The Basic Theory of Plate Tectonics Recognizes Two Ways Continental Margins Can Grow Seaward.

Reading Passage Question

The basic theory of plate tectonics recognizes two ways continental margins can grow seaward. Where two plates move away from a midocean rift that separates them, the continental margins on those plates are said to be passive. Such continental margins grow slowly from the accumulation of riverborne sediments and of the carbonate skeletons of marine organisms. Since most sequences of such accretions, or miogeoclinal deposits, are unreformed, passive margins are not associated with mountain building.

Along active margins continents tend to grow much faster. At an active margin an oceanic plate plunges under a continental plate, fragments of which then adhere to the continental margin. The process is met with extensive volcanism and mountain building. A classic example is the Andes of the west coast of South America.

In the original plate-tectonic model western North America was described as being initially passive and then active. It was assumed that the continent grew to a limited extent along this margin as oceanic rocks accreted in places such as the Coast Ranges of California. The model was successful in explaining such disparate features as the Franciscan rocks of the California Coast Ranges, created by subduction, and the granite rocks of the Sierra Nevada that originated in volcanoes.

The basic plate-tectonic reconstruction of the geologic history of western North America remains unchanged in the light of microplate tectonics, but the details are radically changed. It is now clear that much more crust was added to North America in the Mesozoic era than can be accounted for by volcanism and by the simple accretion of sediments. Further, some adjacent terranes are not genetically related, as would be expected from simple plate tectonics, but have almost certainly travelled great distances from entirely different parts of the world.

“The basic theory of plate tectonics recognizes two ways continental margins can grow seaward.” - is a GMAT reading comprehension passage with answers. Candidates need a strong knowledge of English GMAT reading comprehension.

This GMAT Reading Comprehension consists of 4 comprehension questions. The GMAT Reading Comprehension questions are designed for the purpose of testing candidates’ abilities in understanding, analyzing, and applying information or concepts. Candidates can actively prepare with the help of GMAT Reading Comprehension Practice Questions.

Solution and Explanation

1. Which one of the following best expresses the main idea of the passage?

(A) The margin of the west coast of North America developed through a combination of active and passive mechanisms.
(B) The growth of continental margins is only partially explained by the basic theory of plate tectonics.
(C) Continental margins can grow seaward in two ways, through sedimentation or volcanism.
(D) The introduction of microplate tectonics poses a fundamental challenge to the existing theory of how continental margins are formed.
(E) Continental margins grow more rapidly along active margins than along passive margins.

2. The passage supplies information for answering all of the following questions regarding continental margins EXCEPT:

(A) How have marine organisms contributed to the formation of passive continental margins?
(B) What were some of the processes by which the continental margin of the west coast of North America was formed?
(C) Are miogeoclinal deposits associated with mountain building along continental margins?
(D) How was the continental margin of the west coast of South America formed?
(E) How much crust added to North America in the Mesozoic era can be accounted for by the accretion of sediments from the ocean floor?

3. The author mentions the Franciscan rocks of the California Coast Ranges in order to make which one of the following points?

(A) The basic theory of plate tectonics accounts for a wide variety of geologic features.
(B) The original plate tectonic model falls short of explaining such features.
(C) Subduction processes are responsible for the majority of the geologic features found along the west coast of North America.
(D) Passive margins can take on many geologic forms.
(E) The concept of microplate tectonics was first introduced to account for such phenomena.

4. Which one of the following does the author mention as evidence for the inadequacy of the original plate tectonic model to describe the formation of continental margins?

(A) Nearly flat, unreformed crystal blocks have been found along some continental margins where there are mountains further inland.
(B) Sediments and fragments from the depths of the ocean accumulate along continental margins.
(C) Large pieces of the Earth's crust that appear to be completely unrelated are found in the same area today.
(D) Unreformed miogeoclinal deposits are usually not linked to mountain building.
(E) Oceanic plates drop beneath continental plates along active margins.

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