Post-drift studies and Convectional Current Theory

Holmes linked mantle convection produced by radioactive heat with continental movement. For exams, associate Holmes with the convection current theory.

Oceanic trenches usually form where one plate subducts beneath another. For exams, link trenches with convergence and subduction.

Oceanic rocks become older as distance from ridges increases. This is an important evidence of seafloor spreading.

Parallel magnetic stripes on the ocean floor show magnetic reversals and spreading. In exams, connect this with palaeomagnetism.

Holmes considered heat from radioactive materials as a source of convection. Remember the heat source as the correct reason in exams.

Bathymetry is the study of ocean-floor depth and form. For exams, connect it with ocean-floor mapping.

New crust forms at mid-oceanic ridges, so rocks there are the youngest. Remember the relation between distance and age in exams.

Wegener's theory weakly explained the force moving continents. Later studies helped reduce this weakness.

Direct answer: Option D is correct. Convection is movement caused by differences in temperature and density. When material is heated, its particles generally spread farther apart, so its density decreases. Less-dense hot material rises through denser surrounding material. Higher up, it loses heat, becomes denser and sinks. This upward and downward movement forms a convection current. Option A is wrong because heating normally decreases density rather than increasing it. Option B is wrong because cooling makes material heavier or denser, causing it to sink, not rise. Option C is wrong because seawater is not the basic reason hot material rises in the theory; the essential cause is the density difference produced by heating and cooling. Option D correctly states the direct cause: heating reduces density. In Earth science, convection in the mantle is a model used to explain movement related to plate tectonics, though the material is solid rock that can flow very slowly. Memory cue: hot and light rises; cool and dense sinks.

Abyssal plains are flat plains in deep oceanic areas. In exams, treat them as major ocean-floor features.

The continental slope is the steep part from shelf toward deep ocean. Remember its position between shelf and deep floor in exams.

Palaeomagnetism studies old magnetic records preserved in rocks. It is useful as evidence for plate movement.

At mid-oceanic ridges, plates diverge and shallow earthquakes are common. In exams, link divergent boundaries with shallow earthquakes.

In subduction zones, earthquakes may range from shallow to deep. For exams, link trenches with seismic belts.

Old oceanic crust may sink into the mantle at trenches. Remember the balance of creation and destruction in exams.

Island arcs often form by volcanic activity related to oceanic plate subduction. Remember them with trenches in exams.

A seamount is a volcanic mountain under the sea. Include it among ocean-floor landforms in exams.

A guyot is a submerged mountain with a relatively flat top. In exams, distinguish it from a seamount.

New information from ocean-floor mapping strengthened plate tectonics. Treat mapping as modern evidence in exams.

Symmetrical magnetic stripes prove spreading on both sides of the ridge. In exams, connect symmetry with seafloor spreading.

Cooler and denser material sinks downward, completing the convection cycle. Remember the relation between density and motion in exams.

Harry Hess prominently proposed the idea of seafloor spreading. For exams, link Hess with mid-oceanic ridges.

Mantle convection gave a more scientific basis for continental movement. In exams, link it with the solution to Wegener's key weakness.

Rising material can separate crust and form new crust at mid-oceanic ridges. In exams, link upwelling with divergent boundaries.

Post-drift studies linked ocean-floor evidence with mantle processes and suggested possible causes of movement. In exams, separate evidence from driving force.