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Origin and Evolution of Earth: Research Questions for a Changing Planet
known. Although we know that heat is transported by mantle convection, we do not yet have the capability to exactly describe these convective patterns, calculate with confidence how different they were in the past, or predict how they will change in the future. Resolving the critical questions about planetary evolution will require much more advanced knowledge of planetary materials and how they affect convection (Question 6), better constraints from seismology on the present configuration of mantle flow at both large and small scales, and significant advances in mathematical modeling of convection that is driven by both temperature and chemical variations.
Convection and Heat Flow
About 43 TW (1012 J/s) of heat flows from Earth’s interior through its surface at present, based on global heat flow measurements and thermal models for cooling oceanic lithosphere. Sources of this surface heat flow include the slow cooling of the mantle and core over the history of the planet; heating produced by radioactive decay of U, Th, and K; and minor sources such as tidal heating. The exact contribution of each to the planet’s heat flow is uncertain. For example, we do not know how much U, Th, and K are contained inside Earth and how these elements are distributed (McDonough, 2007). These elements are more effective at keeping Earth hot if they are located deep within the mantle, or even to some degree in the core, rather than near the surface. As a result of these uncertainties, we cannot yet answer the simple question: How fast is Earth cooling?
The primary mechanism for transporting heat within Earth’s interior is convection. It was once believed that mantle convection was impossible because the mantle was demonstrably solid. But much like a glacier, the mantle can behave like both a brittle solid and a liquid: it fractures when deformed rapidly but flows on long timescales. We now know that both the mantle and the outer core circulate in a complex pattern of large- and small-scale flows. In the molten outer core, which has very low viscosity (some estimates suggest a value similar to that of liquid mercury), convection is rapid. Hot liquid metal circulates up to the top of the core where it loses heat to the base of the mantle and then sinks again in a turbulent pattern that is affected by rotation and the magnetic field the flow generates. By contrast, mantle motions are ponderous. Typical velocities are about 5 cm/yr (based on geodetic, magnetic, seismic, and geological measurements), and at this rate the nominal “round-trip” journey of a mantle wide convection cell—across the surface for 5,000 km, down 2,900 km to the bottom of the mantle, and back to the surface again—would take about 300 million years. This rate of travel is consistent with simple thermal convection models that treat the mantle as if it were a liquid with a viscosity (estimated from postglacial rebound rates) of about 1021 Pa-s. The configuration of convection in Earth’s mantle provides the primary control on how Earth cools, mainly because the mantle makes up roughly two-thirds of Earth’s mass and 85 percent of its volume (Figure 2.1).
Mantle motions carry hot material from deep inside Earth toward the surface, where heat is lost to the atmosphere and ultimately to space, and also carry cold surface rocks down to great depths. Unresolved issues concerning mantle convection arise from uncertainties about material properties at high pressures and temperatures. Experiments and field evidence show that mantle rock becomes soft enough to flow over geological time periods at depths of just 30 to 60 km, where the temperature surpasses 700°C and pressure reaches several thousand atmospheres. At higher temperature—above 1200°C—the viscosity of mantle rock is low enough that it behaves much like a thick liquid; almost all of the mantle is hotter than 1200°C. Mantle viscosity exerts the primary control on the form of convection and the efficiency at which heat is moved toward Earth’s surface. However, other factors also are important. For example, viscous dissipation associated with deformation of stiff lithospheric plates at subduction zones strongly affects the form of convection and the relationship between convective vigor and surface heat flow. The largest uncertainties are for the lower mantle. Seismological data suggest that the flow pattern there is complex. Other observations suggest that viscosity increases in the lower mantle, and numerical models indicate that flow velocities in the lower mantle may be much slower than plate velocities such that the overturn time is a billion years or more (Kellogg et al., 1999; Ren et al., 2007).