by Kyle Colville and Mathew Perdue
Mount Vesuvius is located on the west coast of Italy in the Campanian Arc and is associated with potassic alkaline volcanism.9 The historic eruption in 79 CE buried the cities of Pompeii and Herculaneum.3 Geophysical research shows the appearance of a slab window beneath Mount Vesuvius allowing upwelling convection currents to interact with the enriched mantle derived from the subducting slab. The appearance of the slab window is probably due to subduction “rollback” and arc related magmatism.7 Geochemical methods of research have been used to determine the evolution of magma bodies beneath Mount Vesuvius. The chemical compositions of the rocks do not show a clear trend that can simply be answered by fractional crystallization being the only method of magmatic differentiation. These variations can be caused by three different processes: crustal contamination, heterogeneous source composition, and magma mixing.8,9
Lassen Peak is located along the western United States and is the southernmost active volcano in the Cascade Arc.11 Lassen Peak’s first eruption period was 27,000 years ago, creating the original Lassen Peak Dome, with the second eruption period occurring in the early 1900s.12 Lassen Peak is part of the oceanic Gorda Plate that is subducting beneath the North American continental plate. The active volcanism associated with the peak is due to the complexity of the tectonic system associated with the two lithospheric plates.11,12,13 The variability in magma compositions that characterize Lassen Peak are caused by the intermixing of magma bodies from short-lived and long-lived volcanoes dispersed throughout the Cascade Arc.11,13
Earth is home to a wide variety of volcanoes with unique characteristics. Their formations are attributed to plate tectonics, a theory that explains how the rigid outer layer of the planet, or lithosphere, is divided into large sections that glide on top of the molten inner layer of the earth and interact with each other during those movements.1 The two primary classifications of tectonic plates are oceanic plates and continental plates. When these two plate types collide with one another, the dense oceanic plate sinks below the less dense continental plate in a process called subduction, in what are termed “subduction zones.”1 Volcanism typically occurs along subduction zones. The process of volcanism is spurred at depth when high temperatures and pressures force water content from the oceanic plate to percolate upward through the overlying mantle material, resulting in lowering the melting point of the continental plate rocks and thus the formation of magma.1 Depending on the specific environment, magma can exhibit a range in composition from mafic to felsic. Mafic magma has low silica content, erupts effusively, and flows easily due to low viscosity. Andesites, basaltic andesites, and basalts are names for the characteristic mafic magmas.2 In contrast, felsic magma has high silica content, erupts explosively, and flows slowly due to high viscosity. Dacite, rhyodacite, and rhyolite are names for the key types of felsic magmas.2
Two specific volcanoes that have formed at subduction zones are Mount Vesuvius and Lassen Peak. Mount Vesuvius is located on the west coast of Italy and is part of the Campanian volcanic arc, an array of volcanoes in the Campania region of Italy.3 The mountain rises to an elevation of 1,281 meters and is classified as a stratovolcano because of its conical shape, steep slope, and history of plinian eruptions.2,3 Plinian eruptions are highly explosive and characterized by vertically extensive columns of rock fragments, powerful pyroclastic flows, and laterally extensive ash fall.2 Lassen Peak is located in northeastern California and is part of the Cascade volcanic arc along the western coast of the United States. The peak exhibits an elevation of 3,187 meters and is classified as a plug dome because of its relatively round shape, shallow slope, and history of phreatic explosions and subplinean eruptions.4,5 A phreatic explosion is an eruption of steam, water, ash, and rock fragments caused by the rapid heating of groundwater or surface water by magma.2 Subplinian eruptions are similar to plinian eruptions except they are of a lesser magnitude.6
Even though Mount Vesuvius and Lassen Peak both formed at subduction zones under macroscopically similar geologic conditions, examining the finer details of these two volcanoes reveals that they are very different from one another. Each has a particular tectonic setting, which has contributed to a specific lithology, which in turn has resulted in a distinct history of eruptions. These three qualities are significant because they illustrate how nature is never uniform, and because they help to exemplify that every volcano is unique.
