Q4. In one model of Earth\'s formation, the earth\'s crust and mantle were origi
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Q4. In one model of Earth's formation, the earth's crust and mantle were originally composed completely of peridotite (ultramafic igneous rock). Explain in detail four processes that created the earth's crust and transformed it into the diversity of rock types present in the crust (both continental and oceanic) today. Discuss the igneous fractionation process related to melting. What sequence and characteristics govern this process? Where in Earth does this fractionation presently occur (oceanic crust, continental crust, mantle)? In what plate tectonic environment(s) does this process occur? What types of rock can be produced via this process alone? Discuss the igneous fractionation process related to crystallization. What sequence and characteristics govern this process? Where in earth does this fractionation typically occur (oceanic crust, continental crust, mantle)? In what plate tectonic environment(s) does this process occur? How does crustal thickness impact the likelihood and extent of this process? What types of rock can be produced via this process alone? Discuss deformation and metamorphism processes in Earth environments and how they impact the diversity of rocks found. Name and explain at least 3 environments for metamorphism, including the process by which alteration occurs and the type of alteration that would be produced there What new types of rocks can be produced via these processes, and what is their relationship to the original rock type? List and explain at least two protolith-metamorphic rock pairs and describe how metamorphism altered the original rock. Speculate on how long this diversification process might have taken, given what you know about the rate of plate movement and the factors that determine this rate a. b. c. d.Explanation / Answer
The ETH researchers' simulations could not confirm either extreme position, since neither of the two approaches produces a continental crust that is composed as it should be based on field observations.
Temperature and pressure narrowly defined
"The rocks of the original continental crustcould only form under relatively narrowly defined temperature and pressure conditions. In both extremes, these conditions do not exist," explains Rozel. "If a new crust is formed solely by volcanoes, whereby the magma cools immediately on the Earth's surface, the crust would be too cold. Conversely, the crust in the other approach would be hotter than it should be."
By contrast, the ideal situation is when the crust is created through a mixture of the two mechanisms, preferably when around 30 percent of the new crust is formed by volcanism. This results in a rock composition similar to what can be found on the west coast of Greenland, for example.
Two-dimensional and global
?Archean crust could have been built by vulcanism and accumulation of magma remaining warm in the crust. Credit: Antoine Rozel / ETH Zurich
For the researchers to calculate their model, however, they had to make some compromises. Although their model is global, it is only two-dimensional. "If we had wanted a high regional resolution and a three-dimensional model, we would have had to run the calculations on a supercomputer for ten years," says Rozel.
In their model, the researchers considered various quantities, such as temperature, pressure, water content of the rock and its viscosity, and simulated the processes up to 100 times to test the parameters with various values.
Ocean crust is the outermost layer of earth under the oceans. It is separated from the underlying mantle by a seismic transition zone called the Moho. A widely held view is that the Moho represents a petrologic change from basaltic-type rocks to a mantle composed mostly of olivine and pyroxene. According to this view, crust is formed by a steady segregation of basaltic melt, derived from partial melting of the mantle, into a crustal magma chamber wherein cooling and crystallization bring about steady-state accretion to the continuously spreading plates. There is sufficient disagreement between the predictions of this hypothesis and marine geophysical data to cause one to doubt the validity of this formation process. At least two other processes are more compatible with the geophysical data. In one, the crust is formed from the episodic injection of basaltic dikes from a mantle reservoir and the Moho is a primary petrologic boundary. In the other, the crust is treated as a mechanical boundary layer in which thermal contraction results in cracking; by comparison, in the mantle thermal contraction is accommodated by flow. The upper part of the crust is formed from episodic extrusion and intrusion of basaltic melt. The lower crust is formed by rapid hydrothermal alteration of mantle that may be continuously or episodically injected by viscous flow at temperatures below the melting temperature.
a) Igneous Rocks are formed by crystallization from a liquid, or magma. They include two types
Volcanic or extrusive igneous rocks form when the magma cools and crystallizes on the surface of the Earth
Intrusive or plutonic igneous rocks wherein the magma crystallizes at depth in the Earth.
Magma is a mixture of liquid rock, crystals, and gas. Characterized by a wide range of chemical compositions, with high temperature, and properties of a liquid.
Magmas are less dense than surrounding rocks, and will therefore move upward. If magma makes it to the surface it will erupt and later crystallize to form an extrusive or volcanic rock. If it crystallizes before it reaches the surface it will form an igneous rock at depth called a plutonic or intrusive igneous rock. : In continental crust
: It occur in convergent plate tectonic environment
: Types of rock produced via this process - a) intrusive igneous rock b) extrusive igneous rock
b) Charles Darwin in 1844 first suggested the possibility that fractional crystallization playsa role in the formation of igneous rocks.
This is the process by which solids, generally crytals, which form from a liquid are prevented from reacting with the liquid.
Evidence for Fractional Crystallization
Observed changes in bulk composition of liquid, e.g. a single volcanic flow or within a single volcano.
Zoning in minerals - most silicate minerals crystallized in igneous systems exhibit evidence of zonation, which reflect changes in composition.
