Controls on Volcanic Activity
Volcanic eruptions are one of the most fascinating and awe inspiring of all geological phenomena. Catastrophic explosions have shattered civilizations, buried cities, and taken untold numbers of lives since the dawn of humankind. Yet, volcanoes are more than Earthly mechanisms of death and destruction. Additionally, volcanic eruptions shape the lands surface, impact global climate, and form the rocks that comprise the greatest portion of the Earths crust.
While destructive and deadly volcanic eruptions tend to garner the most attention, many eruptions occur in a style that could perhaps be described as benign. That is, not every eruption is explosive many are simply slow and gradual leaks of lava. Why do such differences exist? What geological parameters determine the destructiveness of a particular eruption? Three important physico-chemical properties have the greatest influence upon the nature of a volcanic eruption.
The first of these three parameters is the bulk chemistry of the lava, or alternatively, to put it in terms of the nature of the rock to be formed by the eruption, the compositional class as determined by the mineralogy of the volcanic rock. To the first order, mafic composition lavas are associated with peaceful eruptions, while intermediate and felsic lavas tend to produce explosive style events.
Secondly, the temperature of the lava plays a major role in the resulting eruption. High temperature lavas those erupting at temperatures above 1000 °C tend to be quiescent, while those erupting at lower temperatures (around 600 °C) are typically explosive.
Finally, the abundance of dissolved gasses within the lava at the time of eruption is the most significant of the three parameters. Lavas rich in dissolved gasses (predominantly water vapor, carbon dioxide, and hydrogen sulfide) are extremely dangerous. Conversely, those lavas that are devoid of dissolved gasses, due to bubble formation earlier in the lavas history, are considerably less likely to be explosive.
How are these three different variables interrelated to produce and explosive or peaceful eruption? The underlying physical parameter that links all three is the concept of viscosity. Viscosity is a property of fluids and is defined to be the fluids resistance to flow. That is, viscosity is the result of the internal friction within a fluid. It is a measure of "stickiness". Thus, high viscosity fluids are slow flowing and "sticky" while low viscosity fluids flow freely and are more "fluid-like". It is critical to understand that viscosity is, after a fashion, a negative physical parameter. High viscosity is slow flowing while low viscosity describes a rapidly flowing fluid.
Once a basic understanding of viscosity is achieved, it is still necessary to relate a lavas viscosity to the three previously discussed physico-chemical parameters and, ultimately, then to the explosiveness of the volcanic eruption. Lavas rich in SiO2 (felsic composition lavas) are characterized by high viscosities. This is most easily understood by considering the crystalline structure of the minerals near the bottom of Bowens reaction series. Those minerals found in felsic volcanic rocks are structurally complex due to the high degree of polymerization of the silicon-oxygen tetrahedral. Because these complex crystal structures are difficult to form, the process begins prior to a melts crystallization. Within a felsic lava there exists regions, or domains, of partially crystallized minerals. The result is that this crystalline "slush" is highly viscous. Conversely, mafic lavas form the more simple structures found in minerals near the top of Bowens reaction series and, as such, are significantly less viscous.
Similarly, the temperature of a fluid has a significant control on viscosity. Cold fluids are more viscous than warmer fluids. Numerous every-day examples of the phenomena can be considered motor oil and honey are only two of the most common. As was discussed in Unit III, mafic compositions crystallize at significantly higher temperatures than do intermediate and felsic compositions. Recall that the top of Bowens reaction series was high temperature and the base cooler temperatures of crystallization. This property of temperature control of viscosity serves to amplify the effect of chemical composition discussed above.
Finally, what is the relationship between viscosity and gas content? In this case, the dependent and independent variables are reversed. Previously it was described how chemistry and temperature control viscosity, here it is found that viscosity controls gas content. Dissolved gasses form bubbles within a lava as the intensity of lithostatic pressure is decreased during the lavas ascent towards the Earths surface. Low viscosity lava allows for the bubbles to move freely through the fluid and escape prior to eruption while high viscosity lava tends to trap bubbles until eruption occurs. Thus, high viscosity lavas are characterized by an abundance of trapped gas bubbles.
