Misplaced Pages

#613386

77-748: The Dox Formation contains thick basaltic sills and a number of small, dark dikes . In the area of Desert View and west of Palisades of the Desert, the basaltic sills form very prominent, dark gray cliffs. The Dox Formation takes its name from frontier educator Virginia Dox , the first white woman to explore the Grand Canyon, for whom the Dox Castle butte was named. The strata of the Dox Formation, except for some more resistant sandstone beds, are relatively susceptible to erosion and weathering. The lower member of

154-423: A mantle wedge above the descending slab. The slab releases water vapor and other volatiles as it descends, which further lowers the melting point, further increasing the amount of decompression melting. Each tectonic setting produces basalt with its own distinctive characteristics. The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene . Olivine can also be

231-493: A basalt is diagnostic of how and where it erupted—for example, whether into the sea, in an explosive cinder eruption or as creeping pāhoehoe lava flows, the classic image of Hawaiian basalt eruptions. Basalt that erupts under open air (that is, subaerially ) forms three distinct types of lava or volcanic deposits: scoria ; ash or cinder ( breccia ); and lava flows. Basalt in the tops of subaerial lava flows and cinder cones will often be highly vesiculated , imparting

308-438: A critical temperature window of ~300 °C, the dating of additional samples from other members, and petrographic and microprobe examination of the mica, indicated the ages of source rocks from which the micas were eroded, and support the interpretation that the Dox Formation is younger than 1140 Ma. Additional dating of detrital zircons from the Dox Formation, corroborated the dating of mica grains, and showed that much of

385-619: A delta, and sheet flow environments, of the Solomon Temple Member. At Mile 64 in Carbon Creek, this part of the Dox Formation consists of stacked, fine-grained sandstone channels that are cut by mud-filled, younger channels. These “cut-and-fill” channel structures are regarded to be indicative of estuarine environments where sea level has fallen and subsequently risen. The contact between the Solomon Temple and Comanche Point members of

462-442: A distinctive pillow shape, through which the hot lava breaks to form another pillow. This "pillow" texture is very common in underwater basaltic flows and is diagnostic of an underwater eruption environment when found in ancient rocks. Pillows typically consist of a fine-grained core with a glassy crust and have radial jointing. The size of individual pillows varies from 10 cm up to several metres. When pāhoehoe lava enters

539-427: A flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns . These structures, or basalt prisms , are predominantly hexagonal in cross-section, but polygons with three to twelve or more sides can be observed. The size of the columns depends loosely on

616-465: A high content of plagioclase; these are sometimes described as leucobasalts . It can be difficult to distinguish between lighter-colored basalt and andesite , so field researchers commonly use a rule of thumb for this purpose, classifying it as basalt if it has a color index of 35 or greater. The physical properties of basalt result from its relatively low silica content and typically high iron and magnesium content. The average density of basalt

693-621: A lightweight "frothy" texture to the rock. Basaltic cinders are often red, coloured by oxidized iron from weathered iron-rich minerals such as pyroxene . ʻAʻā types of blocky cinder and breccia flows of thick, viscous basaltic lava are common in Hawaiʻi. Pāhoehoe is a highly fluid, hot form of basalt which tends to form thin aprons of molten lava which fill up hollows and sometimes forms lava lakes . Lava tubes are common features of pāhoehoe eruptions. Basaltic tuff or pyroclastic rocks are less common than basaltic lava flows. Usually basalt

770-623: A lower total content of alkali oxides than trachybasalt and most basanites and tephrites. Basalt generally has a composition of 45–52 wt% SiO 2 , 2–5 wt% total alkalis, 0.5–2.0 wt% TiO 2 , 5–14 wt% FeO and 14 wt% or more Al 2 O 3 . Contents of CaO are commonly near 10 wt%, those of MgO commonly in the range 5 to 12 wt%. High-alumina basalts have aluminium contents of 17–19 wt% Al 2 O 3 ; boninites have magnesium (MgO) contents of up to 15 percent. Rare feldspathoid -rich mafic rocks, akin to alkali basalts, may have Na 2 O + K 2 O contents of 12% or more. The abundances of

