About this video
A reasoning-skills walkthrough of a strengthen-the-argument question. Eight craters run in a straight line, and the argument concludes they were caused by volcanic events rather than meteorites, since the craters are different ages and a mixed cause would be unlikely. The tutor evaluates each answer choice and explains why choice B supplies the missing link that no natural cause would produce a line of meteorite craters of different ages.
You'll be able to
- identify the conclusion and premises in a strengthen-the-argument question.
- explain why a missing assumption, when stated, strengthens an argument.
- eliminate answer choices that weaken an argument or are off-topic.
Key words
- Conclusion
- The claim the argument is trying to establish, here that the craters were caused by volcanic events.
- Premise
- A supporting statement offered as a reason for the conclusion, such as the linear arrangement of the craters.
- Unstated assumption
- A link the argument relies on but does not say, which a strengthening answer often supplies.
- Strengthen
- To make the conclusion more likely to be true when a new statement is added.
- Weaken
- To make the conclusion less likely, often by removing support or raising an alternative explanation.
Transcript
- [Tutor] "Eight large craters run in a long straight line across a geographical region. Although some of the craters contain rocks that have undergone high-pressure shocks characteristic of meteorites slamming into Earth, these shocks could also have been caused by extreme volcanic events.
Because of the linearity of the craters, it is very unlikely that some of them were caused by volcanoes and others were caused by meteorites. Thus, since the craters are all different ages, they were probably caused by volcanic events rather than meteorites. Which one of the following statements, if true, would most strengthen the argument?"
So let's just make sure we understand the argument here. So you have these eight craters that run in a long straight line, so one, two, three, four, five, six, seven, eight, so those are eight craters that run in a long straight line, and they're talking about is it caused by meteorites?
Meteor, let me write that a little bit neater, meteorite versus volcanoes, volcanoes, and they say because of the linearity it's unlikely that some of them were caused by volcanoes and others were caused by meteorites. So it's unlikely to be a mix of them because you know it seems very low probability that hey, that one's a meteorite, that one's a meteorite and a volcano, and that one's a meteorite, and volcanic activity just happened to be on the exact straight line in between them.
So that's what this statement is saying. "Because of the linearity of the craters", "Because of the linearity of the craters, it is very unlikely that some of them were caused by volcanoes and others were caused by meteorites." And then they say, "Thus, since the craters are all different ages, they were probably caused by volcanic activity/events rather than meteorites."
So why does that, why does that seem to rule out meteorites? Well if it was one meteor shower, you could imagine a bunch of meteors all falling and having the same rough direction, and one falls here, and another one's right behind it falls here, and another one falls there, and so they're all part of the same event, you could imagine that multiple meteorites part of the same event at around the same time forming a line as they collide with the Earth, but they're saying they're all different ages.
So if you're talking about craters caught started at different times, the odds of meteor, one meteorite hitting, and then a few hundred or thousand years, or a million years later another meteorite hitting in this exact same line, that seems very unlikely and so that's why the difference in ages is also pointing to volcanic events.
The linearity implies that we're probably not a mix and then the different of ages plus the linearity implies that we're probably dealing, or according to the author, with volcanic events. Now let's see what would further enhance or strengthen the argument. So choice A, "A similar but shorter line of craters that are all the same age is known to have been caused by volcanic activity."
So one, if they're talking about a shorter line of craters that are the same age, well one this doesn't apply that strongly. This is a longer line of craters that are different ages. In some ways the different ages makes it a little bit more interesting to explain, and so the shorter line of craters that are all the same age doesn't really backup what's going on here, and so it doesn't seem to really strengthen the argument.
A shorter line of craters that are same age is less impressive than what we're talking about here. B, "No known natural cause would likely account for eight meteorite craters of different ages forming a straight line." So this is what I had mentioned. If you imagined meteorites, meteorite craters all of the same time, if they're part of the same meteor shower, well then you could imagine them all you know they're all traveling together, and they break up as they or one big rock breaks up as it enters into the Earth's atmosphere into multiple small ones but they're all going the same direction, and then maybe where they collide with Earth they can form a line of craters.
But the fact that there's no natural cause which will likely account for eight meteorites to fall at different times, I implicitly assume that when I try to develop this conclusion but here they're saying it explicitly, so I was kinda doing the work for them but that really helps and in fact I would put this, I would put this you know maybe right over here.
I would say, "And the craters are all different ages, and no known natural cause would likely account for eight meteorite craters of different ages forming a straight line. Therefore, they were probably caused by volcanic events rather than meteorites." That would actually make it more cohesive and you'd be less dependent on someone like me to say hey well, you know what are the odds of different meteorites at different times of causing the straight line.
So I like choice B lot. Let's look at the other choices just to make sure that we don't like them alright? "There is no independent evidence of either meteorites or volcanic activity in the region where the craters are located." Well that definitely wouldn't strengthen the argument, that actually would undermine the argument a little bit.
If you say, if the argument they're saying it's probably volcanic event, but then if we said there's no evidence of volcanic activity in the region, well that would undermine not strengthen so let's cross that out. "There is no independent evidence of a volcanic event strong enough to have created the high-pressure shocks that are characteristic of meteorites slamming into Earth."
So this okay, this would undermine it. They're saying hey some of these have the characteristics of these high-pressure shocks and if there's no evidence that volcanic events can do that, well that would undermine the conclusion that these were probably caused by volcanic events.
"No single meteor shower has created exactly eight impact craters that form a straight line." So this one is interesting because they're talking about a single meteor shower, this is more of the things all happening at the same time, and so maybe if these craters were the same age this could help strengthen the argument that it was volcanic, but we're not talking about all the craters getting created at the same time.
So we don't need to think about, even rule out the single meteor shower, so once again I don't think that does it, that doesn't really strengthen the argument. B I feel very good about.
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