(1) Draw a cross section
of a subduction zone and label: the bulge, the trench, the accretionary prism,
the arc.
Where would the back-arc basin form
(if one was to form).
(2) Why are some collisional
margins characterized by extension in the upper plate and
others by evidence for shortening
in the upper plate? A sketch would help here.
(3) Why does oceanic crust subduct rather than continental crust?
(4) What is the Wadatti-Benioff zone?
(5) What is the accretionary wedge and how does it form? how is material added to and removed from the wedge?
(6) How and why are volcanoes at convergent margins different from divergent and intraplate volcanoes?
(7) How are granites likely to be formed in subduction zones?
(8) What are pyroclastic eruptions? In your answer, make sure you define: ignimbrite, welded tuff, and caldera.
(9) What is a nappe. Show how one might develop in passive margin sediments as India collided with Asia?
(10) Western North America
seems to be a collage of small crustal blocks or terranes.
How were these added (or accreted)
to the margin of North America.
(11) Describe the orogenic cycle.
1) Lesser Antilles A) ocean to ocean B) ocean to continent C) continent to continent
2) Himalaya Mountains A) ocean to ocean B) ocean to continent C) continent to continent 3) Andes Mountains A) ocean to ocean B) ocean to continent C) continent to continent
4) Mediterranean Sea A) ocean to ocean B) ocean to continent C) continent to continent
5) Convergence without subduction. A) ocean to ocean B) ocean to continent C) continent to continent
6) Where island arcs form. A) ocean to ocean B) ocean to continent C) continent to continent
7) Subduction is largely due to the force of A) gravity. B) slab suction. C) hotspots. D) ridge push. E) back-arc spreading.
8) Deep-sea trenches A) have nothing to do with plate tectonics. B) are narrow troughs in the ocean floors. C) are major mountain ranges with rugged topography on the ocean floors. D) are submerged interstates from the lost continent of Atlantis. E) are never found near volcanic island arcs.
9) Features associated with subduction include A) deep earthquake zones. B) deep, elongate trenches. C) curving chains of volcanoes. D) strong earthquakes. E) all of the above
10) East of the Andes, 6 km of sediment infill has accumulated in a(an) A) back-arc basin. B) fore-arc basin. C) eugeosyncline. D) foreland basin. E) trench.
11) Shallow subduction angles are associated with A) extension. B) deep earthquakes. C) narrow fore-arcs. D) extended back-arcs. E) decreased magmatism.
12) Subduction occurs along A) mid-ocean ridges. B) transform faults. C) hot-spots. D) volcanic arcs. E) continental rifts.
13) Silicic volcanic rocks are most common at A) mantle hot spots. B) island arcs. C) continental magmatic arcs. D) divergent margins. E) shallow subduction zones.
14) The most important factor in producing magma at convergent boundaries is A) geothermal gradient. B) the asthenosphere. C) frictional heat. D) low-melting sediments. E) water.
15) Evidence for the production of granite through crustal melting is A) xenoliths. B) migmatites. C) lack of gabbroic differentiate. D) occurrence at continental arcs. E) all of the above
16) Pyroclastic eruptions are most common at A) divergent margins. B) mantle hot spots. C) shield volcanoes. D) volcanic arcs. E) national parks.
17) Accumulation of sediment in an accretionary wedge is unique to A) rift grabens. B) continental shelves. C) back-arc basins. D) deep sea floor. E) subduction zones.
18) Land-derived clastic rock in an accretionary wedge are mostly A) orthoquartzite. B) greywackes. C) pelagic muds. D) siltstone. E) organic ooze.
19) Shearing and mixing of accretionary prism sediments at subduction zones produces A) m accent(e)lange. B) fault gouge. C) sedimentary breccia. D) greywacke. E) partial melting.
20) Continental crust age averages __________ my, while ocean crust averages __________ my. A) 200; 100 B) 100; 200 C) 4,000; 200 D) 100; 50 E) 2,000; 100
21) The Himalayas have no modern magmatic arc because A) orogenesis is over. B) the crust is too thick. C) heat energy is being used for uplift. D) there's no subduction. E) the mantle is cool here.
22) Stress from the Indian-Asian collision is carried to the interior of China along A) a subduction zone. B) strike-slip faults. C) a magmatic arc. D) the Silk Road. E) all of the above 23) Most sedimentary debris stripped from the Himalayas has been deposited A) in the Indian subcontinent. B) in the Indus River Valley. C) in the Tibetan Plateau. D) on Indian beaches. E) in submarine fans.
