Volcanic breccia are rocks comprised mostly of large, angular to semi-angular, broken pieces of rocks or minerals. These fragments or clasts may or may not have a fine-grained matrix and are larger than volcanic ash, i.e., 2 mm.
The meaning of volcanic breccia is broad, i.e., it is not limited to composition or how fragmentation happens. Usually, it refers to any rock with large angular fragments emplaced by or formed from volcanic action.
Also, you can have intrusive breccia if brecciation happens below the surface due to intruding magma or volcanic activities. Volcanic and intrusive kinds collectively form igneous breccia.
That is not all. These rocks parallel other breccia with different origins like sedimentary, tectonic, impact, or hydrothermal. The only difference is that their formation or emplacement involves volcanic activities.
Lastly, rocks composed of large, rounded, ellipsoidal, or aerodynamically streamlined volcanic blocks are known as agglomerates, not breccia.
What is volcanic breccia made of?
Volcanic breccia mostly has large fragments of lapilli or volcanic blocks. However, they may have volcanic ash and other materials of non-volcanic origin, like clay or silt, etc.
These fragments include mineral crystals, lava, volcanic glass like pumice or scoria, and lithic. Lithic fragments are plucked from rock walls or picked up by pyroclastic density currents.
Lastly, the origin of the various fragments that make this rock may be juvenile, cognate, or lithic/accidental clasts.
Juveniles form from erupting magma, while cognates include earlier formed rocks but are genetically related to the magma.
Composition
Volcanic breccia doesn’t infer a specific composition but depends on the magma type involved. Their composition may be felsic, intermediate, mafic, or ultramafic.
You can describe their composition using compositional names such as dacitic, trachytic, kimberlitic, carbonatitic, rhyolitic, or basaltic volcanic breccia.
Usually, chemical analysis, color, or mineral assemblage help determine composition. Also, knowing the magma composition they erupted from can help.
Types of volcanic breccia
There are three main types of volcanic breccia, considering their mode of formation. These types are pyroclastic, hyaloclastite, and autobreccia.
Let us look at each briefly.
1. Pyroclastic breccia
It forms from explosive volcanic eruptions. These eruptions may be magmatic, phreatic, or phreatomagmatic.
These eruptions will eject pyroclasts like ash, lapilli, volcanic blocks, and bombs into the air.
These pyroclasts will then be deposited by falling from the air or by pyroclastic density to form tephra.
If the deposited tephra has mainly large angular clasts and consolidates, i.e., lithifies or welds, it will form pyroclastic breccia.
2. Hyaloclastites
Hyaloclastites form during a submarine basaltic eruption of pillow lavas. The interaction of magma and cold water causes a non-explosive spalling, creating unconsolidated, angular basaltic glass fragments.
These fragments are mostly sideromelane, but some can be tachylite. The former is a clear, yellow, or yellowish-brown glass formed by the rapid cooling of basaltic magma, and the latter is opaque black or brown glass that cools relatively slower.
These unconsolidated volcanic glasses will then be altered to palagonite. Palagonite is a yellowish, yellow-orange, or brownish material formed from glass alteration.
Palagonitization, together with weathering and diagenesis, will form hyaloclastite breccia.
3. Autobreccia
Autobreccia form when lava flows, especially aa and blocky lava fragments, congealing surface forming angular clinkers and blocks.
These blocks and clinkers are common in intermediate to silica-rich magmas like andesitic, dacitic, or rhyolitic. However, some basaltic lava flows form clinkers, too.
Molten lava may pick and incorporate the angular to semi-angular fragments, forming autobreccia or flowtop breccia.
Lastly, autobreccia includes those formed at vents but not ejected.
Note: Besides this classification, we may have other ways to categorize volcanic breccia, which include the environment in which they are deposited, rock type and location, or the landform they form. Also, these rocks may form an embryonic volcano. Such volcanoes have breccia-filled pipes with no surface expression. Phreatic eruptions are one cause.
Frequently asked questions
The formation of a volcanic breccia indicates that magma contained lots of gas, resulting in an explosive eruption. Also, it may mean phreatomagmatic or phreatic eruptions where explosions involve water, wet sediments, or ice.
Additionally, it may indicate viscous magma lava flows that result in brecciation of congealing lava surface.
Volcanic breccia differs from tuff since it is dominated by large angular fragments of rock or crystals more than 2 mm in size. In contrast, tuff has at least 75% volcanic ash, usually less than 2 mm in size.
Also, tuff forms only from explosive eruptions. However, breccia can be from lava flow, submarine basaltic eruptions, or even autobrecciation during lava flows.
References
- Kusky, T. M., & Cullen, K. E. (2005). Encyclopedia of earth and space science. Facts on File.
- Bowes, D. R. (1989). Volcanic breccia. In Bowes, D. R. (ed.). The encyclopedia of igneous and metamorphic petrology (pp. 396-398). New York: Van Nostrand Reinhold.
- Fisher, R. V., & Schmincke, H.U. (1984). Pyroclastic rocks (1st ed.). Springer-Vlg.
- Bonewitz, R. (2012). Rocks and minerals (1st ed.). DK Pub