Until the 1950s, most scientists believed that the geology of the ocea

Question

Until the 1950s, most scientists believed that the
geology of the ocean floor had remained essentially
unchanged for many millions of years. But this idea
became insupportable as new discoveries were made.
  (5)   First, scientists noticed that the ocean floor exhibited
odd magnetic variations. Though unexpected, this was
not entirely surprising, because it was known that
basalt—the volcanic rock making up much of the
ocean floor—contains magnetite, a strongly magnetic
  (10)   mineral that was already known to locally distort
compass readings on land. This distortion is due to the
fact that although some basalt has so-called “normal”
polarity—that is, the magnetite in it has the same
polarity as the earth’s present magnetic field—other
  (15)   basalt has reversed polarity, an alignment opposite
that of the present field. This occurs because in magma
(molten rock), grains of magnetite—behaving like
little compass needles—align themselves with the
earth’s magnetic field, which has reversed at various
  (20)   times throughout history. When magma cools to form
solid basalt, the alignment of the magnetite grains is
“locked in,” recording the earth’s polarity at the time
of cooling.

As more of the ocean floor was mapped, the
  (25)   magnetic variations revealed recognizable patterns,
particularly in the area around the other great oceanic
discovery of the 1950s: the global mid-ocean ridge, an
immense submarine mountain range that winds its way
around the earth much like the seams of a baseball.
  (30)   Alternating stripes of rock with differing polarities are
laid out in rows on either side of the mid-ocean ridge:
one stripe with normal polarity and the next with
reversed polarity. Scientists theorized that mid-ocean
ridges mark structurally weak zones where the ocean
  (35)   floor is being pulled apart along the ridge crest. New
magma from deep within the earth rises easily through
these weak zones and eventually erupts along the crest
of the ridges to create new oceanic crust. Over millions
of years, this process, called ocean floor spreading,
  (40)   built the mid-ocean ridge.

This theory was supported by several lines of
evidence. First, at or near the ridge crest, the rocks are
very young, and they become progressively older
away from the crest. Further, the youngest rocks all
  (45)   have normal polarity. Finally, because geophysicists
had already determined the ages of continental volcanic
rocks and, by measuring the magnetic orientation of
these same rocks, had assigned ages to the earth’s
recent magnetic reversals, they were able to compare
  (50)   these known ages of magnetic reversals with the
ocean floor’s magnetic striping pattern, enabling scientists
to show that, if we assume that the ocean floor moved
away from the spreading center at a rate of several
centimeters per year, there is a remarkable correlation
  (55)   between the ages of the earth’s magnetic reversals
and the striping pattern.  
 1 
 1. Which one of the following most accurately expresses the main idea of the passage?

(A) In the 1950s, scientists refined their theories concerning the process by which the ocean floor was formed many millions of years ago.
(B) The discovery of basalt’s magnetic properties in the 1950s led scientists to formulate a new theory to account for the magnetic striping on the ocean floor.
(C) In the 1950s, two significant discoveries led to the transformation of scientific views about the geology of the oceans.
(D) Local distortions to compass readings are caused, scientists have discovered, by magma that rises through weak zones in the ocean floor to create new oceanic crust.
(E) The discovery of the ocean floor’s magnetic variations convinced scientists of the need to map the entire ocean floor, which in turn led to the discovery of the global mid-ocean ridge.

2 
 2. The author characterizes the correlation mentioned in the last sentence of the passage as “remarkable” in order to suggest that the correlation

(A) indicates that ocean floor spreading occurs at an extremely slow rate
(B) explains the existence of the global mid-ocean ridge
(C) demonstrates that the earth’s magnetic field is considerably stronger than previously believed
(D) provides strong confirmation of the ocean floor spreading theory
(E) reveals that the earth’s magnetic reversals have occurred at very regular intervals

3 
 3. According to the passage, which one of the following is true of magnetite grains?

(A) In the youngest basalt, they are aligned with the earth’s current polarity.
(B) In magma, most but not all of them align themselves with the earth’s magnetic field.
(C) They are not found in other types of rock besides basalt.
(D) They are about the size of typical grains of sand.
(E) They are too small to be visible to the naked eye.

4 
 4. If the time intervals between the earth’s magnetic field reversals fluctuate greatly, then, based on the passage, which one of the following is most likely to be true?

(A) Compass readings are most likely to be distorted near the peaks of the mid-ocean ridge.
(B) It is this fluctuation that causes the ridge to wind around the earth like the seams on a baseball.
(C) Some of the magnetic stripes of basalt on the ocean floor are much wider than others.
(D) Continental rock is a more reliable indicator of the earth’s magnetic field reversals than is oceanic rock.
(E) Within any given magnetic stripe on the ocean floor, the age of the basalt does not vary.

5 
 5. Which one of the following would, if true, most help to support the ocean floor spreading theory?

