Sedimentary rocks are an understated source of information when trying to understand present and past eruptive activity at a volcano. They therefore form a very important focus of much volcano research. Here I will explain some of the reasons why studying and understanding sediments is so important in volcanological research.
Why are sediments important? – the rock cycle
Sediments are an important component of the rock cycle. The rock cycle in general includes sediments which come from the weathering and erosion of pre-existing igneous and metamorphic rocks, in turn these sediments may be compressed and transformed into metamorphic minerals by heat and pressure. They can also be melted and become a component of magma and subsequent igneous rocks. In turn these metamorphic and igneous rocks can become exposed on the surface of the earth and worn away by weather, landslides (gravity), water and ice and end up as sediments.
This is all well and good, but, what about material that isn’t weathered from pre-existing rocks? What about all the volcanic material thrown out during eruptions and deposited across the land and throughout the sea?
In most cases this material, the finest grains in particular, end up within the sedimentary environment. These are environments in which eroded grains of pre-existing rocks are dominant (not to mention all the substances and bits and pieces that come from biological sources!), for example in rivers, the sand on the beach, as well as, the silt and mud in estuaries and the sea. This leaves us with a bit of a dilemma…
Igneous vs. sediment
In general, any rock or pieces of rock produced from magma and intruded below the surface, or erupted onto the surface of the earth are considered to be types of igneous rock. Igneous rocks however can take two forms: intrusive (those that remain below the ground) and extrusive or volcanic (those erupted above ground). Volcanic material is what we will consider here.
When volcanic material (AKA tephra) is deposited onto the ground it is a ‘primary’ volcanic deposit. The material has been erupted and sits where it first touches the ground. However, in most environments this material will be re-mobilised, for example, by wind, water or gravity.
As soon as these grains move, they are technically no longer ‘primary’ volcanic material and no longer form a ‘primary’ volcanic deposit. Technically these re-mobilised grains are now effectively sediment – they will behave like any other sedimentary grains of the same morphology, being influenced by different conditions in the surrounding environment.
Therefore, there is plenty of debate about when volcanic grains become a sediment, and how the deposits that they form should be named. Commonly deposits composed of volcanic grains that are not ‘primary’ are termed volcaniclastic sedimentary deposits.
There are still some complications with this, for example:
- How far does a volcanic grain have to travel before it comes a volcaniclastic sedimentary grain – 1mm? If such a small distance as this is considered for renaming of the grain from ‘primary volcanic’ to ‘volcaniclastic’ then shouldn’t all volcanic deposits be re-named as volcaniclastic sediments?
- How would you be able to tell if a volcanic grain has travelled 1mm or 1km from the place where it was originally deposited? – this is the centre of some important and often very confusing research (including my PhD)!
- And, I’m sure many more complications – feel free to comment with your own thoughts on this.
Understanding sediments is just as important as understanding the volcanic grains present within a volcanic or sedimentary rock sequence.
Layers of volcanic material can often be found within layers of sedimentary material. A full understand of all of these deposits will help piece together the history of the volcanic grains. For example, you might be able to work out the environment in which the volcanic material was deposited (e.g. water and if this was the water of a river, lake or the ocean), as well as aiding identification of ‘primary’ volcanic and volcaniclastic sedimentary deposits. In turn, this can help you when trying to understand the eruptive history of a volcano, including the type of eruption and eruption dynamics. Finally, this can then aid in identifying the hazards posed by different types of eruption at an active or dormant volcano, potentially preventing the loss of life during the next eruptive episode.