847-507: A phenocryst, and when present, may have rims of pigeonite. The groundmass contains interstitial quartz or tridymite or cristobalite . Olivine tholeiitic basalt has augite and orthopyroxene or pigeonite with abundant olivine, but olivine may have rims of pyroxene and is unlikely to be present in the groundmass . Alkali basalts typically have mineral assemblages that lack orthopyroxene but contain olivine. Feldspar phenocrysts typically are labradorite to andesine in composition. Augite

SECTION 10

#1733085470614

924-701: A prominent angular unconformity , which is part of the Great Unconformity . Typically, the surface of this unconformity is a remarkably flat, ancient erosional surface, often argued to be a peneplain , that cuts across units such as the Bass Formation, Hatakai Shale, and Dox Sandstone. Resistant beds within the Unkar Group, such as Cardenas Basalt and Shinumo Quartzite, form ancient hills, called monadnocks , that rise as much as 240 m (790 ft) high above this ancient plain. Thin drapes of Tapeats Sandstone of

1001-523: A significant constituent. Accessory minerals present in relatively minor amounts include iron oxides and iron-titanium oxides, such as magnetite , ulvöspinel , and ilmenite . Because of the presence of such oxide minerals, basalt can acquire strong magnetic signatures as it cools, and paleomagnetic studies have made extensive use of basalt. In tholeiitic basalt , pyroxene ( augite and orthopyroxene or pigeonite ) and calcium -rich plagioclase are common phenocryst minerals. Olivine may also be

1078-447: A thin, discontinuous basaltic lava flow . At various levels within the Dox Formation, dark basalt has been injected as sills . They form very prominent, dark gray cliffs in the area below Desert View and west of Palisades of the Desert. In addition, a number of small, dark basalt dikes also have intruded into the Dox Formation. The lower contact of the Dox Formation with the underlying Shinumo Quartzite appears to be gradational and

1155-510: A variegated appearance to this member. Salt casts, ripple marks , and desiccation cracks are common in the Comanche Point Member. It also contains a few thin beds of stromatolitic dolomite . These stromatolitic dolomite beds occur either within or directly adjacent to the leached beds. In the eastern Grand Canyon, the Comanche Point Member occupies more than half of the Dox outcrop area and

1232-427: Is 2.9 g/cm , compared, for example, to granite ’s typical density of 2.7 g/cm . The viscosity of basaltic magma is relatively low—around 10 to 10 cP —similar to the viscosity of ketchup , but that is still several orders of magnitude higher than the viscosity of water, which is about 1 cP). Basalt is often porphyritic , containing larger crystals ( phenocrysts ) that formed before the extrusion event that brought

1309-611: Is also a common rock on the surface of Mars , as determined by data sent back from the planet's surface, and by Martian meteorites . Solomon Temple (Grand Canyon) Solomon Temple is a 5,121-foot-elevation (1,561-meter) summit located in the Grand Canyon , in Coconino County of northern Arizona , USA . It is situated four miles north of Moran Point, 3.5 miles east of Newberry Butte , and 1.5 mile southeast of Rama Shrine , its nearest higher neighbor. Topographic relief

1386-401: Is also produced by some subglacial volcanic eruptions. Basalt is the most common volcanic rock type on Earth, making up over 90% of all volcanic rock on the planet. The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below the ocean ridges . Basalt is also the principal volcanic rock in many oceanic islands , including

1463-417: Is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low- viscosity lava rich in magnesium and iron ( mafic lava) exposed at or very near the surface of a rocky planet or moon . More than 90% of all volcanic rock on Earth is basalt. Rapid-cooling, fine-grained basalt is chemically equivalent to slow-cooling, coarse-grained gabbro . The eruption of basalt lava

