Volcaniclastics is a collective term for fragmented volcanic materials. This term doesn’t account for how the fragmentation and where they are deposited occurs.
Also, using the term volcaniclastics doesn’t consider the presence of some non-volcanic materials. Some definitions will include those transported from distances.
We call these fragments volcaniclasts. Examples include pyroclasts, alloclasts, epiclasts, and autoclasts.
Usually, volcaniclastics account for more volcanic materials by volume than lava flows.
Subtypes of volcaniclastics are:
1. Pyroclasts
Pyroclasts are a sum of fragments produced during a volcanic eruption. They can be molten, semi-molten, solid magma, or accidental clasts torn from country rock.
Some authors call pyroclasts tephra. Others consider the accumulation of these materials as tephra.
There are also other stricter definitions of tephra. These may include pyroclasts that fall from the air only.
How do they form? Pyroclasts form when rapidly expanding volatiles in magma pulverize and eject lava and country rock.
Also, they can form phreatomagmatic eruptions resulting from magma and water interaction.
Another way is auto brecciation, which occurs when forming lava domes collapse.
Lastly, pyroclasts have different sizes, from ash to lapilli to volcanic bombs and blocks. Their texture is often glassy, solid, or vesiculated, like pumice and scoria.
2. Autoclastic volcanic materials
Autoclastic volcanic materials are fragmented by semi-solid and solid lava movement or gas explosions that don’t involve explosive eruptions. They include intrusion or flow of breccia.
Usually, autoclasts include those formed at the vent but not ejected and fragments from gas explosions.
Examples include the clinkers or blocks formed during lava flow or clasts formed when lava domes and spines collapse without causing explosions.
Also, they include talus piles, which form on vents as highly viscous magma emerges or lava flows into the water.
Lastly, these volcaniclasts involve less energy and are self-inflicting. Also, they usually create single lithologic, poorly sorted angular juvenile blocks and small-volume deposits.
3. Alloclastic volcanic materials
They form when existing igneous rock fragments from subsurface activities that may or may not include fresh magma intrusion. These clasts are common along faults in volcanic rocks.
The name alloclastic is from two Greek words, allos meaning other and klastós broken. It implies other preexisting rocks are broken.
Thus, alloclasts are neither pyroclasts nor autoclasts. Why? Because they don’t form from juvenile magma.
4. Volcanic epiclastic materials (epivolcaniclastics)
Epivolcaniclastics or volcanic epiclastic materials refer to rock fragments from erosion, disintegration, and weathering of volcanic rocks. However, epiclastic fragments may come from older, lithified tuff.
Erosion, disintegration, and weathering are the same epiclastic processes that produce sedimentary rocks. Also, they are transported as muddy volcanic debris flows.
Therefore, you can tell the difference between these reworked volcanic deposits and pyroclasts that never consolidated. Usually, they have fluvial epiclastic deposits like cross-bedding, lenticular beds, or abrasion of clasts.
Some authors consider volcanic epiclastic materials as reworked pyroclasts. However, reworking or recycling pyroclasts by wind or water will not convert them into epiclastic fragments.
Also, these fragments characteristically have little to no glass shards or pumice, as most will have undergone alteration.
Lastly, forming epiclastic materials on slopes during magma intrusion repose will erode the formed volcanic edifices. Edifices are the main volcanic landforms built by erupted volcanic materials.
5. Mixed volcaniclastic materials
These deposits have mingled epiclastic or pyroclastic materials reworked in a lake or stream.
We often classify such materials as volcaniclastics according to fragment size and pyroclastic material percentage.
References
- Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
- Fisher, R. V., & Schmincke, H.U. (1984). Pyroclastic rocks (1st ed.). Springer-Vlg