Post-drift studies and Convectional Current Theory

Magma rises at mid-oceanic ridges to form new crust. In exams, remember ridges as constructive zones and trenches as destructive zones.

Holmes considered mantle convection currents driven by heat as the moving force. In exams, treat it as an improvement over Wegener's weak force explanation.

Hot magma rises near ridges, so heat flow is high there. In exams, connect heat flow with active spreading.

The direct answer is B: in deep ocean trenches at convergent boundaries. Sea-floor spreading occurs when new oceanic crust forms at mid-ocean ridges as magma rises and cools. Step by step: a ridge creates new crust; plates move away from the ridge; with time, older oceanic crust becomes cooler and denser; when it meets another plate at a convergent boundary, the dense oceanic plate can sink beneath the other plate; this process is subduction and commonly occurs at deep-sea trenches, where the old crust is recycled into the mantle. Option B is correct. Option A is wrong because mid-ocean ridges are mainly places where new crust forms, not where old crust is destroyed. Option C is not a general destruction zone. Option D is wrong because continental plateaus are not the usual sites of oceanic-plate subduction. Memory cue: ridges make, trenches take.

Basalt preserves the magnetic direction present when it cools. In exams, link normal and reversed polarity with magnetic stripes.

Wegener gave evidence but could not explain a satisfactory driving force. In exams, convection currents answer this gap.

Sea-floor spreading strengthened the idea of moving plates. In exams, treat it as a link from Wegener to plate tectonics.

A deep seismic belt indicates a descending plate. In exams, connect trenches, subduction, and deep earthquakes together.

Hot material becomes less dense and rises, while cooler material sinks. In exams, explain convection through heat and density differences.

Oceanic rocks were found to be generally young, proving continuous creation and destruction. In exams, use age data as evidence of a dynamic ocean floor.

The sea floor spreads in both directions from the ridge, producing symmetrical ages. In exams, note the relation between distance and age.

Basaltic magma cools at ridges to form new parts of the ocean floor. In exams, connect basalt with oceanic crust.

New crust forms at ridges and old crust is destroyed at trenches. In exams, understand the ocean floor as a balanced cycle.

Magnetic anomalies act like a time-ordered record of spreading. In exams, treat them as records of the ocean floor.

Sea-floor spreading is part of plate motion, which can also cause convergence and collision. In exams, link different boundaries within one plate system.

New crust has less time for sediment accumulation, so sediment is thin near ridges. In exams, relate sediment thickness to age.

Internal heat creates temperature differences in the mantle. In exams, understand heat as the starting point of convection currents.

Later theory showed that continents do not move alone but as parts of plates. In exams, see continents and ocean floors as plate components.

A rift valley shows crustal stretching and separation of plates. In exams, link rifts with divergent boundaries.

Old oceanic crust descends into trenches, so very ancient ocean floor is rare. In exams, explain young ocean floors through subduction.

A systematic increase in age shows the floor is not fixed but spreading. In exams, use it as direct evidence against a fixed ocean floor.

Mid-oceanic ridges are constructive plate boundaries. In exams, treat them not just as heights but as active geological zones.

Cool material becomes denser and sinks, completing the cell. In exams, link rising and sinking with density.

Transform faults show parallel or lateral motion of plates. In exams, link them with the third type of plate boundary.

Drilling samples confirmed that age increases with distance from ridges. In exams, treat drilling as direct sample-based evidence.