|
Accretionary Lapilli
Lapilli is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' (singular: ''lapillus'') is Latin for "little stones". By definition lapilli range from in diameter. A pyroclastic particle greater than 64 mm in diameter is known as a volcanic bomb when molten, or a volcanic block when solid. Pyroclastic material with particles less than 2 mm in diameter is referred to as volcanic ash. Formation Lapilli are spheroid-, teardrop-, dumbbell- or button-shaped droplets of molten or semi-molten lava ejected from a volcanic eruption that fall to earth while still at least partially molten. These granules are not accretionary, but instead the direct result of liquid rock cooling as it travels through the air. Lapilli tuffs are a very common form of volcanic rock typical of rhyolite, andesite and dacite pyroclastic eruptions, where thick layers of lapilli can be deposited during ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Tuff
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock containing 25% to 75% ash is described as tuffaceous (for example, ''tuffaceous sandstone''). Tuff composed of sandy volcanic material can be referred to as volcanic sandstone. Tuff is a relatively soft rock, so it has been used for construction since ancient times. Because it is common in Italy, the Romans used it often for construction. The Rapa Nui people used it to make most of the ''moai'' statues on Easter Island. Tuff can be classified as either igneous or sedimentary rock. It is usually studied in the context of igneous petrology, although it is sometimes described using sedimentological terms. Tuff is often erroneously called tufa in guidebooks and in television programmes. Volcanic ash The material that is expelled in a volcanic ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Tephra
Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism. 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. 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. 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. 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 geolo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Rock Microstructure
Rock microstructure includes the texture and small-scale structures of a rock. The words ''texture'' and ''microstructure'' are interchangeable, with the latter preferred in modern geological literature. However, ''texture'' is still acceptable because it is a useful means of identifying the origin of rocks, how they formed, and their appearance. Textures are ''penetrative fabrics'' of rocks; they occur throughout the entirety of the rock mass on a microscopic, hand-specimen, and often outcrop scale. This is similar in many ways to foliations, except a texture does not necessarily carry structural information in terms of deformation events and orientation information. Structures occur on a hand-specimen scale and above. Microstructure analysis describes the textural features of the rock, and can provide information on the conditions of formation, petrogenesis, and subsequent deformation, folding, or alteration events. Sedimentary microstructures Description of sedimentary rock ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Scoria
Scoria is a pyroclastic, highly vesicular, dark-colored volcanic rock that was ejected from a volcano as a molten blob and cooled in the air to form discrete grains or clasts.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) ''Glossary of Geology'' (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. It is typically dark in color (generally dark brown, black or purplish-red), and basaltic or andesitic in composition. Scoria is relatively low in density as a result of its numerous macroscopic ellipsoidal vesicles, but in contrast to pumice, all scoria has a specific gravity greater than 1, and sinks in water. The holes or vesicles form when gases that were dissolved in the magma come out of solution as it erupts, creating bubbles in the molten rock, some of which are frozen in place as the rock cools and solidifies. Scoria may form as part of a lava flow, typically near its surface, or as fragmental ejecta (lapilli, blocks and bombs), for inst ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Axiolitic
A QAPF diagram is a double ternary diagram which is used to classify igneous rocks based on mineralogic composition. The acronym QAPF stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid (Foid)". These are the mineral groups used for classification in QAPF diagram. Q, A, P and F percentages are normalized (recalculated so that their sum is 100%). Origin QAPF diagrams were created by the International Union of Geological Sciences (IUGS): ''Subcommission on the Systematics of Igneous Rocks'' fostered by Albert Streckeisen (whence their alternative name: Streckeisen diagrams). Geologists worldwide accept the diagrams as a classification of igneous, especially plutonic rocks. Usage QAPF diagrams are mostly used to classify plutonic rocks (phaneritic rocks), but are also used to classify volcanic rocks if modal mineralogical compositions have been determined. QAPF diagrams are not used to classify pyroclastic rocks or volcanic rocks if modal mineralogical composition is not ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Spherulite
In petrology, spherulites () are small, rounded bodies that commonly occur in volcanic glass, vitreous igneous rocks. They are often visible in specimens of obsidian, pitchstone, and rhyolite as globules about the size of millet seed or rice grain, with a duller lustre (mineralogy), luster than the surrounding glassy base of the rock, and when they are examined with a lens they prove to have a radiate fibrous structure. Structure Under the microscope the spherulites are of circular outline and are composed of thin divergent fibers that are crystalline as verified with Polarization (waves), polarized light. Between crossed Nicol prism, Nicols, a black cross appears in the spherulite; its axes are usually perpendicular to one another and parallel to the crosshairs; as the microscope stage is rotated the cross remains steady; between the black arms there are four bright sectors. This shows that the spherulite consists of radiate, Double refraction, doubly refracting fibers that ha ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Volcanic Ash Aggregation
Volcanic ash aggregation occurs when particles of volcanic ash collide and stick together during transport. This process modifies the size distribution of airborne particles, which affects both atmospheric dispersal and fallout patterns on the ground. Aggregation also impacts the dynamics of volcanic plumes, pyroclastic density currents, and their associated hazards. Numerical models There are two main approaches to include the effects of ash aggregation in numerical models of ash injection and dispersal. One is to initialize the model with an aggregated grain size distribution, by moving fractions of the erupted mass into larger size bins (for example, the Cornell model). A second approach is a full theoretical description of aggregate growth through time, based on the Smoluchowski coagulation equation In statistical physics, the Smoluchowski coagulation equation is a population balance equation introduced by Marian Smoluchowski in a seminal 1916 publication, describing the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Eroded
Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust, and then sediment transport, transports it to another location where it is deposit (geology), deposited. Erosion is distinct from weathering which involves no movement. Removal of rock or soil as clastic sediment is referred to as ''physical'' or ''mechanical'' erosion; this contrasts with ''chemical'' erosion, where soil or rock material is removed from an area by Solvation, dissolution. Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres. Agents of erosion include rainfall; bedrock wear in rivers; coastal erosion by the sea and Wind wave, waves; glacier, glacial Plucking (glaciation), plucking, Abrasion (geology), abrasion, and scour; areal flooding; Aeolian processes, wind abrasion; groundwater processes; and Mass wasti ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Fiamme
Fiamme are lens-shapes, usually millimetres to centimetres in size, seen on surfaces of some volcaniclastic rocks. They can occur in welded pyroclastic fall deposits and in ignimbrites, which are the deposits of pumiceous 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-tuffs 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 (andesites to rhyolites) where ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Eutaxitic
In igneous petrology, eutaxitic texture describes the layered or banded texture in some extrusive rock bodies. It is often caused by the compaction and flattening of glass shards and pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular v ... fragments around undeformed crystals. See also * * * Igneous rocks by texture {{petrology-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Rock Microstructure
Rock microstructure includes the texture and small-scale structures of a rock. The words ''texture'' and ''microstructure'' are interchangeable, with the latter preferred in modern geological literature. However, ''texture'' is still acceptable because it is a useful means of identifying the origin of rocks, how they formed, and their appearance. Textures are ''penetrative fabrics'' of rocks; they occur throughout the entirety of the rock mass on a microscopic, hand-specimen, and often outcrop scale. This is similar in many ways to foliations, except a texture does not necessarily carry structural information in terms of deformation events and orientation information. Structures occur on a hand-specimen scale and above. Microstructure analysis describes the textural features of the rock, and can provide information on the conditions of formation, petrogenesis, and subsequent deformation, folding, or alteration events. Sedimentary microstructures Description of sedimentary rock ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
