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Generaldefinition of breccia

1st Oct 2022 14:19 UTCThe Evje og Hornnes geomuseum, Fennefoss Expert

The general definition of breccia at Wikipedia:

(https://en.wikipedia.org/wiki/Breccia)

"Breccia (/ˈbrɛtʃiə, ˈbrɛʃ-/) is a sedimentary rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. "

But somewhat further in the article it is recognized that there are breccia's with a completely different origin: fault, igneous, volcanic, intrusive, impact.

I do not understand how to reconcile this.

Can anybody clarify this to me?

1st Oct 2022 14:59 UTCDavid Von Bargen Manager

Breccias are large chunks of angular rock fragments that are surrounded by a fine grained matrix. They differ from porphyries in that these igneous rocks have large crystals in a finer grained igneous matrix.

Sedimentary breccias are formed when large chunks of rock have not had sufficient time to be well rounded become lithified. Volcanic breccias form when there is a volcano that when it erupts breaks up already solidified volcanic rocks and the matrix is composed of the newly erupted lava.

Impacts and hydrothermal breccias occur when the impact or steam explosion breaks up the preexisting rocks into angular chunks and the finer particles form the matrix.

1st Oct 2022 17:03 UTCDemetrius Pohl Expert

Breccia is a textural term describing rock fragmentation. Breccias are essentially angular broken fragments of any kind of rock cemented together with a fine-grained cement of smaller rock fragments. Fragments of rock are known as clasts, and can be juvenile (immature) or mature. Juvenile clasts are ones that appear sharp and angular because they have not been worn down or rounded by secondary processes like water or wind transport, or milling in a volcanic vent or fault zone. So breccias can be created by sedimentary or volcanic processes, by hydrothermal process or chemical process (collapse breccias in cave formation, or by tectonic processes like faulting.

1st Oct 2022 17:08 UTCRichard Gibson 🌟

I edited the Wikipedia article to remove "sedimentary" from the first line. The rest of the article seems OK.

1st Oct 2022 18:30 UTCThe Evje og Hornnes geomuseum, Fennefoss Expert

OK, thanks, that was my problem. This definition of breccia as primarily a sedimentary rock is repeated on several websites, while other types exist too. I was wondering, was there some special reason that I had overlooked...

2nd Oct 2022 03:44 UTCDon Windeler

I like breccias a lot and have carved out a couple of flats of them in my collection -- even did a show display on them once. I'd never throught about them as a sedimentary rock, but I can see how they could be interpreted that way. There are clasts of some rock / mineral material that are cemented and lithified. You can look at the name in front as a genetic modifier, i.e. description of how the breccia was formed, but if you are hewing to a certain description of sedimentary rocks it could land in that bucket.

Right or wrong, I always looked at breccia as a textural description. Interesting discussion here that's made me think a little about the terminology.

Cheers,

3rd Oct 2022 00:23 UTCEd Clopton 🌟 Expert

Good catch! I would have read right past the "sedimentary" in the first line. I guess they are technically sedimentary as Don concludes, but the fact that they can form in volcanic events obscures their technical sedimentary nature. Interesting idea.

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