Mantle convection structure: layering, plumes and deep piles

7 problems

• Are the deep mantle slow regions denser or lighter than their surroundings?

  Contested

  If the two big regions are denser than normal mantle they are chemically different and long-lived; if lighter they are probably just hot. Different methods give opposite answers.

• What are the two continent-sized slow regions at the mantle base?

  Contested

  Below Africa and the Pacific, seismic waves slow down in two huge regions about 1000 km tall. They could be hot rock, a different rock composition, or even leftovers of the body that hit Earth to form the Moon.

• Is the lower mantle chemically distinct from the upper mantle?

  Contested

  Sinking plates reach the deep mantle, so the mantle is not split into two sealed layers. Yet the deep mantle may still differ in composition, for example holding more of the silicon-rich mineral bridgmanite.

• Are hotspot plumes rooted at the core-mantle boundary?

  Partially resolved

  Volcanic hotspots like Hawaii and Iceland may be fed by hot columns rising from the bottom of the mantle. Seismic images are blurry, so how deep these columns go and how many exist is uncertain.

• Where are the mantle's primordial helium and tungsten signatures stored?

  Open

  Some volcanic rocks carry helium and tungsten isotope ratios that date from Earth's first 100 million years. The hidden storage place that kept them unmixed for 4.5 billion years is unknown.

• How much of Earth's heat flow comes from radioactive decay?

  Open

  Earth loses about $4.6\mathrm{e}20\,\mathrm{erg}$ of heat each second through its surface, partly from decaying uranium, thorium and potassium and partly from heat left over from its formation. The split is measured with geoneutrinos (antineutrinos from radioactive decay inside Earth) and is still uncertain.

• What fraction of subducted mass crosses the 660 km boundary?

  Partially resolved

  Some sinking plates pass straight into the deep mantle while others flatten and stall near 660 km or 1000 km depth. How much material crosses, and for how long stalled plates stay, controls how well the mantle mixes.