Why do (some) silicic bodies behave effusively instead behaving explosively as in common?

Understand the effusive behavior of silicic bodies: Exploring the Factors That Influence Magmatic Eruptions

Magmatism plays a crucial role in shaping the Earth’s surface and is responsible for the formation of various volcanic features. While volcanic eruptions are often associated with explosive events, it has been observed that some silicic bodies exhibit effusive behavior, characterized by relatively gentle extrusion of magma. This intriguing phenomenon has attracted considerable scientific interest, raising the question: why do some silicic bodies behave effusively rather than explosively, as is commonly expected? In this article, we examine the factors that influence the effusive behavior of silicic bodies and shed light on this fascinating aspect of magmatism.

Viscosity and volatile content: Key Influencers of Siliceous Eruption Dynamics

One of the primary factors affecting the eruptive behavior of silicic bodies is their viscosity, which is determined by the composition of the magma. Silicic magmas are rich in silica (SiO2) and exhibit high viscosities due to the polymerization of silica tetrahedra. The high viscosity impedes the upward movement of gas bubbles within the magma, resulting in the gradual release of gas and effusive extrusion of the magma. In contrast, mafic magmas, which have lower silica content, are less viscous and allow gas bubbles to rise efficiently, resulting in explosive eruptions.

Another important factor influencing the dynamics of silicic eruptions is the volatile content of the magma. Volatiles, such as water vapor (H2O) and carbon dioxide (CO2), are dissolved in the magma and play an important role in determining the explosivity of volcanic eruptions. Siliceous magmas tend to have relatively low volatile content compared to mafic magmas. This lower volatile content reduces the explosivity potential of the magma and favors effusive behavior. The lower gas content and higher viscosity of silicic magmas combine to promote effusive extrusion of the magma rather than explosive fragmentation.

Effects of conduit geometry and tectonic setting

The geometry of the volcanic conduit, the path through which magma rises to the surface, also has a significant influence on the eruptive behavior of silicic bodies. Conduits with wider diameters allow for efficient magma and gas ascent, favoring explosive eruptions. On the other hand, narrower conduits promote higher resistance to magma ascent and facilitate the escape of volatiles, resulting in effusive behavior. The shape and dimensions of the conduit are influenced by several factors, including the tectonic setting in which the volcano is located. In extensional tectonic environments, where the Earth’s crust is being pulled apart, wider conduits and explosive eruptions are more common. In contrast, compressional tectonic settings, characterized by converging crustal plates, tend to favor narrower conduits and effusive eruptions.

In addition, the presence of pre-existing fractures and faults in the crust can also influence the eruptive behavior of silicic bodies. These structural weaknesses can provide pathways for magma to rise, reducing the resistance to magma flow and promoting effusive behavior. Conversely, in regions with more intact and impermeable rock, magma ascent is hindered, resulting in more explosive eruptions.

The Role of Crystal Content and Degassing Processes

The crystal content of silicic magmas can significantly affect their eruptive behavior. Silicic magmas often contain abundant crystals such as quartz, feldspar, and mica, which can act as a scaffolding network and increase the viscosity of the magma. Higher crystal content increases resistance to magma flow and promotes effusive behavior. In addition, the presence of crystals can aid degassing processes by providing nucleation sites for the growth of gas bubbles. This gradual degassing allows for a more controlled release of gas, contributing to the effusive extrusion of the magma.

In contrast, magmas with lower crystal content are more prone to rapid degassing and may experience sudden decompression, leading to explosive eruptions. The interplay between crystal content, degassing processes, and magma ascent dynamics is complex and varies with the specific characteristics of each volcanic system.
In summary, the effusive behavior of silicic bodies can be attributed to a combination of factors, including magma viscosity, volatile content, conduit geometry, tectonic setting, crystal content, and degassing processes. Understanding these factors and their interactions is critical to understanding the diverse eruptive behaviors observed in silicic volcanic systems. Further research and interdisciplinary studies combining geology, geochemistry, and geophysics are needed to unravel the complexity of magmatic processes and refine our understanding of volcanic phenomena.

FAQs

Why do (some) silicic bodies behave effusively instead of behaving explosively as is common?

Some silicic bodies behave effusively instead of explosively due to certain factors that influence the viscosity and gas content of the magma. Here are some key reasons:

What is the role of magma viscosity in determining whether a silicic body behaves effusively or explosively?

Magma viscosity plays a crucial role in determining the eruptive behavior of silicic bodies. Silicic magmas have high viscosity due to their high silica content, which inhibits the escape of gases. When the magma is highly viscous, gases get trapped within the magma, leading to pressure buildup. If the magma is too viscous, it hinders the rapid expansion of gases and promotes effusive eruptions, characterized by slow lava flows instead of explosive eruptions.

How does the gas content affect the eruptive behavior of silicic bodies?

The gas content in silicic magmas also influences their eruptive behavior. Silicic magmas typically contain a significant amount of dissolved gases, such as water vapor, carbon dioxide, and sulfur dioxide. When the magma has a high gas content, the pressure exerted by the expanding gases can overcome the viscosity and lead to explosive eruptions. On the other hand, if the gas content is lower, the magma is less likely to erupt explosively, favoring effusive behavior.

Are there other factors that contribute to the effusive behavior of silicic bodies?

Yes, apart from magma viscosity and gas content, other factors can contribute to the effusive behavior of silicic bodies. One important factor is the presence of cracks or conduits in the Earth’s crust that allow the magma to reach the surface more easily. If such pathways exist, the magma can flow more freely, promoting effusive eruptions. Additionally, the presence of external water, such as groundwater or surface water, can interact with the magma, reducing its viscosity and facilitating effusive behavior.

Can the effusive behavior of silicic bodies change over time?

Yes, the eruptive behavior of silicic bodies can change over time. It is possible for a silicic body that has exhibited effusive behavior in the past to transition to a more explosive phase. This can occur if the magma composition changes, leading to an increase in gas content or a decrease in viscosity. Additionally, external factors such as tectonic activity or the injection of fresh magma can influence the eruptive behavior and potentially trigger more explosive eruptions.

Are there any examples of silicic bodies that have exhibited effusive behavior instead of the typical explosive behavior?

Yes, there are examples of silicic bodies that have behaved effusively instead of exhibiting the more common explosive behavior. One notable example is the Yellowstone Caldera in the United States. While it is classified as a supervolcano with the potential for highly explosive eruptions, most of its past eruptions have been effusive in nature, characterized by the slow extrusion of rhyolitic lava flows. This is attributed to the high viscosity of the silicic magma, which impedes rapid gas expansion and promotes effusive behavior.