1540-753: Is considered a key to understanding plate tectonics , its compositions have been much studied. Although MORB compositions are distinctive relative to average compositions of basalts erupted in other environments, they are not uniform. For instance, compositions change with position along the Mid-Atlantic Ridge , and the compositions also define different ranges in different ocean basins. Mid-ocean ridge basalts have been subdivided into varieties such as normal (NMORB) and those slightly more enriched in incompatible elements (EMORB). Isotope ratios of elements such as strontium , neodymium , lead , hafnium , and osmium in basalts have been much studied to learn about

1617-478: Is defined as volcanic rock with a content of between 45% and 52% silica and no more than 5% alkali metal oxides. This places basalt in the B field of the TAS diagram. Such a composition is described as mafic . Basalt is usually dark grey to black in colour, due to a high content of augite or other dark-coloured pyroxene minerals, but can exhibit a wide range of shading. Some basalts are quite light-coloured due to

SECTION 20

#1733085470614

1694-499: Is distinguished from enclosing members by its slope-forming and color variegated character. The upper member of the Dox Formation is the Ochoa Point Member. Within the central Grand Canyon, it also has been removed by pre-Tapeats Sandstone erosion. It consists of micaceous mudstone that grades upward into a predominantly red quartzose, silty sandstone. Sedimentary structures found in this member include, salt crystal casts in

1771-424: Is interpreted to have accumulated as the result of continued tidal-flat sedimentation. The lower part of this member appears to have accumulated in the higher mud flat position of the tidal flat environment where periodic desiccation occurred. The upper part of this member is inferred to have accumulated in the deeper, or more seaward part of the tidal-flat environment. At the time that these tidal flats were covered by

1848-492: Is marked by a change in topographic expression and color. The basal 12 m (39 ft) of the Dox Formation directly overlying Shinumo Quartzite consists of predominantly dark green to black, fissile, slope-forming shale that contains thin sandstone beds. This shale makes a distinct notch between the resistant cliff-forming quartzites of the Shinumo Quartzite underlying them and resistant cliff-forming arkosic sandstones of

1925-747: Is observed by geologists at about 20 volcanoes per year. Basalt is also an important rock type on other planetary bodies in the Solar System . For example, the bulk of the plains of Venus , which cover ~80% of the surface, are basaltic; the lunar maria are plains of flood-basaltic lava flows ; and basalt is a common rock on the surface of Mars . Molten basalt lava has a low viscosity due to its relatively low silica content (between 45% and 52%), resulting in rapidly moving lava flows that can spread over great areas before cooling and solidifying. Flood basalts are thick sequences of many such flows that can cover hundreds of thousands of square kilometres and constitute

2002-579: Is rich in titanium compared to augite in tholeiitic basalt. Minerals such as alkali feldspar , leucite , nepheline , sodalite , phlogopite mica, and apatite may be present in the groundmass. Basalt has high liquidus and solidus temperatures—values at the Earth's surface are near or above 1200 °C (liquidus) and near or below 1000 °C (solidus); these values are higher than those of other common igneous rocks. The majority of tholeiitic basalts are formed at approximately 50–100 km depth within

2079-501: Is significant as it rises 2,500 feet (760 meters) above the Colorado River in less than one mile. Solomon Temple was named after historical king Solomon by geologist François E. Matthes , following Clarence Dutton 's practice of naming geographical features in the Grand Canyon after mythological deities and heroic figures. This feature's name was officially adopted in 1906 by the U.S. Board on Geographic Names . Solomon Temple

2156-402: Is suppressed. Above this depth, submarine eruptions are often explosive, tending to produce pyroclastic rock rather than basalt flows. These eruptions, described as Surtseyan, are characterised by large quantities of steam and gas and the creation of large amounts of pumice . When basalt erupts underwater or flows into the sea, contact with the water quenches the surface and the lava forms

2233-440: Is too hot and fluid to build up sufficient pressure to form explosive lava eruptions but occasionally this will happen by trapping of the lava within the volcanic throat and buildup of volcanic gases . Hawaiʻi's Mauna Loa volcano erupted in this way in the 19th century, as did Mount Tarawera , New Zealand in its violent 1886 eruption. Maar volcanoes are typical of small basalt tuffs, formed by explosive eruption of basalt through

