Misplaced Pages

#526473

51-870: The fallout from the eruption blanketed much of North America, depositing as one of the most widespread air-fall pyroclastic layers, formerly known as the Pearlette type O ash bed in the United States and Wascana Creek ash in Canada. The thick tuff formation resulting from this eruption is well-exposed at various locations within Yellowstone National Park , including Tuff Cliff along the Gibbon River , Virginia Cascade , and along U.S. Highway 20 . Lava Creek Tuff ranges in color from light gray to pale red in some locales. Rock texture of

102-544: A hydrofracture breccia. Hydrothermal clastic rocks are generally restricted to those formed by hydrofracture , the process by which hydrothermal circulation cracks and brecciates the wall rocks and fills them in with veins. This is particularly prominent in epithermal ore deposits and is associated with alteration zones around many intrusive rocks, especially granites . Many skarn and greisen deposits are associated with hydrothermal breccias. A fairly rare form of clastic rock may form during meteorite impact. This

153-431: A biased view of the original mineralogy of the rock. Porosity can also be affected by this process. For example, clay minerals tend to fill up pore space and thereby reducing porosity. In the process of burial, it is possible that siliciclastic deposits may subsequently be uplifted as a result of a mountain building event or erosion . When uplift occurs, it exposes buried deposits to a radically new environment. Because

204-448: A concentrated dispersion of interacting pyroclasts and partly trapped gas). The former type are sometimes called pyroclastic surges (even though they may be sustained rather than "surging") and lower parts of the latter are sometimes termed pyroclastic flows (these, also, can be sustained and quasi steady or surging). As they travel, pyroclastic density currents deposit particles on the ground, and they entrain cold atmospheric air, which

255-410: A considerably lesser portion of framework grains and minerals. They only make up about 15 percent of framework grains in sandstones and 5% of minerals in shales. Clay mineral groups are mostly present in mudrocks (comprising more than 60% of the minerals) but can be found in other siliciclastic sedimentary rocks at considerably lower levels. Accessory minerals are associated with those whose presence in

306-481: A great resistance to decomposition are categorized as stable, while those that do not are considered less stable. The most common stable mineral in siliciclastic sedimentary rocks is quartz (SiO 2 ). Quartz makes up approximately 65 percent of framework grains present in sandstones and about 30 percent of minerals in the average shale. Less stable minerals present in this type of rocks are feldspars , including both potassium and plagioclase feldspars. Feldspars comprise

357-602: A logarithmic size scale. Siliciclastic rocks are clastic noncarbonate rocks that are composed almost exclusively of silicon, either as forms of quartz or as silicates. The composition of siliciclastic sedimentary rocks includes the chemical and mineralogical components of the framework as well as the cementing material that make up these rocks. Boggs divides them into four categories; major minerals, accessory minerals, rock fragments, and chemical sediments. Major minerals can be categorized into subdivisions based on their resistance to chemical decomposition. Those that possess

408-546: A muddy matrix that leaves little space for precipitation to occur. This is often the case for mudrocks as well. As a result of compaction, the clayey sediments comprising mudrocks are relatively impermeable. Dissolution of framework silicate grains and previously formed carbonate cement may occur during deep burial. Conditions that encourage this are essentially opposite of those required for cementation. Rock fragments and silicate minerals of low stability, such as plagioclase feldspar, pyroxenes , and amphiboles , may dissolve as

459-437: A result of increasing burial temperatures and the presence of organic acids in pore waters. The dissolution of frame work grains and cements increases porosity particularly in sandstones. This refers to the process whereby one mineral is dissolved and a new mineral fills the space via precipitation. Replacement can be partial or complete. Complete replacement destroys the identity of the original minerals or rock fragments giving

510-491: A sample's environment of deposition . An example of clastic environment would be a river system in which the full range of grains being transported by the moving water consist of pieces eroded from solid rock upstream. Grain size varies from clay in shales and claystones ; through silt in siltstones ; sand in sandstones ; and gravel , cobble , to boulder sized fragments in conglomerates and breccias . The Krumbein phi (φ) scale numerically orders these terms in

