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answer the questions in the worksheet that i submitted.
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Name: ____________________________________
Geology 101: How the Earth Works
Worksheet 7 – Fall 2020
Magma at plate boundaries is produced through the depressurization or hydration of
peridotite in the mantle, but ultramafic magmas/lavas do not occur at the surface. After
magmas form, they undergo compositional differentiation that controls the igneous rock
types that occur at the surface.
Step 1. Partial Melting
o Consider the rock, peridotite (reference page 96, Fig. 4.4 in your textbook)
● What minerals are present in the rock peridotite?
● What elements are present in the minerals that compose peridotite (See
Textbook Table 4-1, p. 95 and Fig. 4.4; p. 96)?
● What does the term “ultramafic” mean?
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o Now consider these minerals in the context of Bowen’s Reaction Series
(Textbook Fig. 4.6; p. 100)
● Describe the relationship between melting/crystallization temperature and silica
content that is described by Bowen’s Reaction series.
● Which mineral is least likely to become liquid? Why?
● If only the mineral with the lowest melting temperature in a peridotite melts, how is
the overall chemical composition of the resulting magma different from the original
chemical composition of the peridotite?
● What happens to the magma to separate it from the residue (the remaining solid)?
Why?
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● What two types of igneous rocks will form from this magma (partial melt of the
peridotite)? How could you tell them apart, and what accounts for the difference?
o How can the generation of basaltic magma in a subduction zone lead to more felsic
rocks? Explain:
● What is the mechanism of melting (lowered pressure, increased temperature, addition
of water)?
● Is melting partial or complete?
● Use Bowen’s reaction series (page 100) to justify your hypothesis.
Step 2. Fractional Crystallization
o Now consider mafic magma that is injected horizontally between the bedding planes
of sedimentary rocks, forming a sill (see sketch on next page).
● If the magma cools very quickly what rock will form? Explain your answer.
● If the magma cools slowly what rock will form? Explain your answer.
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● Now assume that the magma cools very slowly, so that silicate minerals
crystallize according to Bowen’s Reaction Series, and then are separated from the
remaining magma due to density/gravity.
o Which mineral will crystallize first?
o What will happen to the mineral once it crystallizes?
o Below is a cross-section of the sill. Sketch on the cross-section where this
mineral will be located in the sill. If you’re using the form version of this
worksheet, please reproduce the cross-section and provide your sketch
below.
Depth
sedimentary rocks
Mafic
Sill
sedimentary rocks
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o Which mineral will crystallize next? ________________________
Sketch its location on the cross-section above.
o What is the final mineral to crystallize?______________________
Sketch its location on the cross-section above.
Synthesis questions: Putting it All Together
1. Describe at least two processes that cause magma differentiation.
2.
How do these processes account for the variety of igneous rocks?
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