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Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism. [ 1 ] Tephra horizons in south-central Iceland : The thick and light-coloured layer at the centre of the photo is rhyolitic tephra from Hekla .
Accretionary lapilli form as a result of the cohesive properties of wet ash, causing the particles to bind. They have a circular structure when specimens are viewed in hand and under the microscope. [1] A further control on the morphology and characteristics of a deposit is the water to magma ratio. It is considered that the products of ...
Lapilli (sg.: lapillus) is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. [1] Lapilli is Latin for "little stones". By definition lapilli range from 2 to 64 mm (0.08 to 2.52 in) in diameter. [2]
Pyroclastic flows sweep down the flanks of Mayon Volcano, Philippines, in 2018. A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) [1] is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of 100 km/h (30 m/s; 60 mph) but is capable of reaching speeds up to ...
Tephra may become lithified to a pyroclastic rock by cementation or chemical reactions as the result of the passage of hot gases (fumarolic alteration) or groundwater (e.g. hydrothermal alteration and diagenesis) and burial, or, if it is emplaced at temperatures so hot that the soft glassy pyroclasts stick together at point contacts, and deform ...
Dense-rock equivalent (DRE) is a volcanologic calculation used to estimate volcanic eruption volume. One of the widely accepted measures of the size of a historic or prehistoric eruption is the volume of magma ejected as pumice and volcanic ash, known as tephra during an explosive phase of the eruption, or the volume of lava extruded during an effusive phase of a volcanic eruption.
The ash and tephra are ejected at speeds of several hundred metres per second, and can rise rapidly to heights of several kilometres, lifted by enormous convection currents. Eruption columns may be transient, if formed by a discrete explosion, or sustained, if produced by a continuous eruption or closely spaced discrete explosions.
The Yn tephra is a geologically recent tephra deposit that covers portions of the U.S. state of Washington and the Canadian provinces of British Columbia and Alberta.It was created by the largest known volcanic eruption from Mount St. Helens, having taken place in possibly 1860 BCE as part of the Smith Creek eruptive period.