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In 1968 Shirley Chisholm became the first Black woman elected to the United States Congress. In 1972 she launched a campaign for the Democratic presidential nomination, breaking another political barrier.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
Pyrolite is a term used to characterize a model composition of the Earth's mantle. This model is based on that a pyrolite source can produce mid-ocean ridge basalts (MORB) by partial melting.[1][2] It was first proposed by Ted Ringwood (1962)[3] as being 1 part basalt and 4 parts harzburgite, but later was revised to being 1 part tholeiitic basalt and 3 parts dunite.[1][4] The term is derived from the mineral names pyroxene and olivine.[5] However, whether pyrolite is entirely representative of the Earth's mantle remains debated.[6]
The major elements composition of pyrolite is about 44.9 wt% SiO2, 4.44 wt% Al2O3, 8.03 wt% FeO, 3.54 wt% CaO, 37.71 wt% MgO, 0.36 wt% Na2O.[8]
1) A pyrolitic upper mantle is mainly composed of olivine (~60 volume percent (vol%)), clinopyroxene, orthopyroxene, and garnet.[9] Pyroxene would gradually dissolved into garnet and form majoritic garnet.[10]
2) A pyrolitic mantle transition zone is mainly composed of 60 vol% olivine-polymorphs (wadsleyite, ringwoodite) and ~40 vol% majoritic garnet. The top and bottom boundary of the mantle transition zone are mainly marked by olivine-wadsleyite transition and ringwoodite-perovskite transition, respectively.
3) A pyrolitic lower mantle is mainly composed of magnesium perovskite (~80 vol%), ferroperclase (~13 vol%), and calcium perovskite (~7%). In addition, post-perovskite may present at the bottom of the lower mantle.
The P-wave and S-wave velocities (Vp and Vs) of pyrolite along the 1600 K adiabatic geotherm are shown in Fig. 2,[2] and its density profile is shown in Fig. 3.[2]
At the boundary between the upper mantle and the mantle transition zone (~410 km), Vp, Vs, and density jump by ~6%, ~6%, and ~4% in a pyrolite model,[2] respectively, which are mainly attributed to the olivine-wadsleyite phase transition.[11]
At the boundary between the mantle transition zone and the lower mantle, Vp, Vs, and density jump by ~3%, ~6%, and ~6% in a pyrolite model, respectively.[2] With more elasticity parameters available, the Vp, Vs, and density profiles of pyrolite would be updated.
Whether pyrolite could represent the ambient mantle remains debated.
In the geochemical aspect, it does not satisfy trace elements or isotopic data of mid-ocean ridge basalts because the pyrolite hypothesis is based on major elements and some arbitrary assumptions (e.g. amounts of basalt and melting in the source).[1] It may also violate mantle heterogeneity.[12]
In the geophysical aspect, some studies suggest that seismic velocities of pyrolite do not match well with the observed global seismic models (such as preliminary reference Earth model (PREM)) in the Earth's interior,[6] whereas some studies support the pyrolite model.[13]
There are other rock models for the Earth's mantle:
(1) Piclogite: by contrast to the olivine-enriched pyrolite, piclogite is an olivine-poor model (~20% olivine) proposed to provide a better match to the seismic velocity observations in the transition zone.[15][16] The piclogite phase composition is similar as 20% olivine + 80% eclogite.[17]
(2) Eclogite: is transformed from the mid-ocean ridge basalt at a depth of ~60 km,[citation needed] exists in the Earth's mantle mainly within the subducted slabs. It is mainly composed of garnet and clinopyroxene (mainly omphacite) up to ~500 km depth (Fig. 4).
(3) Harzburgite: mainly exists under the mid-ocean ridge basalt layer of the oceanic lithosphere, and can enter into the deep mantle along with the subducted oceanic lithosphere. Its phase composition is similar as pyrolite, but shows higher olivine proportion (~70 vol%) than pyrolite.[18]
Overall, pyrolite and piclogite are both rock models for the ambient mantle, eclogite and harzburgite are rock models for subducted oceanic lithosphere. Formed from partial melting of pyrolite, the oceanic lithosphere is mainly composed of the basalt layer, harzburgite layer, and depleted pyrolite from top to bottom.[19] The subducted oceanic lithospheres contribute to the heterogeneity in the Earth's mantle because they have different composition (eclogite and harzburgite) from the ambient mantle (pyrolite).[2][14]
Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
Pyrolite is a term used to characterize a model composition of the Earth's mantle. This model is based on that a pyrolite source can produce mid-ocean ridge basalts (MORB) by partial melting. It was first proposed by Ted Ringwood (1962) as being 1 part basalt and 4 parts harzburgite, but later was revised to being 1 part tholeiitic basalt and 3 parts dunite. The term is derived from the mineral names pyroxene and olivine. However, whether pyrolite is entirely representative of the Earth's mantle remains debated.
Peridotite (US: PERR-ih-doh-tyte, pə-RID-ə-) is a dense, phaneritic (coarse-grained) igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole. Peridotite is the dominant rock of the upper part of Earth's mantle. The compositions of peridotite xenoliths found in certain basalts are of special interest along with kimberlite pipe (kimberlite), because they provide samples of Earth's mantle brought up from depths ranging from about 30 km to 200 km or more. Some of the xenoliths preserve isotope ratios of osmium and other elements that record processes that occurred when Earth was formed, and so they are of special interest to paleogeologists because they provide clues to the early composition of Earth's upper mantle and the complexities of the processes that occurred. The word peridotite comes from the gemstone peridot, which consists of pale green forsterite (olivine). Peridotite is typically bright green with some specks of black or brown, although most hand samples tend to be darker green. Peridotitic outcrops typically range from earthy bright yellow to dark green; this is because olivine is easily weathered to iddingsite. While green and yellow are the most common colors, peridotitic rocks may exhibit a wide range of colors including blue, brown, and red.
The terraforming of Venus is the hypothetical planetary engineering procedure that would transform Venus from a planet hostile to life to one that could sustainably host humans and other lifeforms free of protection or mediation. Necessary adjustments to the existing environment of Venus to support human life would require a reduction of the planet's surface temperature and a substantial elimination of the planet's dense atmosphere. At 737 K (464 °C; 867 °F), the surface of Venus is above the melting point of lead. Meanwhile, its 9.2 MPa (91 atm) carbon dioxide and sulfur dioxide atmosphere is almost 100 times as dense as Earth's. These facts are closely interrelated, as Venus's extreme temperature is due to the high pressure of its dense atmosphere and the greenhouse effect. Terraforming would require the removal or conversion of the atmosphere to some other form, alongside the addition of breathable oxygen to the atmosphere. Proposals to modify the planet include "veiling" the planet from the sun, thus dropping the temperature low enough to condense or solidify carbon dioxide, which would then need to be removed or stored in some way. The rapid growth in carbon capture and storage since the beginning of the century presents another plausible catalyst to terraforming.
Earth's crustal evolution involves the formation, destruction and renewal of the rocky outer shell at that planet's surface. The variation in composition within the Earth's crust is much greater than that of other terrestrial planets. Mars, Venus, Mercury and other planetary bodies have relatively quasi-uniform crusts unlike that of Earth which contains both oceanic and continental plates. This unique property reflects the complex series of crustal processes that have taken place throughout the planet's history, including the ongoing process of plate tectonics. The proposed mechanisms regarding Earth's crustal evolution take a theory-orientated approach. Fragmentary geologic evidence and observations provide the basis for hypothetical solutions to problems relating to the early Earth system. Therefore, a combination of these theories creates both a framework of current understanding and also a platform for future study.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Shirley Chisholm, elected in 1968.