2310-455: Is ultimately derived from Late Latin basaltes , a misspelling of Latin basanites "very hard stone", which was imported from Ancient Greek βασανίτης ( basanites ), from βάσανος ( basanos , " touchstone "). The modern petrological term basalt , describing a particular composition of lava -derived rock, became standard because of its use by Georgius Agricola in 1546, in his work De Natura Fossilium . Agricola applied

2387-529: The Clementine mission demonstrate that the lunar maria possess a continuum of titanium concentrations, and that the highest concentrations are the least abundant. Lunar basalts show exotic textures and mineralogy, particularly shock metamorphism , lack of the oxidation typical of terrestrial basalts, and a complete lack of hydration . Most of the Moon 's basalts erupted between about 3 and 3.5 billion years ago, but

Dox Formation - Misplaced Pages Continue

2464-717: The Escalante Creek , Solomon Temple , Comanche Point , and Ochoa Point members . The contacts between members of the Dox Formation are gradational and are based mainly on topographic expression, the sedimentary depositional environment , and color changes. The lowermost member of the Dox formation is the Escalante Creek Member. It consists of over 244 m (801 ft) of light-tan to greenish brown, siliceous quartz sandstone and calcareous lithic and arkosic sandstone overlain by 122 m (400 ft) of dark-brown-to-green shale and mudstone . The sandstones of

2541-1176: The Paraná Traps in Brazil, the Siberian Traps in Russia , the Karoo flood basalt province in South Africa, and the Columbia River Plateau of Washington and Oregon . Basalt is also prevalent across extensive regions of the Eastern Galilee , Golan , and Bashan in Israel and Syria . Basalt also is common around volcanic arcs, specially those on thin crust . Ancient Precambrian basalts are usually only found in fold and thrust belts, and are often heavily metamorphosed. These are known as greenstone belts , because low-grade metamorphism of basalt produces chlorite , actinolite , epidote and other green minerals. As well as forming large parts of

2618-520: The hypabyssal and plutonic equivalents of basalt. During the Hadean , Archean , and early Proterozoic eons of Earth's history, the chemistry of erupted magmas was significantly different from what it is today, due to immature crustal and asthenosphere differentiation. The resulting ultramafic volcanic rocks, with silica (SiO 2 ) contents below 45% and high magnesium oxide (MgO) content, are usually classified as komatiites . The word "basalt"

2695-524: The lanthanide or rare-earth elements (REE) can be a useful diagnostic tool to help explain the history of mineral crystallisation as the melt cooled. In particular, the relative abundance of europium compared to the other REE is often markedly higher or lower, and called the europium anomaly . It arises because Eu can substitute for Ca in plagioclase feldspar, unlike any of the other lanthanides, which tend to only form cations . Mid-ocean ridge basalts (MORB) and their intrusive equivalents, gabbros, are

2772-438: The Dox Formation consists of silty -sandstone and sandstone, and some interbedded argillaceous beds, that form stair-stepped, cliff-slope topography. The bulk of the Dox Formation typically forms rounded and sloping hill topography that occupies an unusually broad section of the canyon. In general, the Dox Formation and associated strata of the Unkar Group rocks dip northeast (10°–30°) toward normal faults that dip 60+° toward

2849-604: The Dox Formation marks a transition from fluvial and coastal environments to marine conditions. The deposition of the Comanche Point Member of the Dox Sandstone marked a return to marginal-marine and tidal flat conditions. Interbedded purplish and red-brown strata of this member appear to reflect accumulation under alternately very shallow water marine and subaerial conditions. The purplish colored beds are interpreted as deposits of tidal flat complexes including mixed-flat and possibly salt-flat environments. The Ochoa Point Member

