Question 1

Which would the Andean-type margin not have more of, in comparison to an Aleutian-type margin?

Accepted Answer

Andean-type margins are characterized by continental volcanic arcs, leading to more felsic pyroclastics and complex crustal structures due to the subduction of an oceanic plate beneath a continental plate. Aleutian-type margins, being oceanic-oceanic plate boundaries, are more likely to have subaqueous mafic pyroclastics close to the volcanoes due to the nature of the oceanic crust and its volcanic activity.

Question 2

Which statement comes closest to describing the isostatic compensation under convergent continental mountain belts?

Accepted Answer

Isostatic compensation refers to the balance between the uplift and subsidence of the Earth's crust following mountain-building events. In convergent continental mountain belts, the extra elevation of mountains is held up by the weight of the denser material that their lower portions displace. This is similar to the way icebergs float in water with a portion of their mass being submerged to maintain balance. This is known as the principle of isostasy.

Question 3

The severed, upper part of a Jurassic seamount might occur as a distinctive, small terrane ________.

Accepted Answer

Seamounts can be accreted to continents at convergent margins, where oceanic crust is being subducted beneath a continental plate. The process can lead to pieces of the oceanic crust, including seamounts, being scraped off and added to the continent in an accretionary wedge. This does not typically happen in the settings described in the other options.

Question 4

Active volcanic arcs above a young oceanic plate are associated with a(n)________ margin.

Accepted Answer

Active volcanic arcs are commonly found at subduction zones where one tectonic plate is being forced beneath another. This occurs at ocean-ocean convergent margins where one oceanic plate subducts beneath another. This process is responsible for the creation of volcanic arcs such as the Ring of Fire around the Pacific Ocean.

Question 5

A(n)________ is a thick accumulation of sediments and small, rootless, tectonic blocks formed of material scraped off a descending, lithospheric plate.

Accepted Answer

The description matches that of an accretionary-wedge complex, which is formed at a convergent plate boundary where sediments and small blocks are scraped off the subducting plate and accumulate to form a thick mass.

Question 6

The ________ is (are)characterized by terrane accretion that has been active throughout most of Mesozoic and Cenozoic time.

Accepted Answer

The western margin of North America is characterized by terrane accretion that has been active throughout most of Mesozoic and Cenozoic time. This process involves the addition of various crustal blocks to the continent's edge, significantly influencing its geological structure.

Question 7

On an Andean-type convergent margin, the continental block contains the ________.

Accepted Answer

On an Andean-type convergent margin, the continental block contains volcanoes, a volcanic arc, and high temperature-moderate pressure metamorphics. Additionally, batholiths may also be present. The accretionary wedge and high pressure-low temperature metasediments and metavolcanics are typically found in the oceanic plate being subducted. Deep ocean trenches are also typically found on the oceanic plate being subducted, not on the continental block. Rotated block faults are not specifically associated with Andean-type convergent margins.

Question 8

Accretionary wedges are formed ________.

Accepted Answer

Accretionary wedges are formed at the edge of the overriding plate against a subduction zone where sediment is scraped off the downgoing plate and accumulates.

Question 9

The ________ have crust about twice as thick as normal.

Accepted Answer

The Himalayan Range and Tibetan Plateau in northern India and SW China have crust about twice as thick as normal due to the collision of the Indian and Eurasian tectonic plates. The other options do not have crust that is significantly thicker than normal.

Question 10

When a subduction zone is established under the edge of a continent, this gives rise to an accretionary wedge, a volcanic arc, and compressional features farther inland that is called ________ margin.

Accepted Answer

When a subduction zone is established under the edge of a continent, it can give rise to an accretionary wedge, a volcanic arc, and compressional features farther inland. This type of margin is known as an Andean margin, named after the Andes mountain range in South America where this type of margin is prominent. The other choices are not directly associated with subduction zones under continents - Aleutian is a subduction zone associated with an oceanic plate, Himalayan is a collisional margin, and passive margin lacks significant tectonic activity.

Question 11

Which of the following best describes the geology of the Pacific coastal regions of western North America including Alaska?

Accepted Answer

The Pacific coastal regions of western North America including Alaska are characterized by geologically different, microcontinent-sized fragments and terranes, formed elsewhere, that were tectonically accreted to North America. This process occurred during the Mesozoic era and is responsible for the diverse geology of the region, including the formation of the North American Cordillera mountain range.

Question 12

How were the Ural Mountains (Russia)formed?

Accepted Answer

The Ural Mountains were formed by the collision and subsequent joining of the former Asian and European plates to form the Eurasian plate. Kilometres of marine strata were squeezed, folded, and uplifted as a result.

Question 13

Prolonged convergence across Aleutian-type margins can result in the ________.

Accepted Answer

Prolonged convergence across Aleutian-type margins can result in the development of thick, continental-type crust. This occurs as sediment from the subducting plate is accreted onto the leading edge of the overriding plate, causing it to thicken and become more continental in nature.

Question 14

A(n)________ is a zone where two continents collide, often preserving slivers of oceanic lithosphere between the colliding plates.

Accepted Answer

A suture zone is a region where two tectonic plates have collided and joined together. This often results in the preservation of fragments of oceanic lithosphere between the two plates, which may be exposed as ophiolites. Terranes are also fragments of lithosphere that have been added to a continent, but they may originate from sources other than suture zones. Passive margins are areas where two tectonic plates are slowly pulling apart, and are not associated with collisions.

Question 15

Thick blocks of crust carried toward a subduction zone ________.

Accepted Answer

Thick blocks of crust carried toward a subduction zone can become accreted terranes by getting sliced off the descending slab and transferred to the accretionary continental margin. This process contributes to the growth of continents over time. The other options (B, C, and D) are not typical outcomes of the subduction of thick crustal blocks.

Question 16

An exotic terrane created by a seamount accreted to a continental margin would be composed ________.

Accepted Answer

Seamounts are volcanic features that are primarily composed of basaltic lava flows. When a seamount accretes to a continental margin, it may bring with it some coral reefs or shallow water limestone deposits, but the dominant rock type of the resulting terrane would still be basaltic lava flows. Therefore, choice D is the best option. Additionally, none of the other choices accurately describe the characteristics of a terrane that formed through the accretion of a seamount.

Question 17

One piece of evidence for the substantial compression accompanying terrane accretion in B.C.and the Yukon is ________.

Accepted Answer

The Jurassic and Cretaceous deformations in the fold and thrust belt of the Canadian Rockies in B.C. and Alberta provide evidence for the substantial compression accompanying terrane accretion in the region.

Question 18

Mountains without active volcanic arcs are associated with a(n)________ margin.

Accepted Answer

Mountains without active volcanic arcs are typically associated with continent-continent collision margins. This occurs when two continental plates collide, pushing the crust upward to form mountains without the volcanic activity that characterizes other types of plate boundaries.

Question 19

Folded, fine grained turbidites that occur high in the Himalayas were originally deposited as sediments ________.

Accepted Answer

Folded, fine-grained turbidites that occur high in the Himalayas were originally deposited as sediments in the accretionary wedge along the southern edge of Asia. This is because the Himalayas were formed by the collision of the Indian and Eurasian plates, which caused the sediments that were deposited in the accretionary wedge to be uplifted and folded.

Question 20

Most of mainland Alaska, over half of B.C.and the Yukon, and most of Washington, Oregon, Nevada and California are ________.

Accepted Answer

The description matches the process of accretion, where different types of rocks and sediments are transported and added to the edge of a continental plate as it moves towards another plate. This process is responsible for the formation of the geology found in the listed regions.

Quiz 13: Mountain Building and Continental Frameworks