Agglomerates refer to the accumulation of coarse volcanic material (clasts, lava) and sometimes country rock fragments with at least 75% bombs set in a finer-grained matrix (lava, volcanic ash, tuff, or tuffaceous matrix).
Volcanic bombs are rounded to spheroidal igneous material more than 64 mm in size. They are formed during an explosive volcanic eruption when viscous, partially molten, or molten lava is ejected. So, they will be rounded or have aerodynamic shapes.
Agglomerates is one of the pyroclastic rocks (made of igneous fragment material, clasts, or pyroclasts). These rocks or accumulations may be unconsolidated or consolidated (welded) and have at least 75% of the pyroclasts measuring more than 64 mm (2.5 inches).
A good example of a welded agglomerate happens in a coarse basaltic ‘spatter’ where the fragments remain airborne for a short time. By the time they fall, they are still liquid or plastic and will cement together. Some of these may also appear as lithified volcanic dust or ash.
Did you know that agglomerate originates from the Latin word agglomerare, which means ‘form into a ball’? It implies that these rocks have mainly rounded or ball-like clasts.
However, no petrologically accepted criteria or qualifiers for agglomerates. But there is a consensus on having a majority of pyroclasts as bombs.
For instance, according to the British Geological Survey (BSG), agglomerates have at least 75% of the pyroclastic fragments larger than 64 mm.
BSG further defines agglomerates as pyroclastic rocks having at least 75% pyroclastic fragments, and 75% of these pyroclasts are bombs (rounded and more than 64 mm in size). Also, these rocks will have less than 25% lapilli (2-64 mm) together with ash (<2 mm) and less than 10% well-sorted, unimodal lithified clasts resulting from volcanic processes.
According to (agglomerate, n.d.), an agglomerate is “a pyroclastic rock in which more than 75% of the pyroclastic fragments exceed 64mm, and at least 50% of these are rounded.” Mindat.org is an outreach project of the Hudson Institute of Mineralogy, a non-profit organization.
Characteristics of agglomerates
Agglomerates have a coarse texture, with at least 75% of clasts over 64 mm (2.5 inches). Their shape varies, and sizes range from small to those 30-40 feet wide, weighing several tons.
These unsorted to poorly sorted fragments set in lava, volcanic dust, ash, tuff, or tuffaceous matrix are poorly bedded and occur closer to the vent or in intrusive volcanic breccia or pyroclastic conduits. Also, they can get thicker, unlike tuff or ash beds that are finer and form thin layers on a wide area.
Lastly, these rocks have mostly rounded to subangular bombs and not the angular block in pyroclastic breccias.
Agglomerate vs. sedimentary conglomerates
Agglomerates resemble sedimentary conglomerate rock. However, they are of volcanic origin, i.e., formed mainly from pyroclastic materials and not sedimentary.
Agglomerates vs. pyroclastic breccia
Agglomerates and pyroclastic breccia differ in the composition of bombs and blocks. In pyroclastic breccia, the angular blocks account for larger clasts. In contrast, agglomerates have more bombs, usually 75% of the fragments.
Composition
Agglomerates will have mainly igneous materials (clasts and lava) and sometimes xenoliths or country rock torn from wall vents. Their composition will vary depending on the pristine magma and can be made of a single or several igneous rocks.
The US Nation Park Service notes volcanic bombs form from mostly basaltic to intermediate lava due to relatively lower viscosity. However, some could be of felsic magma, too.
Besides the fragmented clasts, some agglomerates may have volcanic mineral crystals. These crystals are peculiar, i.e., they resemble those formed in lava but those in plutonic rocks, indicating they are deep inside the Earth’s crust.
A better way to look at them is as agglomerations of crystals forming deep inside the Earth’s crust as magma slowly rose towards the surface. At some point, they were thrown by gases or steam during a violent volcanic explosion.
Examples of these rocks with agglomerations of crystals are the sanidinites of the Eifel. Other volcanoes include Ascension (South Atlantic), Saint Vincent (Caribbean), Vesuvius (Campania, Italy), etc., where they form in a coarse ash bed. The common minerals in these volcanoes are anorthite, olivine, augite, hornblende, leucite, and biotite.
How are agglomerates formed?
Agglomerates form like any other pyroclastic rocks, i.e., during an explosive volcanic eruption. As magma rises, the pressure falls, which results in the exsolution and rapid expansion of gases that will produce and eject rock fragments (pyroclasts) into the air.
Pyroclasts can be ash, lapilli, volcanic blocks, or bombs, mostly from lava. However, they can be from country rock torn from a volcano vent. This volcanic ejected material (tephra) will settle and form various pyroclastic rocks.
If most of the ejected lava is viscous, molten, or semi-solid, some fragment rounding (as they rotate) will occur, forming bombs. These bombs will eventually fall back to the ground and form agglomerates. Of course, other materials like ash, lapilli, or blocks will accompany bombs.
Most agglomerates commonly form in Strombolian eruptions, especially in strongly peralkaline volcanoes.
However, they can be from pyroclastic density current created by explosive eruptions that form calderas. Such will result in massive or large agglomerate and poorly bedded deposits. A good example is the Taal volcano (Philippines), Campi Flegrei volcano (Naples, Italy), and Santorini caldera (Greece)
What is a vent agglomerate
As the name suggests, these agglomerates plug or choke satellite or main vent when a caldera collapses or during an explosive eruption, but more commonly towards the end of volcanic activity.
These pyroclasts will compact into a rock that appears as an outcrop with a limited extent and will be circular when viewed on a geological map. Most once-active volcanic crater tends to have vent agglomerates.
References
- Price, M., & Walsh, K. W. (2005). Rocks and minerals. Dorling Kindersley.
- Bonewitz, R. (2012). Rocks and minerals (1st ed.). DK Publishing.
- Agglomerate. (n.d.). British Geological Survey (BGS). Retrieved October 25, 2022, from https://webapps.bgs.ac.uk/bgsrcs/rcs_details.cfm?code=AGG
- Britannica, The Editors of Encyclopaedia (2016, June 3). Agglomerate. Encyclopedia Britannica. https://www.britannica.com/science/agglomerate
- Agglomerate. (n.d.). Mindat.org. Retrieved October 25, 2022, from https://www.mindat.org/min-48583.html
- Pyroclasts and pyroclastic rocks. (n.d.). National Parks Service. Retrieved October 26, 2022, from https://www.nps.gov/subjects/volcanoes/pyroclasts.htm