2926-411: The Dox Formation overlying them. The change in topographic expression, color, and the facies change, from quartz arenite, to mudstone and fine-grained arkose – is gradational. The contact between the Dox Formation and the Shinumo Quartzite at Mile 74.7, where the quartzite forms a narrow V-shaped gorge below a platform carved on the soft shale of the Dox Formation, can be seen from Mile 74. The contact of

3003-406: The Dox Formation with the overlying Cardenas Basalt is smooth, planar, parallel to bedding, and locally interfingering. In places the sandstones of the Dox Formation have small folds and convolutions that are indicative of soft sediment deformation . In addition, in places, the uppermost 60 cm (2.0 ft) of the Dox Formation is mildly baked. A thin lava flow occurs within the uppermost part of

3080-493: The Dox Formation. Thus, the contact between the Cardenas Basalt and the Dox Formation is conformable and interfingering. This indicates that sands were still being deposited when the first lavas erupted and that deposition occurred during the transition from the accumulation of Dox Formation to Cardenas Basalt. The overlying Tonto Group is separated from the Dox Sandstone and the rest of folded and faulted Unkar Group by

3157-525: The Dox formation represents a change from the accumulation of sediments in nearshore marine and coastal environments, to the accumulation of sediments in terrestrial floodplains and river channels. The Escalante Creek Member consists of sediments that were deposited in shallow river channels, which were as wide as 150 feet (46 m) and 15 feet (4.6 m) deep. Numerous stacked sandstone channels can be observed just above Unkar Rapid (River Mile 73) and within side drainages between River Miles 65 and 73. At

Dox Formation - Misplaced Pages Continue

3234-457: The Earth's crust, basalt also occurs in other parts of the Solar System. Basalt commonly erupts on Io (the third largest moon of Jupiter ), and has also formed on the Moon , Mars , Venus , and the asteroid Vesta . The dark areas visible on Earth's moon , the lunar maria , are plains of flood basaltic lava flows. These rocks were sampled both by the crewed American Apollo program and

3311-514: The Escalante Creek member exhibit small-scale, tabular-planar cross-bedding , and graded bedding . The graded shale beds contain interclasts at the base of this member of the Dox Formation. Two intervals of convoluted bedding , which are the stratigraphically highest occurrence of fluid evulsion structures in the Unkar Group, occur within 30 m of the base of the Escalante Creek Member. The tan to brownish color of this member contrasts sharply with

3388-479: The Solomon Temple Member. Within the central Grand Canyon, it has been removed by pre-Tapeats Sandstone erosion. The strata of this member consist mainly of interbedded fine grained, slope-forming, argillaceous sandstone and sandy argillite , and subordinate claystone . The colors exhibited by these strata are variegated, typically alternating between purplish and red-brown. Five pale green-to-white, leached red beds that are as much as 2 m (6.6 ft) thick give

3465-635: The Tonto Group now cover most of these ancient monadnocks. However, a few of these monadnocks protrude up into the Bright Angel Shale. These monadnocks served locally as sources of coarse-grained sediments that accumulated during the marine transgression to form the Tonto Group. Excellent exposures of the angular unconformity at the top of the Dox Formation and the base of the Tapeats Sandstone can be seen at Mile 71.0 where Tapeats Sandstone rests on

3542-468: The Unkar Group (excluding the Bass Formation) was deposited between 1170 and 1100 Ma, and that the Dox formation was deposited after 1140 Ma. The age of the overlying Cardenas Basalt demonstrates that the Dox Formation accumulated before 1104 Ma. Basalt Basalt ( UK : / ˈ b æ s ɔː l t , - əl t / ; US : / b ə ˈ s ɔː l t , ˈ b eɪ s ɔː l t / )

3619-476: The basalt/andesite field of the QAPF diagram. Basalt is further distinguished from andesite by its silica content of under 52%. It is often not practical to determine the mineral composition of volcanic rocks, due to their very small grain size, in which case geologists instead classify the rocks chemically, with particular emphasis on the total content of alkali metal oxides and silica ( TAS ); in that context, basalt

