Question:
Passage:
The theory of plate tectonics describes the motions of the lithosphere, the comparatively rigid outer layer of the Earth that includes all the crust and part of the underlying mantle. The lithosphere is divided into a few dozen plates of various sizes and shapes; in general the plates are in motion with respect to one another. A mid-ocean ridge is a boundary between plates where new lithospheric material is injected from below. As the plates diverge from a mid-ocean ridge they slide on a more yielding layer at the base of the lithosphere.
Since the size of the Earth is essentially constant, new lithosphere can be created at the mid-ocean ridges only if an equal amount of lithospheric material is consumed elsewhere. The site of this destruction is another kind of plate boundary: a subduction zone. There one plate dives under the edge of another and is reincorporated into the mantle. Both kinds of plate boundary are associated with fault systems, earthquakes and volcanism, but the kinds of geologic activity observed at the two boundaries are quite different.
The idea of sea-floor spreading actually preceded the theory of plate tectonics. In its original version, in the early 1960s, it described the creation and destruction of the ocean floor, but it did not specify rigid lithospheric plates. The hypothesis was substantiated soon afterward by the discovery that periodic reversals of the Earth's magnetic field are recorded in the oceanic crust. As magma rises under the mid-ocean ridge, ferromagnetic minerals in the magma become magnetized in the direction of the geomagnetic field. When the magma cooks and solidifies, the direction and the polarity of the field are preserved in the magnetized volcanic rock. Reversals of the field give rise to a series of magnetic stripes running parallel to the axis of the rift. The oceanic crust thus serves as a magnetic tape recording of the history of the geomagnetic field that can be dated independently. The width of the stripes indicates the rate of the sea-floor spreading.
Question:
What is the core theme of the passage?
Passage:
The theory of plate tectonics describes the motions of the lithosphere, the comparatively rigid outer layer of the Earth that includes all the crust and part of the underlying mantle. The lithosphere is divided into a few dozen plates of various sizes and shapes; in general the plates are in motion with respect to one another. A mid-ocean ridge is a boundary between plates where new lithospheric material is injected from below. As the plates diverge from a mid-ocean ridge they slide on a more yielding layer at the base of the lithosphere.
Since the size of the Earth is essentially constant, new lithosphere can be created at the mid-ocean ridges only if an equal amount of lithospheric material is consumed elsewhere. The site of this destruction is another kind of plate boundary: a subduction zone. There one plate dives under the edge of another and is reincorporated into the mantle. Both kinds of plate boundary are associated with fault systems, earthquakes and volcanism, but the kinds of geologic activity observed at the two boundaries are quite different.
The idea of sea-floor spreading actually preceded the theory of plate tectonics. In its original version, in the early 1960s, it described the creation and destruction of the ocean floor, but it did not specify rigid lithospheric plates. The hypothesis was substantiated soon afterward by the discovery that periodic reversals of the Earth's magnetic field are recorded in the oceanic crust. As magma rises under the mid-ocean ridge, ferromagnetic minerals in the magma become magnetized in the direction of the geomagnetic field. When the magma cooks and solidifies, the direction and the polarity of the field are preserved in the magnetized volcanic rock. Reversals of the field give rise to a series of magnetic stripes running parallel to the axis of the rift. The oceanic crust thus serves as a magnetic tape recording of the history of the geomagnetic field that can be dated independently. The width of the stripes indicates the rate of the sea-floor spreading.
Question:
What is the core theme of the passage?
The theory of plate tectonics describes the motions of the lithosphere, the comparatively rigid outer layer of the Earth that includes all the crust and part of the underlying mantle. The lithosphere is divided into a few dozen plates of various sizes and shapes; in general the plates are in motion with respect to one another. A mid-ocean ridge is a boundary between plates where new lithospheric material is injected from below. As the plates diverge from a mid-ocean ridge they slide on a more yielding layer at the base of the lithosphere.
Since the size of the Earth is essentially constant, new lithosphere can be created at the mid-ocean ridges only if an equal amount of lithospheric material is consumed elsewhere. The site of this destruction is another kind of plate boundary: a subduction zone. There one plate dives under the edge of another and is reincorporated into the mantle. Both kinds of plate boundary are associated with fault systems, earthquakes and volcanism, but the kinds of geologic activity observed at the two boundaries are quite different.
The idea of sea-floor spreading actually preceded the theory of plate tectonics. In its original version, in the early 1960s, it described the creation and destruction of the ocean floor, but it did not specify rigid lithospheric plates. The hypothesis was substantiated soon afterward by the discovery that periodic reversals of the Earth's magnetic field are recorded in the oceanic crust. As magma rises under the mid-ocean ridge, ferromagnetic minerals in the magma become magnetized in the direction of the geomagnetic field. When the magma cooks and solidifies, the direction and the polarity of the field are preserved in the magnetized volcanic rock. Reversals of the field give rise to a series of magnetic stripes running parallel to the axis of the rift. The oceanic crust thus serves as a magnetic tape recording of the history of the geomagnetic field that can be dated independently. The width of the stripes indicates the rate of the sea-floor spreading.
Question:
What is the core theme of the passage?
Show Hint
Notice that the passage starts by naming a theory and that every later paragraph, including the magnetic-stripe evidence, is used to explain or support that same theory.
Updated On: Jul 20, 2026
- Magnetic field reversal
- The formation of magma
- The location of mid-ocean ridges
- Plate tectonic theory
- Oceanic crust sedimentation
Show Solution
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The Correct Option is D
Solution and Explanation
The passage opens by naming its subject directly: "The theory of plate tectonics describes the motions of the lithosphere..." and every paragraph that follows builds on this one idea. The first paragraph explains what the lithosphere and its plates are and how mid-ocean ridges create new material. The second paragraph explains the other half of the mechanism, subduction zones, where old material is destroyed to balance out what is created. The third paragraph brings in the sea-floor spreading hypothesis and the magnetic-stripe evidence, but it introduces this evidence specifically as support that helped substantiate the theory of plate tectonics, not as a topic in its own right. So the passage moves from defining the theory, to explaining its mechanics, to presenting the evidence behind it - all of it organized around plate tectonics as the central subject. That makes option (d), "Plate tectonic theory," the core theme.
The other options each pick out a supporting detail rather than the overall theme. (a) Magnetic field reversal is only discussed in the last paragraph as evidence for the theory, not as the passage's main focus. (b) The formation of magma is mentioned only briefly, in explaining how magnetic stripes form, and is never developed as a topic on its own. (c) The location of mid-ocean ridges covers just one of the two plate-boundary types described (ridges and subduction zones both matter), so it is too narrow to be the theme. (e) Oceanic crust sedimentation is not discussed anywhere in the passage at all; the text talks about magnetic stripes in the crust, not sediment layers. Since the whole passage is structured around explaining and supporting one theory, (d) is the answer.
The other options each pick out a supporting detail rather than the overall theme. (a) Magnetic field reversal is only discussed in the last paragraph as evidence for the theory, not as the passage's main focus. (b) The formation of magma is mentioned only briefly, in explaining how magnetic stripes form, and is never developed as a topic on its own. (c) The location of mid-ocean ridges covers just one of the two plate-boundary types described (ridges and subduction zones both matter), so it is too narrow to be the theme. (e) Oceanic crust sedimentation is not discussed anywhere in the passage at all; the text talks about magnetic stripes in the crust, not sediment layers. Since the whole passage is structured around explaining and supporting one theory, (d) is the answer.
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