A pyroclastic breccia is a volcanic rock with predominantly angular rock fragments larger than 64 mm (volcanic blocks) set in a finer grain matrix such as ash or lapilli formed during an explosive volcanic eruption. However, these rocks may have some volcanic bombs, too.
Some strict definitions will consider a rock a pyroclastic breccia if it has more than 75% volcanic block. Remember, volcanic blocks are angular rock fragments larger than 64 mm (2.5 inches) ejected during an explosive volcanic eruption while already in a solidified form.
If volcanic bombs (subrounded or streamlined volcanic clasts larger than 64 mm ejected while lava is partially or wholly molten) form most of these rocks, you have an agglomerate.
Some types of pyroclastic breccia are Vulcanian, pyroclastic flows, and hydrovolcanic breccias. Two types of hydrovolcanic breccias are recognized, i.e., the phreatomagmatic and phreatic.
It is worth noting that pyroclastic breccia is only a type of volcanic breccia. The other types are autoclastic and epiclastic. We will not be talking about these two kids.
Lastly, breccia often means any rock with angular rock fragments or minerals larger than 2 mm embedded in a finer-grained matrix. These include igneous (intrusive and pyroclastic), impact, sedimentary, tectonic, and hydrothermal breccia. However, for pyroclastic, the angular particles are larger than 64 mm.
How do they form?
Pyroclastic breccias form when rock fragments, predominantly angular rocks larger than 64 mm (2.5 inches) ejected during an explosive volcanic eruption or activity, lithify in a finer-grained matrix.
The explosive volcanic eruption results in brecciation (solidified lava or country rock into smaller, angular fragments, bombs, ash, and lapilli). Any fragments thrown into the air is tephra, while individual clasts or fragments are pyroclasts.
However, it is also possible for magma or heated rock to form steam that will explosively form and eject pyroclasts forming these rocks.
Another way these rocks form is an eruption resulting in a pyroclastic flow current. These flows will cause rock or lava fragmentation, creating angular fragments. If these angular fragments lithify a groundmass, you have pyroclastic breccia.
Types of pyroclastic breccias
Some common types of pyroclastic breccia flow are Vulcanian, pyroclastic, and hydrovolcanic breccias. These names only indicate the process by which the breccia formed and not its composition.
1. Vulcanian and other volcanic eruptions
Pyroclastic breccia commonly forms via volcanic eruptions like the Vulcanian, Pelean, and Strombolian. These violent eruptions will cause the fragmentation of rocks, forming ash, lapilli, blocks, and bombs.
If there is a deposition of mainly the blocks (angular) in ash, lapilli, or finer-grained matrix, you will have a pyroclastic breccia.
2. Pyroclastic flows breccia
Pyroclastic flows may result in the fragmentation of already solidified lava or country rock. These fragments will then lithify in a finer-grained matrix, forming pyroclastic flow breccia if it has major angular blocks larger than 64 mm.
3. Hydrovolcanic breccias
Hydrovolcanic breccias form when magma or lava interacts with water. We further divide this category into two groups as follows:
i). Phreatic breccia
Phreatic breccia is a steam-driven explosion that throws ash, blocks, bombs, or lapilli into the air and forms rocks with mainly angular blocks in a fine-grained matrix.
It occurs when hot rock, magma, lava, or new (still hot) volcanic deposits like pyroclastic flow deposits heat, boil, and flash the surface or water beneath the ground, forming steam.
The steam explosion will cause brecciation and expel pyroclasts or tephra into the air, forming these rocks.
ii). Phreatomagmatic breccia
Phreatomagmatic breccias form when hot magma explosively interacts with water. This explosive interaction will result in steam, ash, and other fragmented rock (pyroclasts) ejection into the air.
Geologists believe the rapid quenching of magma interaction with water results in explosive thermal contraction, contributing to brecciation and ash formation.
Unlike phreatic breccia, phreatomagmatic will have mainly magmatic or juvenile clasts. Also, in the former, magma never reaches the Earth’s surface.
Pyroclastic breccia composition
Pyroclastic breccia describes the texture of extrusive igneous rock with large, angular fragments. It doesn’t tell any information on composition.
The composition of pyroclastic breccia is primarily of igneous origin but varies depending on the magma (lava). In most cases, these rocks have felsic, such as rhyolitic to intermediate, like andesitic composition, since they tend to erupt violently. However, you can have basaltic ones.
Some of the magma may have larger mineral crystals formed deeper in the Earth’s crust, probably as the magma rose.
Also, they may have some country rock or accidental clasts torn from the vent wall. These rocks can be igneous, sedimentary, or even metamorphic.
Lastly, to help define the exact type of rock, you will have a prefix of the rock involved, such as andesitic Vulcanian breccia or basaltic pyroclastic breccia.
Summary
- Description: Soft to hard rock with predominantly large (>64mm) angular fragments set in a fine-grained matrix.
- Rock type: Pyroclastic volcanic rock
- Color: Mostly graying but can be black, brown, reddish, or multi-colored. Color will depend on the composition of volcanic blocks that form these pyroclastic breccias.
- Texture: Pyroclastic, volcaniclastic or brecciated
References
- W., L. M. R. (2002). Igneous rocks: IUGS classification and glossary of terms (1st ed.). Cambridge University Press.
- Bowes, D. R. (1990). Volcanic Breccia. In The encyclopedia of igneous and Metamorphic Petrology (1st ed.). Van Nostrand Reinhold.
- Kusky, T. M. (2005). Encyclopedia of earth science (1st ed.). Facts on File.