3696-454: The base of the Escalante Creek Member, the two intervals of convoluted bedding, which comprise the stratigraphically highest fluid evulsion structures in the Unkar Group, appear to represent the last of the series of earthshocks that began during deposition of the Shinumo Quartzite. Strata containing large sandstone channels of the Escalante Creek Member are overlain by strata containing much smaller channels, such as those of braided streams in

3773-475: The characteristic igneous rocks formed at mid-ocean ridges. They are tholeiitic basalts particularly low in total alkalis and in incompatible trace elements, and they have relatively flat REE patterns normalized to mantle or chondrite values. In contrast, alkali basalts have normalized patterns highly enriched in the light REE, and with greater abundances of the REE and of other incompatible elements. Because MORB basalt

3850-467: The characteristic red and red-brown color of the rest of the Dox Formation. Within the Dox Formation, the Solomon Temple Member overlies the Escalante Creek Member. The Solomon Temple Member consists of cyclical sequences of red mudstone, siltstone , and quartz sandstone. The lower 213 m (699 ft) of this member consist of slope-forming red-to-maroon shaley siltstone and mudstone interbedded with quartz sandstone. The upper 67 m (220 ft) of

3927-447: The crust, forming an apron of mixed basalt and wall rock breccia and a fan of basalt tuff further out from the volcano. Amygdaloidal structure is common in relict vesicles and beautifully crystallized species of zeolites , quartz or calcite are frequently found. During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While

SECTION 50

#1733085470614

4004-426: The dominant fossils reported from the Comanche Point Member of the Dox Formation. These stromatolites, which weather brown to greenish-brown, consist of dolomite, with minor amounts of silt and clay. They typically take the form of laterally linked hemispheroids and are associated with desiccation cracks and birdseye structures. In addition, a few thin dolomite beds having fine laminations, possibly algal, occur beneath

4081-410: The eroded surface of the Dox Formation, and a basalt sill. In these exposures, Dox red beds and a dark gray basaltic sill, dip 8 to 10 degrees to the east. They are covered by nearly horizontal Tapeats Sandstone. The surface of this angular unconformity is quite irregular as differential erosion of the resistant basalt sill formed monadnocks that have been buried by Tapeats Sandstone. Stromatolites are

4158-541: The evolution of the Earth's mantle . Isotopic ratios of noble gases , such as He / He, are also of great value: for instance, ratios for basalts range from 6 to 10 for mid-ocean ridge tholeiitic basalt (normalized to atmospheric values), but to 15–24 and more for ocean-island basalts thought to be derived from mantle plumes . Source rocks for the partial melts that produce basaltic magma probably include both peridotite and pyroxenite . The shape, structure and texture of

4235-457: The initial eruption of the Cardenas Basalt, the eastern grand Canyon region was at or very near sea level. Features found in the lowermost part of the Cardenas Basalt indicate that the basaltic lavas outpoured over unconsolidated sandy and silty Dox sediments at the time they were wet. It is unknown whether these sediments were slightly above or slightly below water level at the time they were buried by lava. The high proportion of altered glass in and

4312-708: The islands of Hawaiʻi , the Faroe Islands , and Réunion . The eruption of basalt lava is observed by geologists at about 20 volcanoes per year. Basalt is the rock most typical of large igneous provinces . These include continental flood basalts , the most voluminous basalts found on land. Examples of continental flood basalts included the Deccan Traps in India , the Chilcotin Group in British Columbia , Canada ,

4389-454: The level of the middle part of the Dox Formation. The missing part of the Dox Formation and overlying Cardenas Basalt and Chuar Group are preserved in a prominent syncline and fault block that is exposed in the eastern Grand Canyon. West of 75-mile Creek in the central Grand Canyon, the strata of the Dox Formation occurs in small, rotated, downfaulted blocks or slivers, and commonly are only partially exposed. In these downfaulted blocks, only

