Some important MCQs with explanations related to Stratigraphy for the Jharkhand Academic Council (JAC) 12th Exam -1

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1. What does stratigraphy study?

a) Rocks formed by volcanic eruptions
b) Layered rocks and their sequence in time
c) Chemical composition of rocks
d) Deformation of rocks

Answer: b) Layered rocks and their sequence in time
Explanation:
Stratigraphy is the branch of geology that deals with the study of rock layers (strata) and their chronological sequence, helping to interpret Earth's history.

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2. What is the principle of superposition in stratigraphy?

a) Older rocks lie above younger rocks
b) Younger rocks lie above older rocks
c) Rock layers form parallel to Earth's surface
d) Rocks deform under pressure

Answer: b) Younger rocks lie above older rocks
Explanation:
The principle of superposition states that in an undisturbed sequence of sedimentary rocks, the youngest layers are on top, and the oldest are at the bottom.

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3. What is an unconformity in stratigraphy?

a) A continuous sequence of rock layers
b) A gap in the geological record
c) A fault plane
d) A volcanic intrusion

Answer: b) A gap in the geological record
Explanation:
An unconformity represents a period during which deposition stopped, erosion removed previously formed rocks, or both occurred, creating a gap in the stratigraphic record.

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4. Which type of unconformity involves parallel sedimentary layers?

a) Angular unconformity
b) Disconformity
c) Nonconformity
d) Paraconformity

Answer: b) Disconformity
Explanation:
A disconformity occurs when there is a gap between parallel layers of sedimentary rocks, indicating a period of non-deposition or erosion.

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5. What is the significance of index fossils in stratigraphy?

a) They help locate mineral deposits.
b) They indicate the environment of deposition.
c) They help in relative dating of rock layers.
d) They indicate tectonic activity.

Answer: c) They help in relative dating of rock layers.
Explanation:
Index fossils are used to identify and correlate the ages of rock layers because they are widespread, abundant, and existed for a short geological time.


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6. What is the principle of lateral continuity?

a) Rock layers extend laterally until they thin out or encounter a barrier.
b) Younger layers are above older layers.
c) Rocks deform laterally under stress.
d) Rocks fold due to compressional forces.

Answer: a) Rock layers extend laterally until they thin out or encounter a barrier.
Explanation:
The principle of lateral continuity states that sedimentary layers initially extend laterally in all directions until they taper off or meet an obstruction.

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7. Which of the following methods is used for absolute dating in stratigraphy?

a) Radiometric dating
b) Cross-bedding analysis
c) Fossil correlation
d) Lithological correlation

Answer: a) Radiometric dating
Explanation:
Radiometric dating measures the decay of radioactive isotopes, providing absolute ages for rocks and helping to build a precise geological timescale.


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8. What is a key bed in stratigraphy?

a) A sedimentary layer rich in fossils
b) A distinctive layer used for correlation
c) A layer of volcanic ash
d) The oldest layer in a sequence

Answer: b) A distinctive layer used for correlation
Explanation:
Key beds are unique, easily recognizable layers (such as volcanic ash or coal seams) used to correlate rock sequences over large areas.


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9. Which principle explains that geological processes occurring today operated in the past?

a) Principle of uniformitarianism
b) Principle of superposition
c) Principle of cross-cutting relationships
d) Principle of lateral continuity

Answer: a) Principle of uniformitarianism
Explanation:
The principle of uniformitarianism states that the processes shaping the Earth today, such as erosion and sedimentation, have operated similarly in the past.


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10. What is lithostratigraphy?

a) The study of rock layers based on their fossil content
b) The study of rock layers based on their lithology
c) The study of rock deformation
d) The study of igneous rocks

Answer: b) The study of rock layers based on their lithology
Explanation:
Lithostratigraphy involves the classification and correlation of rock layers based on their physical and mineralogical characteristics.


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11. What is biostratigraphy?

a) The study of rock layers based on fossils
b) The study of rock layers based on mineral composition
c) The study of Earth's magnetic field in rocks
d) The study of isotopic ratios in rocks

Answer: a) The study of rock layers based on fossils
Explanation:
Biostratigraphy uses the presence, absence, and abundance of fossils to correlate and date rock layers.


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12. What type of stratigraphic unit is defined by time?

a) Lithostratigraphic unit
b) Biostratigraphic unit
c) Chronostratigraphic unit
d) Seismic stratigraphic unit

Answer: c) Chronostratigraphic unit
Explanation:
Chronostratigraphic units represent intervals of geological time, helping to place rock layers in the context of Earth's history.


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13. What is the principle of cross-cutting relationships?

a) Older rocks are cross-cut by younger features like faults or intrusions.
b) Faults are older than the rocks they displace.
c) Rock layers extend laterally in all directions.
d) Rocks deform before deposition.

Answer: a) Older rocks are cross-cut by younger features like faults or intrusions.
Explanation:
This principle states that a geological feature (e.g., fault or intrusion) that cuts through a rock layer is younger than the layer it disrupts.


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14. Which type of unconformity separates sedimentary rocks from igneous or metamorphic rocks?

a) Angular unconformity
b) Disconformity
c) Nonconformity
d) Paraconformity

Answer: c) Nonconformity
Explanation:
A nonconformity occurs when sedimentary rocks are deposited on top of older igneous or metamorphic rocks.


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15. What does a facies in stratigraphy represent?

a) A time gap in the stratigraphic record
b) A faulted rock layer
c) The characteristics of a rock layer reflecting its depositional environment
d) A fossilized organism

Answer: c) The characteristics of a rock layer reflecting its depositional environment
Explanation:
Facies refer to the distinctive features of a rock layer that indicate the conditions under which it was deposited, such as marine, fluvial.

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