Earths mantle lies below the crust and above the core. Scientists can

Question

Earth’s mantle lies below the crust and above the core. Scientists cannot examine the mantle directly but have inferred its structure by measuring how seismic waves from earthquakes are distorted as they traverse it. In laboratory experiments, researchers have also studied mineral formation at the high pressures and temperatures in the mantle. These measurements and experiments indicate that the mantle consists of several concentric layers, with different minerals predominating at different depths. As pressures and temperatures increase toward the core, they force the elements to rearrange into new crystal structures, forming different minerals.

Until 2004, scientists believed that the lower mantle was fairly uniformly composed of a dense form of magnesium silicate belonging to a family of crystals called perovskite that extended all the way down to the boundary between the mantle and the core. But seismic measurements revealed the lower mantle’s bottommost layer to be substantially denser than the rest. Since magnesium silicate perovskite has a tightly packed geometry that seemed to maximize the mass per unit volume, the higher pressures in this bottommost layer were not expected to change the perovskite’s geometry. Thus, scientists assumed that a greater abundance of heavy elements within the crystal must explain the higher density. However, this assumption was also problematic, because convection should stir the lower mantle, mixing the layers and producing a uniform distribution of elements.

In 2004, geophysicists managed to expose magnesium silicate to the extreme pressures and temperatures of this bottommost layer, producing an unexpected new, denser crystalline structure they called postperovskite. Because postperovskite would conduct heat away from Earth’s core more rapidly than perovskite would, scientists hypothesize that the early Earth’s core must have been hotter than previously thought. This suggests that the inner core cooled enough to solidify only about one billion years ago. The solid inner core strengthens Earth’s magnetic field. This field shields the surface from radiation from space, which can cause genetic mutations and would be especially dangerous for life on land. Thus, the growing intensity of the magnetic field about one billion years ago may have first allowed life to expand from the seas onto dry land. So postperovskite's discovery may help explain the timing of developments in life’s evolution.

1. As evidence that postperovskite is present in Earth’s mantle, the passage mentions the

A. timing of developments in the evolution of life on Earth
B. different minerals predominating at different depths in the core
C. results of exposing magnesium silicate to high pressures and temperatures
D. indirectly measured temperature of Earth’s inner core
E. dangers that genetic mutations pose for life on Earth

2. According to the passage, the scientists’ hypothesis that the early Earth’s core must have been hotter than previously thought is based on

A. measurements of seismic waves from earthquakes that establish a layer of Earth’s inner core has been losing heat
B. samples of billion-year-old magnesium silicate crystals whose density is similar to that of postperovskite
C. a theoretical model demonstrating the range of temperatures in which postperovskite can be converted to perovskite
D. the presence of fossils showing that organisms were living on dry land before one billion years ago
E. differences between the heat-conduction properties of perovskite and those of postperovskite in the lower mantle

3. Based on the passage, the implications of the discovery of postperovskite most likely challenged many geophysicists' beliefs about

A. why Earth’s inner core strengthens Earth’s magnetic field
B. when the elements in Earth’s inner core first became solid
C. which elements are present in Earth’s core and mantle
D. whether crystalline magnesium silicate can form in Earth’s lower mantle
E. what causes magnesium silicate crystals to have a tightly packed geometry

4. Based on the passage, which of the following statements about perovskite is most likely true?

A. It could be transformed into postperovskite as it sinks through the mantle.
B. It may have a more tightly packed crystalline geometry than postperovskite does.
C. It has been directly observed to distort slightly when seismic waves traverse it.
D. It contains heavy elements in greater quantities than postperovskite does.
E. It might comprise several concentric layers of different minerals below the mantle.

CHECK HERE THE SAME PASSAGE WITH DIFFERENT SET OF QUESTIONS

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Show Answer Question #1

Official Answer

C

Show Answer Question #2

Official Answer

E

Show Answer Question #3

Official Answer

B

Show Answer Question #4

Official Answer

A

sachi-in · Answer

1. As evidence that postperovskite is present in Earth’s mantle, the passage mentions the

A. timing of developments in the evolution of life on Earth  -  this is presented as a consequence not an evidience 
B. different minerals predominating at different depths in the core  -  Not used in support  of alternate hypothesis
  C. results of exposing magnesium silicate to high pressures and temperatures  -  Experiment proved that, and that evidience was used to suppot this alternate hypothesis  
D. indirectly measured temperature of Earth’s inner core  -  No such expt. conducted
E. dangers that genetic mutations pose for life on Earth  -  No it was a conclusion not evidience

2. According to the passage, the scientists’ hypothesis that the early Earth’s core must have been hotter than previously thought is based on

Passage says: Because  postperovskite would conduct heat away from Earth’s core more rapidly than perovskite would , scientists hypothesize that the early Earth’s core must have been hotter than previously thought.

Hence heat conducting capacity was responsible for this conclusion.

