mechanism of thermal transport changes from conduction, as within the rigid tectonic plates, to convection, in the portion of Earth's mantle that convects. Despite its solidity, most of Earth's mantle behaves over long time-scales as a fluid, and heat is transported by advection, or material transport. Second, radioactive heat production is concentrated within the crust of Earth, and particularly within the upper part of the crust, as concentrations of uranium, thorium, and potassium are highest there: these three elements are the main producers of radioactive heat within Earth. Thus, the geothermal gradient within the bulk of Earth's mantle is of the order of 0.5 kelvin per kilometer, and is determined by the adiabatic gradient associated with mantle material (peridotite in the upper mantle). Negative geothermal gradient Negative geothermal gradients occur where temperature decreases with depth. This occurs in the upper few hundreds of meters near the surface. Because of the low thermal diffusivity of rocks, deep underground temperatures are hardly affected by diurnal or even annual surface temperature variations. At depths of a few meters, underground temperatures are therefore similar to the annual average surface temperature. At greater depths, underground temperatures reflect a long-term average over past climate, so that temperatures at the depths of dozens to hundreds of meters contain information about the climate of the last hundreds to thousands of years. Depending on the location, these may be colder than current temperatures due to the colder weather close to the last ice age, or due to more recent climate change. Negative geothermal gradients may also occur due to deep aquifers, where heat transfer from deep water by convection and advection results in water at shallower levels heating adjacent rocks to a higher temperature than rocks at a somewhat deeper level. Negative geothermal gradients are also found at large scales in subduction zones. A subduction zone is a tectonic plate boundary where oceanic crust sinks into the mantle due to the high density of the oceanic plate relative to the underlying mantle. Since the sinking plate enters the mantle at a rate of a few centimeters per year, heat conduction is unable to heat the plate as quickly as it sinks. Therefore, the sinking plate has a lower temperature than the surrounding mantle, resulting in a negative geothermal gradient. See also Temperature gradient Earth's internal heat budget Geothermal power Hydrothermal circulation PANGAEA Global Heat Flow Database data set with