Pyroclastic Rocks

Volcanic eruptions are spectacular natural events that have piqued man’s curiosity since prehistoric times. On the one hand they can be of great benefit to man, but on the other can cause great harm and thereby provide a major impetus to their study. Broader aspects make use of volcanoes as windows into the interior of the earth. Magma, molten rock, provides one of the major clues to the earth’s origin and to the evolution of its mantle and crust. Our planet’s hydrosphere and atmosphere — and thus the origin and evolution of life — owe their origin to degassing of the earth, a process largely accomplished by volcanic eruption. Periods of especially intense volcanic activity can affect climate and thus the world’s flora and fauna. Volcanic rocks are the source material from which many sedimentary and metamorphic rocks are derived.

Volcanoes are hills, mounds or sheets of relatively localized igneous rock assemblages made up of pyroclastic rocks, lava flows, and intrusions in varying proportions. Volcanoes differ notably in their geometry, volume, and relative amounts of pyroclastic rocks and lava flows, with differences mostly dependent upon eruptive mechanisms and rates of extrusion. These in turn depend chiefly upon the magma composition. Chemical composition of magma is also responsible for, or can be correlated with, physical properties such as volatiles and viscosity, which govern to a large extent the nature of many pyroclastic eruptions.

There are many questions about magmas that pertain to the origin of pyroclastic rocks. Why are some magmas quietly effusive, others violently explosive, and still others alternate between both characteristics? Why do some magmas build high-standing pyroclastic cones, some build small cinder cones, and still others enormous shield volcanoes or lava plateaus? Some of the most scenic volcanoes are towering cones composed primarily of pyroclastic debris knit together by a skeletal framework of lavas and dikes that help maintain the high-standing edifice. Is the commonly observed change from initial explosive to later more quiet eruptive activity within the same volcano caused by a decrease in the amount of dissolved volatiles, or is it caused by differences in the way rising magma interacts with its environment during its ascent? Which are the most important processes that cause magma to break up into particles?

Volcanic activity takes many forms, ranging from quiet lava emissions to extremely violent and explosive bursts, many of which can be related to magma composition as discussed in Chapter 3. The kinds of eruptions can be correlated to volcano shapes and sizes, and in this chapter we explore the connection between pyroclastic systems, eruptive mechanisms and their influences upon juvenile particles.

Pyroclastic fragments, also known as pyroclasts (Schmid, 1981), are produced by many processes connected with volcanic eruptions. They are particles expelled through volcanic vents without reference to the causes of eruption or origin of the particles. Hydroclastic fragments are a variety of pyroclasts formed from steam explosions at magma-water interfaces, and also by rapid chilling and mechanical granulation of lava that comes in contact with water or water-saturated sediments (Chaps. 4 and 9). We focus upon pyroclastic material in this chapter, but much of the discussion also applies to hydroclastic deposits.

The transport modes of subaerial fallout tephra are (1) by ballistic trajectory and (2) by turbulent suspension. Energy is supplied initially to fragments by the eruption and later by wind. Tephra that falls from the atmosphere onto land is called subaerial fallout or airfall tephra. Tephra deposited in standing water is called subaqueous fallout tephra and includes (1) submarine or marine tephra, and (2) sub- lacustrine tephra. Subaqueous fallout tephra originating from eruptions on land is discussed separately (Chap. 7) from pyroclastic materials originating wholly beneath water (Chap. 10). Island arc and oceanic island flank environments generally include both kinds.

Following classic studies of deep sea exploration in the 1940’s (Bramlette and Bradley, 1942; Neeb, 1943; Norin, 1948), a great deal of modern research has been done on marine ash layers, much of which is summarized by Kennett (1981). Marine ash layers were originally studied for their value as widespread stratigraphic markers, but deep penetration of the sea floor by drilling from the Glomar Challenger, compared to piston cores or dredging, has allowed assessment of paleovulcanicity extending as far back as Jurassic time, with major implications for understanding rates of sea floor spreading and the evolution of island arcs. Additionally, marine ash layers have supplied information about the cyclicity of volcanism, volcanic production rates and volumes, and the influence of large explosive eruptions on climate.

Pyroclastic flows are volcanically produced hot, gaseous, particulate density currents. Their deposits offer unparalleled opportunities to estimate minimum volumes of near-surface magma chambers as well as vertical chemical, mineralogical, and thus temperature and pressure distributions within the magma columns immediately prior to eruption. The modes of origin and transport of pyroclastic flows have been the subject of intense debate ever since the 1902 eruption of Mt. Pelée produced nuées ardentes which destroyed the town of St. Pierre. Because pyroclastic flows are emplaced rapidly and may flow for long distances, they are particularly useful for intrabasinal correlations.

