Tectonics — Platforms, Basement, Shields, and Cratons In continental geology, the platform together with its underlying basement rocks collectively constitute which large, stable unit?

Difficulty: Medium

Correct Answer: craton

Explanation:


Introduction / Context:
Continental interiors contain very old, stable cores made of ancient crystalline basement overlain in places by younger, relatively undeformed sedimentary cover. Distinguishing the terms shield, platform, basement, and craton is essential for structural geology and tectonics questions.


Given Data / Assumptions:

  • Basement: Ancient crystalline (igneous–metamorphic) rocks.
  • Platform: Regions where basement is overlain by flat-lying sedimentary cover.
  • Shield: Areas where basement is exposed at the surface (little or no cover).
  • Craton: The long-lived stable block including both shields and platforms.


Concept / Approach:
A craton is the fundamental, mechanically stable portion of continental lithosphere. Where the basement is uncovered, it forms a shield; where the basement is covered by relatively undeformed sediments, that surface expression is termed a platform. Together, these expressions make up the craton. Therefore, “platform + basement” is most accurately named a craton, not a shield. A shield is a special case (exposed basement) and does not require an overlying platform.


Step-by-Step Solution:

Define each term precisely (basement, platform, shield, craton).Relate surface expression (shield/platform) to the same deep, stable core.Recognize that the integrated unit (basement ± cover) is the craton.Select “craton.”


Verification / Alternative check:
Geology texts consistently portray shields and platforms as components of cratons. Maps label Canadian Shield (exposed basement) and adjacent platforms as parts of the North American craton.


Why Other Options Are Wrong:

  • shield: Exposed basement only; does not include sedimentary platform cover.
  • both (a) and (b): Incorrect since a shield is not equivalent to platform + basement.
  • None of the above: Incorrect because “craton” is correct.


Common Pitfalls:
Using “shield” and “craton” interchangeably. A shield is a type-area within a craton; the craton includes both shield and platform regions.


Final Answer:
craton

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