A measure of the heat being conducted through the surface rocks of the Earth. It is usually determined by taking two or more temperature readings at different depths down a borehole or sampling tube, and measuring or estimating the thermal conductivities of the rocks in between. On land, measurements are usually taken at depths greater than 200 m to avoid temperature changes associated with climatic oscillations. The largest uncertainties are due to local or regional heat transference by convective circulation. In oceanic sediments the observed heat flow decreases in proportion to t^0.5, where t is the age of the oceanic crust. Near the oceanic ridge crests the heat flow averages 100 mW/m², decreasing to less than 50 mW/m² for crust older than 100 Ma. In the continents, the older crustal areas appear to have a lower heat flow, averaging about 38 mW/m², than the younger crust; in Mesozoic and Tertiary orogenic areas it is 60–75 mW/m². The heat flow in the cratonic shields (see craton) is almost entirely accounted for by radiogenic heat production within the surface rocks themselves, implying depletion of radiogenic heat-producing elements in the crust and upper mantle beneath these areas. The annual global heat loss by the Earth is somewhat uncertain because of heat loss by convection, but is about (4.1–4.3) × 10¹³ W (1.3 × 10²¹ J/year), about 75% of which is lost through the oceanic crust. Internal heat sources within the Earth are: (a) radiogenic heat-producing elements; (b) gravitational energy released by core formation; (c) exothermic chemical reactions; and (d) heat remaining from the initial accretion of the Earth, comprising the original heat of the materials, kinetic heat from early meteoritic and planetismal bombardment, and exothermic chemical reactions. The proportion of each source is not clear, although radiogenic heat is probably the major component; gravitational energy release may have been dominant during core formation.