# What is "Heat Flow"?

Heat flow is the movement of heat (energy) from the interior of Earth to the surface.  The source of most heat comes from the cooling of the Earth’s core and the radioactive heat generation in the upper 20 to 40 km of the Earth’s crust.  Radioactive heat generation is a product of crustal rocks containing high concentrations of the naturally occurring radioactive elements: thorium, potassium, and/or uranium.  Heat flow is higher in areas with either high radioactivity or where the Earth’s crust is thinner, such as the mid-oceanic ridges or the Basin and Range Province of the Western United States.  Additionally, there are areas with heat flow ‘anomalies’ that have higher than average crustal heat flow without a clearly identified tectonic or radioactive explanation, usually related to fluid flow such as in South Dakota.

Heat flow is calculated using the rock thermal conductivity multiplied by the temperature gradient.  The standard units are mW/m2 = milli Watts per meter squared. Thus, think of a flat plane 1 meter by 1 meter and how much energy is transferred through that plane is the amount of heat flow.

Thermal conductivity is determined using rock cores or cutting on a devise that measures the amount of energy the rock sample can transfer.  Examples of devices used in a laboratory are a divided bar or needle probe.  Thermal conductivity units are typically in W/mK = Watts per meter Kelvin.  Thermal conductivity values of a rock (mineral) will change as the temperature increases, which is why the units include Kelvin.

The temperature gradient of Earth at the measurement site is determined from collecting the temperature in a well at specific depths.  Often gradient units are either °C/km or °F/100 ft. If the temperature measurements are taken after the well is no longer impacted from the drilling fluid, it is considered at equilibrium.  These values are of the highest quality and include a series of data points to assist in understanding the changes in the geology/structure of Earth. There is a tutorial on temperature logging with examples explaining why the gradient changes.

Temperature measurements are also collected while drilling wells, especially oil and gas wells.  These data values are referred to as bottom-hole temperatures, because they are taken at the bottom of the interval the well was drilled to at that time.  Thus these values need to have corrections added to them to compensate for the drilling fluid either heating (shallow wells) or cooling (deeper wells).  Also one well location can have multiple bottom-hole temperatures (BHT).  While less temperature information is collected for each site than equilibrium sites, an oil and gas field usually has numerous BHT values available for comparison; the ability to compare temperatures improves on the accuracy of the single value.

In order for a heat flow value to be fully calibrated, after the thermal conductivity and gradient are calculated, there are corrections that may be required based on where the well was drilled.  Examples of them are for steep topography (the north facing slope of a mountain is colder than a south facing slope) and the geologic structure (a fault creating a sharp change in rock type with very different thermal conductivities).

For global heat flow data, visit the International Heat Flow Commission.