Tectonic Setting of Mount Vesuvius
Mount Vesuvius, a volcano within the Campanian Arc, formed due to the convergence of the oceanic portion of the African plate subducting beneath the continental Eurasian plate.3 Geophysical research has consistently indicated the presence of a slab window, a geological feature creating a unique volcanic setting.7 Under Mount Vesuvius, the lower portion of the subducting African plate has torn and detached from the upper portion, forming a gap or “slab window.” This complex interaction can be seen in Figure 1. Because upwelling convection currents moving through the lithospheric gap can interact with the enriched mantle from the subducting slab, lithologies unique to Mount Vesuvius are different from the rest of the Arc.3
Subduction of varying velocities along the length of the subduction zone typically causes plates to tear. The slab window beneath Mount Vesuvius is bounded by two faults that were torn along the margin due to stresses caused by these changes in velocity. These slab tear faults probably connect two adjacent sections of the subduction zone, and accommodate horizontal movements if the two sections move at different velocities.7 Arc related magmatism also had an appreciable effect on the eventual detachment of the lithospheric segment along the subduction zone. Upwelling convection currents worked in conjunction with varying subduction velocities, and this is believed to have caused the detachment and tear, creating the slab window.7 The distinct physical processes associated with the tectonic setting of Mount Vesuvius are specific to the region and are important because they have influenced the lithology of the volcano.
Lithology of Mount Vesuvius
Over the last 19,000 years, Mount Vesuvius has erupted ~ 50 km3 of magma with varying compositions. The many chemical compositions of the resulting rocks do not show a clear trend in contrast to typical volcanic products, which have compositions that can simply be explained by fractional crystallization. Analysis of geochemical data from a borehole drilled to a depth of 240 m along the southern slopes of Mount Vesuvius through rock layers known as the CdT sequence has provided insight into the causes of compositional variation.8
Borehole evidence shows that three different processes cause the variations in rock types: heterogeneous source composition, magma mixing, and crustal contamination.8 The heterogeneous mantle and the magma bodies that develop here have potassium-rich compositions. Mantle xenoliths that are brought to the surface in ascending magma bodies are evidence of the chemical alterations beneath Mount Vesuvius.9 Crustal contamination and magma mixing also evolve magma bodies beneath Mount Vesuvius. Strontium isotope analyses have been used to determine the sources of contamination and the varying degrees of differentiation. Because radiogenic Strontium ratios were not consistent within the magmas, magma mixing has sourced variations in rock type. In addition, a deep magma chamber was contaminated by Mesozoic limestone, which altered the magmas’ Strontium contents.8 These three magmatic processes are significant not only because they affect the evolution of magma at Mount Vesuvius, but also because they contributed to the specific eruption history of Mount Vesuvius.
Eruption History of Mount Vesuvius
Mount Vesuvius is well known for its 79 CE eruption, which buried the cities of Pompeii and Herculaneum. Mount Vesuvius has erupted several times since 79 CE, and beginning in 1631, Mount Vesuvius entered a long period of frequent volcanic activity continuing until 1944. The 1631 eruption, the largest eruption since 79 CE, destroyed neighboring cities with pyroclastic flows. In March of 1944, Vesuvius sustained a plinian eruption, destroying many aircrafts involved in World War II; however, no one was killed or injured during this eruption.3 A photo of the eruption can be seen in Figure 2.
Tectonic Setting of Lassen Peak
Lassen Peak is the southernmost active volcano in the Cascade Arc, a volcanic arc along the western United States that has been slowly migrating northwest due to North American plate movement.12 The volcano is part of the Gorda Plate, shown in Figure 3, which is an oceanic plate currently subducting beneath the North American continental plate with a shallow, eastward dip.12 The peak is located north of the San Andreas Fault system and Sierra Nevada Mountains, and west of the Basin and Range tectonic province.14 In addition, it is surrounded by various mountain ranges, valleys, and minor fault systems. The complex interrelated tectonic forces associated with each of the aforementioned plates and their associated orogenic events are unique to Lassen Peak, and are significant because they have contributed to its volcanic activity and characteristic lithology.
