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Eruptions Word of the Day: Peperite

The next Eruptions Word of the Day describes what happens when hot magma and cool sediment get too close.

Continuing my series where I try to define words of volcanic interest, the new Eruptions Word of the Day is a favorite of mine, mostly because my undergraduate thesis on Vinalhaven Island in Maine ended up dealing with a lot of these types of deposits … so, without further ado, the word is peperite!

Now, what exactly is a “peperite”?

Well, a picture is worth 1,000 words, right (so that will save me some time):

Peperite in the Vinalhaven Diabase, Vinalhaven Island, Maine. Image by Erik Klemetti. Click on the image to see a larger version.

Let’s make some observations:

  1. The rock is full of red-to-black clasts of various sizes and shapes.
  2. The clasts are almost all basaltic andesite in composition.
  3. Some of the clasts look like they have a “chilled” margin, where hot molten material came into contact with colder material.
  4. Some of the clasts look like you could piece them back together – as if they were shattered.
  5. The material between them (the matrix) is mostly light grey/tan, looks to be fine grained.
  6. On closer inspection, the matrix looks like mud or sand.
  7. In some spots, the matrix looks “baked”, like it was exposed to high temperatures.
  8. In other spots, the matrix looks like it was squeezed into cracks and open spaces in the basaltic andesite clasts.
  9. So, what does this suggest?

    First off, it looks like sediment (the muddy matrix) is mixed with magma (basaltic andesite clasts). The chilled margins on the clasts and the baked areas of the sediment suggest that the magma was still hot and the sediment was cool (relatively speaking). This is supported by shattered clasts of basaltic andesite in the deposit (thermal shock). The sediment looks to have squeezed its way into spaces between clasts, so the sediment was likely not solid – unconsolidated sediment – and like wet, as mud should be. First order conclusion: It appears that hot basaltic andesite magma interacted with wet, unconsolidated muddy sediment.

    And there you have it: peperite – magma interacting, sometimes explosively, with wet, unconsolidated sediment, producing a mixed rock with magmatic clasts and a sediment matrix.

    The image above is from Vinalhaven Island in Maine, where the Siluro-DevonianVinalhaven Diabase, a thick basaltic andesite sill intruded the Seal Cove Formation (a sedimentary unit). Along the contact in places you find these peperites where the sill intruded into this wet sediment. This likely means that the sill was being intruded fairly shallowly in the crust as sediment begins to compact and loose pore water quickly as it is buried. Peperites have been recognized in locales all over the world and has been interpreted as one of the steps towards explosive hydrovolcanism – and experiments have been tried to recreate a peperite in the lab. There is some evidence that even lava flows on the surface can form peperite deposits as well. However, the thing to remember is that when you find peperites in the rock record, what you are seeing is evidence of shallow emplacement of magma in the crust where it can interact with wet, soft sediment – a current analog might be if magma intruded underneath Mono Lake in California or in the shallow seas along an oceanic arc like the Marianas or Tonga (think about the Hunga Tonga-Hunga Ha`apai eruption).

    Another example of a peperite, courtesy of Highly Allochthonous.

    And from where is the name “peperite” derived? It comes from the type locality of peperites, Peper Limagne d’Au- vergne region of central France, where the rocks reminded George Scropes in 1827 of ground pepper.


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