Post-drift studies and Convectional Current Theory

Palaeomagnetism studied old magnetic signals recorded in rocks. Exam tip: place it among evidences of plate movement.

Later studies carried Wegener's idea toward plate tectonics. Exam tip: remember continuity of ideas.

Heat from radioactive decay can create temperature differences in the mantle. Exam tip: connect heat source with convection.

After rising, material spreads and can cause divergence of plates. Exam tip: connect sideward flow with divergence.

Magma rises at mid-oceanic ridges to form new crust. Exam tip: treat ridges as crust creation zones.

New rocks form near the ridge and older rocks are found farther away. Exam tip: treat age pattern as spreading evidence.

Spreading creates new crust and subduction takes old crust back. Exam tip: remember crustal recycling.

Later evidences strengthened the idea of moving lithospheric plates. Exam tip: remember the sequence from drift to plate tectonics.

Plates are rigid parts of the lithosphere including crust and upper mantle. Exam tip: connect lithosphere with plates.

At an oceanic divergent boundary new crust and a ridge form. Exam tip: connect oceanic divergence with ridges.

When plates come together, one may go beneath the other. Exam tip: connect convergence with subduction.

Wegener lacked a clear sufficient driving force while Holmes proposed mantle currents. Exam tip: focus on the word mechanism.

Plate tectonics explained combined movement of continents and ocean floors. Exam tip: treat continental drift as an earlier base of plate tectonics.

New lava rocks solidify on both sides with matching magnetic records. Exam tip: treat symmetry as strong evidence of spreading.

This theory showed that ocean floor also forms and spreads. Exam tip: understand the ocean floor as active, not passive.

Collision or sliding of plates can release energy and cause earthquakes. Exam tip: connect earthquake distribution with plate boundaries.

Volcanoes occur at boundaries due to magma and plate processes. Exam tip: treat volcanic belts as evidence of plate tectonics.

A convection current is a continuous cycle caused by heating and cooling. Material heated at depth becomes less dense and rises. Near the upper part, it spreads sideways and gradually loses heat. After cooling, it becomes denser and sinks again. At depth it may be heated once more, allowing the cycle to continue. Option A presents this sequence in the correct order: heating and rising, spreading, cooling, and sinking.

The other choices omit an essential part or reverse the physical process. Cooling generally makes the material denser, so cooling and rising is not the normal sequence. Material does not simply remain fixed, and a convection cycle cannot consist only of upward movement because the return downward flow is necessary. In the mantle, these movements are very slow and occur over long geological periods, but the order remains the same. The supplied answer A is therefore consistent with the basic convectional-current theory.

Active geological structures were found on ocean floors. Exam tip: understand ocean basins as dynamic relief.

Wegener proposed continental drift and Holmes added convection currents as a mechanism. Exam tip: write scientist contributions separately.

Near ridges the floor is relatively new, so sediment is thinner. Exam tip: connect sediment thickness with age evidence.

Harry Hess popularized the concept of sea-floor spreading. Exam tip: connect Hess with sea-floor spreading.

Convection currents linked internal energy with plate movement. Exam tip: remember internal heat as a driving force.

These events may be linked with interaction of plates. Exam tip: connect multiple evidences with plate boundaries.

The direct answer is A: magma rises, cools and forms new oceanic crust. At a mid-ocean ridge, tectonic plates move apart. Hot material from below rises into the opening. When magma reaches or approaches the ocean floor, it loses heat, solidifies and becomes new basaltic oceanic crust. Continued upwelling and cooling push older crust away from the ridge, producing seafloor spreading. This process also helps explain symmetrical magnetic stripes on both sides of many ridges, because minerals record Earth’s magnetic direction as the lava cools. Option A is correct because it gives the essential sequence: rising magma, cooling, and crust formation. Option B is wrong because magma does not remove all seawater; water remains in the ocean, although volcanic activity can interact with it locally. Option C is wrong because clouds form mainly through atmospheric water vapour and weather processes, not because magma only makes clouds. Option D is wrong because magma movement and related mantle processes help plates move; they do not keep every plate fixed. Magma does not create all crust everywhere, but at divergent boundaries it is central to new oceanic crust formation. Memory cue: ridge opens, magma rises, lava cools, new crust spreads.