Fiamme
Encyclopedia
Fiamme are lens-shapes, usually mm to cm in size, seen on surfaces of some volcaniclastic rocks. They can occur in welded pyroclastic fall deposits and in ignimbrite
s, which are the deposits of pumice
ous pyroclastic density currents
. The name fiamme comes from the Italian word for flames, describing their shape. The term is descriptive and non-genetic.
Fiamme are most typical of welded lapilli
-tuff
s and are commonly found in association with eutaxitic textures, best seen under the microscope.
Some fiamme represent fragments of volcanic ejecta, often pumice lapilli that have been flattened by compaction and/or shear. Some fiamme are formed from flattened hot, relatively low viscosity
, high porosity
fragments of volcanic glass
or pumice
. But this is not the only way they can form: they can also form when pumice lapilli are altered to clay
and compact during diagenesis
; and fiamme are also widely reported in viscous lavas (andesite
s to rhyolite
s) where they form by shear-induced autobrecciation
of pumiceous or obsidian
zones, followed by shear and annealing of the fragments. Fiamme can also result from patchy alteration and recrystalisation of volcanic rocks, or by patchy revesiculation of welded tuff matrix
(especially in rheomorphic peralkaline
tuffs).
Ignimbrite
An ignimbrite is the deposit of a pyroclastic density current, or pyroclastic flow, a hot suspension of particles and gases that flows rapidly from a volcano, driven by a greater density than the surrounding atmosphere....
s, which are the deposits of pumice
Pumice
Pumice is a textural term for a volcanic rock that is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid...
ous pyroclastic density currents
Pyroclastic flow
A pyroclastic flow is a fast-moving current of superheated gas and rock , which reaches speeds moving away from a volcano of up to 700 km/h . The flows normally hug the ground and travel downhill, or spread laterally under gravity...
. The name fiamme comes from the Italian word for flames, describing their shape. The term is descriptive and non-genetic.
Fiamme are most typical of welded lapilli
Lapilli
Lapilli is a size classification term for tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. Lapilli means "little stones" in Latin. They are in some senses similar to ooids or pisoids in calcareous sediments.By definition lapilli range...
-tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
s and are commonly found in association with eutaxitic textures, best seen under the microscope.
Some fiamme represent fragments of volcanic ejecta, often pumice lapilli that have been flattened by compaction and/or shear. Some fiamme are formed from flattened hot, relatively low viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
, high porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
fragments of volcanic glass
Volcanic glass
Volcanic glass is the amorphous product of rapidly cooling magma. Like all types of glass, it is a state of matter intermediate between the close-packed, highly ordered array of a crystal and the highly disordered array of gas...
or pumice
Pumice
Pumice is a textural term for a volcanic rock that is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid...
. But this is not the only way they can form: they can also form when pumice lapilli are altered to clay
Clay minerals
Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations. Clays have structures similar to the micas and therefore form flat hexagonal sheets. Clay minerals are common weathering products and low...
and compact during diagenesis
Diagenesis
In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
; and fiamme are also widely reported in viscous lavas (andesite
Andesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...
s to rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
s) where they form by shear-induced autobrecciation
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
of pumiceous or obsidian
Obsidian
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth...
zones, followed by shear and annealing of the fragments. Fiamme can also result from patchy alteration and recrystalisation of volcanic rocks, or by patchy revesiculation of welded tuff matrix
Matrix (geology)
The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of...
(especially in rheomorphic peralkaline
Peralkaline
Peralkaline rocks include those igneous rocks which have a deficiency of aluminium such that sodium and potassium are in excess of that needed for feldspar. The presence of aegerine and riebeckite are indicative of peralkaline conditions....
tuffs).
See also
- TuffTuffTuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
and volcanic rockVolcanic rockVolcanic rock is a rock formed from magma erupted from a volcano. In other words, it is an igneous rock of volcanic origin... - AgglomerateAgglomerateAgglomerates are coarse accumulations of large blocks of volcanic material that contain at least 75% bombs...
and ignimbriteIgnimbriteAn ignimbrite is the deposit of a pyroclastic density current, or pyroclastic flow, a hot suspension of particles and gases that flows rapidly from a volcano, driven by a greater density than the surrounding atmosphere.... - List of rock textures
- Rock microstructureRock microstructureRock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature...