Bunge, Peter. "Mantle Convection." Earth

oceaniccrustplumeMantlesubductingplatespreading platescrustcontinentalCoreOver short periods of time the mantle seems rigid, but overmillions of years the rocks of the mantle flow. Hot, lessdense rock rises up from near the core towards the surfacewhere it cools and eventually sinks back towards the core.This density-driven movement of material is calledconvection, the primary way in which heat is transferredfrom deep within Earth to its surface.Mantle ConvectionPeter BungeDo you believe that the Earth’s interior is in constant motion?Could I convince you that it contains currents that are tracked bysupercomputers? Well, you might say, I know about currents inthe oceans—but currents inside the Earth? After all, most of theEarth is made up of solid rock. (There is also a big chunk of ironin the innermost Earth, which we call the core.) Perhaps we areboth right. On a human time scale, rocks are solid, but if youwait for a long time—millions of years—rocks slowly changetheir shape. They flow.Peter Bunge is an Assistant Professor of Geophysics in the Department of Geosciences atPrinceton University.

In my work, I use supercomputers to track thecurrents that churn beneath the surface of thisdynamic planet.The Theory of Continental DriftGeologists knew this well before computersexisted. The idea so fascinated Arthur Holmes,a British geologist, that he envisioned the rocksslowly deforming, or moving, deep inside theEarth, and in the process pushing continentsapart on the surface of the globe. Holmes wasnot alone in his search to understand themotion of the continents. In fact, he wasmotivated by another great earth scientist, theGerman meteorologist Alfred Wegener, who hadsuggested that continents drift. At first, likeothers before him, Wegener was simplyimpressed by how the coastlines of all thecontinents around the Atlantic Ocean looked likethey fit together. But Wegener did not stopthere. Instead, he began a careful study ofrocks and fossils from the edges of Africa,Australia, and North and South America. Hefound similar rock types along the coasts of allthe continents that ring the Atlantic Ocean. Hefound evidence of glaciers that had originated inpresent-day South Africa and spread into SouthAmerica and India. And he found fossils in thesouthern continents that must have inhabitedsimilar ecosystems. In order to explain thesesimilarities and connections, Wegenerhypothesized that the continents must havebeen together at some point in the past.What Propels the Continents?In 1944, Holmes proposed an outrageous idea.If all continents had once been joined, they musthave covered a vast region of the underlyingmantle. Like a blanket, the continents wouldinsulate this region deep inside the Earth. Theregion would then warm up over an expanse ofgeologic time. Because these warm rockswould be much lighter than the rocks on thesurface, they would rise and push the overlyingcontinents apart. This mechanism, calledthermal convection, had been applied to thebehavior of fluids but not to the behavior of thesolid mantle. (For more about Holmes’theories, read the profile of Arthur Holmes inSection One). Convection in the mantleprovided an elegant way to propel Wegener’scontinental drift, and the field of mantleconvection was born.How Geophysicists Use Computer ModelsMore than fifty years have passed, and thegeophysicists who study convection in themantle have significantly advanced Holmes’theories. They have set up mathematicalequations to describe mantle convection, andthey have turned to computers to help solvethem. Geophysicists have long tried to usecomputers to better understand the Earth.Computers are useful because manycomplicated processes inside the Earth can beunderstood only by solving detailedmathematical equations over and over again.We begin by breaking up sections of the mantleinto discrete areas, or cells. We track severalvariables in one cell, then compare theseresults to what is occurring in neighboring cells.These results are projected onto relatively shortperiods of time (maybe 100,000 years). Whenall these complex equations are computed, wecan begin to make assumptions about theprocesses in individual areas over time. Basedon what we know about how one area’s activityrelates to another, we can then makeassumptions about what will happen in largerareas. Fortunately, this tedious taskcan now be carried out by computers. As aresult, detailed computer models of the mantlenow exist.

