Post-drift studies and Convectional Current Theory

Later studies connected ocean-floor features and convection currents with movement. Exam tip: separate mechanism from evidence.

Radioactive decay produces internal heat and can drive mantle flow. Exam tip: do not confuse this with solar energy.

In sea-floor spreading, new crust forms near ridges and older crust lies farther away. Age pattern is a key evidence.

Rising convection currents beneath ridges are linked with spreading and new crust formation. Treat ridges as upwelling zones.

Deep trenches are generally related to convergence and subduction zones. Exam tip: do not reverse the roles of trenches and ridges.

Matching magnetic stripes on both sides of ridges support sequential formation of oceanic crust. It is strong evidence for spreading.

A symmetrical age pattern indicates bilateral spreading. In hard questions, read distance and age together.

Wegener gave evidence but the driving force was weak; later ideas explained a possible driving force. Exam tip: remember this key criticism.

Mantle flow can provide energy and direction for plate motion. Exam tip: treat it as a major mechanism, not a simple isolated cause.

Arthur Holmes advanced the continental-drift discussion by suggesting a possible internal force that could move continents. Wegener had correctly collected evidence such as matching coastlines and fossils, but he could not convincingly explain what power moved huge landmasses. Holmes connected movement with convection currents in the mantle, making the explanation more physically meaningful.

In a convection current, heated material becomes less dense and rises, while cooler material becomes denser and sinks. This continuous circulation could exert movement-producing forces beneath the crust. Therefore, option A is correct. Options B, C, and D contradict the scientific ideas associated with Holmes and do not explain his contribution.

New crust forms at ridges and old crust may sink at trenches. Creation and destruction operate together.

Internal heat drives mantle flow and new crust may form at ridges. In sequence questions, track the direction of process.

Oceanic crust is generally younger than continental crust. This supports formation and recycling of crust.

A descending limb is linked with cooler, denser material moving downward. It may be related to trenches and subduction.

Ridges may create new crust and trenches may destroy old crust. Together they explain a dynamic ocean floor.

Paleomagnetic stripes record past magnetic reversals on both sides of ridges. This strengthened sea-floor spreading.

At ridges, mantle material can rise, cool, and form new crust. It is the central zone of spreading.

Wegener mentioned tidal and polar forces, but they were considered inadequate. Later explanations emphasized internal forces.

Ocean-floor mapping revealed hidden relief features. These strengthened explanations of plate movement and spreading.

Cooling changes the density of a material. When a substance cools, its particles generally move less vigorously and become more closely packed, so the same amount of material occupies less space. Its density therefore increases. Under gravity, the denser material tends to move downward while warmer, less-dense material rises.

This difference in density is the basis of a convection cycle: hot material rises, loses heat, becomes cooler and denser, and then sinks. The sinking is not caused by colour, zero salinity, or the ending of magnetism. Thus option A gives the physical reason asked in the question. In a real Earth system, details can be complex, but the density explanation is correct for this school-level concept.

Sea-floor spreading showed that oceanic crust is dynamic. This made the explanation of continental drift more scientific.

Youngest rocks near ridges directly indicate spreading. Older rocks generally occur farther from ridges.

Plates are rigid lithospheric units, and mantle flow can interact with them. This is a basis of the modern explanation.

The direct answer is A: older ocean floor had more time for deposition. A mid-ocean ridge is a place where new oceanic crust forms. Fresh basalt at the ridge is very young, so only a thin layer of marine sediment has had time to collect on it. As the crust moves away from the ridge by sea-floor spreading, it becomes older. During this longer period, dust, shells, clay and other particles settle from seawater and build a thicker layer. Therefore sediment thickness generally increases with distance from the ridge. Option A is correct because it gives the age-and-time explanation. Option B is wrong: distant crust is not necessarily always free from volcanic activity, and volcanism is not the main reason for sediment thickness. Option C is wrong because water is present over oceanic crust away from the ridge. Option D is wrong because ocean-floor sediment is not made of continental ice. Memory cue: young ridge means thin sediment; older, farther crust means thicker sediment.

Ridges, magnetic stripes, and trenches are evidence of a dynamic floor. This weakened the idea of permanent oceans.