3. Based on the passage, the implications of the discovery of postperovskite most likely challenged many geophysicists' beliefs about

A. why Earth’s inner core strengthens Earth’s magnetic field  -  why it stengthens is not explained in passage
B. when the elements in Earth’s inner core first became solid  -  It was not challenged, belief that life formed 1M yrs ago reinforced
C. which elements are present in Earth’s core and mantle  -  Conception about element in Mantle changed , not about Core 
D. whether crystalline magnesium silicate can form in Earth’s lower mantle  -  They knew it can form ( only belief about their geometry changed )
  E. what causes magnesium silicate crystals to have a tightly packed geometry  -  Yes ! earlier they believed metals were responsible not they know it's a special type of structure / arrangement that does it

4. Based on the passage, which of the following statements about perovskite is most likely true?

A. It could be transformed into postperovskite as it sinks through the mantle.  -  probably becuase more heat and pressure may transform it
B. It may have a more tightly packed crystalline geometry than postperovskite does.  -  NO postperovskite are denser
C. It has been directly observed to distort slightly when seismic waves traverse it.  -  NO mention of wheather distortion can occur
  D. It contains heavy elements in greater quantities than postperovskite does.  -  We don't know that, the presense of metals were used to support a hypothesis which was proven wrong by the postperovskite theory, so atleast we know it's not the metals that cause the higher density hence metals may or may not be present , no expreiment conducted to prove the presense or the lack of that  
E. It might comprise several concentric layers of different minerals below the mantle. - below the mantle there is core, we don't know anything about that from passage

Starburst10 · Answer

This question was in the same passage -- can someone explain it?

The statement "the higher pressures in this bottommost layer were not expected to change the perovskite’s geometry" functions in the passage as
A. support for a claim about magnesium silicate perovskite's tightly packed geometry
B. a premise supporting the claim that scientists made an incorrect assumption about the abundance of heavy elements within perovskite
C. an explanation for the fact that a certain scientific assumption was found to be problematic
D. a description of a prediction for which the preceding statement about magnesium silicate perovskite provides the grounds
E. an experimental result on which a subsequent statement about postperovskite is intended to cast doubt

carcass · Answer

Do you mean that the passage is missing one question? I do not see it above

an2639 · Answer

Hi for my mock test I have got a different set of questions for this passage - Any reference link for those questions?

Bunuel · Answer

Please post the screenshots of the passage and all the questions. Thank you!

an2639 · Answer

Bunuel · Answer

Thank you very much! That question is here: https://gmatclub.com/forum/earth-s-mant ... 27355.html

Leo_1511 · Answer

Hi Bunuel, can you explain Q3 please?

SergejK · Answer

Bunuel :  Here is 1 additional questions from official mock 3.

Bunuel · Answer

Added the question to the topic here: https://gmatclub.com/forum/earth-s-mant ... l#p3375350 Thank you!

callingTardis · Answer

Some of your green colored answers dont match the official answers above

PardeepGandhi · Answer

3. Based on the passage, the implications of the discovery of postperovskite most likely challenged many geophysicists' beliefs about

A. why Earth’s inner core strengthens Earth’s magnetic field
B. when the elements in Earth’s inner core first became solid
C. which elements are present in Earth’s core and mantle
D. whether crystalline magnesium silicate can form in Earth’s lower mantle
E. what causes magnesium silicate crystals to have a tightly packed geometry

For this question, I believe the answer should be E. Since, there is a mention of what scientists believed before 2004 in the 2nd paragraph. And in the last one, the geophysicists proved that the previous assumptions were untrue.

Please correct me if I'm wrong.

JuniqueLid · Answer

I think B as the OA for Q3 is a stretch.

The question asks: "...most likely challenged many geophysicists’ beliefs...". So the correct answer must be something that the passage explicitly says was previously assumed or believed — and then overturned.

In the case of Option B (when the inner core first became solid): The passage introduces the new hypothesis that the core solidified about one billion years ago, suggested by the discovery because postperovskite conducts heat more rapidly which caused the core to cool faster. It also says: "the early Earth’s core must have been hotter than previously thought." This can be seen as a revision of prior assumptions about the temperature and cooling rate, but the passage never says there was a specific belief about the exact timing of solidification.

After all, there may not have been a specific prior belief — scientists may simply not have known — so nothing was "challenged" in terms of specific timing.

I actually think (C) is more reasonable. There was an explicit prior hypothesis that "higher density was caused by heavy elements" but that hypothesis is "problematic" althought there has been no concrete evidence to disprove it...until (the next paragraph) 2004 when they found postperovskite. Postperovskite explained the density, not elements distribution.

Can someone help explain this question as there seem to be many different views from different people here, despite the OA is marked as (B). Thank you.

ppoomm · Answer

I chose C in my mock too. After reviewing my mock, I see that C is incorrect in two main things:

1) "Which elements are present in Earth's core and   MANTLE  " -> Postperovskite is present in the bottommost layer (inner core). The passage never mentioned about it at the mantle layer in regard of Postperovskite. And we do not care about mantle because we already know that perovskite magnesium silicate predominate in that layer.

2) Geophysicists (or even "scientists" in paragraph 2 - if referring further back to what this choice supposes to convey) did not believe (or THOUGHT) about the elements in Earth's core and   MANTLE.   They believed that the cores entirely composed of magnesium silicate. The passage later revealed that their belief already was correct. However, they got it wrong at the crystaline structure, perovskite vs postperovskite, leading to different heat conductivity property.

So, here is the thing: BOTH perovskite and postperovskite are made from magnesium silicate  (and geophysicists never thought they are from differrent elements).  They only differ in their structure.