Earth's mantle lies below the crust and above the core. Scientists cannot examine the mantle directly but have inferred its...
GMAT Reading Comprehension : (RC) Questions
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.
The statement that "the higher pressures in this bottommost layer were not expect to change the perovskite's geometry" functions in the passage
1. Passage Analysis:
Progressive Passage Analysis
| Text from Passage | Analysis |
|---|---|
| Earth's mantle lies below the crust and above the core. | What it says: The Earth has three layers stacked like a sandwich - crust on top, mantle in the middle, core at the bottom. What it does: Sets up basic geography/structure that everything else will build on Source/Type: Basic scientific fact Connection to Previous Sentences: Opening sentence - establishes foundation Visualization: Think of Earth like an apple: skin (crust), flesh (mantle), core (pit). We're focusing on the "flesh" layer. Reading Strategy Insight: This is pure setup - don't worry about complexity yet. Just picture the basic structure. |
| Scientists cannot examine the mantle directly but have inferred its structure by measuring how seismic waves from earthquakes are distorted as they traverse it. | What it says: Scientists can't dig down to see the mantle, so they use earthquake waves like X-rays to "see" inside Earth. What it does: Explains HOW scientists study something they can't directly access Source/Type: Scientific methodology fact Connection to Previous Sentences: Builds on sentence 1 by explaining the challenge: now that we know WHERE the mantle is, here's the problem with studying it Visualization: Like using ultrasound to see a baby in the womb - you can't see directly, but waves tell you the shape and structure inside. Reading Strategy Insight: This explains the detective work aspect - everything we'll learn came from indirect evidence. |
| In laboratory experiments, researchers have also studied mineral formation at the high pressures and temperatures in the mantle. | What it says: Scientists also recreate mantle conditions in labs to see what happens to rocks under extreme pressure and heat. What it does: Adds a second research method to complement the seismic waves Source/Type: Scientific methodology fact Connection to Previous Sentences: This builds on sentence 2's "how do we study it" theme. Now we have TWO ways: earthquake waves + lab experiments Visualization: Think of a pressure cooker that can squeeze rocks with the force of 400 miles of Earth weight while heating them to 4000°F. Reading Strategy Insight: The author is being thorough but not complex - just listing the tools scientists use. We now have the complete toolkit. |
| These measurements and experiments indicate that the mantle consists of several concentric layers, with different minerals predominating at different depths. | What it says: Both methods (waves + experiments) show the mantle isn't uniform - it has layers like an onion, with different rock types at different depths. What it does: Gives us the first major finding from all that research Source/Type: Scientific conclusion based on evidence Connection to Previous Sentences: This is the payoff! Sentences 2-3 told us HOW scientists study the mantle. NOW we get the first answer to WHAT they found. Visualization: Like a jawbreaker candy - multiple colored layers, each made of different materials, with the layers getting more compressed toward the center. Reading Strategy Insight: Feel relieved here - this is a clear, simple conclusion after the setup. The complexity isn't increasing. |
| As pressures and temperatures increase toward the core, they force the elements to rearrange into new crystal structures, forming different minerals. | What it says: This explains WHY there are different layers - as you go deeper, the intense pressure squeezes atoms into new arrangements, creating new types of rocks. What it does: Provides the mechanism/explanation for the layered structure Source/Type: Scientific explanation of physical process Connection to Previous Sentences: This EXPLAINS sentence 4. Sentence 4 said "different minerals at different depths" - this sentence tells us WHY that happens Visualization: Like Play-Doh being squeezed - same ingredients, but extreme pressure reshapes them into totally different forms. Coal becomes diamond under pressure. Reading Strategy Insight: This is helpful elaboration, not new complexity. The author is explaining the "why" behind the "what." |
| 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. | What it says: Before 2004, scientists thought the bottom half of the mantle was mostly one type of rock (perovskite) all the way down to the core. What it does: Sets up the "old thinking" that's about to be challenged Source/Type: Historical scientific belief (now known to be wrong) Connection to Previous Sentences: This seems to contradict sentence 4's "different minerals at different depths" - but notice the key word "Until 2004" signals this old view was wrong Visualization: Imagine thinking the bottom 1800 miles of the mantle was like a thick, uniform peanut butter layer made of one rock type. Reading Strategy Insight: The phrase "Until 2004" is a huge signal - this is old, wrong information that's about to be corrected. Don't get attached to these details. |