561-498: A sediment is deposited, it becomes subject to cementation through the various stages of diagenesis discussed below. Eogenesis refers to the early stages of diagenesis. This can take place at very shallow depths, ranging from a few meters to tens of meters below the surface. The changes that occur during this diagenetic phase mainly relate to the reworking of the sediments. Compaction and grain repacking, bioturbation , as well as mineralogical changes all occur at varying degrees. Due to

SECTION 10

#1732764969527

612-403: A type of volcaniclastic deposit, which are deposits made predominantly of volcanic particles. 'Phreatic' pyroclastic deposits are a variety of pyroclastic rock that forms from volcanic steam explosions and they are entirely made of accidental clasts. 'Phreatomagmatic' pyroclastic deposits are formed from explosive interaction of magma with groundwater . The word pyroclastic is derived from

663-585: A volcanic conduit, volcanic jet, or pyroclastic density current. Pyroclasts are transported in two main ways: in atmospheric eruption plumes, from which pyroclasts settle to form topography-draping pyroclastic fall layers, and by pyroclastic density currents (PDCs) (including pyroclastic flows and pyroclastic surges ), from which pyroclasts are deposited as pyroclastic density current deposits, which tend to thicken and coarsen in valleys, and thin and fine over topographic highs. During Plinian eruptions , pumice and ash are formed when foaming silicic magma

714-504: Is called a lava fountain or 'fire-fountain'. If sufficiently hot and liquid when they land, the hot droplets and clots of magma may agglutinate to form 'spatter' ('agglutinate'), or fully coalesce to form a clastogenic lava flow . Clastic rock Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus , chunks, and smaller grains of rock broken off other rocks by physical weathering . Geologists use

765-544: Is cemented together and lithified it becomes known as sandstone. Any particle that is larger than two millimeters is considered gravel. This category includes pebbles , cobbles and boulders. Like sandstone, when gravels are lithified they are considered conglomerates. Conglomerates are coarse grained rocks dominantly composed of gravel sized particles that are typically held together by a finer grained matrix. These rocks are often subdivided into conglomerates and breccias. The major characteristic that divides these two categories

816-412: Is compaction. As sediment transport and deposition continues, new sediments are deposited atop previously deposited beds, burying them. Burial continues and the weight of overlying sediments causes an increase in temperature and pressure. This increase in temperature and pressure causes loose grained sediments become tightly packed, reducing porosity, essentially squeezing water out of the sediment. Porosity

867-598: Is emplaced at temperatures so hot that the soft glassy pyroclasts stick together at point contacts, and deform: this is known as welding . One of the most spectacular types of pyroclastic deposit is an ignimbrite , which is the deposit of a ground-hugging pumiceous pyroclastic density current (a rapidly flowing hot suspension of pyroclasts in gas). Ignimbrites may be loose deposits or solid rock, and they can bury entire landscapes. An individual ignimbrite can exceed 1000 km in volume, can cover 20,000 km of land, and may exceed 1 km in thickness, for example where it

918-467: Is fragmented in the volcanic conduit, because of rapid shear driven by decompression and the growth of microscopic bubbles. The pyroclasts are then entrained with hot gases to form a supersonic jet that exits the volcano, admixes and heats cold atmospheric air to form a vigorously buoyant eruption column that rises several kilometers into the stratosphere and cause aviation hazards . Particles fall from atmospheric eruption plumes and accumulate as layers on

969-514: Is further reduced by the precipitation of minerals into the remaining pore spaces. The final stage in the process is diagenesis and will be discussed in detail below. Cementation is the diagenetic process by which coarse clastic sediments become lithified or consolidated into hard, compact rocks, usually through the deposition or precipitation of minerals in the spaces between the individual grains of sediment. Cementation can occur simultaneously with deposition or at another time. Furthermore, once

1020-608: Is ponded within a volcanic caldera. Pyroclasts include juvenile pyroclasts derived from chilled magma, mixed with accidental pyroclasts, which are fragments of country rock . Pyroclasts of different sizes are classified (from smallest to largest) as volcanic ash , lapilli , or volcanic blocks (or, if they exhibit evidence of having been hot and molten during emplacement, volcanic bombs ). All are considered to be pyroclastic because they were formed (fragmented) by volcanic explosivity, for example during explosive decompression, shear, thermal decrepitation , or by attrition and abrasion in

