Plate Tectonics and Orogeny Questions and Answers

Q: Arrange correctly the following continental assembly according to ages :(a) Ur1. &sim;3.0 Ga(b) Nena2. &sim;1.5 Ga(c) Rodina3. &sim;1.8 Ga

The keyed matching is (a) - 1, (b) - 3, (c) - 2: Ur about 3.0 Ga, Nena about 1.8 Ga and Rodinia (printed Rodina) given about 1.5 Ga.In the scheme of Rogers, Ur is the earliest proposed continent, assembled around 3.0 Ga. Nena (North America, Europe and Asia, the core of Columbia) was assembled around 1.8 Ga. Rodinia is the youngest of the three; it is usually dated to about 1.1 to 1.0 Ga, so the 1.5 Ga in the list is not its standard age, but it is the youngest age offered and the only one left for it once Ur and Nena are placed.The other options put Nena or Rodinia earlier than Ur, or give Nena the youngest age, which reverses the order of assembly.

Q: The development of Gondwana land assembly, coinciding with .............. orogeny, took place during ............. through fusion of ....................... .

The answer is Pan-African, 500-600 Ma, East and West.Gondwana was assembled at the end of the Neoproterozoic and in the Cambrian, when the blocks of West Gondwana (South America and Africa) collided with those of East Gondwana (India, Antarctica, Australia and Madagascar). The collisional belts formed at that time, such as the East African Orogen and the Mozambique belt, are grouped as the Pan-African orogeny, of roughly 500 to 600 Ma.The Hercynian or Variscan orogeny is Late Palaeozoic (about 380-280 Ma) and built Pangaea in Europe, not Gondwana; 1000-12000 Ma is not even a geological range. Columbia (Nuna) is a much older, Palaeoproterozoic supercontinent.

Q: The Neotethys ocean opened during Early Permian as a result of rifting of ................... with Indian continent.

The Neotethys opened by the rifting of the Cimmerian terranes.In the Early Permian a long ribbon of continental fragments, the Cimmerian blocks (including parts of present-day Turkey, Iran, Afghanistan, Tibet and Sibumasu), rifted from the northern margin of Gondwana, including the margin next to India. The new ocean that opened behind them was the Neotethys, while the Palaeotethys ahead of them closed as they drifted north to collide with Eurasia in the Triassic.The Variscan terranes belong to the earlier Palaeozoic assembly of Europe; the Russian continent (Baltica) lay north of Palaeotethys; and the Hindu Kush blocks are only part of the Cimmerian collage, not the term for the whole.

Q: The correct order of succession of supercontinent formation amongst the following is :

The correct succession is Columbia-Rodinia-Pannotia.The supercontinent cycle runs: Columbia (also called Nuna), assembled about 2.0-1.8 Ga; Rodinia, assembled about 1.1-0.9 Ga; the short-lived Pannotia, about 0.6 Ga at the end of the Proterozoic; and then Pangea, assembled in the late Palaeozoic around 0.3 Ga.The other orders fail. Gondwana is a Palaeozoic southern landmass that became part of Pangea, so it cannot come before Rodinia or after Pangea. Arctica and Atlantica are older Archaean-Palaeoproterozoic continental nuclei that went into Columbia, so they do not follow it.

Q: The segments of oceanic crust between island arcs and continents are termed :

These segments are Back-arc basins.Behind a volcanic island arc, on the side away from the trench, extension of the overriding plate opens small ocean basins floored by new oceanic crust, such as the Sea of Japan and the Mariana Trough. They lie between the arc and the continent.Fore-arc basins lie on the other side of the arc, between the arc and the trench. 'Island arc margin' is not a term for a segment of oceanic crust, and seamounts are isolated submarine volcanoes.

Q: The stress regime at the mid-ocean ridges is tensional in character. This is supported by earthquakes due to :

The tension is shown by earthquakes due to Normal faulting.Focal mechanisms of earthquakes under the crests of mid-ocean ridges show normal faulting, with the tension axis perpendicular to the ridge. Normal faults form when the crust is pulled apart, which confirms the tensional stress regime of sea-floor spreading.Strike-slip earthquakes occur on the transform faults that offset ridge segments, not on the spreading crest itself. Reverse faulting indicates compression. 'Opening of rift' describes the process, not a type of faulting that earthquakes reveal.

Q: What is the spreading rate of slow spreading ridge ?

A slow spreading ridge opens at about 10-40 mm/year.Mid-ocean ridges are grouped by full spreading rate. Slow ridges, such as the Mid-Atlantic Ridge, spread at a few centimetres a year, roughly 10-40 or 50 mm/year, and have a deep axial rift valley. Intermediate ridges spread at about 50-90 mm/year, and fast ridges, such as the East Pacific Rise, at about 90-160 mm/year or more, with a smooth axial high.So 40-80 mm/year is intermediate, 8-16 cm/year is fast, and 16-24 cm/year is faster than any ridge spreads today.

