Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism.^[1]

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.

Tephrochronology is a geochronological technique that uses discrete layers of tephra—volcanic ash from a single eruption—to create a chronological framework in which paleoenvironmental or archaeological records can be placed. Often, when a volcano explodes, biological organisms are killed and their remains are buried within the tephra layer. These fossils are later dated by scientists to determine the age of the fossil and its place within the geologic record.

Tephra is any sized or composition pyroclastic material produced by an explosive volcanic eruption and precise geological definitions exist.^[2] It consists of a variety of materials, typically glassy particles formed by the cooling of droplets of magma, which may be vesicular, solid or flake-like, and a varying proportions of crystalline and mineral components originating from the mountain and the walls of the vent. As the particles fall to the ground, they are sorted to a certain extent by the wind and gravitational forces and form layers of unconsolidated material. The particles are further moved by ground surface or submarine water flow.^[3]

The distribution of tephra following an eruption usually involves the largest boulders falling to the ground quickest, therefore closest to the vent, while smaller fragments travel further – ash can often travel for thousands of miles, even circumglobal, as it can stay in the stratosphere for days to weeks following an eruption. When large amounts of tephra accumulate in the atmosphere from massive volcanic eruptions (or from a multitude of smaller eruptions occurring simultaneously), they can reflect light and heat from the sun back through the atmosphere, in some cases causing the temperature to drop, resulting in a temporary "volcanic winter". The effects of acidic rain and snow, the precipitation caused by tephra discharges into the atmosphere, can be seen for years after the eruptions have stopped. Tephra eruptions can affect ecosystems across millions of square kilometres or even entire continents depending on the size of the eruption.^[4]

Tephra fragments are classified by size:

• Ash – particles smaller than 2 mm (0.08 inches) in diameter
• Lapilli or volcanic cinders – between 2 and 64 mm (0.08 and 2.5 inches) in diameter
• Volcanic bombs or volcanic blocks – larger than 64 mm (2.5 inches) in diameter

The use of tephra layers, which bear their own unique chemistry and character, as temporal marker horizons in archaeological and geological sites, is known as tephrochronology.^[3]

The word "tephra" and "pyroclast" both derive from Greek: The word τέφρα (téphra) means "ash",^[5] while pyroclast is derived from the Greek πῦρ (pyr), meaning "fire",^[6] and κλαστός (klastós), meaning "broken in pieces".^[7] The word τέφραv (means "ashes") is used in broad context within an account by Aristotle of an eruption on Vulcano (Hiera) in Meteorologica.^[8]

The release of tephra into the troposphere affects the environment physically and chemically. Physically, volcanic blocks damage local flora and human settlements. Ash damages communication and electrical systems, coats forests and plant life, reducing photosynthesis, and pollutes groundwater.^[9] Tephra changes below- and above-ground air and water movement. Chemically, tephra release can affect the water cycle. Tephra particles can cause ice crystals to grow in clouds, which increases precipitation. Nearby watersheds and the ocean can experience elevated mineral levels, especially iron, which can cause explosive population growth in plankton communities.^[4] This, in turn, can result in eutrophication.

In addition to tephrochronology, tephra is used by a variety of scientific disciplines including geology, paleoecology, anthropology, and paleontology, to date fossils, identify dates within the fossils record, and learn about prehistoric cultures and ecosystems. For example, carbonatite tephra found at Oldoinyo Lengai (a volcano in the East African Rift Valley) has buried and preserved fossilized footprints of humans near the site of the eruption.^[10] Under certain conditions, volcanic blocks can be preserved for billions of years^{[citation needed]} and can travel up to 400 km away from the eruption.^{[citation needed]} Volcanic eruptions around the world have provided valuable scientific information on local ecosystems and ancient cultures.^{[citation needed]}

The Waw an Namus volcano is surrounded by an apron of dark tephra, which has a notable color contrast to the surrounding Sahara Desert.

