Enormous rivers of hot rock spanning hundreds of miles across and reaching all the way down into the planet's metal-rich core have been seen for the first time.The searing-hot plumes, which feed volcanoes on the surface, are likely themselves fed by two "superblobs" beneath Africa and the Pacific Ocean, the researchers said.The new finding comes from supercomputer visualizations of the Earth's interior derived from seismic data on hundreds of earthquakes over the last several decades.The new results may settle a long-standing debate about whether these molten jets of magma, called mantle plumes, trigger volcanic eruptions. Mystery plumes For decades, scientists have debated the existence of mantle plumes, or hot columns of magma that rise in the Earth's mantle, the layer between the crust and the molten iron and nickel outer core.Earth's crust floats on a layer of molten rock known as magma. The dominant theory is that volcanoes form where one tectonic plate dives beneath another, squeezing magma up through fissures in the Earth's crust.But volcanoes, such as those that form the Hawaiian Islands and Iceland, often erupt far from any plate boundary. In 1971, geologist W. Jason Morgan proposed a completely different mechanism for the formation of these off-plate volcanoes: deep jets of magma coming straight from the mantle-core boundary.