Q4. In one model of Earth\'s formation, the earth\'s crust and mantle were origi
ID: 294015 • Letter: Q
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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 whiclh 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. a. b. c. d. 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.Explanation / Answer
Ans:
The 4 processes which created the Earth's crust and transformed it into the current state are:-
1)Igneous fractionation related to melting of mantle generated magma.
2)Igneous fractionation related to crystallization of the melt.
3) Sedimentary processes.
4) Metamorphic processes.
A) The Igneous fractionation process in relation to melting can be described that with the concept of Partial melting.When a multicomponent rock melts , unless it has a Eutectic composition,it melts over a range of temperature at a given pressure,and during this melting the composition of the liquid melt changes.
The initial composition of the mantle rocks had the composition of Peridotite(ultramafic rock).The partial melting of this rock resulted in generation of lighter melts first.The lighter melts had intermediate and felsic composition. The Earth's crust is mostly composed of lighter melts of felsic and intermediate composition.The incompatible elements can be analysed to infer this.The incompatible elements have concentrated in the crust of the Earth owing to more lighter melts.The partial melting occurs in the upper mantle.Partial melting of the mantle melts can generate rocks of wide ranging compositions from Peridotite to basalts to granites.
B) Igneous fractionation related to crystallization is the Igneous process by which the melt composition changes due to crystallization of minerals as the melt temperature is lowered.Following the mineral formation series as depicted in the Bowen's reaction series, the mafic minerals will crystallize first and settle down in the melt,and as more and more of the minerals crystallize ,the melt becomes more and more felsic.
The final melt is mostly a quartz rich felsic melt.This finally crystallizes to form granitic and quartz Igneous bodies.
The fractionation typically occurs at the site of crust formation,ie.Mid oceanic ridges along the oceanic crust.
Rocks that get formed as a result of fractionation crystallization and decompression melting and as a result of slow cooling will have large grain sizes,exg Gabbro; and faster cooling would result in smaller grain size crystals,exg Basalts.The composition of the final melt is granitic so rocks like Pegmatites , Granodiorites and Granites are formed.
C)The final process of rock formation is defined as metamorphism.Metamorphic rocks are formed when the original rock(protolith) is subjected to increased pressure and temperature conditions ,which results in a change in the structure of rocks.The Metamorphic environments can be described on the basis of Metamorphic gardes using the Metamorphic facies diagram.There are 4 type of environments on the basis of Metamorphic grade:low temperature low pressure(exg zeolite facies),high pressure low temperature(exg Blueschist facies),high temperature low pressure(exg Sanidinite facies) and high pressure high temperature environments.(exg eclogite facies)
D) Speculation from the age of oldest rocks known,suggests that these rock forming processes are there from the origin of Earth.The oldest oceanic crustal rocks are merely 200million years old,as the oceanic crust undergoes subduction.The oldest rocks are found in the continental crust,the continental crust being lighter than oceanic crust doesn't undergo subduction.
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