1071-456: Is reserved for mudrocks that are laminated, while mudstone refers those that are not. Siliciclastic rocks initially form as loosely packed sediment deposits including gravels, sands, and muds. The process of turning loose sediment into hard sedimentary rocks is called lithification . During the process of lithification, sediments undergo physical, chemical and mineralogical changes before becoming rock. The primary physical process in lithification

SECTION 20

#1732764969527

1122-429: Is the amount of rounding. The gravel sized particles that make up conglomerates are well rounded while in breccias they are angular. Conglomerates are common in stratigraphic successions of most, if not all, ages but only make up one percent or less, by weight, of the total sedimentary rock mass. In terms of origin and depositional mechanisms they are very similar to sandstones. As a result, the two categories often contain

1173-427: Is then heated and thermally expands. Where the density current becomes sufficiently dilute to loft, it rises into the atmosphere as a 'phoenix plume' (or 'co-PDC plume'). These phoenix plumes typically deposit thin ashfall layers that may contain little pellets of aggregated fine ash. Hawaiian eruptions such as those at Kīlauea produce an upward-directed jet of hot droplets and clots of magma suspended in gas; this

1224-454: The Dott scheme , which uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of muddy matrix between these larger grains. Rocks that are classified as mudrocks are very fine grained. Silt and clay represent at least 50% of the material that mudrocks are composed of. Classification schemes for mudrocks tend to vary, but most are based on the grain size of

1275-481: The chemical and mineralogic make-up of the single or varied fragments and the cementing material ( matrix ) holding the clasts together as a rock. These differences are most commonly used in the framework grains of sandstones. Sandstones rich in quartz are called quartz arenites , those rich in feldspar are called arkoses , and those rich in lithics are called lithic sandstones . Siliciclastic sedimentary rocks are composed of mainly silicate particles derived from

1326-489: The Greek [πῦρ] Error: {{Lang}}: invalid parameter: |links= ( help ) , meaning fire; and κλαστός , meaning broken. Unconsolidated accumulations of pyroclasts are described as tephra . 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

1377-474: The abundance of phenocrysts differs between the members. Hornblende is relatively abundant in member A but rare in other members. Unit 3 is distinguished from unit 1 and 2 by higher crystal content and more plagioclase. Member A is distinguished from member B primarily by the presence of the mineral amphibole in the former. The zircon and phenocrysts rims recorded that the magma of the Lava Creek Tuff

1428-547: The activity of organisms. Despite being close to the surface, eogenesis does provide conditions for important mineralogical changes to occur. This mainly involves the precipitation of new minerals. Mineralogical changes that occur during eogenesis are dependent on the environment in which that sediment has been deposited. For example, the formation of pyrite is characteristic of reducing conditions in marine environments. Pyrite can form as cement, or replace organic materials, such as wood fragments. Other important reactions include

1479-581: The composition of sandstone. They generally make up most of the gravel size particles in conglomerates but contribute only a very small amount to the composition of mudrocks . Though they sometimes are, rock fragments are not always sedimentary in origin. They can also be metamorphic or igneous . Chemical cements vary in abundance but are predominantly found in sandstones. The two major types are silicate based and carbonate based. The majority of silica cements are composed of quartz, but can include chert , opal , feldspars and zeolites . Composition includes

1530-866: The field, it may at times be difficult to distinguish between a debris flow sedimentary breccia and a colluvial breccia, especially if one is working entirely from drilling information. Sedimentary breccias are an integral host rock for many sedimentary exhalative deposits . Clastic igneous rocks include pyroclastic volcanic rocks such as tuff , agglomerate and intrusive breccias , as well as some marginal eutaxitic and taxitic intrusive morphologies. Igneous clastic rocks are broken by flow, injection or explosive disruption of solid or semi-solid igneous rocks or lavas . Igneous clastic rocks can be divided into two classes: Clastic metamorphic rocks include breccias formed in faults , as well as some protomylonite and pseudotachylite . Occasionally, metamorphic rocks can be brecciated via hydrothermal fluids, forming