Volcanism and Lithology in the Lassen Region
In contrast to the majority of volcanism in the Cascade Arc, which is predominantly calc- alkaline in nature, volcanism of the southern Arc is compositionally diverse.15 Relatively small and short-lived overlapping regional volcanoes that produce mafic magmas of basaltic to andesitic composition dominate the southern portion of the Arc. Larger and longer-lived lithologically complex volcanic centers are interspersed among these regional volcanoes and produce rocks of varying composition ranging from mafic and felsic andesite to felsic dacite and rhyolite.16 In these centers, mafic magmas derived from the molten portion of the earth periodically intrude relatively stable felsic sub-surface bodies formed during earlier magmatism.15 The Lassen Volcanic Center (LVC), home to Lassen Peak, became active ~ 825,000 years ago, and thus is the youngest such center in the Cascade Arc.4 Since its initial onset, the LVC has gone through several stages of magmatic evolution that represent a complex transition from mafic to felsic volcanism. Lassen Peak belongs to the latest of these stages, which has produced intermediate andesites alongside domes of dacite and rhyodacite. Even though Lassen Peak and Mount Vesuvius have both experienced magmatic evolution throughout their active lives, the processes that spurred the evolution are not exactly the same, and therefore each volcano has its own lithological composition. The compositional variation of Lassen Peak is important because it has defined the volcano’s history of eruptions.
Eruption History of Lassen Peak
Lassen Peak has had two primary eruptive periods in its lifetime. The first, a single felsic eruption, occurred ~27,000 years ago and contributed to the formation of the original Lassen Peak dome.13 The second took place in the early 1900s and involved a series of events that transpired over a span of three years. Beginning on May 30, 1914, volcanic activity commenced at Lassen Peak through numerous steam blasts and increased hydrothermal activity. On May 14, 1915, a lava dome of black dacite with andesitic inclusions began to accumulate. On May 19, a phreatic explosion interrupted the growth of the dome, causing it to collapse and sending dome rocks 3 km down the western and northeastern faces of the volcano. Three days later, a subplinean eruption ejected compositionally banded pumice with unbanded light dacite and resulted in a variety of volcanic hazards, including a vertical column of volcanic material that rose 9.5 km high (Figure 4), a pyroclastic flow that extended 6 km to the northeast of the peak, lahar flows that reached 20 km down creek systems, and ash deposits that spanned up to 300 km to the east. The events of May 1915 are significant not only because of the devastation they caused, but also because they were initiated by a magma mixing event between a felsic dacite host magma and an intruding andesite magma. This led to the production of black dacite, light dacite, dark andesite, and andesitic inclusions. Between 1915 and 1917, several additional intermittent eruptions transpired at Lassen Peak, though none were as powerful or compositionally diverse as the events of 1915.4,13,16,17
We have provided a comprehensive literature review detailing the tectonic setting, lithology, and historic eruptions of Mount Vesuvius and Lassen Peak. Mount Vesuvius is located in the Campanian Arc of Italy and is ~ 4,000 ft tall and ~ 19,000 years old.3,9 The larger and older Lassen Peak is located in the Cascade Arc in California and is 10,498 ft tall and ~ 27,000 years old.13 Mount Vesuvius and Lassen Peak are characterized by particular tectonic settings, specific lithologies, and distinct eruption histories. These factors are interrelated, and even though some of the physical and chemical processes may be similar, the results are different. The presence of the slab window beneath Mount Vesuvius allows upwelling convection currents to move through the lithospheric gap and interact with the enriched mantle from the subducting slab, producing the potassium rich magmas that are unique to Mount Vesuvius.3,7,8 Lassen Peak’s location in the Cascade Arc is dominated by the intermixing of small, short-lived volcanoes and large, long-lived volcanoes that produce varying magma compositions.12 The younger mafic magmas have intruded upon the older felsic plutonic bodies causing magmatic differentiation to occur.15 For this reason, once Lassen Peak became active, magmatic bodies had already gone through several stages of evolution, giving the lithology of Lassen Peak its compositional variability. Ultimately, all of these qualities related to Mount Vesuvius and Lassen Peak portray that no two volcanoes are exactly alike.