4466-471: The lower marker bed in the Comanche Point Member. The Dox Formation is a complex sequence of marine , coastal , estuarine , and fluvial mud -dominated deposits that represent deposition at the prograding margin of the Unkar Basin. The Escalante Creek and Solomon Temple members preserve the record of fluvial, estuarine, and deltaic sedimentation . The contact between the underlying Shinumo Quartzite and

4543-479: The lower two members, the Escalante Creek and Solomon Temple members, are preserved as the remainder of the Dox Formation and Unkar Group has been removed by pre-Tapeats Sandstone erosion. The only complete section of the Dox Formation is exposed in the eastern Grand Canyon. In that area, the Dox formation, which is the thickest unit of the Unkar Group , has been subdivided into four members. In ascending order, they are

4620-604: The magma as it decompresses during its approach to the surface; the erupted lava then solidifies before the gases can escape. When vesicles make up a substantial fraction of the volume of the rock, the rock is described as scoria . The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarser) groundmass are more properly referred to either as diabase (also called dolerite) or—when they are more coarse-grained (having crystals over 2 mm across)—as gabbro . Diabase and gabbro are thus

4697-416: The magma to the surface, embedded in a finer-grained matrix . These phenocrysts are usually made of augite, olivine , or a calcium-rich plagioclase, which have the highest melting temperatures of any of the minerals that can typically crystallize from the melt, and which are therefore the first to form solid crystals. Basalt often contains vesicles ; they are formed when dissolved gases bubble out of

SECTION 60

#1733085470614

4774-704: The mantle. Many alkali basalts may be formed at greater depths, perhaps as deep as 150–200 km. The origin of high-alumina basalt continues to be controversial, with disagreement over whether it is a primary melt or derived from other basalt types by fractionation. Relative to most common igneous rocks, basalt compositions are rich in MgO and CaO and low in SiO 2 and the alkali oxides, i.e., Na 2 O + K 2 O , consistent with their TAS classification . Basalt contains more silica than picrobasalt and most basanites and tephrites but less than basaltic andesite . Basalt has

4851-407: The member consists primarily of maroon quartz sandstone that exhibits numerous channel features, and contains low-angle, tabular, and channel-like festoon cross beds. The Solomon Temple Member is about 280 m (920 ft) thick in the eastern Grand Canyon. It is so named because of exposures 2.4 kilometers northeast of Solomon Temple . Within the Dox Formation, the Comanche Point Member overlies

4928-406: The most voluminous of all volcanic formations. Basaltic magmas within Earth are thought to originate from the upper mantle . The chemistry of basalts thus provides clues to processes deep in Earth's interior . Basalt is composed mostly of oxides of silicon, iron, magnesium, potassium, aluminum, titanium, and calcium. Geologists classify igneous rock by its mineral content whenever possible;

5005-422: The mudstone, and asymmetrical ripple marks and small-scale cross beds, in the sandstones. The Ochoa Point Member is 53 to 92 m (174 to 302 ft) thick and forms steep slopes and cliffs below the Cardenas Basalt. The Dox Formation that directly underlies the Cardenas Basalt consists of brick-red to vermilion well-bedded sandstone, with parallel bedding and shaly partings, forming smooth slopes. It also contains

5082-410: The oldest samples are 4.2 billion years old, and the youngest flows, based on the age dating method of crater counting , are estimated to have erupted only 1.2 billion years ago. From 1972 to 1985, five Venera and two VEGA landers successfully reached the surface of Venus and carried out geochemical measurements using X-ray fluorescence and gamma-ray analysis. These returned results consistent with

5159-418: The pervasive fracturing (hyaloclastite) of the basal Cardenas Basalt support this interpretation. The high sodium content of the basal Cardenas Basalt indicates spilitization due to reaction with hypersaline water. Hematitic alteration of the sediment that resulted from baking by the lavas is minimal, amounting to a few centimeters or less. Indirect dating of mica grains from the Dox Formation indicate that it

