Pyroclastic rocks

Pyroclastic rocks form from the accumulation of fragmental debris ejected during explosive volcanic eruptions. Pyroclastic debris may accumulate either on land or under water. Volcanic eruptions that generate pyroclastic debris are extremely high-energy events and are potentially dangerous if they occur near populated areas.

Formation of Pyroclastic Rocks

Pyroclastic rocks form as a result of violent volcanic eruptions such as that of Mount St. Helens in 1980 or Mount Vesuvius in 79 CE. Molten rock, or magma, within the earth sometimes makes its way to the surface in the form of volcanic eruptions. These eruptions may produce lava or, if the eruption is highly explosive, fragmental debris called tephra or pyroclasts. The term “pyroclastic” is from the Greek roots pyros (“fire”) and klastos (“broken”). Dissolved water and gases (volatiles) are the source of energy for these explosive eruptions. All molten rock contains dissolved fluids such as water and carbon dioxide. When the molten rock is still deep within the earth, the confining pressure of the overlying rock keeps these volatiles from being released. When the magma rises to the surface during an eruption, the pressure is lowered, and the gases and water may be violently released, causing fragmentation of the molten rock and some of the rock surrounding the magma. This type of explosive eruption is more common in rocks rich in silica, which are more viscous (flow more thickly) than those that are silica-poor. External sources of water, such as a lake or groundwater reservoir, may also provide the necessary volatiles for an explosive eruption.

Pyroclastic debris can be produced from any of three different types of volcanic eruptions: magmatic explosions; phreatic, or steam, explosions; and phreatomagmatic explosions. Magmatic explosions occur when magma rich in dissolved volatiles undergoes a decrease in pressure such that the volatiles are rapidly released or exsolved. The solubility of volatiles in magma is partially controlled by confining pressure, which is a function of depth. Solubility decreases as the magma rises toward the surface. At a certain depth, carbon dioxide and water begin exsolving and become separate fluid phases. At this point, the magma may undergo explosive fragmentation either through an open vent or by destroying the overlying rock in a major eruptive event.

As a magma rises toward the surface, it may encounter a groundwater reservoir or, in a subaqueous vent, interact with surface water. In both cases, the superheating and boiling of water followed by its explosive expansion to gas may fragment the magma and the surrounding country rock. The ratio between the mass of water and the mass of magma controls the type of eruption. If there is little water in relation to magma, the explosive activity may be confined to the eruption of steam and is called a phreatic explosion. If the magma contains significant quantities of dissolved volatiles and encounters a large amount of water, the resulting explosion is termed phreatomagmatic.

Composition of Pyroclastic Deposits

Pyroclastic deposits are composed of tephra, or pyroclasts. These fragments can have a wide range of sizes. Particles less than 2 millimeters in diameter are termed ash; those between 2 and 64 millimeters are called lapilli; and those greater than 64 millimeters are called blocks, or bombs. There are three principal components that make up pyroclastic debris: lithic fragments, crystals, and vitric, or juvenile, fragments. Lithic fragments can be subdivided into pieces of the surrounding rock explosively fragmented during an eruption (accessory lithics), pieces of already solidified magma (cognate lithics), and particles picked up during transport of eruptive clouds down the flanks of a volcano (accidental lithics). Crystal fragments are whole or fragmented crystals that had solidified in the magma before eruption. Vitric or juvenile fragments represent samples of the erupting, still molten, magma. They may be either partly crystallized or uncrystallized (glass). Pumice is a type of juvenile fragment that contains many vesicles, or holes, as a result of the rapid exsolution of gases during eruption. Small, very angular glass fragments are called shards.

Types of Pyroclastic Deposits

Three types of pyroclastic deposit can be distinguished based on the type of process that forms the deposit: pyroclastic-fall deposits, pyroclastic-flow deposits, and pyroclastic-surge deposits. These types can all be formed by any of the previously described different types of volcanic eruption. Any of these deposits may be termed a tuff if the predominant grain size is less than 2 millimeters.

