Volcanic blocks are angular to sub-angular rock fragments measuring more than 64 mm (2.5 inches) in size blasted into the air while in solid form during an explosive volcanic eruption. Since these rock fragments are solids at the time of ejection, they don’t have rounded or streamlined shapes like volcanic bombs.
These blocks can be from solidified material formed during a previous eruption or accidental clasts (xenoliths) torn from vent walls. Also, they can be from magma involved in the volcanic activity itself.
Lastly, it is part of tephra. Tephra is the total sum of fragmented materials blasted into the air during a volcanic eruption. They are ash (< 2mm), lapilli (2-64 mm), bombs, and blocks, while individual fragments are pyroclasts.
How do volcanic blocks differ from bombs
The volcanic bombs and blocks are at least 64 mm (2.5) inches, and their composition can be similar.
However, volcanic blocks have angular to sub-angular shapes (from shattering) because their ejection occurs when they are already solids. In contrast, volcanic bombs have rounded to streamlined aerodynamic shapes and have smoother surfaces as they are ejected when lava is molten or semi-solid. They acquire a rounded or streamlined shape during the flight in the air.
Lastly, an accumulation of rocks composed predominantly of blocks is known as pyroclastic breccia. If it mainly has bombs, you will have an agglomerate.
Characteristics
Volcanic bombs are angular to sub-angular rock fragments. However, some accidental clasts plucked deeper in the Earth’s crust can appear as cobble-like pebbles due to attrition or fluidization during upward movement.
Depending on their formation, these rock fragments will have different textures, including bread crust and colors (black, gray, brown, reddish, etc.). Also, it is possible to have slab-like or platy blocks if they form from foliated metamorphic rocks or lava flows. Even banded ones do occur if from a country rock with bands.
Their sizes will vary from at least 2.5 inches to as large as several dozens of feet and will way a few pounds to tens of tonnes. However, most will be nearly equal in dimensions.
A notable example happened in May 1924 at Kīlauea Volcano (Hawaii, USA), where Halema‘uma‘u crater threw over 14 tons of a volcanic block from the crater. Also, an eruption at Mount Vesuvius (Italy) expelled blocks weighing 2-3 tons as far as 100-200 meters from the vent.
Lastly, due to their large size, most blocks form a few miles to meters from the vent. However, some may travel a longer distance down valleys or, if fed to pyroclastic density currents or form part of
How do these blocks form?
Volcanic blocks form when an explosive eruption ejects solid rock fragments >64mm into the air and form mainly on Plinian (vesuvian), Vulcanian, and Strombolian volcanoes. The explosion may cause fragmentation (shattering) and is what throws these resultant pieces into the air.
The rock fragments can be from disruption of lava from older volcanic activities, including lava domes, old magma tunnels, or country rocks on the vent. For instance, a lava dome.
Also, blocks can be from juvenile, essential, or cognate clasts created when part of the magma or lava involved in the eruption solidifies. This will include any lava domes formed over the vent.
A last possible way that blocks form is when the breakage of volcanic bombs, especially on impact. You will end up with angular rocks.
What is the composition of volcanic blocks?
Since blocks form from country rock torn on vents, materials from previous volcanic activities, or even the volcanic eruption itself, the exact composition will greatly vary. Why? Because it depends on the magma involved that differs and the kind of country rock torn. Also, there will be variations from one place to another.
Most of these blocks will have felsic to intermediate compositions. However, you can have basaltic blocks. However, the low viscosity doesn’t favor block formation. Instead, they will erupt in semi-solid or molten form with little fragmentation.
Also, they may have phenocrysts (coarse-grained minerals or crystals formed deep in the Earth’s crust as magma rose to the surface) in a glassy or finer-grained matrix. Such a porphyritic texture is also possible.
On the other hand, if the blocks are accidental clasts (xenoliths). Such clasts can be igneous, metamorphic, or sedimentary rocks. Here, the possibility is unlimited and can have a composition like any of these rocks.
Hazards
Volcanic blocks can damage man-made infrastructure and potentially cause injury and loss of life to animals or humans. However, their effect occurs near the vents since they hardly. So, they are not as hazardous as pyroclastic flow.
References
- Fisher, R. V., & Schmincke, H.-U. (1984). Pyroclastic rocks. Springer-Vlg.
- Selley, R. C., M., C. L. R., & Plimer, I. R. (2005). Pyroclastics. In Encyclopedia of geology (1st ed.). Elsevier.