Plate Tectonics and Orogeny Questions and Answers

Answer: B

Continental convergent zones are related to compression and thick crust.

When two continents collide, neither subducts easily because continental crust is buoyant. The crust is shortened by compression through folding and thrust stacking, and it thickens, reaching about 70 km beneath the Himalaya and Tibet, roughly twice the normal thickness.

Thin crust results from extension, as in rifts and passive margins, and extension is the opposite of the stress regime at a convergent boundary, so the other combinations do not fit.

Answer: B

The key's answer is 150 million years ago.

Gondwana began to break apart in the Jurassic. India, then joined to Madagascar, Antarctica and Australia, started to separate from the rest of Gondwana in the Late Jurassic to Early Cretaceous, roughly 150-130 million years ago, and then moved north across the Tethys. Of the options, 150 million years is the one that falls in this interval.

50 million years ago is roughly when India collided with Asia, the end of the drift rather than its start. 300 and 400 million years ago are before Pangaea had even finished assembling, so no break-up could have begun then.

Answer: A

The plates are the African, Indo-Australian and Antarctic.

The Rodrigues Triple Junction in the southern Indian Ocean, east of Madagascar, is where three spreading ridges meet: the Central Indian Ridge, the Southwest Indian Ridge and the Southeast Indian Ridge. They separate the African (Somalian) plate, the Indo-Australian (Capricorn and Australian) plates and the Antarctic plate, making it a ridge-ridge-ridge triple junction.

The North American, Pacific and Eurasian plates do not reach the southern Indian Ocean, so the other options are wrong.

Answer: A

The sequence from south to north is MFT-MBT-MCT-STD-ITS.

1. Main Frontal Thrust (MFT): the southern edge of the Siwaliks against the Ganga plain.
2. Main Boundary Thrust (MBT): separates the Siwaliks from the Lesser Himalaya.
3. Main Central Thrust (MCT): carries the Greater Himalayan crystallines over the Lesser Himalaya.
4. South Tibetan Detachment (STD): a north-dipping normal fault at the top of the Greater Himalaya, below the Tethyan sediments.
5. Indus-Tsangpo Suture (ITS): the collision boundary with the Asian plate in southern Tibet.

The other sequences misplace the MFT, which is the youngest and southernmost thrust, or put the MBT north of the MCT and STD, or run from north to south.

Answer: A

The drift of the Cimmerian continents opened the Neotethys.

The Cimmerian blocks, a ribbon of continental fragments including parts of present-day Turkey, Iran, Afghanistan, Tibet and Malaysia, rifted from the northern margin of Gondwana in the Permian. As they moved north towards Eurasia, the Palaeotethys ahead of them closed, and a new ocean, the Neotethys, opened behind them, between the Cimmerian blocks and Gondwana.

The Palaeotethys was the ocean that closed, not the one that opened. The Prototethys is an older, early Palaeozoic ocean. The southern Indian Ocean opened much later, in the Cretaceous break-up of Gondwana.

Answer: C

The order is 117 Ma, 65 Ma, 90 Ma.

Each hotspot is dated by the flood basalt it produced on or near the Indian plate:

1. Kerguelen: about 117 Ma, the Rajmahal Traps of eastern India and the Kerguelen Plateau.
2. Reunion: about 65-66 Ma, the Deccan Traps.
3. Marion: about 90 Ma, the Madagascar flood basalts and the St Mary's Islands rhyolites of Karnataka, when India separated from Madagascar.

The option 90, 65, 117 Ma swaps Kerguelen and Marion. The options 60, 90, 117 and 120, 95, 65 Ma assign the Deccan's age to the wrong hotspot.

Answer: D

The Columbia River flood basalts are linked with the Yellowstone hotspot.

The Columbia River Basalt Group in Washington, Oregon and Idaho erupted mainly between about 17 and 15 million years ago, when the North American plate lay above the head of the Yellowstone plume. As the plate moved south-west over the hotspot, it left the track of the Snake River Plain, ending at the present Yellowstone caldera.

Kerguelen is linked with the Rajmahal Traps and Kerguelen Plateau, Tristan with the Parana-Etendeka basalts of South America and Africa, and Hawaii with the Hawaiian-Emperor seamount chain in the Pacific.

Answer: B

These features characterise continental rifts.

Where continental lithosphere is stretched, as in the East African Rift, the Rio Grande Rift or the Basin and Range, the crust thins, normal faults bound grabens and horsts, and earthquakes are shallow. Melting of the upwelling mantle gives basalt, and that basalt in turn melts the continental crust to give rhyolite, producing the bimodal basalt-rhyolite suite typical of rifts.

Mid-ocean ridges also have normal faults and shallow earthquakes, but their magmatism is basalt only, since there is no continental crust to melt. Convergent boundaries have thick crust, thrust faults and andesitic magmatism. Transform boundaries have strike-slip faults and little magmatism.