Many volcanic eruptions result from the interaction of magma and external water (Table 9-1), but few volcanologists (e.g., Jaggar, 1949) have emphasized the importance of nonmagmatic water in volcanic eruptions. In our view, the importance of external water in explosive eruptions is still underestimated. Wood (personal communication) even holds that maars, which most commonly develop from hydroclastic eruptions, are the second most common volcanic landform on earth next to scoria cones.

In this chapter we are concerned with underwater volcaniclastic eruptions and products, as well as pyroclastic flows and debris generated on land and transported by mass flowage into the sea. Submarine fallout deposits derived from land-based eruptions are treated in Chapter 7. Clastic materials redistributed as mass flows or turbidity currents originating from the rapid build-up of tephra and epiclastic volcanic debris along coastal parts of volcanoes are also discussed in various places throughout the text (e.g. Chap. 13).

The name lahar is Indonesian for volcanic breccia transported by water (van Bemmelen, 1949, p. 191) but has come to be synonymous in geological literature with volcanic debris flow, a mass of flowing volcanic debris intimately mixed with water. The term lahar refers both to the flowing debris-water mixture, and also to the deposit thus formed. A classic review of the various origins of lahars is that of Anderson (1933). A more recent discussion of lahar deposits by Parsons (1969) is included in a review of volcanic breccias. Crandell (1971) gives an account of the origin and characteristics of post-glacial lahars from the slopes of Mount Rainier volcano (Washington), and Neall (1976) has prepared a bibliography of their global occurrences. The recent eruptive phases of Mount St. Helens (Washington) produced lahars in a variety of ways (Christiansen, 1980; Janda et al., 1980; Janda et al., 1981; Harrison and Fritz, 1982) but investigations of these have not been completed to date.

There are few rock types that offer better opportunities for alteration studies (weathering, diagenesis, hydrothermal) than glassy volcanic and volcaniclastic rocks. The main reason is that volcanic glass is thermodynamically unstable and decomposes more readily than nearly all associated mineral phases. Volcanic glass is a super-cooled silicate liquid with a poorly ordered internal structure consisting of loosely linked SiO₄ tetrahedra with considerable intermolecular space. Hydration and concomitant breakdown of glass results in fluxes of some elements out of the glass into interstitial pore waters. Precipitation of secondary (authigenic) minerals from such solutions, replacement of glass shards by new minerals and filling of pore space created by dissolution of glassy particles during alteration are some of the most rapid low temperature lithification processes known. Moreover, changes in composition of pore solutions and elevation in temperatures during hydration and burial result in variable mineral compositions and rapidly changing mineral assemblages because of the restricted P/T stabilities of some of the alteration products.

Stratigraphic methods used to study volcanic rocks are similar to those used to study sedimentary rocks and have similar purposes: establishing correlations, vertical time sequences, determining facies changes and the like. Additional aid in pyroclastic stratigraphy comes from igneous petrology, studies of magma evolution, and studies of the growth history of volcanoes. We stress that stratigraphic analysis — (1) mapping and subdivision of volcanic sequences into members, formations and groups, (2) determining vertical and lateral facies changes and (3) interpreting the types of eruptions, and the origin and manner of transport of rock types and the environments of deposition — provides the necessary framework for petrological and geochemical work, and that pyroclastic and epiclastic volcanic rocks often provide the most important parameters for establishing the stratigraphic framework in volcanic areas. Because of the many ways that volcanic sequences originate, however, different stratigraphic and petrologic approaches may be necessary in different situations such as, for example, in areas of plateau basalts, ignimbrite plateaus, clusters of scoria cones, composite andesite volcanoes, deep-sea ash layers or thick nonmarine tuff accumulations. Moreover, stratigraphic problems on active or dormant volcanoes commonly differ from those that must be solved in ancient volcanic regions, such as greenstone belts, or zeolitized volcaniclastic sediments, where the volcanic record may be best preserved in sedimentary accumulations derived by erosion of primary volcanic deposits.

The close proximity of continental borderlands, active and ancient tectonic regions, and modern and ancient volcanoes has long been known. Pre-plate tectonic ideas about this association culminated in Kay’s (1951) masterful synthesis showing the relationship between volcanic island arcs and ancient eugeosynclinal sedimentary basins containing abundant volcaniclastic rocks. This association is now considered to develop within convergent plate margins. Wide acceptance of plate tectonic processes began during the late-1960’s, and has resulted in a continuing and accelerating flood of literature concerned with reinterpreting a vast amount of descriptive geology in addition to new studies. Many formerly difficult geologic problems appear amenable to interpretation based on plate tectonic theory, but we add a note of caution: it is far from certain whether or not plate tectonic theory adequately explains tectonism and volcanism in Archean time which includes nearly three-quarters of earth history.