题目 1 · Stimulus Analysis & Resource Interpretation (Part a)
8 分OPTION A: PLATE TECTONICS
With reference to named places for illustration, describe and explain the formation of (i) a rift valley at a constructive margin within continental crust, and (ii) a deep sea trench and island arc at a destructive oceanic-oceanic plate margin. [8]
With reference to named places for illustration, describe and explain the formation of (i) a rift valley at a constructive margin within continental crust, and (ii) a deep sea trench and island arc at a destructive oceanic-oceanic plate margin. [8]
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解题
(i) Rift valley: at most constructive margins, new oceanic crust forms as plates move apart beneath the ocean. However, where a constructive margin develops within continental crust, the same divergent, tensional forces instead stretch and thin the thicker continental crust. As tension builds, the crust fractures along a series of roughly parallel normal faults, and the central block of crust between these faults subsides relative to the blocks either side, producing a long, steep-sided valley — a rift valley. The East African Rift Valley is a well-known example, formed as the African Plate is slowly splitting into the larger Nubian and smaller Somali sub-plates; if this rifting continues over geological time, the valley could eventually widen enough to flood and form a new ocean basin, in the same way the Red Sea (further north along the same rift system) has already begun to do.
(ii) Deep sea trench and island arc: where two oceanic plates meet at a destructive margin, the denser (usually older, cooler) of the two plates is forced to bend downward and subduct beneath the other. This bending of the subducting plate creates a long, narrow, very deep depression on the ocean floor at the point of subduction — a deep sea trench, such as the Mariana Trench, formed where the Pacific Plate subducts beneath the Philippine Sea Plate, and which contains the deepest known point in the Earth's oceans. As the subducting plate descends, it heats up and partially melts; the resulting magma, being less dense than the surrounding mantle and crust, rises up through the overriding oceanic plate. Where this magma reaches the seafloor and builds up over time, it forms a curved chain of volcanic islands — an island arc — running parallel to, and on the landward side of, the trench, as seen in the Mariana Islands.
Final answer: rift valley (e.g. the East African Rift Valley): where a constructive margin develops within continental crust rather than beneath an ocean, tensional forces as plates diverge stretch and thin the crust, causing it to fracture along parallel fault lines and the central block to subside relative to the sides, forming a steep-sided rift valley (which may eventually widen into a new ocean basin if rifting continues); deep sea trench and island arc (e.g. the Mariana Trench and Mariana Islands, where the Pacific Plate subducts beneath the Philippine Sea Plate): the denser oceanic plate bends and subducts beneath the other oceanic plate, creating a deep depression (trench) at the point of subduction, while partial melting of the subducted plate at depth generates magma that rises through the overriding oceanic plate to build a curved chain of volcanic islands (an island arc) parallel to the trench
(ii) Deep sea trench and island arc: where two oceanic plates meet at a destructive margin, the denser (usually older, cooler) of the two plates is forced to bend downward and subduct beneath the other. This bending of the subducting plate creates a long, narrow, very deep depression on the ocean floor at the point of subduction — a deep sea trench, such as the Mariana Trench, formed where the Pacific Plate subducts beneath the Philippine Sea Plate, and which contains the deepest known point in the Earth's oceans. As the subducting plate descends, it heats up and partially melts; the resulting magma, being less dense than the surrounding mantle and crust, rises up through the overriding oceanic plate. Where this magma reaches the seafloor and builds up over time, it forms a curved chain of volcanic islands — an island arc — running parallel to, and on the landward side of, the trench, as seen in the Mariana Islands.
Final answer: rift valley (e.g. the East African Rift Valley): where a constructive margin develops within continental crust rather than beneath an ocean, tensional forces as plates diverge stretch and thin the crust, causing it to fracture along parallel fault lines and the central block to subside relative to the sides, forming a steep-sided rift valley (which may eventually widen into a new ocean basin if rifting continues); deep sea trench and island arc (e.g. the Mariana Trench and Mariana Islands, where the Pacific Plate subducts beneath the Philippine Sea Plate): the denser oceanic plate bends and subducts beneath the other oceanic plate, creating a deep depression (trench) at the point of subduction, while partial melting of the subducted plate at depth generates magma that rises through the overriding oceanic plate to build a curved chain of volcanic islands (an island arc) parallel to the trench
评分标准
Level 1 (1-2 marks): basic statement that rift valleys/trenches form from plate movement, with little accurate process detail and/or no named example.
Level 2 (3-5 marks): sound description of ONE landform's formation with reasonable accuracy, or both covered only briefly; named example(s) present but underdeveloped.
Level 3 (6-8 marks): detailed, accurate explanation of BOTH landforms' formation (continental rifting/faulting/subsidence for the rift valley; oceanic-oceanic subduction/trench/magma generation/island arc for the second), integrating named examples (e.g. East African Rift Valley; Mariana Trench/Islands) to illustrate the processes described.
Level 2 (3-5 marks): sound description of ONE landform's formation with reasonable accuracy, or both covered only briefly; named example(s) present but underdeveloped.
Level 3 (6-8 marks): detailed, accurate explanation of BOTH landforms' formation (continental rifting/faulting/subsidence for the rift valley; oceanic-oceanic subduction/trench/magma generation/island arc for the second), integrating named examples (e.g. East African Rift Valley; Mariana Trench/Islands) to illustrate the processes described.