Download Chapter 7B. Oceanic crust - The University of Southern Mississippi

The University of Southern Mississippi
Department of Marine Science
Study Questions
Chapter 7: Ocean Crust
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From what sorts of localities can geologists directly observe the composition and
structure of oceanic crust?
What is the Bowen Reaction Series? Describe it.
Describe the process of fractionation of a magma and how it pertains to formation of the
various kinds of rock found in the crust.
A parent magma undergoes fractionation, which alters its composition. What other
process does it likely undergo that alters its composition?
Igneous rocks are classified using criteria, some chemical and some mineralogical.
Briefly, what are the criteria?
There are two widely used approaches to analysis of igneous rocks. What are they?
What is the objective of normative analysis, and why is it used in preference to modal
analysis?
What are the three kinds of basalt found on the ocean floor?
What rock type is found commonly in volcanic arc settings above subduction zones?
Draw and describe the petrologic equivalents of seismic layers 1, 2 and 3 of the ocean
crust. Also show the petrologic subdivisions of the layers. What are the characteristic
velcity ranges of p-waves through seismic layers 1-3?
What is a gabbro, an alkali basalt, a tholeiitic basalt, MORB, EMORB, an andesite and a
peridotite on the basis of chemistry and mineralogy?
What is the difference between a pillow basalt and a sheeted basalt? Discuss, in
particular, differences in appearance and mechanism of emplacement.
What was the environment of emplacement of pillow basalts found at the base of pillow
basalts immediately above the sheeted dykes in the diagram below, a section exposed at
a transorm fault of a rift valley?
a. Are they intrusive or extrusive?
b. Did they form in contact with sea water or hydothermal waters?
c. Did they form at the surface and were subsequently buried by more pillow basalts, or
were they formed intrusively as a sill, a layer of magma intruded horizonally?
d. The diagram shows a cross section
through the crust of a rift valley with A
pillow basalts above and sheeted
dikes below. Is the pillow basalt
B
younger at Point A or point B?
MAR581/481. Geological Oceanography
Study Questions, Chapters 7, Page 1
The University of Southern Mississippi
Department of Marine Science
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Describe the petrographic character of the rocks in the region of the Moho. Where does
the seismic Moho lie in relation to the petrographic units at the base of the crust?
How were layered and massive gabbro emplaced within Layer 3?
Between what layers is the magma chamber found that is the source for basalt at the
spreading center?
How thick are pillow basalts and sheeted dykes? How thick is the gabbro layer, massive
and layered combined?
What is the greater constituent of ocean crust by volume, basalt or gabbro?
What part of the oceanic crust holds the strongest part of the magnetic signal of magnetic
anomalies?
Which is most homogeneous in chemical composition, the basalt at ocean ridges or the
basalt at hot spot islands? Why?
Rock in the mantle should be solid under the theoretical temperature and pressure regime.
Under what conditions can this solid rock be made to melt?
What is an ophiolite?
What is obduction?
How is the composition of the asthenosphere likely to differ from that of the crust and
lower mantle? (Hint: think about what happens to the asthenosphere chemically as a
consequence of formation of the crust.
MAR581/481. Geological Oceanography
Study Questions, Chapters 7, Page 2