1. United States Geologic Survey. (2008). This Dynamic Planet: A Teaching Companion [PDF]. Retrieved July 25, 2016 from http://volcanoes.usgs.gov/vsc/file_mngr/file 139/This_Dynamic_Planet-Teaching_Companion_Packet.pdf
4. United States Geological Survey. (2012). USGS: Volcano Hazards Program CalVO Lassen Volcanic Center. Retrieved April 18, 2016, from https://volcanoes.usgs.gov/volcanoes/lassen_volcanic_center/
5. National Park Service. (2005). Geology Field Notes: Lassen Volcanic National Park, California. Retrieved July 25, 2016, from http://nature.nps.gov/geology/parks/lavo/index.cfm
6. Cioni, R. (2014). Plinian and Subplinean Eruptions. Lecture presented at MeMoVolc workshop: The dynamics of volcanic explosive eruptions. University of Geneva, Switzerland. Retrieved July 25, 2016, from http://www.unige.ch/sciences/terre/mineral/CERG/MeMoVolc-Workshop/Program/10_Cioni.pdf
7. Rosenbaum, G., Gasparon, M., Lucente, F., Peccerillo, A., and Miller, M. (2008). Kinematics of slab tear faults during subduction segmentation and implications for Italian magmatism. AGU Journals, 27, 1-5 and 12. doi:10.1029/2007TC002143.
8. Di Renzo, V., Di Vito, M. A., Arienzo, I., Carandente, A., Civetta, L., D’antonio, M., Giordano, F., Orsi, G., Tonarini, S. (2007). Magmatic History of Somma–Vesuvius on the Basis of New Geochemical and Isotopic Data from a Deep Borehole (Camaldoli della Torre). Journal of Petrology, 48, 775-781. doi: 10.1093/petrology/egl081.
9. Frost, B., Frost, C., 2014. Essentials of Igneous and Metamorphic Petrology. 32 Avenue of the Americas, New York, NY 10013-2473, USA. Cambridge University Press. 118-122.
10. Kaiser, D., The Mount Vesuvius Eruption of March 1944. Retrieved May 2016 from http://www.warwingsart.com/12thAirForce/Vesuvius.html
11. Winter, J. (2014). Principles of Igneous and Metamorphic Petrology (2nd ed.). Upper Saddle River, New Jersey 07458. Pearson Education Inc. 69-70.
12. Bullen, T. D., & Clynne, M. A. (1990). Trace Element and Isotopic Constraints on Magmatic Evolution at Lassen Volcanic Center. Journal of Geophysical Research, 95 (B12), 19671- 19691. doi:10.1029/JB095iB12p19671
13. Salisbury, M. J., Bohrson, W. A., Clynne, M. A., Ramos, F. C., & Hoskin, P. (2008). Multiple Plagioclase Crystal Populations Identified by Crystal Size Distribution and in situ Chemical Data: Implications for Timescales of Magma Chamber Processes Associated with the 1915 Eruption of Lassen Peak, CA. Journal of Petrology, 49(10), 1755-1780. doi:10.1093/petrology/egn045
14. Janik, C. J., & McLaren, M. K. (2010). Seismicity and fluid geochemistry at Lassen Volcanic National Park, California: Evidence for two circulation cells in the hydrothermal system. Journal of Volcanology and Geothermal Research, 189, 257-277. doi:10.1016/j.jvolgeores.2009.11.014
15. Klemetti, E. W., & Clynne, M. A. (2014). Localized Rejuvenation of a Crystal Mush Recorded in Zircon Temporal and Compositional Variation at the Lassen Volcanic Center, Northern California. PLoS ONE, 9(12). doi:10.1371/journal.pone.0113157
16. Clynne, M. A. (1999). A Complex Magma Mixing Origin for Rocks Erupted in 1915, Lassen Peak, California. Journal of Petrology, 40(1), 105-132. doi:10.1093/petroj/40.1.105
17. National Park Service. (2016). The Eruption of Lassen Peak. Retrieved April 18, 2016, from https://www.nps.gov/lavo/learn/nature/eruption_lassen_peak.htm
Kyle Colville is a geology student at the Franklin College of Arts and Sciences at the University of Georgia. He is working towards his bachelor’s of science degree and wants to continue his education by earning his master’s after completion of his undergraduate degree. His primary interest is in petroleum geology and he hopes to pursue a career in this field in the future. In his free time he enjoys playing soccer and mountain biking.
Mathew Perdue recently graduated from the University of Georgia with a bachelor’s degree in geology, and from Lanier Technical College with a degree in architectural drafting. He is currently pursuing an internship opportunity out of state with an engineering firm, but plans to return to school in the future to obtain a master’s degree.
Acknowledgments: We would like to thank Kelsey Crane, our Writing Intensive Program teaching assistant, for her help, guidance, dedication, and support throughout the construction of this paper. We could not have done this without her.
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