Volcanic bomb is pyroclastic rock that is a cooling of a mass of lava it flies thorough the air after eruption. If it is to be called a bomb, a specimens must be larger than 2, 5 inch diameter. Smaller specimens are known as Lapilli. Specimens up to 20 ft. (6 m) in diameter are known. Volcanic bombs are usually brown or red, weathering to a yellow-brown color. Specimens can become rounded as they fly through the air, although they may also be twisted or pointed. They may have a cracked, fine-grained, or glassy surface. There are several types of volcanic bomb, which are named according to their outward appearance and structure.
Color: Dark shades of red, brown, or green
Group: Extrusive
Minerals: Volcanic bombs commonly possess a basaltic or similar mafic composition.
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Volcanic Bomb Classification
Bombs are named according to their shape, which is determined by the fluidity of the magma from which they are formed.
Ribbon or cylindrical bombs form from highly to moderately fluid magma, ejected as irregular strings and blobs. The strings break up into small segments which fall to the ground intact and look like ribbons. Hence, the name “ribbon bombs”. These bombs are circular or flattened in cross section, are fluted along their length, and have tabular vesicles.
Spherical bombs also form from high to moderately fluid magma. In the case of spherical bombs, surface tension plays a major role in pulling the ejecta into spheres.
Spindle, fusiform, or almond/rotational bombs are formed by the same processes as spherical bombs, though the major difference being the partial nature of the spherical shape. Spinning during flight leaves these bombs looking elongated or almond shaped; the spinning theory behind these bombs’ development has also given them the name ‘fusiform bombs’. Spindle bombs are characterized by longitudinal fluting, one side slightly smoother and broader than the other. This smooth side represents the underside of the bomb as it fell through the air.
Cow pie bombs are formed when highly fluid magma falls from moderate height, so the bombs do not solidify before impact (they are still liquid when they strike the ground). They consequently flatten or splash and form irregular roundish disks, which resemble cow dung.
Bread-crust bombs are formed if the outside of the lava bombs solidifies during their flights. They may develop cracked outer surfaces as the interiors continue to expand.
Cored bombs are bombs that have rinds of lava enclosing a core of previously consolidated lava. The core consists of accessory fragments of an earlier eruption, accidental fragments of country rock or, in rare cases, bits of lava formed earlier during the same eruption.
Volcanic Bomb Formation
A volcanic bomb is a type of volcanic projectile that forms during explosive eruptions. It is typically a rounded to elongated mass of molten rock (lava) that is ejected from a volcano while still semi-liquid or plastic. Volcanic bombs can vary in size from a few centimeters to several meters in diameter, and they can travel significant distances away from the vent of the volcano before landing.
The formation of volcanic bombs involves a combination of processes related to the nature of the erupting magma and the explosive dynamics of the eruption itself. Here’s an overview of how volcanic bombs form:
- Magma Composition: The composition of the magma plays a crucial role in the formation of volcanic bombs. The magma needs to be sufficiently viscous (thick and sticky) to resist fragmentation into small particles during the eruption. This viscosity is often influenced by factors such as the silica content of the magma.
- Gas Content: Magma contains dissolved gases, primarily water vapor and carbon dioxide. As the magma rises toward the surface, the decreasing pressure allows these dissolved gases to come out of solution and form bubbles. The accumulation of gas bubbles within the magma increases its internal pressure.
- Explosive Eruption: During an explosive volcanic eruption, the pressure from the expanding gas bubbles within the magma becomes significant. When this pressure exceeds the strength of the surrounding rock, it can lead to the fragmentation of the magma into smaller particles, forming a mixture of fragmented lava, volcanic ash, and gases known as a pyroclastic flow or pyroclastic surge.
- Ejection of Molten Fragments: In addition to the fine ash and rock fragments, larger, semi-liquid or plastic globs of magma can also be expelled from the vent. These globs are volcanic bombs. The bombs are often shaped by their aerodynamic interaction with the surrounding air as they are ejected, which can give them a characteristic streamlined or teardrop shape.
- Solidification: As volcanic bombs are expelled into the atmosphere, they start to cool rapidly due to the lower temperature at higher altitudes. The outer layer of the bomb solidifies, forming a crust, while the interior remains partially molten. This can create a distinctive “bread crust” appearance.
- Landing: The solidified outer crust of the bomb helps it retain its shape as it travels through the air and lands on the ground. Depending on the size, shape, and initial velocity of the bomb, it may either bury itself partially or completely in the ground or create impact craters upon landing.
In summary, volcanic bombs form during explosive volcanic eruptions when semi-liquid or plastic magma is ejected from the vent due to the buildup of gas pressure. The bombs cool and solidify as they travel through the air before landing on the ground, often displaying distinctive shapes and textures due to their aerodynamic interactions and rapid cooling.