Africa's volcanoes have had an impact on the fossil record. Geographically a part of Africa, El Hierro is a shield volcano and the youngest and smallest of the Canary Islands. The most recent El Hierro eruption occurred underwater, in 2011, and caused earthquakes and landslides throughout the Canary Islands. Instead of ash, floating rocks, 'restingolites' were released after every eruption.^[11] After the 2011 eruption, fossils of single-celled marine organisms were found in the restingolites verifying the origin theory that Canary Island growth comes from a single buoyant jet of magma from the Earth's core instead of cracks in the ocean floor. This is reflected in the decreasing age of the islands east to west from Fuerteventura to El Hierro.^[12]

There are about 60 volcanoes in Ethiopia, located in east Africa. In Southern Ethiopia, the Omo Kibish Rock Formation is composed of layers of tephra and sediment. Within these layers, several fossils have been discovered. In 1967, 2 Homo sapiens fossils were discovered in the Omo Kibish Formation by Richard Leaky, a paleoanthropologist. After radiocarbon dating, they were determined to be 195 thousand years old.^[13] Other mammals discovered in the formation include Hylochoerus meinertzhageni (forest hog) and Cephalophus (antelope).^[14]

In Asia, several volcanic eruptions are still influencing local cultures today. In North Korea, Paektu Mountain, a stratovolcano, first erupted in 946 AD and is a religious site for locals. It last erupted in 1903. In 2017, new fossil evidence was discovered that determined the date of Paektu Mountain's first eruption, which had been a mystery. A team of scientists directed by Dr. Clive Oppenheimer, British volcanologist, discovered a larch trunk embedded within Paektu Mountain. After radiocarbon dating, the larch was determined to be 264 years old which coincides with the 946 AD eruption. Its tree rings are being studied and many new discoveries are being made about North Korea during that time.^[15]

In northeastern China, a large volcanic eruption in the early Cretaceous caused the fossilization of an entire ecosystem known as the Jehol Biota when powerful pyroclastic flows inundated the area. The deposits include many perfectly preserved fossils of dinosaurs, birds, mammals, reptiles, fish, frogs, plants, and insects.^[16]

Europe's volcanoes provide unique information about the history of Italy. One example is Mount Vesuvius, a stratovolcano located in southern Italy,^[17] which last erupted in March 1944. Earlier, in 79 AD, in an eruption which lasted 12 to 18 hours, Vesuvius had covered the city of Pompeii in molten lava, ash, pumice, volcanic blocks, and toxic gases. Much of the town was preserved and organic materials fossilized by the volcanic ash, and that has provided valuable information about the Roman culture.^[18] Also, in Italy, Stromboli volcano, a stratovolcano, last erupted in July 2019.

Several volcanic eruptions have been studied in North America. On 18 May 1980, Mount St. Helens, a stratovolcano in Washington state, erupted, spreading five hundred million tons of tephra ash across Washington, Oregon, Montana and Idaho causing earthquakes, rockslides, and megatsunami which severely altered the topography of nearby areas.^[19] In Yellowstone National Park, eruption-related flooding caused trees to collapse and wash into lake beds where they fossilized. Nearby forests were flooded, removing bark, leaves, and tree limbs.^[20] In 2006, the Augustine Volcano in Alaska erupted generating earthquakes, avalanches, and projected tephra ash approximately two hundred and ninety kilometers away. This dome volcano is over forty thousand years old and has erupted 11 times since 1800.^[21]

In South America, there are several historic active volcanoes. In southern Chile, the Chaitén volcano erupted in 2011 adding 160 meters to its rim. Prehistoric weapons and tools, formed from obsidian tephra blocks, were dated at 5,610 years ago and were discovered 400 km away.^[22] Due to the location of the subduction zone of the eastern Pacific's Nazca Plate, there are twenty one active volcanoes in southern Peru.^[23] In 2006, fossils, found under a layer of volcanic ash in Peru, were excavated by a team of paleontologists led by Mark D. Uhen, professor at George Mason University. The fossils were identified as 3 different types of archaeocetes, prehistoric whales, and are older than 36.61 million years which, as of 2011, makes them the oldest whale fossils discovered.^[24]

• Media related to Tephra at Wikimedia Commons
• Volcanic materials identification