| But seismic measurements revealed the lower mantle's bottommost layer to be substantially denser than the rest. | What it says: BUT earthquake wave measurements showed the very bottom layer was much heavier/denser than expected. What it does: Presents the evidence that contradicted the old theory Source/Type: Observational evidence from seismic measurements Connection to Previous Sentences: This directly CONTRADICTS sentence 6. The word "But" signals the correction of the old thinking. Visualization: Like expecting uniform peanut butter but finding a layer of lead at the bottom - way heavier than it should be. Reading Strategy Insight: This is classic RC structure - old theory, then contradicting evidence. The "But" makes this connection crystal clear. |
| 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. | What it says: Scientists thought that perovskite was already packed as tightly as possible, so even more pressure shouldn't change its structure. What it does: Explains why the density finding was so surprising Source/Type: Scientific reasoning about crystal structure Connection to Previous Sentences: This explains WHY sentence 7's density discovery was unexpected. If the rock was already maximally packed, extra pressure shouldn't make it denser. Visualization: Like a suitcase that's already packed to bursting - you wouldn't expect sitting on it to make it any smaller. Reading Strategy Insight: This is helpful context explaining the scientists' surprise, not new complexity to memorize. |
| Thus, scientists assumed that a greater abundance of heavy elements within the crystal must explain the higher density. | What it says: So scientists figured the bottom layer must have more heavy atoms mixed in to explain why it's denser. What it does: Presents the first attempted explanation for the density puzzle Source/Type: Scientific hypothesis/assumption Connection to Previous Sentences: Logical follow-up to sentence 8. If structure can't change, then composition must be different - more heavy elements. Visualization: Like finding a chocolate chip cookie that's surprisingly heavy and assuming it must have lead chocolate chips instead of regular ones. Reading Strategy Insight: This shows scientific reasoning - when one explanation fails, try another. The word "Thus" shows clear logical connection. |
| However, this assumption was also problematic, because convection should stir the lower mantle, mixing the layers and producing a uniform distribution of elements. | What it says: BUT this "heavy elements" explanation had a problem too - the mantle churns like boiling soup, so heavy elements should get mixed evenly, not concentrated in one layer. What it does: Shoots down the first attempted explanation Source/Type: Scientific reasoning about mantle dynamics Connection to Previous Sentences: Directly contradicts sentence 9's assumption using the word "However" as a clear signal Visualization: Like stirring chocolate chips into cookie dough - they get distributed evenly, not concentrated at the bottom. Reading Strategy Insight: Another classic RC move - proposed solution, then why it doesn't work. We're building toward the real answer. |
| 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. | What it says: In 2004, scientists finally recreated the extreme conditions and discovered that perovskite DOES change structure under that much pressure, becoming a new, denser form called postperovskite. What it does: Presents the breakthrough discovery that solves the mystery Source/Type: Experimental discovery/breakthrough Connection to Previous Sentences: This is the answer we've been building toward! Remember sentence 6 said "Until 2004" - here's what happened IN 2004 that changed everything. Visualization: Like finally getting enough pressure to compress that "fully packed" suitcase after all - it turns out there WAS a way to make it smaller. Reading Strategy Insight: This is the climax - the mystery from sentences 7-10 is solved! Scientists were wrong about the structure not being able to change. |
| 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. | What it says: Since this new rock form conducts heat better, scientists think Earth's core used to be much hotter than they previously believed. What it does: Introduces the first implication of the discovery Source/Type: Scientific hypothesis based on the discovery Connection to Previous Sentences: This builds on sentence 11's discovery by exploring what it means for Earth's history Visualization: Like discovering your house's insulation is better than you thought, which means your furnace must have been running hotter to keep you warm. Reading Strategy Insight: We're now in "implications" mode - seeing what this discovery means for bigger questions. |