Volcanic Bomb Distribution Area
The distribution area of volcanic bombs, or the area where they can be found after being ejected from a volcano during an eruption, can vary widely depending on several factors. These factors include the type of eruption, the size of the volcano, the type of magma involved, prevailing wind conditions, and the strength of the explosive event. Here are some general considerations for the distribution area of volcanic bombs:
- Eruption Type: Different types of volcanic eruptions can lead to varying distributions of volcanic bombs. Explosive eruptions, such as Plinian or Vulcanian eruptions, are more likely to eject volcanic bombs over larger distances compared to effusive eruptions, where lava flows out relatively gently.
- Volcano Size: Larger volcanoes tend to have greater explosive potential, which can result in the ejection of volcanic bombs over larger areas. Smaller volcanoes might have more localized distributions.
- Magma Properties: The viscosity and gas content of the magma play a significant role. More viscous magmas are more likely to form volcanic bombs and can carry them greater distances due to their resistance to fragmentation.
- Wind Patterns: Prevailing wind patterns at the time of eruption can carry volcanic bombs in specific directions. Wind can greatly influence the distribution area, potentially carrying volcanic bombs far downwind from the eruptive vent.
- Eruption Intensity: The intensity of the eruption, including factors like the height of the eruption column, the rate of magma discharge, and the explosiveness of the event, can influence how far volcanic bombs are ejected.
- Topography: The local terrain and topography can affect the distribution of volcanic bombs. Mountains, hills, and valleys can deflect or funnel the trajectory of ejected material.
- Geographical Location: The location of the volcano, its proximity to populated areas, and the presence of natural barriers can influence where volcanic bombs are distributed.
- Eruption History: Previous eruptions of the same volcano can provide insight into the potential distribution area of volcanic bombs. Patterns from past eruptions may be used to estimate the range of distribution for future events.
It’s important to note that while volcanic bombs can travel significant distances from the eruptive vent, they are often found closer to the volcano itself. The distribution area can extend from the immediate vicinity of the vent to several kilometers away, depending on the factors mentioned above.
Researchers and volcanologists often study the distribution of volcanic bombs and other volcanic ejecta to gain a better understanding of the eruptive processes and hazards associated with volcanic activity. This information can be crucial for hazard assessment and risk mitigation in volcanic regions.
Physical Properties of Volcanic Bombs
The physical properties of volcanic bombs are influenced by their formation, flight through the air, and subsequent cooling and solidification processes. Here are the key physical properties of volcanic bombs:
- Shape and Size: Volcanic bombs can exhibit a wide range of shapes and sizes. Their forms can include spherical, elliptical, streamlined, or irregular shapes, depending on their aerodynamic interaction with the air during flight. Sizes can vary from centimeters to several meters in diameter, with larger bombs often having elongated or teardrop shapes.
- Exterior Crust: As volcanic bombs are ejected from the volcano and travel through the air, their outer layers cool and solidify rapidly due to exposure to lower temperatures at higher altitudes. This results in the formation of a solid crust on the surface of the bomb. The exterior crust can be rough or smooth and is often darker in color compared to the molten interior.
- Interior Texture: The interior of a volcanic bomb may remain partially molten or contain pockets of semi-molten material. The interior texture can range from glassy or crystalline to vesicular (containing gas bubbles) depending on the cooling rate and the mineral composition of the magma.
- Vesicles: Many volcanic bombs contain vesicles, which are small gas bubbles that were present in the molten magma before ejection. These vesicles often collapse or partially close as the bomb cools and solidifies, leaving voids or cavities in the interior.
- Weight and Density: The weight and density of a volcanic bomb are determined by its size, shape, and composition. Larger bombs tend to have greater mass and density. The crust of the bomb contributes to its overall weight and density, while the vesicles may reduce overall density.
- Impact Features: When volcanic bombs land, they can create impact craters or depressions in the ground due to their kinetic energy upon impact. The shape and depth of these features can provide insights into the angle of impact and the velocity of the bomb.
- Color: The color of volcanic bombs can vary based on the mineral composition of the magma. Bombs may be dark-colored if they contain iron-rich minerals or lighter-colored if they have a higher proportion of silicate minerals.
- Surface Features: The exterior surface of a volcanic bomb can exhibit various features, including flow lines, grooves, and ridges. These features result from the bomb’s interaction with the air and its rotational motion during flight.
- Cooling Rate: The rate at which a volcanic bomb cools influences its internal crystallinity and texture. Rapid cooling at the surface may lead to a glassy texture, while slower cooling in the interior can promote the growth of crystals.
Understanding the physical properties of volcanic bombs provides valuable information about the eruption dynamics, magma behavior, and volcanic processes. These properties can be studied to decipher the conditions under which the bombs formed and traveled through the atmosphere before landing, contributing to our knowledge of volcanic hazards and eruption mechanisms.
References
- Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
- Wikipedia contributors. (2018, October 18). Volcanic bomb. In Wikipedia, The Free Encyclopedia. Retrieved 15:22, May 14, 2019, from https://en.wikipedia.org/w/index.php?title=Volcanic_bomb&oldid=864612411
