. In general the more silica in magma, the greater is its viscosity. The flow of magma is impeded because silicate structures link together into long chains, even before crystallization begins. Consequently, because of high silica content, granitic lavas are very viscous and tend to form comparatively short, thick flows. By contrast, basaltic lava, which contain less silica, tend to be more fluid.
In Hawaiian eruptions, the magmas are hot and basaltic, so they are extruded with ease. By contrast, highly viscous granitic magmas are more difficult to force through a vent. On occcasion, the vent may become plugged with viscous magma, which results in a buildup of gases and a great pressure increase, so a potentially explosive eruption may result. However, a viscous magma is not explosive by itself. It is the gas content that puts the bang into a violent eruption.
Pahoehoe lava is characterized by a smooth, billowy, or ropy surface. A ropy surface develops when a thin skin of cooler lava at the surface of the flow is pushed into folds by the faster moving, fluid lava just below the surface. Pahoehoe lava flow.
There are three types of lava and lava flows: pillow, pahoehoe (pronounced pah-hoy-hoy), and aa(pronounced ah-ah) . Pillow lavas are volumetrically the most abundant type because they are erupted at mid-ocean ridges and because they make up the submarine portion of seamounts and large intraplate volcanoes, like the Hawaii-Emperor seamount chain. Pahoehoe is the second most abundant type of lava flow.
Aa is characterized by a rough, jagged, spinose, and generally clinkery surface. Aa flows advance much like the tread of a bulldozer. This photo is looking across an aa channel. Note the character of the aa that makes the wall of the channel.
Dissolved gases in magma provide the force that extrudes molten rock from the vent. These gases are mostly water vapor and carbon dioxide. As magma moves into a near-surface environment, such as within a volcano, the confining pressure in the uppermost portion of the magma body is greatly reduced. This reduced confining pressure allows dissolved gases to be released suddenly, just as opening a soda bottle allows dissolved carbon dioxide gas to bubble out of the soda.
At high temperatures and low, near-surface pressures, these gases will expand to occupy hundreds of times their original volume. Very fluid basaltic magmas allow the expanding gases to bubble upward and escape from the vent with relative ease. As they escape, the gases will often propel incandescent lava hundreds of meters into the air, producing lava fountains. Although spectacular, such fountains are mostly harmless and not generally associated with major explosive events that cause great loss of life and property. Rather, eruptions of fluid basaltic lavas, such as those that occur in Hawaii, are relatively quiet.
At the other extreme, highly viscous magmas impede the upward migration of expanding gases. The gases collect in bubbles and pockets that increase in size and pressure until they explosively eject the semimolten rock from the volcano. The result is a Mount St. Helens or Mt. Pinatubo.
To summarize, the viscosity of magma, plus the quantity of dissolved gases and the ease with which they can escape, determines the nature of a volcanic eruption. We can now understand the gentle volcanic eruptions of hot, fluid lavas in Hawaii and the explosive, violent, dangerous eruptions of viscous lavas in some volcanoes.
The gases in highly viscous magmas become superheated, and upon release they expand a thousandfold as they blow pulverized rock, lava, and glass fragments from the vent. The particles produced by these processes are called pyroclastics (meaning “fire fragments”). These ejected lava fragments range in size from very fine dust and sand-sized volcanic ash, to large pieces.
The fine ash particles are produced when the extruded lava contains so many gas bubbles that it resembles the froth flowing from a newly opened bottle of champagne. As the hot gases expand explosively, the lava is disseminated into very fine glassy fragments. When the hot ash falls, the glassy shards often fuse to form welded tuff. Sometimes the frothlike lava is ejected in larger pieces called pumice. This material has so many voids (air spaces) that it is often light enough to float in water.
Pyroclastics the size of walnuts, called lapilli (“little stones”) and pea-sized particles called cinders are also very common. Cinders contain numerous voids and form when ejected lava blocks are pulverized by escaping gases. Particles larger than lapilli are called blocks when they are made of hardened lava and bombs when they are ejected as incandescent lava. Since volcanic bombs are semimolten upon ejection, they often take on a streamlined shape.
Successive eruptions from a central vent result in a mountainous accumulation of material known as a volcano. Located at the summit of many volcanoes is a steep-walled depression called a crater. The crater is connected to a magma chamber via a pipelike conduit, or vent. Some volcanoes have unusually large summit depressions that exceed one kilometer in diameter and are known as calderas.
When fluid lava leaves a conduit, it is often stored in the crater or caldera until it overflows. On the other hand, viscous lava forms a plug in the pipe. It rises slowly or is blown out, often enlarging the crater. However, lava does not always issue from a central crater. Sometimes it is easier for the magma or escaping gases to push through fissures on the volcano’s flanks.
The eruptive history of each volcano is unique. Consequently, all volcanoes are somewhat different in form and size. Nevertheless, volcanologists recognize three general eruptive patterns and characteristic forms: shield volcanoes, cinder cones, and composite cones.
Mauna Kea, Hawaiʻi, a shield volcano on the Big Island of Hawaii.
When fluid lava is extruded, the volcano takes the shape of a broad, slightly domed structure called a shield volcano. They are so called because they roughly resemble the shape of a warrior’s shield. Shield volcanoes are built primarily of basaltic lava flows and contain only a small percentage of pyroclastic material.
Cinder cones are built from ejected lava fragments. Loose pyroclastic material has a high angle of repose (between 30 and 40 degrees), the steepest angle at which the material remains stable. Thus volcanoes of this type have very steep slopes. Cinder cones are rather small, usually less than 300 meters high, and often form as parasitic cones on or near larger volcanoes. In addition, they frequently occur in groups.
Composite Cones Earth’s most picturesque volcanoes are composite cones. Most active composite cones are in a narrow zone that encircles the Pacific Ocean, appropriately named the Ring of Fire.
A composite cone or stratovolcanois a large, nearly symmetrical structure composed of interbedded lava flows and pyroclastic deposits, emitted mainly from a central vent.
Just as shield volcanoes owe their shape to the highly fluid nature of the extruded lavas, so too do composite cones reflect the nature of the erupted material. Composite cones are produced when relatively viscous lavas of andesitic composition are extruded.
A composite cone may extrude viscous lava for long periods. Then, suddenly, the eruptive style changes and the volcano violently effects pyroclastic material. Most of the ejected pyroclastic material falls near the summit, building a steep-sided mound of cinders. In time, this debris becomes covered by lava. Occasionally, both activities occur simultaneously, and the resulting structure consists of alternating layers of lava and pyroclastics.
