Plate tectonics is a scientific theory that describes the large-scale motions of Earth's lithosphere, the hard outer shell of the planet. The model builds on the concepts of continental drift, developed during the first few decades of the 20th century. The geoscientific community accepted the theory of plate tectonics only after the concepts of seafloor spreading were developed in the late 1950s and early 1960s.
Tectonic plates are able to move because the Earth's lithosphere has a higher strength than the underlying asthenosphere (the upper portion of the mantle). Lateral density variations in the mantle result in convection and plate movement is thought to be driven by a combination of the motion of the seafloor away from the spreading ridge (due to variations in topography and density of the crust) and drag, also known as downward suction, at the subduction zones. Another explanation lies in the different forces generated by the rotation of the globe and the gravitational forces of the Sun and the Moon. The relative importance of each of these factors is unclear, but scientists are sure that each plays some part in the underlying movement.
On Earth, the lithosphere is broken up into seven or eight major tectonic plates (depending on how they are defined) and many minor plates. Where plates meet and their relative motion determine the type of boundary: convergent, divergent, or transform. Earthquakes, volcanic activity, mountain building, and oceanic trench formation occur along these plate boundaries. On convergent boundaries, subduction carries plates into the mantle; the material lost is roughly balanced by the formation of new oceanic crust along divergent margins by seafloor spreading. In this way, the total surface of the globe remains the same - this prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories, which still have some supporters, proposed gradual shrinking or gradual expansion of the globe but more recent evidence suggests this is not the case. Regardless, it is clear that plate tectonics, while firmly entrenched as a theory in the field of geoscience, is still evolving and will continue to change in the years to come.
1. Which of the following situations is most analogous to the “conveyor belt principle” described in the 3rd paragraph?A. The total amount in a savings account increases this year because interest earned on the total savings more than offsets the several withdrawals throughout the year.
B. The food stockpiles at a local charity remain the same over the past month because no one has requested any food supplies and there have been no new donations.
C. The number of crops on a farm goes up and down throughout the year as some crops are harvested and new crops are planted.
D. The price of a product remains the same because supply and demand remain balanced throughout the year.
E. Inventory in a retail store remains constant because items are automatically replaced when they are purchased by consumers.
Solution: E. Two things are important to note from the passage about the lithosphere: 1) Its movement (not creation or elimination) causes the plates on the earth's surface to move; and 2) it is responsible for oceans and landmasses alike (the third paragraph discussed this). So choices A and B are irrelevant. Choices C and D are similar to each other, and both would actually weaken that theory because they would show that the balance of lithosphere formed and lost would have been disturbed. Choice E is correct, as the earth's radius remaining constant would explain its surface area - which is covered by lithosphere - remaining constant, too.
A. The amount of the Earth’s surface area covered by oceans has not changed substantially in its history.
B. New continents have not been formed over the course of Earth’s history.
C. The amount of lithosphere formed in ocean divergent zones is greater than that lost in deep subduction zones.
D. The amount of lithosphere lost in subduction zones is not the same as that created in divergent zones.
E. The radius of the earth has not increased over the Earth’s history.
Solution: B In the first paragraph, the author states that "The geoscientific community accepted the theory of plate tectonics ONLY after the concepts of seafloor spreading were developed in the late 1950s and early 1960s." From this you know that seafloor spreading is an essential component to the theory of plate tectonics so B is correct. Choice A is incorrect as there is no evidence the theory is controversial and the author certainly doesn't think it lacks evidence. Choices C and D are too strong - he does mention that the theory will evolve, but we can't infer that he thinks "most" disagreements will be resolved or that any new explanations are necessary (it's more likely, given that he spent so much time discussing plate tectonics based on seafloor spreading, that he's more interested in confirmation and clarification of the current theory). And E is also false - he rarely mentions gravity but has a lot to say about convection.
A. plate tectonics is a controversial theory that lacks key evidence
B. seafloor spreading is an essential component of plate tectonic theory
C. most disagreements in plate tectonic theory will be resolved in the years to come
D. new explanations for plate movement are necessary
E. gravity is more important in explaining plate movement than convection and differences in topography.
Solution: D. The author is describing the theory of plate tectonics and does so without inserting any opinion. He is not "arguing" anything and at no point does he mention any controversy, so choice A is incorrect, and although he mentions that the theory will evolve he doesn't make any specific predictions, so E is incorrect. For C, there is no discussion or focus on shortcomings of the theory so this is incorrect. For E, any "evidence" he gives is part of the narrative for how the theory came to be - he doesn't provide the support himself, but rather discusses how the theory has evolved into what it is. The goal of the passage is to make the reader better understand the theory of plate tectonics, so D is correct.
A. argue that plate tectonics is a controversial theory
B. present support for an evolving scientific theory
C. explain some of the shortcomings of the plate tectonic theory
D. describe an important scientific theory
E. predict future developments in the theory of plate tectonics
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19 Jan 2021, 03:00
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(03:35) wrong 
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