Reaction Rims - produced by chemical reaction between crystals and liquid or sudden changes in T and P.
e.g. pyroxene rims on olivine result when the liquid containing the olivine becomes saturated with respect to silica as a result of the growth of olivine.
Mg2SiO4 + SiO2 ===> 2MgSiO3
In fractional crystallization the solids are removed or isolated from the liquid, resulting in the remaining or residual liquid having a new composition.
e.g. Basalt liquid crystallizes olivine (Mg2SiO4) which is undersaturated with respect to SiO2, causing the initial liquid in to become depleted in Mg and enriched in Si, resulting in less Mg and more Si in the liquid, after forming the olivine.
If the olivine is now removed from the system, the residual liquid is now depleted in Mg and enriched in Si compared with the parent liquid.
Many mechanisms of fractional crystallization have been proposed.Gravitational Effects
This is the most often suggested mechanism which is interpreted to indicate that fractional crystalization has occurred. This is dependant on the density of the solid phase(s) and the density of the liquid phase from which the solids are crystillzing.
May have crystal settling or floatation as a means of fractionation.
Crystal Settling
Most often cited gravitational effect in the recorded in the published literature.
Early formed minerals olivine (3.3-3.4 g/cc) and pyroxene (3.2-3.5 g/cc) are generally denser than the liquid (3.0 g/cc) from which they crystallize.
Due to the density contrast between liquid and solid, the solids settle out of the liquid. Evidence for settling has been observed in a variety of environments from a single lava flow, ~ 1 m thick, to plutons, 1,000's of metres thick.One example concerns crystallization of melts that form mafic and ultramaficrocks. MgO and SiO2 concentrations in melts are among the variables that determine whether forsterite olivine orenstatite pyroxene is precipitated, but the water content and pressure are also important. In some compositions, at high pressures without water crystallization of enstatite is favored, but in the presence of water at high pressures, olivine is favored. Convergent type of tectonic plate.
c) Deformation and Metamorphism
The Earth’s crust is slowly moving. Did you know that the Atlantic Ocean is getting wider at about the rate your fingernails grow, or that India is barging its way slowly northward into the continent of Asia?
The huge forces that move continents stretch and squash parts of the Earth’s crust., generating earthquakes and building mountains. They cause rocks near the surface to be fractured and faulted. At greater depth, the heat and pressure involved can cause folding and/or metamorphism.
Dickson Fjord in Greenland: This huge cliff (over 2,000 metres high!) is made of a metamorphic rock called gneiss.
This was once a series of both igneous and sedimentary rocks, which were deeply buried, folded by huge forces, and completely recrystallised at high temperature and pressure to form this beautifully banded and folded rock. Types of rock produced by metamorphism and deformation - a) Example - metamorphism of a shale (made up initially of clay minerals and quartz) b) Gneiss As metamorphic grade increases, the sheet silicates become unstable and dark colored minerals like hornblende and pyroxene start to grow. C) Granulite - At the highest grades of metamorphism most of the hydrous minerals and sheet silicates become unstable and thus there are few minerals present that would show a preferred orientation. The resulting rock will have a granulitic texture that is similar to a phaneritic texture in igneous rocks.
A protolith is the original, unmetamorphosed rock from which a given metamorphic rock is formed (proto-: first; lithos: rock; both Greek).[1]
For example, the protolith of a slate is ashale or mudstone. Metamorphic rocks can be derived from any other kind of non-metamorphic rock and thus there is a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.
Protoliths are non-metamorphic rocks and have no protoliths themselves. The non-metamorphic rocks fall into two classes: sedimentary rocks, formed from sediment, and igneous rocks, formed from magma. The source of the sediment of a sedimentary rock is termed its provenance.
d) the "RATE" of churn" within the mantle, and resulting heaving and swirling of plate movement are derivatives of:
the amount of heat within the earth,
primordial,gravity pressure,radioactive decay,tidal friction),combined withthe gravitational tugs of the Sun and Moon on Earth,the resulting "jitter" within the earth caused by the Milankovitch cyclesand the physical dynamics, properties and characteristics ofheat generation rates,and transfer rates through varied mediums,sphere dynamics,the patterns within the movements caused by existing physical geographical patternsplus the random interaction between the Earth and various celestial bodies that have happened into our path over time, creating such geological features as the gulf of Mexico, the pool of nickel under Sudbury, etc. .Current plate movement can be tracked directly by means of ground-based or space-based geodetic measurements; geodesy is the science of the size and shape of the Earth. Ground-based measurements are taken with conventional but very precise ground-surveying techniques, using laser-electronic instruments. However, because plate motions are global in scale, they are best measured by satellite-based methods. The late 1970s witnessed the rapid growth ofspace geodesy, a term applied to space-based techniques for taking precise, repeated measurements of carefully chosen points on the Earth's surface separated by hundreds to thousands of kilometers. The three most commonly used space-geodetic techniques -- very long baseline interferometry (VLBI), satellite laser ranging (SLR), and the Global Positioning System (GPS) -- are based on technologies developed for military and aerospace research, notably radio astronomy and satellite tracking.
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