1581-654: The formation of chlorite , glauconite , illite and iron oxide (if oxygenated pore water is present). The precipitation of potassium feldspar, quartz overgrowths, and carbonate cements also occurs under marine conditions. In non marine environments oxidizing conditions are almost always prevalent, meaning iron oxides are commonly produced along with kaolin group clay minerals. The precipitation of quartz and calcite cements may also occur in non marine conditions. As sediments are buried deeper, load pressures become greater resulting in tight grain packing and bed thinning. This causes increased pressure between grains thus increasing

Lava Creek Tuff - Misplaced Pages Continue

1632-450: The ground, which are described as fallout deposits. Pyroclastic density currents arise when the mixture of hot pyroclasts and gases is denser than the atmosphere and so, instead of rising buoyantly, it spreads out across the landscape. They are one of the greatest hazards at a volcano, and may be either 'fully dilute' (dilute, turbulent ash clouds, right down to their lower levels) or 'granular fluid based' (the lower levels of which comprise

1683-436: The instantaneous age of volcanic eruption as recorded by sanidine. Two samples from ignimbrite visually closely similar to unit 1 or 2, the oldest ignimbrite units of the Lava Creek Tuff, have Ar/Ar ages of 634.5 ± 6.8 kyr and 630.9 ± 4.1 kyr. Ar/Ar dating experiments on sanidine from member B have yielded eruption ages of 627.0 ± 1.7 kyr, 631.3 ± 4.3 kyr, and 630.9 ± 2.7 . U–Pb dating for zircon crystals from both

1734-431: The major constituents. In mudrocks, these are generally silt, and clay. According to Blatt, Middleton and Murray mudrocks that are composed mainly of silt particles are classified as siltstones. In turn, rocks that possess clay as the majority particle are called claystones. In geology, a mixture of both silt and clay is called mud. Rocks that possess large amounts of both clay and silt are called mudstones. In some cases

1785-588: The member A and B yields an age of 626.5 ± 5.8 kyr, which is indistinguishable from the Ar/Ar date of sanidine. Another team reported U–Pb ages of 626.0 ± 2.6 kyr and 629.2 ± 4.3 kyr for zircon from member A and member B, respectively. The ignimbrite sheet was formed from rhyolite magma and contains phenocrysts of quartz , sanidine, and subordinate sodic plagioclase , along with minor proportions of magnetite , ilmenite , ferroaugite, fayalite , iron-rich hornblende , zircon, chevkinite, and allanite . However,

1836-405: The pores between grain of sediment. The cement that is produced may or may not have the same chemical composition as the sediment. In sandstones, framework grains are often cemented by silica or carbonate. The extent of cementation is dependent on the composition of the sediment. For example, in lithic sandstones, cementation is less extensive because pore space between framework grains is filled with

1887-424: The process brings material to or closer to the surface, sediments that undergo uplift are subjected to lower temperatures and pressures as well as slightly acidic rain water. Under these conditions, framework grains and cement are again subjected to dissolution and in turn increasing porosity. On the other hand, telogenesis can also change framework grains to clays, thus reducing porosity. These changes are dependent on

1938-479: The reservoir and volatile exsolution from crystallizing magma. The eruption of the Lava Creek Tuff has been reconstructed through geological analysis of the deposits. Proximal ignimbrite units of member A and B have been studied in detail and correlated with distal air-fall. Meanwhile, the newly identified units 1, 2, 3, and 4 are only known at a few locales, nonetheless, they indicate that the Lava Creek eruption

1989-410: The rock are not directly important to the classification of the specimen. These generally occur in smaller amounts in comparison to the quartz, and feldspars. Furthermore, those that do occur are generally heavy minerals or coarse grained micas (both muscovite and biotite ). Rock fragments also occur in the composition of siliciclastic sedimentary rocks and are responsible for about 10–15 percent of

2040-433: The same sedimentary structures. Sandstones are medium-grained rocks composed of rounded or angular fragments of sand size, that often but not always have a cement uniting them together. These sand-size particles are often quartz but there are a few common categories and a wide variety of classification schemes that classify sandstones based on composition. Classification schemes vary widely, but most geologists have adopted

