New studies show that the Earth's interior is much more complex than the traditional picture of a thin crust surrounding a thick, solid-rock mantle that in turn surrounds a molten-iron core.
"The boundary layer between the core and the overlying mantle may be forming, reforming and shifting, much like the plates of Earth's crust," which carry the continents and sea floors of the planet's surface, the California Institute of Technology said in a news release.
The studies were presented at the recent fall meeting of the American Geophysical Union.
The latest maps and related studies suggest that there are two other layers trapped between the liquid-iron core and the solid-rock mantle, at a depth of roughly 1,800 miles beneath Earth's surface, said Don Anderson, director of Caltech's seismology laboratory and president-elect of the Geophysical Union.
Because these layers are like an upside-down image of continents and oceans on the planet's surface, Caltech geophysicist Brad Hager and other scientists call them continents and oceans or sometimes "anti-continents" and "anti-oceans."
"The boundary between the hot, molten-iron core and the rocky mantle is like what happens in a blast furnace," where various materials settle or rise depending on their density, Anderson said.
Molten iron is most dense, forming Earth's core. A lighter mix of molten rock and iron rises to form the oceans or anti-oceans atop the core. The next layer up consists of the semi-solid underground continents or anti-continents, which are less dense than the molten oceans.
Last year, scientists from Harvard University and Caltech announced that they had used computer analysis of seismic waves generated by earthquakes to make blurry maps of Earth's interior, much as X-rays make pictures of the inside of the human body.
The crude maps showed that the core isn't a smooth sphere but has molten-iron mountains as tall as Mt. Everest poking into the overlying mantle, and valleys six times deeper than the Grand Canyon.
When scientists discovered mountains and valleys on Earth's core, they said friction from sloshing of molten iron across those features might explain why the planet rotates with a slight jerkiness that makes a day five-thousandths of a second longer or shorter than 24 hours every decade.
But studies by Hager and University of Colorado scientist John Wahr showed that features that big at the core-mantle boundary would cause 10 times more variation in day length than actually occurs.
So Hager concluded that molten rock-and-iron oceans above the core's molten iron mountains would smooth out the roughness of the core-mantle boundary, reducing friction so the variation in the length of a day matches the five-thousandths of a second that actually is observed.