Figure 1: The results of the model show the temperaturedistribution for mantle convection assuming incompressible,internally heated isoviscous mantle with a Rayleigh numberof Ra=10 7 . The surface part is not shown, so that we cansee the lighter colored upwellings and darker coloreddownwellings (the planform). The planform is dominated bydownwelling plumes, rather than by linear zones.These models work by laying a threedimensionalgrid with millions of points over theentire mantle. The models keep track of severalvariables—including the velocity, temperature,and pressure at each of the grid points—bysolving the appropriate mathematical equationsrepeatedly. After doing this many millions oftimes, the computer acquires enoughinformation to form a portrait of convection inthe mantle. It takes a day to two weeks to runsuch a simulation on a supercomputer.The Role of Plate TectonicsMantle convection has turned out to be farmore exciting than Holmes originally envisioned.Holmes thought only of drifting continents, but itturns out that the entire surface of the Earth ismoving, a process now known as platetectonics. The Earth’s outer, rigid shell is brokenup into pieces called tectonic plates. The platesare propelled by the mantle convection currentsbelow. The continents are embedded in theseplates, and are carried along as the platesmove. Tectonic plates form slowly at thesurface at undersea valleys, called mid-oceanspreading centers, where the mantle loses itsheat to the exterior. These are places where thetectonic plates spread apart and magma, ormolten rock from the mantle, wells up. Themagma freezes in the gap formed byspreading, causing the plates to grow. Theplates cool off as they move away from thespreading centers, and eventually they sinkback into the mantle at places called subductionzones. They sink because they become colder,and therefore denser, than the mantle below.The formation of new plates and theconsumption of old plates are directly coupledwith mantle convection (Figure 1).How Does Mantle Convection Work?The nature of mantle convection can beexplored in computer simulations, whichsometimes pose questions as well as answerthem. For example, earlier simulations predicteda complex pattern of mantle flow involving alarge number of relatively small subductionzones spread across the Earth (Figure 2). In

MANTLE CONVECTIONFigure 2: For this model the viscosity of the lower mantlehas been increased by a factor of 30. Linear downwellingsheets dominate the planform.fact, we know that there are only two majorsubduction zone systems. One system runsfrom Australia to Japan along the westernmargin of the huge Pacific plate. The other liesoff the western coasts of middle and SouthAmerica, where the smaller Cocos and Nazcaplates return into the mantle. Until recently,mantle convection simulations have rarelyresulted in such a simple flow.Why are Mantle Convection Cells So Wide?Why are there so few subduction zones,especially in view of the physics of convectioncells? A convection cell describes the motion inwhich hot material circulates upward, then sinksback down as it cools. Subduction zones arethe downwelling part of a convection cell; theircounterparts are the mid-oceanic ridges. Ingeneral, convection cells are as wide as theyare deep, and their size decreases as the vigorof convection increases. Abundant heat, whichdrives convection, is generated throughout themantle by radioactivity. Small convection cellsare thus predicted for the mantle, since it is hotand therefore vigorously convects. However, inthe 3,000-kilometer-deep mantle, the cells aremuch wider than they are deep, averaging asmuch as 10,000 kilometers across. Forgeophysicists, this raises the question of whythe mantle prefers such long convection cellsrather than the smaller cells predicted based onthe simple laws of physics.Greater understanding of this puzzle has arisenthrough recent large numerical convectionsimulations based on our knowledge of thephysical parameters of the mantle. The answerlies in the fact that rocks in the lower mantlemore than 600 kilometers below the surfaceare stiffer, or more viscous, than rocks in thesofter upper mantle (Figure 2). This is becausepressure in the mantle increases with depth. Theever-increasing pressure causes mantle mineralsto undergo a series of structural transformationsso that they can be packed more closely. Suchhigh-pressure minerals—like the mineralperovskite, which makes up more than ninetypercent of the lower mantle volume and thus isprobably the most abundant mineral on Earth—are stiffer and mechanically stronger than theirupper-mantle counterparts. It is this contrast in