Title:Global heat flow

Seth Stein (Dept. of Geological Sciences, Northwestern University, Evanston, IL 60208, email: seth@earth.northwestern.edu)

Carol A. Stein (Dept. of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, IL 60607-7059; email: cstein@uic.edu)

Abstract: It is said that "heat is the geological lifeblood of planets". Planets are great heat engines, whose nature and history govern their thermal, mechanical, and chemical evolution. The most direct constraint on how the heat engine operated at depth through time is the present outward heat flow at the surface. On Earth, this heat flow is primarily associated with the plate tectonic cycle whereby hot material upwells at spreading centers and then cools. Because the strength of rock decreases with temperature, the cooling material forms the strong plates of the lithosphere. The cooling oceanic lithosphere moves away from the ridges, and eventually reaches subduction zones where it descends in downgoing slabs, reheating as it goes. The other important component of surface heat flow is that conducted through the continents, which are not subducted.

Measuring the temperature at several depths and the thermal conductivity gives the heat flow. Such measurements began in 1939 on land and in 1952 at sea. By now, heat flow has been measured at about 30,000 sites worldwide. This is done at sea using a probe that penetrates into the soft sediment on the sea floor, and on land using boreholes drilled for oil or other purposes.

In general, oceanic heat flow data varies with the age of the lithosphere. Values greater than 100 mW/m**2 occur near the ridges, and decrease smoothly to about 50 mW/m**2 in the oldest oceanic lithosphere. Similarly, ocean depth is about 2500 m at the ridges, and increases to about 5600 m for the oldest sea floor. These variations can be described using a simple model for the formation of the lithosphere by hot material at the ridge, which cools as the plate moves away. The modeled depth to a given temperature increases as the square root of lithospheric age, predicting that ocean depth should increse with the square root of age and heat flow should similarly decrease. Because ocean depth seems to ``flatten'' at about 70 Myr, we often use a modification called a plate model, which assumes that the lithosphere evolves toward a finite plate thickness with a fixed basal temperature. The flattening reflects the fact that heat is being added from below, so the predicted sea floor depth and heat flow behave for young ages like in the halfspace model, but evolve asymptotically toward constant values for old ages.

Comparison with depth, heat flow, and geoid (gravity) data shows that the plate thermal model is a good, but not perfect, fit to the average data, because processes other than this simple cooling also occur. In particular, heat flow in lithosphere younger than about 50 Ma is lower than the model's predictions. This is generally assumed to reflect water flow in the crust transporting some of the heat, as shown by the spectacular hot springs at midocean ridges. If so, the observed heat flow is lower than the model's predictions, which assume that all heat is transferred by conduction. Unfortunately this hydothermal heat transport is hard to quantify, so based on the model predictions global oceanic heat flow is generally assumed to be 50% larger than directly measured (geophysical dark energy?).

Combining the oceanic and continental heat flow estimates gives a total global heat loss of 44 TW . Of this loss, 70% is in the oceans and 30% in the continents, reflecting both the higher heat flow and larger area of the oceans.