Post-drift studies and Convectional Current Theory

Similarity of continental margins points to past connection. Exam tip: write fit with fossil and rock evidence.

Later studies gave evidences and convection explained a possible energy mechanism. Exam tip: write both evidence and mechanism.

The direct answer is B: rising magma at a mid-oceanic ridge cools and forms new basaltic oceanic crust. A mid-oceanic ridge is an underwater mountain chain located where oceanic plates move apart. Step by step: plates diverge; hot mantle material rises into the opening; the magma reaches or approaches the seabed; it cools and becomes solid basalt; repeated cooling adds fresh oceanic crust on both sides of the ridge. B is correct because basalt is the common volcanic rock of newly formed ocean floor. A is wrong because old crust is not immediately destroyed at a ridge; destruction commonly occurs at some subduction zones. C is wrong because ridge spreading does not mean that continents simply shrink. D is wrong because cyclones are atmospheric weather systems, not the direct result of magma forming seafloor. Memory cue: divergent ridge means rising magma and new seafloor.

Rocks away from ridges are older, so more sediment can accumulate on them. In exams, link sediment thickness with age.

Mantle convection is considered an important driving process for plate movement. In exams, connect it with the cause of plate motion.

Sonar helped measure sea depth and map ocean-floor features better. Treat it as a technology of ocean-floor study in exams.

A mid-oceanic ridge is an example of a divergent boundary where plates move apart. In exams, link ridges with divergent motion.

At oceanic trenches, old and dense crust may sink downward. In exams, connect trenches with subduction.

Post-drift studies explained the process of movement better. Remember it as an answer to Wegener's weakness in exams.

The age and structure of oceanic crust indicate plate movement. In exams, consider the ocean floor dynamic.

In convectional current theory, a temperature difference produces differences in density within a material. Warmer material generally expands, becomes less dense, and tends to rise. Cooler material becomes relatively denser and tends to sink. The repeated upward and downward movement forms a convection current. In discussions of the mantle, this idea is used to explain how internal heat may drive slow movements of mantle material and contribute to large-scale tectonic processes.

Option C is correct because it directly states the rising and sinking movement produced by the temperature difference. Tides are mainly related to the gravitational effects of the Moon and Sun, while sound waves and rain clouds are not the specific result asked about here. The important chain is temperature difference, density difference, and then upward and downward movement. That chain identifies convection clearly.

The direct answer is Option D. A dynamic ocean floor is one that is continuously created, moved and destroyed. At mid-ocean ridges, magma rises and cools to form new oceanic crust; this is called sea-floor spreading. In other places, old, dense oceanic crust sinks beneath another plate at a subduction zone and is recycled into the mantle. This complete process proves that the ocean floor is not fixed. Option A, sand on beaches, is only a coastal material and does not show global crustal movement. Option B, river erosion, changes land near rivers but does not explain ocean-floor renewal. Option C, forest distribution, concerns vegetation and climate, not plate motion. Option D gives both creation and destruction, so it is the strongest evidence. Remember: ridges make; trenches consume.

Heat flow is high near ridges because magma rises there. In exams, connect heat flow with new crust formation.

Oceanic crust forms anew and old crust may be destroyed by subduction. So its average age is lower.

Subduction can cause melting and magma formation, leading to volcanism. In exams, link volcanic arcs with subduction.

New crust forms near the ridge, and crust becomes older away from it. In exams, relate distance from ridge with age.

Mid-oceanic ridges are long mountain chains under the sea. Count them among the most important oceanic landforms in exams.

Convection currents can provide direction and force to plate movement. In exams, write them as a driving process of motion.

Parallel magnetic stripes on the ocean floor are found symmetrically on both sides of mid-oceanic ridges. New oceanic crust forms at these underwater ridges when magma rises, cools, and hardens. Iron-bearing minerals in the cooling rock record the direction of Earth’s magnetic field at that time. Because Earth’s magnetic field has reversed repeatedly, alternating magnetic bands are preserved in the crust as it moves away from the ridge.

Thus, option C is correct. The matching pattern on both sides is important evidence for sea-floor spreading and supports the theory of plate tectonics. Continental deserts, river valleys, and glacial lakes do not continuously create this symmetrical ocean-floor record. The ridge is the place where new crust is produced and then carried outward.

New findings showed that the ocean floor is created and destroyed. Thus, the idea of a fixed ocean floor weakened.

The direct answer is A: at a convergent boundary, a dense oceanic plate usually sinks beneath another plate. Convergence means that plates move toward one another. Oceanic lithosphere is commonly denser than continental lithosphere because it is made largely of dense basaltic rocks. Step by step: two plates approach; the denser oceanic plate cannot remain above the less-dense plate easily; it bends downward into the mantle; this process is called subduction. Subduction can produce trenches, earthquakes, volcanoes and magma. A is correct because it describes the usual movement. B is wrong because a dense plate does not always float above another plate. C is wrong because plates move within Earth’s lithosphere, not into the atmosphere. D is wrong because convergence causes change rather than complete fixed stability. ‘Usually’ matters: the exact result depends on the plates involved. Memory cue: convergence plus dense oceanic plate means subduction.

Volcanism is often related to plate boundaries and rising magma. Remember both ridges and subduction in exams.

Both ideas relate to Earth's internal energy and plate motion. In exams, connect them with the concept of a dynamic Earth.

Deep trenches and seismicity can indicate a subduction zone. In exams, connect these signs with plate convergence.

The distribution of earthquakes helps identify active plate boundaries. In exams, link seismic belts with plate boundaries.