Pyroclastic rock in the context of "Stratovolcano"

Rhyolite (/ˈraɪ.əlaɪt/ RY-ə-lyte) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral assemblage is predominantly quartz, sanidine, and plagioclase. It is the extrusive equivalent of granite.

Its high silica content makes rhyolitic magma extremely viscous. This favors explosive eruptions over effusive eruptions, so this type of magma is more often erupted as pyroclastic rock than as lava flows. Rhyolitic ash-flow tuffs are among the most voluminous of continental igneous rock formations.

Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism.

Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, they remain as tephra unless hot enough to fuse into pyroclastic rock or tuff. When a volcano explodes, it releases a variety of tephra including ash, cinders, and blocks. These layers settle on the land and, over time, sedimentation occurs incorporating these tephra layers into the geologic record.

In volcanology, an explosive eruption is a volcanic eruption of the most violent type. A notable example is the 1980 eruption of Mount St. Helens. Such eruptions result when sufficient gas has dissolved under pressure within a viscous magma such that expelled lava violently froths into volcanic ash when pressure is suddenly lowered at the vent. Sometimes a lava plug will block the conduit to the summit, and when this occurs, eruptions are more violent. Explosive eruptions can expel as much as 1,000 kg (2,200 lb) per second of rocks, dust, gas and pyroclastic material, averaged over the duration of eruption, that travels at several hundred meters per second as high as 20 km (12 mi) into the atmosphere. This cloud may subsequently collapse, creating a fast-moving pyroclastic flow of hot volcanic matter.

The 1669 eruption of Mount Etna is the largest-recorded historical eruption of that volcano on the east coast of Sicily, Italy. After several weeks of increasing seismic activity that damaged the town of Nicolosi and other settlements, an eruption fissure opened on the southeastern flank of Etna during the night of 10–11 March. Several more fissures became active during 11 March, erupting pyroclastics and tephra that fell over Sicily and accumulated to form the Monti Rossi scoria cone.

Lava disgorged from the eruption fissures and flowed southwards away from the vent, burying farmland and a number of towns during March and April, eventually covering 37–40 square kilometres (14–15 sq mi). The inhabitants of the towns fled to the city of Catania and sought refuge there; religious ceremonies were held in the city to implore the end of the eruption. In early April a branch of the lava flow advanced towards the city, and on the 1 or 16 April it reached the city walls, provoking the flight of many of its inhabitants. The city walls held up the lava, which began to flow into the Ionian Sea. More than two weeks later, parts of the flow surmounted the walls and penetrated Catania but did not cause much damage. The eruption ended in July.

Herculaneum is an ancient Roman town located in the modern-day comune of Ercolano, Campania, Italy. Herculaneum was buried under a massive pyroclastic flow in the eruption of Mount Vesuvius in 79 AD.

Like the nearby city of Pompeii, Herculaneum is famous as one of the few ancient cities to be preserved nearly intact, as the solidified material from the volcano that blanketed the town protected it against looting and the elements. Although less known than Pompeii today, it was the first and, for a long time, the only discovered Vesuvian city (in 1709). Pompeii was revealed in 1748 and identified in 1763. Unlike Pompeii, the mainly pyroclastic material that covered Herculaneum carbonized and preserved more wooden objects such as roofs, beds, and doors, as well as other organic-based materials such as food and papyrus.

A volcanic dam is a type of natural dam produced directly or indirectly by volcanism, which holds or temporarily restricts the flow of surface water in existing streams, like a man-made dam. There are two main types of volcanic dams, those created by the flow of molten lava, and those created by the primary or secondary deposition of pyroclastic material and debris. This classification generally excludes other, often larger and longer lived dam-type geologic features, separately termed crater lakes, although these volcanic centers may be associated with the source of material for volcanic dams, and the lowest portion of its confining rim may be considered as such a dam, especially if the lake level within the crater is relatively high.

Volcanic dams generally occur worldwide, in association with former and active volcanic provinces, and are known to have existed in the geologic record, in historic times and occur in the present day. Their removal or failure is similarly recorded. The longevity, and extent varies widely, having periods ranging from a few days, weeks or years to several hundred thousand years or more, and dimensions ranging from a few meters to hundreds, to several thousand.

Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash lithifies into solid rock. Rock that contains greater than 75% ash is considered tuff, while rock containing 25% to 75% ash is described as tuffaceous (for example, tuffaceous sandstone). A pyroclastic rock containing 25–75% volcanic bombs or volcanic blocks is called tuff breccia. Tuff composed of sandy volcanic material can be referred to as volcanic sandstone.

Tuff is a relatively soft rock, so it has been used for construction since ancient times. Because it is common in Italy, the Romans used it often for construction. The Rapa Nui people used it to make most of the moai statues on Easter Island.