Historical Geology/Volcanic ash
Volcanic ash is the name given to the fine particles of igneous rock produced by a volcano. As such, the name "volcanic ash" is a misnomer, since "ash" really means a residue left after incineration: the name is left over from times when people thought that volcanic ash really was ash.
In this article we shall discuss the origin, transport, and lithification of volcanic ash, and as usual we shall consider how we can recognize volcanic ash, and its lithified counterpart (tuff) in the geological record. It will be helpful for the reader to have already read the main article on igneous rocks before reading this article.
Origin and dispersal of volcanic ash
A volcanic eruption can throw off a fine spray of lava into the atmosphere as well as or instead of producing a cohesive lava flow. Because it originates as a fine spray, the droplets of lava often cool too rapidly to form a crystal structure; instead, they form an amorphous solid (a glass), although sometimes cooling may be slow enough for minerals to form. Sometimes the ejecta will also include particles of material torn by the eruption from the solid rock forming the volcano. Collectively, these fine airborne particles are the constituents of volcanic ash.
These particles can be distributed in two different ways. If the volcano is sufficiently forceful, it can throw the volcanic ash high into the air, where it will be distributed by wind. Particles transported in the way can travel great distances: for example, ash from the eruption of Krakatoa was carried all the way to the island of Sumatra.
Alternatively, the ejected material can be transported in a pyroclastic flow. In this case, the ash-filled air is denser than the surrounding air because of its load of ash, and for this reason mixes poorly with the surrounding air, and so can travel over great distances (200 kilometers or more) before finally dissipating. The current can travel particularly far if it finds its way into a pre-existing channel such as a river valley.
Lithified volcanic ash is known as tuff. This comes in two varieties.
Welded tuff has its origins when the beds of volcanic ash, when they form, are hot enough for the clasts to compact and weld together.
Volcanic ash: the igneous sediment
Volcanic ash is, obviously, an igneous rock: it comes out of volcanoes, and you can hardly get more igneous than that. As such, you might be surprised to find an article on volcanic ash dropped into a chapter which is otherwise about sediment and sedimentary rocks. However, in many ways volcanic ash behaves like a sediment. As geologists say, it's "igneous when it goes up, and sedimentary when it comes down."
For one thing, volcanic ash consists of small clasts, just like other sediments; also, as we have seen, it can undergo cementation just like the process that cements together sand grains to make sandstone. The image to the right is a scanning electron microscope image of a single clast from the eruption of Mt. St. Helens.
Another feature that makes volcanic ash sedimentary in nature is that it can form graded beds like a sediment, with smaller particles on top. If the ash is shot high into the air, then the larger particles, the mass of which is better able to overcome air resistance, will settle first. If, on the other hand, the ash is transported by pyroclastic flow, then again the ash will still grade upwards from coarse to fine: as the current disperses and fails, the size of the particle that it can transport decreases, and so at any particular point in its path the particles that it deposits will decrease in size. (However occasionally, depending on the behavior of the volcano, the flow can also increase in speed, leading to inverse grading.)
Finally, ash beds resemble other sediments because they are a good place to find fossils. Inhaling volcanic ash tends to shred the lungs of animals, and if the ash bed is reasonably thick the bodies will undergo rapid burial, leaving them well-preserved.
However, volcanic ash is also igneous, and for this reason displays a number of features that make it of special interest to geologists. A particular volcanic eruption is a single event at a particular point in time: when we see a horizontally continuous layer of volcanic ash, we know that it was all laid down at the same time, and whatever is immediately beneath it was a snapshot of the landscape as it was just before the eruption. Such a geological feature is known as an event horizon.
This is by no means true of sedimentary rocks in general. Just because you have a continuous horizontal stratum of, for example, sandstone, this does not necessarily mean that the beds at one end were laid down at the same time as the beds at the other end, as we shall discuss in a later article.
Furthermore, each volcanic eruption has its own chemical signature: the exact mix of chemicals in one volcanic eruption will be different from another. This means that even if a layer of volcanic ash is not exposed everywhere, so that we cannot directly observe that it is a single layer, we can still check whether two exposed areas of volcanic ash do or don't belong to the same eruptive event.
Finally, unlike most sedimentary rocks, volcanic ash and the rocks that form from it can be dated directly.
Volcanic ash and tuff: how do we know?
It takes no subtle chain of reasoning to know that volcanic ash or tuff found in the geological record is indeed volcanic ash or tuff. For when a volcano erupts, we can watch volcanic ash beds being formed: and the beds in the geological record look like that.
This appearance is even more evident under the microscope. The photograph to the right shows a photomicrograph of welded tuff, where the clasts have been squashed and welded together (the larger, white fragments are broken crystals included in the ash). Note the distinctive shape of the clasts: no-one looking at welded tuff under a microscope could mistake it for anything but welded tuff.
Finally, we may note that since ash and tuff are igneous in origin they have a chemical composition that lies on the usual felsic-ultramafic spectrum of igneous rocks: so we may speak of "rhyolite ash" or "basaltic tuff". So chemically there can be no doubt that volcanic ash and tuff are igneous in origin: but the fact that they are clastic means that we can't confuse them for any other sort of igneous rock.
These considerations mean that geologists can be confident of identifying volcanic ash and tuff when they find them in the geological record.