The wonders of volcanic ash – an introduction

Volcanic ash includes minute fragments of volcanic glass, crystals and rocks that are ejected out of a volcano during highly explosive eruptions. Lots of information can be gained about eruption dynamics from the study of this ash, and this forms a major part of my PhD research. In this article I will show you what volcanic ash looks like close up, I will share some ideas on how different characteristics of ash form and introduce why studying these is so important.

Volcanic ash under a microscope

Volcanic ash includes the finest material that is produced in volcanic eruptions. Therefore, it is often so small that you can barely see it with the naked eye. One way of getting a closer look is to put some grains of ash beneath a microscope.

Basalt ash under the microscope
Black basaltic ash under a binocular microscope (left) and in thin-section (right). Credit: S.Wanmer 2017

In general volcanic ash is <1mm in size. This material is the most buoyant and can be transported to great distances away (up to thousands of km’s) from its volcanic source. Ash can contain fragments of rock which was blasted apart in the vent of the volcano and broken apart within the eruption cloud, these particles are known as lithics (basically rocks) and are referred to as ‘accidental’ components of a volcanic deposit as they were ‘accidentally’ picked up during an eruption. Ash also includes fresh volcanic glass produced by particles of magma that cooled during an eruption and subsequently interacted with each other in the eruption column.  This glass is referred to as a ‘juvenile’ component of a volcanic deposit as it is composed of fresh material produced directly from the eruption and fragmentation of magma.

Volcanic glass, or ‘juvenile’ particles, can contain a lot of information about the eruption processes in which it was formed. For example, the overall shape of the particles can give some indication as to the presence or absence of water during an eruption. If the particles have lots of jagged edges then they may have interacted with lots of other particles within the eruption column prior to deposition. Some particles of ash may have smooth concave edges suggesting that they were once the walls of vesicles (bubbles of magmatic gas). An absence of vesicles can suggest that an eruption involved only a limited amount of magmatic fragmention, and that the explosions that produced the ash were instead controlled by interactions with water (e.g. hydrovolcanic eruptions). The glass may have some even smaller vesicles trapped within it, these can be studied in greater depth to understand the eruption dynamics that were present during a particular eruption.

Once these observations have been understood in modern-day examples from known eruptions, it may be possible to identify similar features within ancient volcanic deposits. This can help us to gain more information about pre-historic eruptions which may also help us to better prepare for the hazards posed by ash from eruptions under various conditions in the future.

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