Plate Tectonics and Orogeny Questions and Answers

Answer: B

The correct matching is P-ii; Q-i; R-iv; S-iii.

• Iceland: a mantle plume sits beneath the Mid-Atlantic Ridge, so it is the classic case of plume-ridge interaction (ii).
• Afar: the Red Sea, Gulf of Aden and East African rifts meet there in a triple junction (i).
• Hawaii: an intraplate hotspot chain over a plume in the middle of the Pacific plate (iv).
• Aleutian Islands: an island arc above the subduction of the Pacific plate under the North American plate (iii).

Options pairing Iceland with the triple junction, Hawaii with subduction, or Afar with the plume all swap two of these settings.

Answer: B

The correct statement is that the seismic Moho lies at shallower depths relative to the petrologic Moho.

The seismic Moho is where P wave velocity jumps to mantle values of about 8 km/s. The petrologic Moho is the base of the rocks that crystallised from crustal magma. In ophiolites the lowest crustal rocks are ultramafic cumulates, dunite and pyroxenite settled out of the magma chamber; they are crust by origin but have mantle-like seismic velocities. So the velocity jump comes at the top of these cumulates, while the petrologic Moho lies below them, at the contact with tectonized mantle harzburgite.

That rules out the statements that the two coincide or that the petrologic Moho is the shallower one. The layered gabbro sits above the ultramafic cumulates, so the petrologic Moho lies below the layered gabbro, not above it.

Answer: A

The decreasing order of duration is S, R, P, Q.

• Supercontinent cycle (S): about 300 to 500 million years from one assembly to the next.
• Milankovitch cycle (R): orbital cycles of about 23,000, 41,000 and 100,000 years.
• Rainfall induced landslide (P): the failure itself takes seconds to hours, and a slow slide may move for days.
• Bolide impact (Q): the impact and crater excavation last seconds to a few minutes.

Every option begins correctly with S except one, so the question turns on the last three. Option D puts the landslide before the Milankovitch cycle, and option C puts the impact before both, which inverts events lasting minutes against cycles lasting tens of thousands of years.

Answer: C

Seismicity is most frequent at Subduction zones.

Convergent margins where one plate sinks beneath another produce the great majority of the world's earthquakes, and almost all of the largest ones, including every event of magnitude 9. They also host intermediate and deep earthquakes down to about 700 km along the sinking slab (the Wadati-Benioff zone).

Mid-ocean ridges (where MORB is erupted) and other extensional plate boundaries produce many but mostly shallow, moderate earthquakes. Volcanoes produce local swarms, far less in total than subduction zones.

Answer: B

The untrue statement is that both are located on respective mid-oceanic ridges.

Kilauea, on the island of Hawaii, sits in the middle of the Pacific plate thousands of kilometres from any ridge, and Reunion's Piton de la Fournaise sits on the African plate in the western Indian Ocean, away from the Central Indian Ridge. Both are intraplate hotspot volcanoes.

The other statements are true: both are attributed to mantle plumes, both are among the world's most active volcanoes, and both erupt basaltic (basic) magma, which the paper calls 'basic magnetism'.

Answer: A

The Hercynian orogeny took place during the late Carboniferous.

The Hercynian (Variscan) orogeny was the mountain building that accompanied the assembly of Pangaea, as Gondwana collided with Laurussia. Its main phase was in the late Carboniferous, continuing into the early Permian, and it formed the old mountain belts of central and western Europe and, in North America, the Alleghanian phase of the Appalachians.

The Cambrian is far too early, the Devonian is the time of the earlier Caledonian-Acadian events, and the Precambrian predates the Palaeozoic orogenies altogether.

Answer: D

The Cimmerian orogeny is characterised by the closure of the Paleotethys.

During the Late Triassic and Early Jurassic a string of microcontinents (the Cimmerian blocks, including parts of Turkey, Iran, Afghanistan and Tibet) rifted away from Gondwana and drifted north. Their collision with Eurasia closed the Paleotethys Ocean behind them and produced the Cimmerian orogeny.

As they moved north, the Neotethys opened behind them; its closure came much later and produced the Alpine-Himalayan orogeny. The Paleotethys had opened in the Palaeozoic, and the Cimmerian events have no direct link with the opening of the Pacific.

Answer: C

The answer given is a shortening of the order of 1,000 km.

India and Asia have converged by roughly 2,500 km since the collision. Plate-reconstruction estimates, such as that of Dewey, Cande and Pitman (1989), credit about 1,000 km of this to the thrusting and folding of the sediments of the old Indian margin together with deformation of Tibetan crust; the rest is attributed to other processes such as subduction of Indian crust and lateral extrusion of blocks.

10,000 km is several times the whole convergence, and 2,400 km is about the whole convergence rather than the part taken by thrusts and folds. A shortening of 500 m would be a trivial fraction of a single fault's displacement, so it cannot describe a mountain belt.

Note that this is a figure from plate reconstructions. Shortening measured by balancing cross-sections is smaller, commonly a few hundred kilometres, so the size of the number is debated; among the options only 1,000 km fits the textbook statement.

Answer: B

The key's answer is shallow earthquakes (less than 100 km).

The Alpine-Himalayan belt is a zone of continental collision. Continental crust is too buoyant to sink deep into the mantle, so there is no long, deep slab of the kind that produces deep earthquakes. Most earthquakes along the belt, including the great Himalayan thrust earthquakes, occur within the crust and upper lithosphere at shallow depths.

Deep earthquakes of 400 to 700 km occur only along subducting oceanic slabs, as in the western Pacific. Intermediate-depth earthquakes occur locally (as beneath the Hindu Kush) but do not characterise the belt, and a limit of 5 km is far too shallow for its seismicity.

Answer: B

Deep-focus earthquakes are common in island arcs.

Island arcs, such as Japan, the Marianas, Tonga and Indonesia, lie above subduction zones where cold oceanic lithosphere sinks into the mantle. Earthquakes occur within the sinking slab along the inclined Wadati-Benioff zone, from shallow depths near the trench to about 700 km far behind it.

Mid-oceanic ridges and continental rifts are zones of extension with only shallow earthquakes, and the Himalayan belt is a continental collision zone whose earthquakes are mostly shallow.