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- Sat Jun 03, 2017 1:04 pm
#35678
Complete Passage Discussion
Scientists in the 1950s made two significant discoveries in the field of ocean geology, suggesting that
the ocean floor changes over time.
Paragraph 1 Overview
The first paragraph introduces the outdated view that the geology of the ocean floor has remained
essentially unchanged for many millions of years. Whenever a passage begins by describing an
established theory or method, you can be sure that the author would argue in favor of a new theory
or method. And indeed, the main point follows as a rebuttal to the old view: “this idea became
insupportable as new discoveries were made” (lines 3-4).
The first major discovery supporting the opposite view is that the ocean floor exhibits odd magnetic
variations (lines 5-6). These variations result from the polarity of the basalt that makes up much the
ocean floor: some basalt has so-called “normal” polarity, whereas other has “reversed” polarity. This
occurs because every time basalt rock is formed from molten lava, the grains of magnetite inside it
align themselves with the earth’s magnetic field at the time of cooling, and the earth’s magnetic field
has reversed throughout history.
There is a preponderance of key definitions in this paragraph (e.g. basalt, magnetite, magma, normal
and reversed magnetic polarity). Make a notation of them and expect to be questioned on your
understanding.
Paragraph 2 Overview
The second paragraph addresses the second major oceanic discovery of the 1950s: the global midocean
ridge. The ridge consists of alternating stripes of rock with differing polarities, which supports
the theory of “ocean floor spreading.” According to that theory, the ridges mark structurally weak
zones where the ocean floor is being pulled apart as new magma rises to the top, eventually erupting
to create new oceanic crust. This process also corroborates the view that the ocean floor has changed
over time.
Paragraph 3 Overview
The third and final paragraph examines in detail the evidence supporting the theory of ocean floor
spreading:
to be questioned on your knowledge of those reasons. That said, do not get buried in the language
of this paragraph. Focus instead on the underlying meaning, especially when dealing with highly
complex sentences such as the one on lines 45-56.
Summary
Two discoveries corroborate the view that the geology of the ocean floor has changed over time. This
passage does not contain an excessive number of interrelated viewpoints: there is the outdated view
of the scientists in line 1-4, and the new viewpoint shared by everyone else (including the author).
The main challenge is to fully understand the arguments that form the new viewpoint.
Here is a summary of each argument:
I. Odd magnetism (1st paragraph)
distilling the most relevant information supporting various arguments. Do not get so caught up in
absorbing the details that you fail to mentally “organize” the arguments at hand. Argumentation
is key, both in the Logical Reasoning and in the Reading Comprehension section of the test, and
especially when dealing with causal relationships in science-oriented passages and stimuli. In
addition, awareness of the general structure would allow you to reduce the time you spend searching
for information when you need to refer back to the passage to answer questions.
Scientists in the 1950s made two significant discoveries in the field of ocean geology, suggesting that
the ocean floor changes over time.
Paragraph 1 Overview
The first paragraph introduces the outdated view that the geology of the ocean floor has remained
essentially unchanged for many millions of years. Whenever a passage begins by describing an
established theory or method, you can be sure that the author would argue in favor of a new theory
or method. And indeed, the main point follows as a rebuttal to the old view: “this idea became
insupportable as new discoveries were made” (lines 3-4).
The first major discovery supporting the opposite view is that the ocean floor exhibits odd magnetic
variations (lines 5-6). These variations result from the polarity of the basalt that makes up much the
ocean floor: some basalt has so-called “normal” polarity, whereas other has “reversed” polarity. This
occurs because every time basalt rock is formed from molten lava, the grains of magnetite inside it
align themselves with the earth’s magnetic field at the time of cooling, and the earth’s magnetic field
has reversed throughout history.
There is a preponderance of key definitions in this paragraph (e.g. basalt, magnetite, magma, normal
and reversed magnetic polarity). Make a notation of them and expect to be questioned on your
understanding.
Paragraph 2 Overview
The second paragraph addresses the second major oceanic discovery of the 1950s: the global midocean
ridge. The ridge consists of alternating stripes of rock with differing polarities, which supports
the theory of “ocean floor spreading.” According to that theory, the ridges mark structurally weak
zones where the ocean floor is being pulled apart as new magma rises to the top, eventually erupting
to create new oceanic crust. This process also corroborates the view that the ocean floor has changed
over time.
Paragraph 3 Overview
The third and final paragraph examines in detail the evidence supporting the theory of ocean floor
spreading:
- (1) the rocks near the ridge crest are young, and become progressively older away from the
crest;
(2) the youngest rocks all have normal polarity;
(3) there is a remarkable correlation between the ages of the earth’s magnetic reversals and
the striping pattern on the ocean floor.
to be questioned on your knowledge of those reasons. That said, do not get buried in the language
of this paragraph. Focus instead on the underlying meaning, especially when dealing with highly
complex sentences such as the one on lines 45-56.
Summary
Two discoveries corroborate the view that the geology of the ocean floor has changed over time. This
passage does not contain an excessive number of interrelated viewpoints: there is the outdated view
of the scientists in line 1-4, and the new viewpoint shared by everyone else (including the author).
The main challenge is to fully understand the arguments that form the new viewpoint.
Here is a summary of each argument:
I. Odd magnetism (1st paragraph)
- Premise: Basalt, the volcanic rock making up the ocean floor, contains a magnetic
mineral called magnetite.
Premise: Some of the basalt has normal polarity (i.e. the same polarity as the earth’s
present magnetic field), and some of it has reversed polarity.
Premise: The earth’s magnetic field has reversed at various times throughout history.
Conclusion: When magma cools to form solid basalt, the magnetite in it becomes aligned
with the magnetic field present at the time of cooling.
- Premise: Alternating stripes of rock with differing polarities are found on either side of
the mid-ocean ridge.
Premise: Near the ridge crest, the rocks are very young. They become progressively
older away from the crest.
Premise: The youngest rocks have normal polarity.
Premise: There is a correlation between the ages of the earth’s magnetic reversals and
the striping pattern on the mid-ocean ridge.
Conclusion: The ridges are structurally weak zones in the ocean floor through which
magma rises to create new oceanic crust. This pulls the ocean floor apart along
the ridge crest in a process called “ocean floor spreading.”
distilling the most relevant information supporting various arguments. Do not get so caught up in
absorbing the details that you fail to mentally “organize” the arguments at hand. Argumentation
is key, both in the Logical Reasoning and in the Reading Comprehension section of the test, and
especially when dealing with causal relationships in science-oriented passages and stimuli. In
addition, awareness of the general structure would allow you to reduce the time you spend searching
for information when you need to refer back to the passage to answer questions.