Post-drift studies and Convectional Current Theory

The direct answer is option A: post-drift studies advanced Wegener's idea through evidence about the ocean floor, mantle and plate movement. Wegener proposed continental drift, but later work supplied stronger evidence and a more complete mechanism. Ocean-floor mapping revealed ridges and trenches; seafloor-spreading evidence showed that new crust can form; mantle processes and plate tectonics explained how large plates move. Option A is correct because it includes the evidence, the internal mechanism and the later development of the theory. Option B is wrong because these studies are not a biography of Wegener. Option C is wrong because meteorology studies the atmosphere and weather, not this main subject. Option D is wrong because later evidence supported, rather than rejected, continental movement. A useful exam chain is: Wegener's idea, ocean-floor evidence, mantle and plate mechanism, modern plate tectonics.

Ridges are linked with creation and trenches with sinking, so the ocean floor is considered dynamic. Exam tip: write creation and recycling together.

Later studies focused on causes of movement and the role of ocean floors. Exam tip: separate Wegener's evidences from later mechanisms.

Convection currents are believed to occur in the mantle due to temperature differences. Exam tip: link mantle with the base of plate movement.

Arthur Holmes linked mantle convection currents with continental movement. Exam tip: remember the scientist with the idea.

Internal heat makes material rise when hot and sink when cool. Exam tip: treat temperature difference as the main cause.

Heating changes the density of mantle material. When mantle material becomes hot, it generally expands and becomes less dense than the cooler material around it. The less-dense material experiences an upward tendency, so it rises. This rising movement is one part of a convection cycle. Option B correctly describes rising because of decreased density. Option A describes the opposite stage: cooler material can become denser and sink, but it does not explain the rising hot material asked about here.

The sequence can be understood through the relation between temperature and density. Heating usually lowers density, and cooling usually raises density, although the mantle is solid and moves extremely slowly over long periods. Thus, the question is not about ocean water freezing or continents simply remaining fixed. It asks what the upward movement represents, and the correct process is buoyant rise caused by lower density. The supplied answer B is scientifically appropriate and the explanation correctly connects heat, density, and upward motion.

Cooler material becomes relatively heavier and sinks. Exam tip: connect sinking with higher density.

Mapping of the ocean floor revealed mid-ocean ridges and new crust. Exam tip: treat ocean floors as active zones.

The direct answer is A: sea-floor spreading. Mid-oceanic ridges are long underwater mountain chains found mainly where tectonic plates move apart. Hot material rises from the mantle, cools and becomes new oceanic crust. As more material rises, older crust is pushed away on both sides, so the ocean floor spreads outward. This is why ridges are called spreading centres. Option A is correct because it names the process that creates new crust at these ridges. Option B, desert expansion, concerns dry-land environments and does not produce underwater mountain chains. Option C, glacial erosion, can carve valleys and alter land or sea-floor surfaces locally, but it is not the main process forming mid-oceanic ridges. Option D, atmospheric circulation, describes movement of air and affects weather and climate, not plate separation and crust formation. The reasoning chain is: plates diverge → mantle material rises → magma cools → new oceanic crust forms → ridge develops. Memory cue: at a mid-ocean ridge, plates move apart and the ocean floor is born.

Sea-floor spreading creates new crust and moves plates apart. Exam tip: connect spreading with new crust.

Magnetic stripes were recorded symmetrically on both sides of new crust. Exam tip: link magnetic evidence with spreading.

Cooling lava rocks can record the magnetic direction of that time. Exam tip: treat palaeomagnetism as an ancient magnetic record.

Earthquakes and volcanoes are often concentrated along plate boundaries. Exam tip: treat these belts as indicators of plate boundaries.

Wegener gave evidences but could not clearly explain a strong mechanism of movement. Exam tip: connect the weakness with driving force.

According to Holmes cyclic mantle currents can move the overlying crust. Exam tip: understand mantle circulation as a basis for plate motion.

Rising material can pull crust apart and create new crust. Exam tip: connect rising current with spreading.

Sinking currents may bring plates together and relate to subduction. Exam tip: connect sinking current with convergence.

Dense oceanic crust may sink at a convergent boundary. Exam tip: link subduction with destruction of old crust.

New oceanic crust keeps forming and old crust can return to the mantle. Exam tip: remember recycling of oceanic crust.

The direct answer is A: sonar surveys were useful for learning about the depth and structure of the ocean floor. Sonar means sending sound waves through water and measuring the returning echo. If the echo returns quickly, the surface is relatively near; if it takes longer, the surface is deeper. Repeated measurements from ships helped scientists prepare maps showing underwater ridges, trenches, plains and other features. These maps became important evidence in post-drift studies because they revealed the shape of mid-ocean ridges and helped support ideas about seafloor spreading. Option A is correct because sonar is an actual marine surveying technique. Option B is wrong because a plough is an agricultural tool used on land. Option C is wrong because a loom is used to make textiles. Option D is wrong because a sundial measures time from the position of the Sun and cannot map deep ocean surfaces. Sonar does not directly show every internal rock detail, but it is highly useful for measuring depth and mapping seafloor relief. Memory cue: sound echo in water = sonar; sonar helps map the ocean floor.

Mapping showed the ocean floor as a varied and active geomorphic area. Exam tip: remember both ridge and trench.

Trenches form where one plate may go beneath another. Exam tip: connect trenches with convergent margins.

Similar fossils on separated continents indicate past connection. Exam tip: connect fossils with fit and past connection.

Glacial marks suggest continents may have been at different latitudes earlier. Exam tip: connect climate evidence with latitudinal position.