5236-475: The rate of cooling; very rapid cooling may result in very small (<1 cm diameter) columns, while slow cooling is more likely to produce large columns. The character of submarine basalt eruptions is largely determined by depth of water, since increased pressure restricts the release of volatile gases and results in effusive eruptions. It has been estimated that at depths greater than 500 metres (1,600 ft), explosive activity associated with basaltic magma

5313-411: The relative volume percentages of quartz (crystalline silica (SiO 2 )), alkali feldspar , plagioclase , and feldspathoid ( QAPF ) are particularly important. An aphanitic (fine-grained) igneous rock is classified as basalt when its QAPF fraction is composed of less than 10% feldspathoid and less than 20% quartz, and plagioclase makes up at least 65% of its feldspar content. This places basalt in

5390-595: The robotic Russian Luna program , and are represented among the lunar meteorites . Lunar basalts differ from their Earth counterparts principally in their high iron contents, which typically range from about 17 to 22 wt% FeO. They also possess a wide range of titanium concentrations (present in the mineral ilmenite ), ranging from less than 1 wt% TiO 2 , to about 13 wt.%. Traditionally, lunar basalts have been classified according to their titanium content, with classes being named high-Ti, low-Ti, and very-low-Ti. Nevertheless, global geochemical maps of titanium obtained from

5467-403: The rock at the landing sites being basalts, including both tholeiitic and highly alkaline basalts. The landers are thought to have landed on plains whose radar signature is that of basaltic lava flows. These constitute about 80% of the surface of Venus. Some locations show high reflectivity consistent with unweathered basalt, indicating basaltic volcanism within the last 2.5 million years. Basalt

5544-455: The sea it usually forms pillow basalts. However, when ʻaʻā enters the ocean it forms a littoral cone , a small cone-shaped accumulation of tuffaceous debris formed when the blocky ʻaʻā lava enters the water and explodes from built-up steam. The island of Surtsey in the Atlantic Ocean is a basalt volcano which breached the ocean surface in 1963. The initial phase of Surtsey's eruption

5621-512: The southwest. This can be seen at the Palisades fault in the eastern part of the main Unkar Group outcrop area (below East Rim). Elsewhere, within the central Grand Canyon, these Unkar strata (Bass, Hakatai, and Shinumo), occur in small, rotated, downfaulted blocks or slivers where they commonly are only partially exposed. Within this part of the Grand Canyon, the Unkar Group is incomplete because pre- Tapeats Sandstone erosion has removed strata above

5698-648: The term "basalt" to the volcanic black rock beneath the Bishop of Meissen's Stolpen castle , believing it to be the same as the "basaniten" described by Pliny the Elder in AD ;77 in Naturalis Historiae . On Earth, most basalt is formed by decompression melting of the mantle . The high pressure in the upper mantle (due to the weight of the overlying rock ) raises the melting point of mantle rock, so that almost all of

5775-580: The upper mantle is solid. However, mantle rock is ductile (the solid rock slowly deforms under high stress). When tectonic forces cause hot mantle rock to creep upwards, pressure on the ascending rock decreases, and this can lower its melting point enough for the rock to partially melt , producing basaltic magma. Decompression melting can occur in a variety of tectonic settings, including in continental rift zones, at mid-ocean ridges , above geological hotspots , and in back-arc basins . Basalt also forms in subduction zones , where mantle rock rises into

5852-422: Was deposited over a relatively short time span, between 1140 and 1104 Ma. Individual mica grains from the Escalante Creek Member of the Dox, were collected and dated by 40Ar/39Ar mass spectrometry. This dating of mica grains yielded a distribution of ages ranging from ca. 1260–1120 Ma, with a well-defined peak at 1140 Ma. Although it was suspected that these dates recorded the age at which the mineral cooled through

5929-400: Was highly explosive, as the magma was quite fluid, causing the rock to be blown apart by the boiling steam to form a tuff and cinder cone. This has subsequently moved to a typical pāhoehoe-type behaviour. Volcanic glass may be present, particularly as rinds on rapidly chilled surfaces of lava flows, and is commonly (but not exclusively) associated with underwater eruptions. Pillow basalt

#613386