The last step in the description of physico-chemical controls on eruption style demands an understanding of the role of dissolved gasses and gas bubbles in volcanic emissions. Deep within the Earth, prior to eruption, magmas contain large quantities of dissolved gasses. As the magma ascends higher in the crust it experiences a decrease in lithostatic pressure. This drop in pressure allows the dissolved gasses to coalesce into bubbles. The presence of bubbles within the magma acts to significantly lower the effective bulk density of the fluid. Therefore, the magma rises even quicker, following paths of crustal weakness to the Earths surface. Thus, the formation of bubbles within magma is one of the critical factors that drive eruptions forward. In many cases, without bubble formation, an eruption of any significant volume could not occur.
The high viscosity of intermediate and felsic composition lavas trap gas bubbles that form during magma ascent and thus the bubbles are not able to escape prior to the magma reaching the surface. Consequently, the lava experiences an abrupt depressurization and the trapped bubbles expand extremely rapidly. Explosive volcanic eruptions occur mainly due to the near instantaneous bursting of innumerable gas bubbles within the erupting lava. The force of the gas expansion shatters the lava, and the surrounding rocks in a violent and often deadly eruption.
Characteristics of Volcanic Eruptions
As discussed above, there are particular physico-chemical characteristics associated with explosive and peaceful volcanic eruptions. It is useful to now summarize the characteristics of mafic and felsic eruptions, as well as describe the physical properties of the rocks formed by such eruptions.
Basaltic composition lavas erupt at temperatures between 1000 and 1200 °C and are characterized by extremely low viscosity. As such, basaltic lavas tend to flow freely for great distances over the Earths surface. Due to the ease with which gas bubbles escape from the low viscosity lava, basaltic eruptions are typically peaceful. However, if the hot lava comes in contact with a reservoir of fluid once it has erupted (lake or ocean water most commonly) the resulting super heating of water to produce steam can cause large post-eruption explosions.
Conversely, intermediate and felsic eruptions occur at temperatures ranging from 600 to 1000 °C. The high degree of polymerization, coupled with cooler temperatures, results in lavas of very high viscosity. The high viscosity acts to trap gas bubbles within the lava resulting in extremely explosive eruptions.
The Nature of Volcanic Deposits
From these composition-specific characteristics of eruption style, it is possible to describe the physical characteristics of both mafic and felsic eruptions. Basaltic lava flows from volcanoes at the Earths surface. While cooling, the viscosity of the lava changes increasing as temperature decreases. The viscosity of the lava at the time of crystallization has an important control on the surface texture of the resulting rock. Basalt formed from highly fluid lavas near the volcanic vent has a ropy, folded, texture and is known by the Hawaiian name Pahoehoe (pa-hoi-hoi not pa-ho-ho). Conversely, lava that crystallizes at lower temperatures and more viscous conditions produces a crumbly, blocky texture known as Aa (ah-ah not a-a). Generally, within any single lava flow, regions near the vent will exhibit mainly pahoehoe textures while more distal parts of the flow will have the aa texture. Importantly, these are textures of the surface of the flow, not of the constituent mineral grains. In both cases the basaltic rock is very fine grained.
Most basaltic eruptions occur not at the Earths surface but rather deep underwater. When hot basalt interacts with cold ocean water along the mid-ocean ridges the hydrostatic pressure of the overlying column of water prevents an explosive eruption. Rather, the basalt that is extruded onto the seafloor form a unique "blob-like" texture known as pillow basalt. These curved blobs of basalt range up to a meter across and are coated with a thin layer of basaltic glass. The extremely rapid cooling of the basaltic lava produces a thin skin over the red hot lava. When this skin ruptures, a new blob of lava squirts out of the crack. Thus, pillow basalt textures are only associated with underwater eruptions and as such are diagnostic of those conditions during eruption.
While basaltic eruptions are often calm and quiet, intermediate and felsic composition lavas erupt with explosive force. The high viscosity and resulting retention of gas bubbles until late in the eruption triggers this destructive style of volcanism. Fragments of volcanic rock form pyroclastic (broken by fire) deposits. These fragments (pyroclasts) are either derived from small blobs and bits of lava that cool quickly during the explosion, or from fragments of preexisting volcanic rock that were part of the volcanic mountain prior to the eruption. In either case, the pyroclasts occur in a wide range of sizes, the smallest of which are know as volcanic ash, while the coarser grained pyroclasts are referred to as cinder.