(A) There are types of rock other than basalt that are known to distort compass readings.
(B) The ages of the earth’s magnetic reversals have been verified by means other than examining magnetite grains in rock.
(C) Pieces of basalt similar to the type found on the mid-ocean ridge have been found on the continents.
(D) Along its length, the peak of the mid-ocean ridge varies greatly in height above the ocean floor.
(E) Basalt is the only type of volcanic rock found in portions of the ocean floor nearest to the continents.

6 
 6. Which one of the following is most strongly supported by the passage?

(A) Submarine basalt found near the continents is likely to be some of the oldest rock on the ocean floor.
(B) The older a sample of basalt is, the more times it has reversed its polarity.
(C) Compass readings are more likely to become distorted at sea than on land.
(D) The magnetic fields surrounding magnetite grains gradually weaken over millions of years on the ocean floor.
(E) Any rock that exhibits present-day magnetic polarity was formed after the latest reversal of the earth’s magnetic field.

samrat123456 · Answer

6. Which one of the following is most strongly supported by the passage?

(A) Correct: Submarine basalt found near the continents is likely to be some of the oldest rock on the ocean floor. because in the passage it says there are global mid-ocean ridges and at or near the ridge crest, the rocks are very young, and they become progressively older away from the crest. So, near continents that are very far from the mid point, there are likely(possibly) some of the oldest rocks. True 
(B) Incorrect: The older a sample of basalt is, the more times it has reversed its polarity. The passage never talks about the age of basalt and the times its polarity is reversed correlation.
(C) Incorrect: Compass readings are more likely to become distorted at sea than on land. No fact or claim is given in the passage to accept this as the answer.
(D) Incorrect: The magnetic fields surrounding magnetite grains gradually weaken over millions of years on the ocean floor. The passage never talks about the weakening of the surrounding magnetic field, it only talks about the polarity, not the strength of the magnetic field.
(E) Incorrect: Any rock that exhibits present-day magnetic polarity was formed after the latest reversal of the earth’s magnetic field. Here consider a scenario where earth has changed its magnetic polarity three times First North to South then South to north and then again north to south, if a rock has a polarity of north to south, that means it was either formed the first time of N to S or second time when polarity was again N to S. So, this option is not always true -Incorrect Choice.

BottomJee · Answer

Always use P.O.E (Process of Elimination) for answering RC questions, because it is easier to weed out the wrong options as supposed to justify an option to be correct.

1. Which one of the following most accurately expresses the main idea of the passage?

(A) In the 1950s, scientists refined their theories concerning the process by which the ocean floor was formed many millions of years ago. -  Incorrect ,  Scientists did refine their theories in the 1950s but it was about the geology of the ocean floor and not about the process by which ocean floor was formed.

(B) The discovery of basalt’s magnetic properties in the 1950s led scientists to formulate a new theory to account for the magnetic striping on the ocean floor. -  Incorrect ,  Scientists were already aware of the basalt's magnetic properties.  (Lines 7-9)

(C) In the 1950s, two significant discoveries led to the transformation of scientific views about the geology of the oceans. -  HOLD ,  because I am not able to find anything to negate this option from the passage.

(D) Local distortions to compass readings are caused, scientists have discovered, by magma that rises through weak zones in the ocean floor to create new oceanic crust. -  Incorrect ,  Local distortions are also caused by basalt rocks which are being formed from magma eruption. This option is extremely detailed oriented, so we shouldn't consider it for a question which is asking us about the  MAIN IDEA  of the passage.

(E) The discovery of the ocean floor’s magnetic variations convinced scientists of the need to map the entire ocean floor, which in turn led to the discovery of the global mid-ocean ridge. -  Incorrect ,   (Lines 24-27)  There is no mention of a need to map the entire ocean floor by scientists rather scientists were expanding their search area in order to find recognizable patterns in magnetic distortions. And this pattern was concentrated near the ridge.

Left with choice C, mark it and move on.

bv8562 · Answer

GMATNinja   Sajjad1994  In question 4, how is option C supported by the passage?

Sajjad1994 · Answer

Explanation

4. If the time intervals between the earth’s magnetic field reversals fluctuate greatly, then, based on the passage, which one of the following is most likely to be true?

Difficulty Level:   700

Explanation

In the final paragraph, the passage correlates the earth’s magnetic reversals with the ocean floor’s magnetic striping pattern. So, if the earth’s magnetic field reversals did not happen on a regular basis, this suggests that the striping pattern of the ocean floor would not be regular, either.

A. No.  The basalt near the peaks of the ridge is the youngest and would have the same polarity as the earth’s magnetic field, and it is basalt with reversed polarity that causes distorted compass readings.

B. No.  The passage never explains why the ridge winds around the earth like the seams on a baseball, just that it does. It cannot be inferred that the time intervals between the earth’s magnetic field reversals had any impact on the shape of the mid-ocean ridge.

C.  Yes.   A variation in the widths of the stripes of basalt on the ocean floor would be consistent with the correlation drawn in final sentence.

