TUFF IS A BROAD TERM used to include any relatively soft, porous rock made of lithified pyroclastic minerals (ash and other sediments ejected from volcanic vents). Tuff should not be confused with the sedimentary rock tufa. Tuffs originate when foaming magma wells to the surface as a mixture of hot gases and incandescent particles, and is ejected from a volcano. After the loose pyroclastic material is deposited, it eventually lithifies to become a volcanic tuff. The conditions under which the ejected pyroclastics lithify determine the final nature of the tuff. If the pyroclastic material is hot enough to fuse, a welded tuff forms at once. Other tuffs lithify slowly through compaction and cementation with calcite or silica-like sedimentary rocks. Because of variations in the conditions of their formation and of the ejected material, tuffs can vary greatly both in texture and in chemical and mineralogical composition. Most tuff formations include a range of fragment sizes and varieties. These range from fine-grained dust and ash (ash tuffs), to medium-sized fragments called lapilli (lapilli tuffs), to large volcanic blocks and bombs (bomb tuffs). Vitric tuffs are mainly composed of ash-size fragments of volcanic glass. Lithic tuffs contain a variety of crystalline rock fragments, which may be of rhyolitic, trachytic, or andesitic composition. In crystal tuffs, crystal fragments originating from partially solidified magmas are more abundant than lithic or vitric fragments.
- Rock type Volcanic, igneous
- Fossils Generally molds of nonmarine plants and animals, including humans
- Major minerals Glassy fragments
- Minor minerals Crystalline fragments
- Color Light to dark brown
- Texture Fine
A nuée ardente (French for “fiery cloud”) is an incandescent mass of volcanic particles suspended in a cloud of volcanic gases. It can move very quickly down even slight inclines and may gain speeds of 100 miles (160km) per hour. A nuée ardente can reach temperatures of 1,300°F (700°C), and virtually no living thing in its path survives. It is characteristic of Pelean eruptions – violent, explosive volcanic eruptions, which most often occur in the Pacific Ring of Fire. A nuée ardente finally sealed the fate of Pompeii, two millennia ago.
In 1902 a nuée ardente swept down from the erupting Mount Pelée in the West Indies, killing all but two of a population of 30,000 in the town of St. Pierre, Martinique.
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