Question

fill in the blank question surface currents are usually generated by \boxed{}. deep - ocean currents usually result from differences in \boxed{} between water masses.

Explanation:

1. For surface currents: Surface ocean currents are mainly driven by wind. Wind blowing over the ocean surface exerts a force (wind stress) that causes the water to move, creating surface currents.
2. For deep - ocean currents: Deep - ocean currents, also known as thermohaline circulation, are caused by differences in water density. Water density is affected by temperature (thermo - ) and salinity (haline - ), so differences in temperature and salinity (which lead to density differences) drive deep - ocean currents.

Answer:

First blank: wind
Second blank: density (or temperature and salinity, as density differences are due to temperature and salinity variations)