What are the two primary heat sources that drive mantle convection?
ATidal heating from the Moon and chemical reactions between silicate minerals
BRadioactive decay of isotopes and primordial heat left over from Earth's formation and accretion
CSolar radiation absorbed at the surface and magnetic friction from core rotation
DPressure-release melting at ridges and latent heat from crystallization
Mantle convection is driven by heat from within: radioactive decay of long-lived isotopes (uranium, thorium, potassium-40) continually generates heat, while primordial heat from accretion and core formation is slowly leaking out. Surface cooling creates the temperature contrast that sustains the convective overturn.
Question 2 True / False
A higher Rayleigh number in the mantle indicates more vigorous convective flow.
TTrue
FFalse
Answer: True
The Rayleigh number (Ra) is the dimensionless ratio of buoyancy forces driving convection to viscous and thermal diffusive forces resisting it. A larger Ra means buoyancy dominates — the fluid overturns more vigorously. Earth's mantle has a very high Ra (~10^7–10^8), which is why convection occurs despite the mantle's enormous viscosity.
Question 3 Short Answer
We cannot observe mantle convection directly. Name two independent lines of evidence that it occurs.
Think about your answer, then reveal below.
Model answer: Seismic tomography reveals slow (hot, buoyant) and fast (cold, dense) regions in the mantle consistent with upwelling and downwelling flow. Plate tectonics itself — the existence of spreading ridges, subduction zones, and hotspot tracks — is the surface expression of mantle circulation.
Seismic tomography infers temperature structure from P- and S-wave velocity anomalies; cold slabs appear fast and hot plumes appear slow. The geometry and kinematics of plates — including their velocities and the pattern of mid-ocean ridges — match the expected surface manifestation of underlying convection cells.