Convection Current
A convection current is a circulating flow in a fluid caused by differences in temperature and density, as warm material rises and cooler material sinks.
Definition
A convection current is a loop of moving material that forms when something warm rises, cools off, sinks, and gets heated again, and it happens in liquids, gases, and even in the slowly flowing rock of the mantle. Convection is heat transfer by the movement of a fluid: when a fluid is heated from below it expands, becomes less dense, and rises, and as it cools it becomes denser and sinks, forming a circulating cell. Convection currents drive weather in the atmosphere, currents in the ocean, and the slow flow of the mantle that moves tectonic plates, while heat moves through true solids only by conduction. Convection occurs when the buoyancy force from thermal expansion overcomes viscous resistance and thermal diffusion, a balance expressed by the Rayleigh number, $Ra = \frac{\rho g \alpha \Delta T d^3}{\kappa \mu}$, with convection beginning above a critical value near $1{,}000$. Earth's mantle has $Ra$ on the order of $10^6$ to $10^8$, so it convects vigorously despite viscosities of $10^{21}\ \text{Pa}\cdot\text{s}$, at velocities of centimeters per year. Mantle convection is heated partly from below (core heat) and partly from within (radioactive decay of uranium, thorium, and potassium), producing narrow hot plumes and broad cold downwellings (subducting slabs) rather than symmetric cells.
Example
In a pot of soup on the stove, hot soup rises in the middle, spreads out, cools, and sinks at the edges, and the same thing happens in the mantle, just much more slowly. Afternoon thunderstorms over the Carolinas form as sun-heated air rises in convection currents, and in the mantle, hot rock rising beneath the Mid-Atlantic Ridge and cold rock sinking beneath Japan are parts of one enormous convection system. The Hadley cell in the atmosphere is convection at planetary scale: air rises at the equator, flows poleward aloft, sinks near $30^\circ$ latitude, and returns along the surface as the trade winds. Thermohaline circulation in the ocean is convection driven by both temperature and salinity.
Key Insight
Warm things rise because they are less dense, not because heat itself goes up; hot air balloons and mantle rock work on the same rule. The mantle is solid, yet it convects, because over millions of years solid rock under intense heat and pressure creeps like a fluid, moving only a few centimeters per year. The classic textbook diagram of tidy mantle convection cells pushing plates is oversimplified. Subducting slabs are the cold, sinking limbs of mantle convection, and their negative buoyancy (slab pull) is the main force moving plates, so plates are participants in convection, not passengers on it.