| This suggests that the inner core cooled enough to solidify only about one billion years ago. | What it says: This means Earth's solid inner core only formed about 1 billion years ago (much more recently than previously thought). What it does: Provides specific timing consequence of the hotter early core Source/Type: Scientific conclusion about Earth's history Connection to Previous Sentences: Direct follow-up to sentence 12. If core was hotter, it took longer to cool and solidify. Visualization: Like a lava cake that stayed molten in the center much longer than expected because the oven was hotter than you knew. Reading Strategy Insight: This gives us a concrete timeline - we're moving from abstract physics to specific Earth history. |
| The solid inner core strengthens Earth's magnetic field. | What it says: Having a solid inner core makes Earth's magnetic field stronger. What it does: Introduces the connection between solid core and magnetic field Source/Type: Scientific fact about planetary physics Connection to Previous Sentences: Since sentence 13 said the core only solidified 1 billion years ago, this tells us what happened when it did solidify Visualization: Like how a steel rod makes an electromagnet stronger than a liquid metal core would. Reading Strategy Insight: We're building a chain: postperovskite discovery → hotter early core → later solidification → stronger magnetic field. Each step flows logically. |
| This field shields the surface from radiation from space, which can cause genetic mutations and would be especially dangerous for life on land. | What it says: Earth's magnetic field protects us from space radiation that damages DNA and would be especially harmful to land creatures. What it does: Explains why the magnetic field matters for life Source/Type: Scientific fact about magnetic field's protective function Connection to Previous Sentences: Builds on sentence 14 by explaining WHY a stronger magnetic field matters - it's about protecting life Visualization: Like Earth wearing an invisible shield that deflects harmful space rays, similar to how sunscreen protects your skin. Reading Strategy Insight: We're connecting deep Earth physics to surface biology - the chain is getting longer but still logical. |
| 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. | What it says: So when the magnetic field got stronger 1 billion years ago (when core solidified), it may have made land safe enough for life to leave the oceans. What it does: Connects the timing to a major evolutionary milestone Source/Type: Scientific hypothesis connecting geology to biology Connection to Previous Sentences: This completes the logical chain: stronger field (sentence 14) protects from radiation (sentence 15), so life could move to land when field strengthened (1 billion years ago from sentence 13) Visualization: Like marine life finally being able to venture onto land once Earth's "radiation umbrella" became strong enough to protect them. Reading Strategy Insight: The word "Thus" signals we're completing a major logical sequence. This is a satisfying conclusion to the chain of reasoning. |
| So postperovskite's discovery may help explain the timing of developments in life's evolution. | What it says: Therefore, discovering this new rock form deep in Earth might explain when and why major evolutionary changes happened. What it does: Provides the final, big-picture conclusion tying everything together Source/Type: Author's summary of the discovery's significance Connection to Previous Sentences: This is a restatement of the entire argument's significance. We started with a mineral discovery (sentence 11) and ended with implications for life's evolution. Visualization: Like how discovering one small puzzle piece deep underground helped solve a completely different puzzle about life on Earth's surface. Reading Strategy Insight: Classic RC ending - broad restatement showing why the whole topic matters. This should feel like a satisfying summary, not new complexity. What We Know Now: Complete story from deep Earth mineral discovery to surface life evolution, all connected through logical steps. |
2. Passage Summary:
Author's Purpose:
To explain how a breakthrough discovery about deep Earth minerals solved a geological mystery and revealed surprising connections to the evolution of life on Earth's surface.
Summary of Passage Structure:
The author builds their explanation through a clear scientific detective story:
- First, they establish the basic setup by explaining what Earth's mantle is, how scientists study it, and what they initially found about its layered structure.
- Next, they present a scientific mystery by describing how new evidence contradicted old beliefs about the mantle's bottom layer, creating a puzzle that existing theories couldn't solve.