2091-399: The shallow depths, sediments undergo only minor compaction and grain rearrangement during this stage. Organisms rework sediment near the depositional interface by burrowing, crawling, and in some cases sediment ingestion. This process can destroy sedimentary structures that were present upon deposition of the sediment. Structures such as lamination will give way to new structures associated with

Lava Creek Tuff - Misplaced Pages Continue

2142-542: The solubility of grains. As a result, the partial dissolution of silicate grains occurs. This is called pressure solutions. Chemically speaking, increases in temperature can also cause chemical reaction rates to increase. This increases the solubility of most common minerals (aside from evaporites). Furthermore, beds thin and porosity decreases allowing cementation to occur by the precipitation of silica or carbonate cements into remaining pore space. In this process minerals crystallize from watery solutions that percolate through

2193-479: The specific conditions that the rock is exposed as well as the composition of the rock and pore waters. Specific pore waters, can cause the further precipitation of carbonate or silica cements. This process can also encourage the process of oxidation on a variety of iron bearing minerals. Sedimentary breccias are a type of clastic sedimentary rock which are composed of angular to subangular, randomly oriented clasts of other sedimentary rocks. They may form either: In

2244-494: The term clastic to refer to sedimentary rocks and particles in sediment transport , whether in suspension or as bed load , and in sediment deposits. Clastic sedimentary rocks are rocks composed predominantly of broken pieces or clasts of older weathered and eroded rocks. Clastic sediments or sedimentary rocks are classified based on grain size , clast and cementing material ( matrix ) composition, and texture. The classification factors are often useful in determining

2295-419: The term can also be used to refer to a family of sheet silicate minerals. Silt refers to particles that have a diameter between .062 and .0039 millimeters. The term mud is used when clay and silt particles are mixed in the sediment; mudrock is the name of the rock created with these sediments. Furthermore, particles that reach diameters between .062 and 2 millimeters fall into the category of sand. When sand

2346-405: The term shale is also used to refer to mudrocks and is still widely accepted by most. However, others have used the term shale to further divide mudrocks based on the percentage of clay constituents. The plate-like shape of clay allows its particles to stack up one on top of another, creating laminae or beds. The more clay present in a given specimen, the more laminated a rock is. Shale, in this case,

2397-406: The tuff ranges from fine-grained to aphanitic and is densely welded . The maximum thickness of the tuff layer is approximately 180–200 m (590–660 ft). Ash flows of the Lava Creek Tuff are divided among six members , informally named unit 1, unit 2, member A and B from bottom to top, with unit 3 and unit 4 having unspecified stratigraphic positions. The emplacement of the Lava Creek Tuff

2448-421: The weathering of older rocks and pyroclastic volcanism. While grain size, clast and cementing material (matrix) composition, and texture are important factors when regarding composition, siliciclastic sedimentary rocks are classified according to grain size into three major categories: conglomerates , sandstones , and mudrocks . The term clay is used to classify particles smaller than .0039 millimeters. However,

2499-405: Was generated from a mix of mantle, Archean crust, and shallow hydrothermally altered intra-caldera rocks. Member A and B were sourced from separate magma reservoirs prior to eruption, at a depth range of 3–6 km (1.9–3.7 mi) and a temperature of 790–815 °C (1,454–1,499 °F). The eruption of member B was probably triggered by a combination of an injection of new silicic magma into

2550-559: Was much more complex than previously thought. These ignimbrite units represent the earliest known eruptive events of the Lava Creek episode. This article about a specific stratigraphic formation in Wyoming is a stub . You can help Misplaced Pages by expanding it . Pyroclastic rock Pyroclastic rocks are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroclasts . Pyroclastic rocks are

2601-443: Was not instantaneous and continuous, but rather, there were multiple pauses, and the members were erupted at different times. To date the timings of their eruptions, two common methods of radiometric dating are employed: Ar/Ar on sanidine and U–Pb on zircon . The interpretation of the two techniques differs in that zircon crystallization occurs early and progressively during magma evolution; therefore, U–Pb ages must predate

SECTION 50

#1732764969527
#526473