24) Among the highest grade of metamorphic rock formed during orogenesis is A) granulite. B) mélange. C) granite. D) limestone. E) nappe rock.
25) Granite plutons in orogens contain more __________ and __________ than magmatic arc granites. A) silica; magnesium B) xenoliths; dikes C) silica; alumina D) silica; olivine E) all of the above
26) Orogeny is another term for A) earthquake. B) continental drift. C) liquefaction. D) mountain building. E) erosion.
27) In ancient mountain belts, outcrops of __________ indicate the suture zones of two plates. A) migmatite B) granite C) marine sediments D) greywacke E) ophiolite
28) Some fragments of continental crust are as much as __________ of the age of Earth. A) 87% B) 53% C) 99% D) 27% E) 33%
29) A craton's history is frequently obscured by A) complex metamorphism. B) deep erosion. C) multiple intrusions. D) sedimentary cover. E) all of the above
30) Scientists once proposed to drill to the Moho. Which of the following would be the best place to drill? A) in the middle of a craton B) in the middle of the Appalachian mountains C) over a negative gravity anomaly D) in oceanic crust near the edge of a major continent E) in oceanic crust near a diverging plate boundary
31) Cratons are generally older than __________ million years. A) 5,000 B) 500 C) 100 D) 1,000 E) 50
32) Thin-skinned tectonics is characterized by A) folded foreland rocks. B) thrusted sedimentary cover. C) undeformed basement. D) a d accent(e)collement. E) all of the above
33) The accretion of exotic terranes to an existing plate or to each other is called A) microplate assembly. B) exotic orogens. C) patchwork accretion. D) collage tectonics. E) arc collapse.
34) At convergent margins, the subducting plate is always oceanic.
35) Older oceanic lithosphere forms shallower slab dips. 36) Magmatic arcs include volcanic and plutonic activity.
37) Both fore-arc regions and back-arc basins are sites of sediment accumulation.
38) Convergent margins may be zones of either compression or extension.
39) Slab suction generates compression in a convergent zone.
40) Seismic hypocenters are located on the overriding plate.
41) Deeper earthquakes in subduction zones are caused by tension in the descending slab.
42) Greater-than-average P-wave velocities characterize subducting slabs.
43) Trenches are areas of high heat flow and low Bouguer gravity anomalies.
44) A steep slab dip will result in a wide fore-arc region.
45) A magmatic arc may have both island and continental components.
46) The Atlantic Ocean is almost encircled by volcanic arcs.
47) Magmatic arc magma is generated when a subducting slab melts.
48) Mantle-derived magma rises more easily at a divergent boundary than at a convergent one.
49) Most granites have been produced by fractional crystallization of mantle-derived magma.
50) Rapid subduction rates produce deeper trenches than slower rates.
51) Slow subduction rates tend to produce smaller accretionary wedges.
52) New lithosphere is constantly being created by volcanic activity at plate boundaries, but old lithosphere is never destroyed or recycled.
53) High temperatures in subduction zones lead to the wholesale production of migmatites.
54) Continental crust is old because it is buoyant.
55) The Himalayas are being uplifted at the rate of about 1 mm annually.
56) Orogenesis is the process of ore deposit formation at convergent boundaries.
57) The Himalayan uplift exemplifies Airy isostasy.
58) Erosion of the land by streams and glaciers may increase the rate of uplift.
59) The Appalachians are lower than the Himalayas because their mode of formation was quite different.
60) Granite plutons are only formed in magmatic arcs.
61) Oceanic plate interiors are young compared to continental plate interiors.
62) The composition of the continental crust varies both horizontally and vertically.
63) Cratons may be both above and below sea level.
64) The exposed portion of a shield is called the craton.
65) The continents are mostly made up of rocks less than 500 million years old.
66) Continental masses grow laterally through accretion of arcs and terranes.
67) The Appalachian Mountain range is currently being built up by tectonic activity.
68) A given continent may have more than one craton.
69) The southern European Alps show evidence of greater former height than the modern Himalayas.
70) Both the eastern and western margins of North America have been shaped by collage tectonics.
71) Some geologists think that subducting slabs come to rest on the outer core.
72) Oceanic lithosphere can be subducted,
but island arcs generally accrete.