Pyroclastic-fall deposits form from the settling of particles out of a plume of volcanic ash and gases erupted into the atmosphere that form an eruption column. Tuffs formed in this way are coarsest near the eruption center and become progressively finer farther away. Ash falls can also be derived from the top of more dense pyroclastic flows, as the finer-grained material is turbulently removed from the upper portion of the pyroclastic flow and then settles to the ground.

Two types of pyroclastic deposits result from the formation of dense clouds of ash during an eruption and the subsequent transport of debris, in the form of a hot cloud of ash, lapilli, and gases. Pyroclastic flows have a relatively high particle concentration and in some areas—the western United States, for example—these flows form enormous deposits with volumes as large as 3,000 cubic kilometers. Pyroclastic-flow deposits rich in pumice are termed ignimbrites. Pyroclastic surges are generally turbulent, low-particle concentration density currents. Surge deposits are volumetrically less important than those of pyroclastic flows but can be very destructive. Both flows and surges may have emplacement temperatures of up to 800 degrees Celsius. Because of their lower density and turbulent nature, pyroclastic surges may attain velocities up to 700 kilometers per hour. It is this combination of speed and temperature that makes pyroclastic surges so dangerous.

Study of Pyroclastic Rocks

Geologists study pyroclastic rocks using field techniques, laboratory analyses, and theoretical considerations of eruption processes. Observations of deposits in the field remain a significant cornerstone of geologic interpretation. Pyroclastic deposits form essentially as sedimentary material—that is, as fragments or clasts moved in air or water and deposited in layers. As such, many of the techniques used by sedimentologists (geologists interested in the formation and history of sedimentary rocks) are employed in the study of pyroclastic deposits. Careful examination of a variety of sedimentary features within pyroclastic deposits can aid in the interpretation of the processes of transport and deposition. This information, studied over as wide a geographic area as possible to ascertain systematic changes in the deposits, will assist in understanding the geologic history of the region.

Much can be learned through analysis of the composition of pyroclastic rocks, which is generally done using a variety of laboratory techniques. The use of specialized microscopes allows geologists to examine very thin sections of rock in order to observe textures and to discern mineral composition. Geochemical techniques have become very popular and powerful in the study of all kinds of rocks, including pyroclastic rocks. By looking at the amounts of certain elements that occur in extremely low abundances and also at relative proportions of certain types of isotopes (naturally occurring forms of the same element that differ only in the number of neutrons in the nucleus), scientists can understand more about the processes taking place deep within the earth that lead to the formation of magma and eventually to the eruption of pyroclastic debris. Scanning electron microscopes have been used to study in detail the surface features and textures of fine volcanic ash particles. This information can lead to better understanding of eruptive and transport processes that formed and deposited the pyroclastic particles.

Theoretical studies associated with pyroclastic rocks revolve primarily around considerations of the mechanics of high-temperature, high-velocity eruption clouds and their transport and deposition. This type of reasoning allows a geologist to infer certain conditions of eruption from an analysis of the deposits. The geologic rock record has abundant pyroclastic deposits, and it is through this type of inference that geologists interpret the geologic history of a region. A comprehensive understanding of pyroclastic deposits must include a thorough understanding of the processes by which the deposit forms.

Association with Violent Eruptions

Most pyroclastic rocks are associated with stratovolcanoes, also called composite volcanoes. These volcanic edifices are built by a combination of extrusive and explosive processes and thus are formed of both pyroclastic debris and lava. Well-known examples of stratovolcanoes include Mount St. Helens, Mount Fuji, and Mount Vesuvius. Volcanoes such as those found on the Hawaiian Islands are of a less energetic variety (with less explosive eruptions) called shield volcanoes; this variety produces insignificant quantities of pyroclastic material. Stratovolcanoes are located around the world and are associated with the global process of plate tectonics, the slow motion of large slabs of crust in response to flow in the earth’s mantle. It is at the zone of plate destruction that rock is melted and makes its way to the surface, sometimes producing pyroclastic eruptions.