D. No.  The continental rock was used to determine the ages of the earth’s recent magnetic reversals, but this does not imply that if the reversals did not happen on a regular basis that continental rock would somehow be superior to oceanic rock in determining the earth’s magnetic field.

E. No.  Whether the reversals happen on a regular basis will have no impact on whether the age of the basalt varies within a stripe of basalt on the ocean floor. The passage does not supply any information about whether the age of the basalt in a single stripe can vary, just that the basalt in each stripe has the same polarity.

Answer: C

Bobsled · Answer

Hi Experts, I am really confused about question 3, please help me.

Option A: "In the youngest basalt, they are aligned with the earth’s current polarity."  
How can we say that the youngest basalt has Earth's current polarity? It can also have reversed polarity if its age is old enough for that, but still being the youngest among all basalts.

Option B: "In magma, most but not all of them align themselves with the earth’s magnetic field." 
In this option, however, we can safely say that when basalt is in magma form then magnetite grains it consists can align themselves with the earth's magnetic field.

Sajjad1994 · Answer

The characteristics of magnetite grains are discussed within the first paragraph.

The first paragraph states that when magma cools, the magnetite grains align themselves with the earth’s polarity and get  “locked in”  to the earth’s polarity at that time. This means that the youngest basalt rocks would have the same polarity as the earth does now. Hence  (A)  is correct.

(B)  is incorrect because this choice contradicts the passage, the passage states that magnetite grains align themselves with the current polarity of the earth.

ChandlerBong · Answer

Hi Sajjad1994 , Can you please post the OE for Q5 and Q6? Thanks in advance

Sajjad1994 · Answer

Explanation

5. Which one of the following would, if true, most help to support the ocean floor spreading theory?

Difficulty Level:   Hard

Explanation

The credited response will strengthen the ocean floor spreading theory, which is explained in the second paragraph. The theory holds that mid-ocean ridges are weak spots in the earth’s crust where the ocean floor is being pulled apart, and magma rises through these zones and creates new crust.

A. No.  Other rocks that distort compass readings have no bearing on how the mid-ocean ridge is formed.

B.  Yes.   If the ages of the earth’s magnetic reversals were confirmed by a source other than magnetite, that would be further proof that the ocean floor spreading theory is correct, since there is a correlation between the magnetic striping of the ocean floor and the earth’s magnetic reversals.

C. No.  There could be numerous reasons that similar basalt is found in the ocean and on the continents. This does not directly support the ocean floor spreading theory.

D. No.  There could be numerous reasons to explain why the height of the mid-ocean ridge varies. This does not directly support theocean floor spreading theory.

E. No.  There could be numerous reasons that basalt is the only type of volcanic rock found nearest the continents. This does not directly support the ocean floor spreading theory.

Answer: B

Sajjad1994 · Answer

Explanation

6. Which one of the following is most strongly supported by the passage?

Difficulty Level:   Very Hard

Explanation

A.  Yes.   The beginning of the final paragraph explains that the youngest rocks in the ocean are near the ridge crest, and they get progressively older the further they are from the crest. Since the ridge is the mid-ocean ridge, this suggests that at least some of the basalt farther from the ridge and closer to the continents is older.

B. No.  According to the first paragraph, the polarity of the basalt doesn’t change over time, as it is “locked in” when the magma that forms it cools.

C. No.  The passage states that magnetite on land was known to distort compass readings, but it does not compare the strength of distortion between magnetite found on land and magnetite found in the ocean.

D. No.  As described at the end of the first paragraph, the alignment of the magnetite grains is “locked in.” There is nothing in the passage to suggest that the magnetic fields gradually weaken.

E. No.  The passage, which contains evidence about basalt rocks, would not support an inference about all rocks.

Answer: A

Apoorva0406 · Answer

GMATNinja  can you please explain ?

Abhi310 · Answer

Can you please provide explanation for Q2.

Sajjad1994 · Answer

Explanation

2. The author characterizes the correlation mentioned in the last sentence of the passage as “remarkable” in order to suggest that the correlation

Explanation

The author describes the correlation between the ages of the earth’s magnetic reversals and the striping pattern of the of the ocean floor as  “remarkable”  as further evidence that the ocean floor has changed over time.

A.  The author does not render any judgment about the rate at which the ocean floor is spreading, but rather sets the rate of the spreading as a condition upon which the correlation is based.

B.  The author does not use the correlation between magnetic reversals and striping to explain the existence of the global midocean ridge.

C.  The passage does not discuss the strength of the the earth’s magnetic field.

D.   The author uses the  “remarkable correlation”  as further evidence to support the theory of ocean floor spreading.

E.  The correlation is remarkable because it shows the striping pattern in the ocean floor aligns with the known times of the earth’s magnetic reversals. The passage does not state that these reversals occurred at regular intervals.

Answer: D

Until the 1950s, most scientists believed that the geology of the ocea

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Reading Comprehension (RC) Questions

Until the 1950s, most scientists believed that the geology of the ocea

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards