**2. Definitions**

Before starting with the identification criteria for paleovolcanic rocks, it is necessary to review some definitions referring to the processes and products of paleovolcanism. The first aspect that requires our attention is the proper definition of paleovolcanism.

There is no specific age from which the limit between ancient volcanism (or paleovolcanism) and modern volcanism can be distinguished, since we can find relatively recent terrains that have been strongly altered and eroded (e.g., some volcanic islands or active calderas hosting geothermal fields), and ancient terrains that preserve a large part of the original characteristics of their volcanic materials (e.g., Permo-Carboniferous volcanism in some areas). For this reason, and although there is no specific definition for the term paleovolcanism, we will consider it as the volcanism recorded in the stratigraphic record of regions that have undergone erosion, diagenesis, tectonics, hydrothermal, and/or metamorphic processes, thus causing significant changes in the original volcanic facies.

The second aspect that needs our attention is the difference between processes and products (**Figure 2**). Processes refer to those aspects concerning the origin as well as the transport and emplacement mechanisms of volcanic materials, while the term products should be understood as the result of these processes. While this is not particularly problematic when referring to a lava flow—and in this case, both the process and the product will receive the same name, *lava flow*—the situation is much more complex when we refer to any type of clastic volcanic deposits (e.g., covering the full spectrum of primary, eruption-fed products to secondary, epiclastic successions). For

#### **Figure 2.**

*Difference between processes and products in volcano-sedimentary environments: a) formation of an eruption column that will rise into the atmosphere and will disperse horizontally controlled by the predominant winds and initiation of a PDC that will run away from the vent on the volcano slopes controlled by gravity (Mount Saint Helens 1980 eruption, (photo by H. Glicken-U.S. Forrest Service. Credit: USGS). b) Fallout deposit formed by the deposition of pumice fragments from the eruption column and of an ignimbrite deposited from a PDC (Tenerife, Canary Islands) (credit: Joan Martí).*

this reason, it is necessary to distinguish here between process and product and to use an appropriate nomenclature that allows them to be differentiated. As an example, we can use the term pyroclastic density current (PDC), which refers to the flow processes of transport and deposition of primary pyroclastic material (e.g., [3, 4]). It is therefore incorrect to use this term as a particular type of fragmentary volcanic deposit. The term PDC deposit, as a general term to refer to an indeterminate type of deposit produced by such a process, or the terms ignimbrite, block-and-ash flow, dilute PDC deposit, dense PDC, deposit, etc., applied to specific deposits derived from PDCs with different characteristics, are appropriate to refer to the products of this process.

This complexity increases when referring to paleovolcanic materials where the discrimination between primary and secondary products (derived from the weathering and erosion of the former) is not always straightforward (e.g., [1, 5]). Likewise, terms such as ignimbrite, block-and-ash, Plinian fall deposit, although they refer to a deposit, imply a specific type of process in each case. In paleovolcanism, it is not always straightforward to identify the genetic characteristics of a deposit based on its lithological features, hence I recommend using purely descriptive terms (see below), even though this description may fit for the products of different processes, and there will be time to apply more precise terms if the information obtained eventually allows it.

To clarify the nomenclature of clastic volcanic materials, Fisher [6, 7] established two groups of definitions, the first group is non-genetic, based on the lithological characteristics of volcanic materials in order to differentiate the different products, and the second group comprises definitions focused on differentiating between their genetic mechanisms. Some of these definitions were reviewed by Fisher and Schmincke [8] and Fisher and Smith [9]. According to Fisher's definitions, the term volcaniclastic includes the entire spectrum of clastic materials composed in part or entirely of volcanic fragments originating from any particle formation mechanism (i.e., pyroclastic, hydroclastic, epiclastic, autoclastic), transported by any mechanism, deposited in any physiographic environment, or mixed with any other volcaniclastic type, or with any type of non-volcanic fragments in any proportion whatsoever. This non-genetic term allows products to be identified without the need to attribute origins or processes to them.

The main fragmentation processes that generate volcaniclastic deposits are pyroclastic, hydroclastic, autoclastic, and epiclastic (**Figure 3**) [7, 9]. Pyroclasts are formed by direct fragmentation of magma due to the rapid exsolution and explosive expansion of the gases it contains. Hydroclasts are formed by explosive or nonexplosive water-magma interactions that result in frozen glass particles. Autoclastic

*Volcano Geology Applications to Ancient Volcanism-Influenced Terrains: Paleovolcanism DOI: http://dx.doi.org/10.5772/intechopen.108770*

#### **Figure 3.**

*Examples of products originated by different fragmentation processes. a) Pumice-rich ignimbrite resulting from pyroclastic fragmentation of the erupting magma at the conduit (Cerro Galán ignimbrite, Central Andes, Argentina, 2 Ma). b) Hyaloclastites originated by hydrofragmentation of a subglacial basaltic lava flow (precaldera deposits, Deception Island, Antarctica, unknown age). c) Autobrecciated sub-aerial andesitic lava flow (Coll de Vanses andesites, Catalan Pyrenees, NE Spain, 300 Ma). d) Epiclastic deposit (ep) formed by water erosion and redeposition of a previous ignimbrite (ig). (Castellar de N'Hug Permian red beds, Catalan Pyrenees, NE Spain, 285 Ma) (credits: Joan Martí).*

fragmentation is caused by the mechanical friction of lava flows that are being emplaced or by the gravitational collapse of domes or spines. Finally, epiclastic fragments are lithic fragments and crystals derived from any type of preexisting rock by weathering and erosion—in this case, volcanic or volcaniclastic. Fisher and Smith [9] suggested that to understand volcanic facies and sedimentation differences between volcanic and non-volcanic areas, fragmentation processes must be clearly separated from particle transport processes (e.g., wind, pyroclastic flows, flowing water, ice, avalanches), since these terms refer to the processes that create the particles and that cannot change from one type of particle to another simply by changing the transport agent.

In this chapter, I follow Fisher's conceptual recommendations, and therefore, an attempt is made to clearly differentiate between processes (fragmentation, transport, and deposition mechanisms) and their products (rocks and deposits), bearing in mind that before deducing the process, we must describe and correctly interpret the products, which is not always possible.
