Within the earth's core, which is iron, pressure increases with depth

Question

Which of the following most logically completes the argument below?

Within the earth's core, which is iron, pressure increases with depth. Because the temperature at which iron melts increases with pressure, the inner core is solid and the outer core is molten. Physicists can determine the melting temperature of iron at any given pressure and the pressure for any given depth in the earth. Therefore, the actual temperature at the boundary of the earth's outer and inner cores—the melting temperature of iron there—can be determined, since_______.

(A) the depth beneath the earth's surface of the boundary between the outer and inner cores is known
(B) some of the heat from the earth's core flows to the surface of the earth
(C) pressures within the earth's outer core are much greater than pressures above the outer core
(D) nowhere in the earth's core can the temperature be measured directly
(E) the temperatures within the earth's inner core are higher than in the outer core

ID: 500251

chaitanya87 · Accepted Answer

This question is an Assumption type . If we keep P for pressure, T for melting temperature and D for depth from the surface, we have
Higher D --> Higher P,(1st line)
Higher P --> Higher T,( 1st line)
and they can determine P and T for any depth (D).

Think like this. If they knew exact temperature, they should know the pressure --> they should have knowledge on the depth. So there's your answer.

Give kudos if you find the explanation useful.

Senthil7 · Answer

Argument:- Within the earth's core, which is iron, pressure increases with depth. Because the temperature at which iron melts increases with pressure, the inner core is solid and the outer core is molten. Physicists can determine the melting temperature of iron at any given pressure and the pressure for any given depth in the earth. Therefore, the actual temperature at the boundary of the earth's outer and inner cores--the melting temperature of iron there--can be determined, since_______.

The correct answer choice is "A" and my explanation as follows:

Option A states "a) the depth beneath the earth's surface of the boundary between the outer and inner cores is known"
The highlighted portion above in the argument states that for a "given" pressure / depth it is possible to determine temperature therefore the assumption that would logically complete this argument is that either or both of these parameters are "given" or in other words "known".

Kudos if you think this explanation most logically completes the choice of answer :-)

CAMANISHPARMAR · Answer

Official Answer:-

Argument Construction

Situation : Pressure within the earth's iron core increases with depth. Because the melting temperature of iron increases with pressure, the inner core is solid and the outer core molten. Physicists can determine iron's melting temperature at any pressure and the pressure it is under at any depth.

Reasoning  : What further premise, combined with the information provided, would support the conclusion that physicists can determine the temperature at the boundary between the outer and inner cores? Since physicists can determine iron's melting temperature at any pressure and the pressure it is under at any depth, they must be able to determine its melting temperature at any depth. The temperature at the boundary between the inner and outer cores must exactly equal the melting temperature there, since that is the boundary between the molten and solid parts of the core. To determine the temperature at the boundary, therefore, it would suffice to know the depth of the boundary.
 
Correct Answer is option A:  If physicists know the depth of the boundary between the inner and outer cores, they can determine the temperature at the boundary.

jabhatta2 · Answer

Hi  avigutman  - I did select A but my question is the significance of the two dashes in the red. Specifically -

When you read the yellow statement , is the author himself/ herself telling us that the temperatures ARE THE SAME (At the boundry) ? OR is that an  inference   to be made by the reader ?

I couldn't tell if the author himself/herself was stating plainly if these two temperatures were the same or not (at the boundry)

avigutman · Answer

jabhatta2  A bit before your yellow highlight we learned that:
 the inner core is solid and the outer core is molten

From that we can infer that the temperature at the boundary is the melting temperature of iron.
The author used the dashes (and the stuff in between the dashes) to help us along with that inference. It's a bit like saying "as you may have already deduced"

So we can replace the dashes like so:
the actual temperature at the boundary of the earth's outer and inner cores , which, as you may have already deduced, is the melting temperature of iron there,  can be determined...

swikrityC · Answer

GMATNinja   KarishmaB  please help me with the question. I couldn't understand the question stem at all and was confused amongst these three answer options A, C, and E.

miag · Answer

Hi,

I would recommend making scratch notes as you read this so you can follow along especially since there are variables and relationships involved.

My scratch notes:  
Earth’s core = iron
Pressure ∝ Depth (depth inc => pressure inc)
Inner core: solid, outer: molten
Temp ∝ pressure 
P = k1D
T = k2P
So for a known depth, you can determine pressure. Using this pressure, you can determine the pressure.

This is consistent with A) and hence A) is the right answer. 
=> One can determine actual temperature at the boundary, if the depth of that boundary is known.
C) - Just knowing the pressures inside core > outer core, will not help us in determining the exact temperature. We need the actual value.
E) - Again, just knowing which is higher doesn't help us to get the actual value

Hope this helps!

KarishmaB · Answer

Take some simplistic numbers.

Earth's core is made of iron. 
- Pressure increases with depth so say at the surface of the core, the pressure is 1 pascal, then as you go inside at 10 m depth, pressure increases to 2 pascal and so on.

- Temperature at which iron melts increases with pressure. If pressure is higher, melting point is higher. So say at 1 pascal, melting point of iron is 100 degree C and at 1 pascal, the melting point is 110 degree Celsius. If the temperature of the entire core is same say 105 degree celsius, then iron at the surface will be melted while iron at 10 m depth will be solid.

- So the inner core is solid and the outer core is molten (Earth's core has two regions - inner and outer core. Because pressure at the inner core is higher, the melting point of iron is higher there. That temperature is not reached and hence the inner core is solid. The outer core has lower pressure and hence melting point is lower there. That temperature is reached and hence iron is in molten form there)

- We know the pressures at different depths and all melting points of iron.

So we can determine the actual temperature at the boundary of the earth's outer and inner cores .. (Here they are talking about the boundary of the two regions where outer core ends and inner core begins. At that point, there would be some temperature. That is the transition pressure - as it reduces toward outer core, iron melts at that temperature. As pressure increases toward inner core, iron does not melt)
How can we determine the temperature at that boundary? The temperature there is the melting point of iron at the pressure there. If we know the pressure there, we can find what temperature is there. 
How can we find the pressure there? We know the pressures at various depths. So if we know the depth of the boundary of inner and outer core, then we can find the pressure there. Based on the pressure there, we know the melting point of iron. So temperature there would be the melting point of iron at that pressure.

That is why option (A) is correct.

We can determine the actual temperature at the boundary if we know the depth of the boundary.

None of the other options even talk about the boundary.

Within the earth's core, which is iron, pressure increases with depth

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 Critical Reasoning (CR) Questions

Within the earth's core, which is iron, pressure increases with depth

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