Question

what two forces are balanced when a system is in isostasy? a. downward gravity and upward buoyancy b. upward gravity and downward buoyancy c. upward accretion and downward buoyancy d. downward gravity and upward heat

Explanation:

Isostasy is a state of gravitational equilibrium between the Earth's crust and the underlying mantle. The downward force of gravity (acting on the crustal material) is balanced by the upward buoyant force (from the mantle). Buoyancy here is related to the density differences and the support from the asthenosphere. Gravity acts downward as it pulls the crustal blocks (like mountain ranges or ocean basins) towards the Earth's center. Buoyancy (upward force) counteracts this, similar to how an iceberg floats in water - the weight (gravity - like force) of the iceberg is balanced by the upward buoyant force from the water. Options B has incorrect direction for gravity (gravity is downward, not upward). Option C has accretion (which is related to adding material, not a force in isostasy balance) and incorrect buoyancy direction. Option D has heat (not a force in the isostasy force balance concept).

Answer:

A. Downward gravity and upward buoyancy