Violent volcanic eruptions that may produce pyroclastic deposits are among the most powerful events occurring on Earth. Historically, many of the most destructive volcanic eruptions have involved pyroclastic surges. The eruption in 79 CE of Mount Vesuvius generated pyroclastic debris that buried the towns of Pompeii and Herculaneum, killing large numbers of people. In 1902, on the island of Martinique in the Caribbean, the violent eruption of Mount Pelée produced a pyroclastic surge that swept down on the city of St. Pierre, killing all but a handful of a population of about thirty thousand. The eruption of Mount St. Helens in 1980 and of El Chichón in Mexico in 1982 both produced pyroclastic surges. Two thousand people were killed as a result of the El Chichón eruption. Pyroclastic surges and flows generally do not present a hazard beyond a radius of about 20 kilometers. In 1991, Mount Pinatubo in the Philippines unleashed the largest eruption in nearly a century. In 2009, Alaska's Redoubt Volcano, which is about 161 kilometers from Anchorage, erupted, spewing ash 18,200 meters into the air. The eruption also had a large pyroclastic flow.

Pyroclastic deposits form a major portion of some volcanic terrains. Some of these deposits are enormously extensive, indicating that the eruptions that produced them were much larger than any witnessed in modern times. It is not clear whether these deposits reflect an overall increase in volcanic activity in Earth’s past or whether this type of titanic eruption occurs sporadically throughout geologic time. Titanic eruptions inject so much debris into the upper atmosphere that global weather can be affected. Earth experienced brilliant red sunsets and lowered temperatures because dust blocked the sun for several years following the 1883 eruption of Krakatau in the strait between Java and Sumatra, which completely destroyed an island and discharged nearly 20 cubic kilometers of debris into the air. The explosion was heard nearly 5,000 kilometers away in Australia, and darkness fell over Jakarta, 150 miles away. In 1815, an even larger eruption of Tambora, Indonesia, vented so much ash that global climate was significantly cooled. Some geologists speculate that enormous volcanic eruptions in Earth’s past even led to extinctions, including that of the dinosaurs, by producing so much ash that the amount of sunlight Earth received was reduced and the entire food chain disrupted. The largest pyroclastic eruptions of all are those in which the roof of a magma chamber simply caves in and hundreds, even thousands, of cubic kilometers of ash are erupted. Such eruptions, sometimes called “supervolcanoes,” include several in Yellowstone National Park (in the last 2 million years); Toba, Sumatra (70,000 years ago); Long Valley, California (700,000 years ago); and the Jemez Caldera, New Mexico (1.2 million years ago).

Principal Terms

ash: fine-grained pyroclastic material less than 2 millimeters in diameter

ignimbrite: pyroclastic rock formed from the consolidation of pyroclastic-flow deposits

lapilli: pyroclastic fragments between 2 and 64 millimeters in diameter

pumice: a vesicular glassy rock commonly having the composition of rhyolite; a common constituent of silica-rich explosive volcanic eruptions

pyroclastic fall: the settling of debris under the influence of gravity from an explosively produced plume of material

pyroclastic flow: a highly heated mixture of volcanic gases and ash that travels down the flank of a volcano; the relative concentration of particles is high

pyroclastic surge: a turbulent, low-particle concentration mixture of volcanic gases and ash that travels down the flank of a volcano

stratovolcano: a volcanic cone consisting of lava, mudflows, and pyroclastic rocks

tephra: fragmentary volcanic rock materials ejected into the air during an eruption; also called pyroclasts

tuff: a general term for all consolidated pyroclastic rocks

volatiles: fluid components, either liquid or gas, dissolved in a magma that, upon rapid expansion, may contribute to explosive fragmentation

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