- Then, they reveal the breakthrough solution when scientists discovered a new mineral form called postperovskite in 2004, which finally explained the mysterious density readings.
- Finally, they trace the surprising chain of implications from this deep Earth discovery to major questions about when and why life was able to move from oceans onto land.
Main Point:
The discovery of postperovskite deep in Earth's mantle not only solved a geological puzzle, but also revealed that Earth's core stayed hot much longer than previously thought, which means the magnetic field that protects life only became strong enough about one billion years ago to allow creatures to safely live on land.
3. Question Analysis:
This question asks us to identify the function of a specific statement within the passage's logical structure. We need to understand how "the higher pressures in this bottommost layer were not expected to change the perovskite's geometry" fits into the author's argument and what role it plays in advancing the scientific reasoning.
Connecting to Our Passage Analysis:
From our analysis, we know this statement appears in sentence 8, right after sentence 7 revealed that seismic measurements showed unexpected density in the bottommost layer. Our passage analysis shows this section is building up the mystery that will later be solved by the postperovskite discovery.
Specifically, the passage analysis reveals:
- Sentence 7 presented contradictory evidence (higher density than expected)
- Sentence 8 explains WHY this was surprising using scientific reasoning about crystal structure
- The statement functions to explain the scientists' surprise and set up their subsequent assumptions
Prethinking:
Looking at the logical flow, the statement about pressures not being expected to change perovskite's geometry serves to explain WHY the density finding was surprising. It's based on the preceding information about perovskite having "tightly packed geometry that seemed to maximize mass per unit volume." This creates a prediction: if the structure is already maximally packed, then higher pressure shouldn't change it. The question is asking us to identify this predictive relationship.
Why It's Wrong:
• This reverses the logical relationship - the statement about pressures not changing geometry is based ON the claim about tightly packed geometry, not supporting it
• The tightly packed geometry claim comes BEFORE and serves as the foundation for the pressure prediction
• This choice confuses cause and effect in the logical sequence
Common Student Mistakes:
- Did I mix up which statement supports which?
→ Trace the logical flow: tight packing → prediction that pressure won't change structure - Am I reading the sentence structure correctly?
→ The word "Since" shows that tightly packed geometry is the reason FOR the prediction
Why It's Wrong:
• This choice mischaracterizes the logical relationship - the statement isn't supporting the claim that scientists made an incorrect assumption
• The statement appears BEFORE the assumption about heavy elements, not after it as support
• The statement actually helps explain WHY scientists made that assumption, rather than proving it wrong
Common Student Mistakes:
- Am I confusing the timeline of the scientific reasoning?
→ First came the pressure prediction, then the heavy elements assumption based on that prediction - Does this choice accurately describe what happened in the passage?
→ The incorrectness of the assumption was shown later by experimental evidence, not by this statement
Why It's Wrong:
• The assumption about heavy elements became problematic due to convection mixing, not because of the pressure statement
• This choice confuses different parts of the scientific reasoning chain
• The pressure statement actually supports the heavy elements assumption rather than explaining why it's problematic
Common Student Mistakes:
- Am I tracking which assumption had which problem?
→ The heavy elements assumption was problematic due to convection, not pressure considerations - What made the assumption problematic?
→ Convection would mix elements evenly, contradicting the idea of concentrated heavy elements
Why It's Right:
• The statement directly follows and builds upon the information about perovskite's "tightly packed geometry"
• It represents a logical prediction based on that geometric property
• The word "Since" clearly establishes the preceding statement as the foundation for this prediction
• This prediction then drives the subsequent scientific reasoning about heavy elements
Key Evidence: "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."
Why It's Wrong:
• This statement appears much earlier in the passage, before postperovskite is even mentioned
• The statement is not an experimental result but a theoretical prediction
• Postperovskite information comes later and actually confirms doubts about this prediction, rather than the statement casting doubt on postperovskite
Common Student Mistakes:
- Am I confusing the chronological order of information in the passage?
→ This statement comes before postperovskite is discovered and discussed - What type of information is this statement?
→ It's a prediction based on theory, not an experimental result