Much of the ash is volcanic glass. Natural glass forms when lava cools so quickly that crystals do not have time to form. In the case of ash, the microscopic shards of glass are created from the thin coating of lava that surrounded the frothy bubbles within the exploding lava. Therefore, volcanic ash grains have extremely sharp edges and points that make breathing the powder hazardous to our lungs as well as extremely damaging to internal combustion engines, electronic devices, and other forms of power equipment.
The most impressive form of pyroclastic eruptions are associated with an interesting and distinctive form of volcanic explosion. Nuee Ardents (new-A are-don-ts) are massive volcanic eruptions that explode with the force of a nuclear weapon. Always associated with felsic to intermediate composition magmas, these eruptions produce an unique volcanic feature the caldera. Calderas are broad shallow bowl-like depressions that exist after a massive nuee ardent eruption. These types of eruptions produce columns of ash that reach into the atmosphere to heights of 20 km above sealevel (higher than jet airplanes fly). Erupting volumes of ash and rock on the order of 10 to 100 km3, these nuee ardent deposits can cover over 10,000 km2. While no major caldera-forming eruption has occurred during recorded history, there is ample evidence for frequent catastrophic eruptions throughout the western United States over the past several million years. Interestingly, these most explosive of all volcanic events never form a volcanic mountain. Rather, caldera forming eruptions produce broad, domal, uplifts prior to their explosive eruption.
Consideration of volcanic rocks brings to mind the famous volcanic mountains of the Earth Mount Rainier, Mount Fuji, Mount Vesuvius. However, not all volcanic eruptions emanate from mountains such as those. Volcanic eruptions on Earth occur in two general styles, the central vent eruptions that produce volcanic mountains of various form, and the fissure eruptions associated with large volumes of basaltic lava.
Central vent volcanoes are characterized by topographic relief and a central conduit or opening from which lava flows. The largest central vent volcanoes and in fact the largest volcanic structures on Earth are the shield volcanoes. These large mountains are hundreds to thousands of kilometers across and tens of kilometers high. Formed by numerous flows of low viscosity basaltic lava, these mountains are very broad compared to their width, hence the name shield. The most well known of the Earths shield volcanoes are located on the Big Island of Hawaii Mauna Kea and Mauna Loa. The largest known volcano in the solar system is Olympus Mons located in the northern hemisphere of Mars.
The most recognizable of the central vent volcanoes are the composite or strato volcanoes. These mountains have the classical conical form so commonly associated with volcanoes. Formed from both lava flows and pyroclastic material, these central vent volcanoes are composed of intermediate to felsic compositions. Smaller than shields, these mountains rise up to tens of kilometers and are ones to tens of kilometers across, thus giving these mountains their steeper sided cone-like form.
The smallest of the central vent structures are the cinder and splatter cones. These small steep sided hills are formed by pyroclastic eruptions and are kilometers across and hundreds of meters tall. Somewhat unexpectedly, these small volcanoes are commonly basaltic in composition.
While technically not a volcanic deposit, there is an important by-product of many volcanic eruptions volcanic mudflows. These large slurries of mud and water form deposits known as lahars. Commonly, these mud flows occur after the initial eruption of a composite volcano. Water is derived from the melting of snow and ice high on the volcano as well as from large thunderstorms that commonly accompany a major pyroclastic eruption. The sudden flooding picks up the newly deposited ash and creates a high density mud flow that can travel rapidly down stream valleys for hundreds of kilometers. The lahar associated with the eruption of Mount Saint Hellens in 1980 traveled over 100 kilometers downstream to the Columbia River. In 1985, the lahars that followed an eruption in Nevado del Ruiz, Columbia killed thousands of people. These events are extremely dangerous because they can strike quickly with little warning in regions far removed from the actual eruption.
While central vent volcanoes are the most famous and easily recognized product of volcanism, the most significant style of eruption on Earth produces no volcanic mountain at all. Rather, the most volumous eruptions are the fissure eruptions associated with mid-ocean ridges and continental flood basalts. Instead of the formation of a central vent, these types of eruptions occur along large linear cracks in the Earths surface. The low viscosity basaltic lava oozes slowly from these cracks and with time covers the Earths surface with a series of basaltic lava flows. In addition to the formation of oceanic crust along the mid-ocean ridges, this process is responsible for the formation of the largest volcanic provinces on the continents. Known as trapps (Sanskrit for steps) or basalt plateaus, these broad flat regions are formed from repeated fissure eruptions. Interestingly, the mare (the dark colored "seas") of the Moon are covered by basalts that erupted along fissures.