Tephra

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.

Overview

Tephra is unconsolidated pyroclastic material produced by a volcanic eruption. 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.^[2]

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.^[3]

Classification

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 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.^[2]

Etymology

The word "tephra" and "pyroclast" both derive from Greek: The word τέφρα (téphra) means "ash",^[4] while pyroclast is derived from the Greek πῦρ (pyr), meaning "fire",^[5] and κλαστός (klastós), meaning "broken in pieces".^[6] The word τέφραv (means "ashes") is used in broad context within an account by Aristotle of an eruption on Vulcano (Hiera) in Meteorologica.^[7]

Environmental impacts

The release of tephra into the

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.^[14]

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.^[15]

Europe

Europe's volcanoes provide unique information about the history of

North America

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.^[18] 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.^[19] 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.^[20]

South America

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.^[21] Due to the location of the subduction zone of the eastern Pacific's Nazca Plate, there are twenty one active volcanoes in southern Peru.^[22] 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.^[23]

References