Q: The heights and widths of the mid-oceanic ridges with reference to the abyssal plane is of the order :

Mid-ocean ridges stand about 2.5 km above the abyssal plain and are about 3000 km wide.The ridge crest typically lies at about 2.5 km depth, while the abyssal plains on either side lie at about 5 km, so the ridge rises some 2-3 km. Its flanks slope down gently over great distances, giving a total width of the order of 1000-4000 km; the Mid-Atlantic Ridge occupies roughly the middle third of the Atlantic.A height of 6 km is too great and a width of 500 km too narrow, and 3000 km is a width, not a height, so the other options are wrong.

Q: The triple junction between African, Indo-Australian and Antarctic plates is of the type :

The junction is of the RRR type.The African, Indo-Australian and Antarctic plates meet at the Rodrigues Triple Junction in the Indian Ocean, where three spreading ridges meet: the Central Indian Ridge, the Southwest Indian Ridge and the Southeast Indian Ridge. A meeting of three ridges is a ridge-ridge-ridge (RRR) junction, the type that is stable in any geometry.RTR, RTF and FTR all require a trench or a transform fault among the three arms, and no subduction zone takes part in this junction.

Q: Plate boundary zones are characterized by the presence of :

Plate boundary zones show both (A) and (B): high heat flow and high seismic and magmatic activity.At divergent boundaries rising magma and thin lithosphere give very high heat flow, shallow earthquakes and volcanism. At convergent boundaries subduction produces deep earthquakes and arc volcanoes, and transform boundaries are seismically very active. Most of the world's earthquakes and volcanoes are concentrated along plate boundaries.Choosing only one of the two features leaves out the other, and 'Neither' contradicts the basic observations on which plate tectonics rests.

11.

Arrange correctly the following continental assembly according to ages :

(a) Ur	1. ∼3.0 Ga
(b) Nena	2. ∼1.5 Ga
(c) Rodina	3. ∼1.8 Ga

A.(a) - 1, (b) - 2, (c) - 3
B.(a) - 3, (b) - 2, (c) - 1
C.(a) - 2, (b) - 1, (c) - 3
D.(a) - 1, (b) - 3, (c) - 2

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: D

The keyed matching is (a) - 1, (b) - 3, (c) - 2: Ur about 3.0 Ga, Nena about 1.8 Ga and Rodinia (printed Rodina) given about 1.5 Ga.

In the scheme of Rogers, Ur is the earliest proposed continent, assembled around 3.0 Ga. Nena (North America, Europe and Asia, the core of Columbia) was assembled around 1.8 Ga. Rodinia is the youngest of the three; it is usually dated to about 1.1 to 1.0 Ga, so the 1.5 Ga in the list is not its standard age, but it is the youngest age offered and the only one left for it once Ur and Nena are placed.

The other options put Nena or Rodinia earlier than Ur, or give Nena the youngest age, which reverses the order of assembly.

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12.

The development of Gondwana land assembly, coinciding with .............. orogeny, took place during ............. through fusion of ....................... .

A.Pan-African, 500-600 Ma, East and West
B.Hercinian, 450 Ma, East and West
C.Variscan, 1000-12000 Ma, North and South
D.Columbian, 300-500 Ma, East and West

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: A

The answer is Pan-African, 500-600 Ma, East and West.

Gondwana was assembled at the end of the Neoproterozoic and in the Cambrian, when the blocks of West Gondwana (South America and Africa) collided with those of East Gondwana (India, Antarctica, Australia and Madagascar). The collisional belts formed at that time, such as the East African Orogen and the Mozambique belt, are grouped as the Pan-African orogeny, of roughly 500 to 600 Ma.

The Hercynian or Variscan orogeny is Late Palaeozoic (about 380-280 Ma) and built Pangaea in Europe, not Gondwana; 1000-12000 Ma is not even a geological range. Columbia (Nuna) is a much older, Palaeoproterozoic supercontinent.

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13.

The Neotethys ocean opened during Early Permian as a result of rifting of ................... with Indian continent.

A.Variscan Terranes
B.Hindukush Terranes
C.Cimmerian Terranes
D.Russian Continent

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: C

The Neotethys opened by the rifting of the Cimmerian terranes.

In the Early Permian a long ribbon of continental fragments, the Cimmerian blocks (including parts of present-day Turkey, Iran, Afghanistan, Tibet and Sibumasu), rifted from the northern margin of Gondwana, including the margin next to India. The new ocean that opened behind them was the Neotethys, while the Palaeotethys ahead of them closed as they drifted north to collide with Eurasia in the Triassic.

The Variscan terranes belong to the earlier Palaeozoic assembly of Europe; the Russian continent (Baltica) lay north of Palaeotethys; and the Hindu Kush blocks are only part of the Cimmerian collage, not the term for the whole.

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14.

The correct order of succession of supercontinent formation amongst the following is :

A.Rodinia-Pangea-Gondwana
B.Columbia-Arctica-Atlantica
C.Columbia-Rodinia-Pannotia
D.Columbia-Gondwana-Rodinia

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: C

The correct succession is Columbia-Rodinia-Pannotia.

