'Dynamic duo' develops framework for Earth's inaccessible interior

The pair paints a story for a chemically complex inner earth, a model that sharply contrasts the heavily relied upon paradigm of the past few decades that the mantle is all one thing and well mixed. The original model was composed of simple concentric spheres representing the core, mantle and crust – but the inner Earth isn’t that simple.

What lies beneath

Earth is made up of several layers. Its skin, the crust, extends to a depth of about 40 kilometers (25 miles). Below the crust is the mantle area, which continues to roughly halfway to the center of Earth. The mantle is the thick layer of silicate rock surrounding the dense, iron-nickel core, and it is subdivided into the upper and lower mantle, extending to a depth of about 2,900 km (1,800 miles). The outer core is beneath that and extends to 5,150 km (3,200 mi) and the inner core to about 6,400 km (4,000 mi).

The inner Earth is not a static storage space of the geologic history of our planet – it is continuously churning and changing. How a mantle convects and how the plates move is very different depending on whether the mantle is isochemical (chemically homogenous made entirely of only one kind of material) or heterogeneous, composed of different kinds of compounds.

Garnero and McNamara’s framework is based upon the assumption that the Earth’s mantle is not isochemical. Garnero says new data supports a mantle that consists of more than one type of material.

“Imagine a pot of water boiling. That would be all one kind of composition. Now dump a jar of honey into that pot of water. The honey would be convecting on its own inside the water and that’s a much more complicated system,” McNamara explains.

Observations, modeling and predictions have shown that the deepest mantle is complex and significantly more anomalous than the rest of the lower mantle. To understand this region, seismologists analyze tomographic images constructed from seismic wave readings. For 25 years they have been detecting differences in the speeds of waves that go through the mantle.

This difference in wave speeds provides an “intangible map” of the major boundaries inside the mantle – where hot areas are, where cold areas are, where there are regions that might be a different composition, etc. The areas with sluggish wave speeds seem to be bounded rather abruptly by areas with wave speeds that are not sluggish or delayed. An abrupt change in wave speed means that something inside the mantle has changed.

If the mantle is all the same material, then researchers shouldn’t be observing the boundary between hot and cold in the mantle as a super sharp edge and the temperature anomalies should also be more spread out. The abrupt change in velocity was noticeable, yet they didn’t know what caused it.