D.1 Geophysical systems

The question asks which specific isotopes drive radioactive decay heating within Earth's mantle. This requires knowledge of which unstable isotopes are geographically significant to Earth's interior thermal budget.

Uranium-238, thorium-232, and potassium-40 are the principal heat-producing radioactive isotopes found in the mantle and crust. Their continuous decay releases heat energy that sustains mantle temperatures high enough for convection to occur.

Carbon-14 is used in radiometric dating of organic material and decays far too quickly to be a significant mantle heat source. Silicon, iron, magnesium, calcium, sodium, and aluminum are all stable elements found abundantly in Earth's crust and mantle but do not undergo radioactive decay that releases significant heat.

Only uranium-238, thorium-232, and potassium-40 are radioactive isotopes present in the mantle in concentrations sufficient to sustain long-term heat production over geological timescales, making this the correct answer.

The question targets the specific radioactive isotopes responsible for internal Earth heating — a factual but conceptually important point distinguishing radiogenic heat from leftover formation heat.

Carbon-14 has a very short half-life (~5,730 years) and is irrelevant to geological timescale heat production. Nitrogen-15 and oxygen-18 are stable isotopes that do not produce heat through decay.

Silicon-28, iron-56, and magnesium-24 are the most abundant stable isotopes in Earth's crust and mantle respectively, but being stable, they undergo no radioactive decay and produce no heat.

While calcium-40 is actually the daughter product of potassium-40 decay, calcium itself is stable. Sodium-23 and aluminum-27 are also stable isotopes and contribute nothing to radiogenic heating.

Uranium-238, thorium-232, and potassium-40 are the correct answer — all three are genuinely radioactive isotopes with half-lives long enough (billions of years) to have been releasing heat continuously since Earth's formation 4.6 billion years ago.

The question asks for the causal explanation of why one specific plate descends at a convergent boundary. The answer must identify the physical property that determines which plate subducts.

Oceanic crust is composed primarily of basalt with a density of approximately 3.0 g/cm³, while continental crust is composed primarily of granite with a density of approximately 2.7 g/cm³. At a convergent boundary, the denser plate is forced downward into the mantle because it is less buoyant.

Buoyancy in the asthenosphere depends on density contrast. Just as a denser material sinks in a fluid, oceanic crust sinks into the semi-plastic mantle because it is heavier relative to the surrounding material, while the less dense continental crust remains buoyant and stays at the surface.

The answer that correctly identifies the density contrast (oceanic ≈3.0 g/cm³ vs continental ≈2.7 g/cm³) as the causal mechanism for subduction is the correct choice.

This question tests understanding of the specific physical mechanism — density contrast — that drives subduction, not just the general fact that one plate goes under the other.

Ridge-push contributes to plate motion generally but does not explain which plate subducts at a specific convergent boundary. Speed of plate movement is not the determining factor in subduction — density is.

This is factually incorrect — oceanic crust is actually thinner (typically 5–10 km) than continental crust (typically 30–70 km). Thickness does not determine subduction; density does.

While radioactive decay does occur in both crust types, the suggestion that differential heating of the continental plate explains subduction is a distortion of the actual mechanism. The key physical variable is compositional density, not heat distribution.

Oceanic crust is denser (≈3.0 g/cm³) than continental crust (≈2.7 g/cm³) is the only option that correctly identifies density contrast as the direct mechanical cause of subduction.