The supercontinent cycle runs: Columbia (also called Nuna), assembled about 2.0-1.8 Ga; Rodinia, assembled about 1.1-0.9 Ga; the short-lived Pannotia, about 0.6 Ga at the end of the Proterozoic; and then Pangea, assembled in the late Palaeozoic around 0.3 Ga.

The other orders fail. Gondwana is a Palaeozoic southern landmass that became part of Pangea, so it cannot come before Rodinia or after Pangea. Arctica and Atlantica are older Archaean-Palaeoproterozoic continental nuclei that went into Columbia, so they do not follow it.

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15.

The segments of oceanic crust between island arcs and continents are termed :

A.Fore-arc basins
B.Island arc margins
C.Back-arc basins
D.Sea mounts

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: C

These segments are Back-arc basins.

Behind a volcanic island arc, on the side away from the trench, extension of the overriding plate opens small ocean basins floored by new oceanic crust, such as the Sea of Japan and the Mariana Trough. They lie between the arc and the continent.

Fore-arc basins lie on the other side of the arc, between the arc and the trench. 'Island arc margin' is not a term for a segment of oceanic crust, and seamounts are isolated submarine volcanoes.

Discuss in Forum

16.

The stress regime at the mid-ocean ridges is tensional in character. This is supported by earthquakes due to :

A.Normal faulting
B.Strike-slip faulting
C.High angle reverse faulting
D.Opening of rift at the ridge crest

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: B

The tension is shown by earthquakes due to Normal faulting.

Focal mechanisms of earthquakes under the crests of mid-ocean ridges show normal faulting, with the tension axis perpendicular to the ridge. Normal faults form when the crust is pulled apart, which confirms the tensional stress regime of sea-floor spreading.

Strike-slip earthquakes occur on the transform faults that offset ridge segments, not on the spreading crest itself. Reverse faulting indicates compression. 'Opening of rift' describes the process, not a type of faulting that earthquakes reveal.

Discuss in Forum

17.

What is the spreading rate of slow spreading ridge ?

A.8-16 cm/year
B.40-80 mm/year
C.10-40 mm/year
D.16-24 cm/year

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: C

A slow spreading ridge opens at about 10-40 mm/year.

Mid-ocean ridges are grouped by full spreading rate. Slow ridges, such as the Mid-Atlantic Ridge, spread at a few centimetres a year, roughly 10-40 or 50 mm/year, and have a deep axial rift valley. Intermediate ridges spread at about 50-90 mm/year, and fast ridges, such as the East Pacific Rise, at about 90-160 mm/year or more, with a smooth axial high.

So 40-80 mm/year is intermediate, 8-16 cm/year is fast, and 16-24 cm/year is faster than any ridge spreads today.

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18.

The heights and widths of the mid-oceanic ridges with reference to the abyssal plane is of the order :

A.6 km and 1500 km, respectively
B.2.5 km and 3000 km, respectively
C.3 km and 500 km, respectively
D.3000 km and 1000 km, respectively

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: B

Mid-ocean ridges stand about 2.5 km above the abyssal plain and are about 3000 km wide.

The ridge crest typically lies at about 2.5 km depth, while the abyssal plains on either side lie at about 5 km, so the ridge rises some 2-3 km. Its flanks slope down gently over great distances, giving a total width of the order of 1000-4000 km; the Mid-Atlantic Ridge occupies roughly the middle third of the Atlantic.

A height of 6 km is too great and a width of 500 km too narrow, and 3000 km is a width, not a height, so the other options are wrong.

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19.

The triple junction between African, Indo-Australian and Antarctic plates is of the type :

A.RTR
B.RTF
C.RRR
D.FTR

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: C

The junction is of the RRR type.

The African, Indo-Australian and Antarctic plates meet at the Rodrigues Triple Junction in the Indian Ocean, where three spreading ridges meet: the Central Indian Ridge, the Southwest Indian Ridge and the Southeast Indian Ridge. A meeting of three ridges is a ridge-ridge-ridge (RRR) junction, the type that is stable in any geometry.

RTR, RTF and FTR all require a trench or a transform fault among the three arms, and no subduction zone takes part in this junction.

Discuss in Forum

20.

Plate boundary zones are characterized by the presence of :

A.High heat flow
B.High seismic and magmatic activity
C.Both (A) and (B)
D.Neither (A) nor (B)

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: C

Plate boundary zones show both (A) and (B): high heat flow and high seismic and magmatic activity.

At divergent boundaries rising magma and thin lithosphere give very high heat flow, shallow earthquakes and volcanism. At convergent boundaries subduction produces deep earthquakes and arc volcanoes, and transform boundaries are seismically very active. Most of the world's earthquakes and volcanoes are concentrated along plate boundaries.

Choosing only one of the two features leaves out the other, and 'Neither' contradicts the basic observations on which plate tectonics rests.

Discuss in Forum

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