Science

Seismic Signals Confirm Existence Of Earth’s Innermost Core

Traditionally we’ve been taught the Earth has three main layers: the crust, the mantle, and the core. Observations of how seismic waves are reflected and scattered, and experiments based on how minerals react to high pressure and temperatures, show that Earth’s inner structure is more complicated than previously thought.

Earth’s mantle is composed mostly of magnesium, iron, and silicon-dioxide, but depending on temperature and pressure, different minerals will form. The upper mantle is composed of silicates common in magmatic and metamorphic rocks, like olivine, pyroxene, and garnet. In a transition zone at depths of 400 to 700 kilometers, these common minerals become unstable and form more exotic silicates, like ringwoodite, wadsleyite, and majority. The most common mineral in absolute is bridgmanite, known also as silicate-perovskite, making up 38 percent of Earth’s volume. Bridgmanite is stable only under high temperature and pressure as found in the Earth’s mantle. Samples were first found in a meteorite (believed to be remains of a fragmented planetoid) that fell from space in 1879, but only in 2014 described as a distinct mineral.

A transition zone, referred to as D” layer by seismologists, marks the contact to Earth’s core at 3,000 kilometers. The exact origin of the D” layer is unknown. Some researchers think this layer is formed by the remains of partially molten tectonic plates, sinking from Earth’s surface to the bottom of the mantle. Others argue that this layer is formed by large crystals, grown to meter-size over millions of years at constant pressure and heat.

Earth’s core is composed of an outer layer, likely a liquid iron alloy with a radius of approximately 2,200 kilometers, and an inner core of solid iron alloy with a radius of 1,300 kilometers.

The idea of another distinct layer in the inner core was proposed a couple of decades ago, but the data has been very unclear. Now a study by researchers from the Australian National University (ANU) has confirmed the existence of the Earth’s “innermost inner core.” The team used a search algorithm to compare thousands of models of the inner core with observed data across many decades about how long seismic waves take to travel through Earth, gathered by seismograph stations all over the world.

The lead author of the study, Ph.D. researcher Joanne Stephenson, says while this new layer is difficult to observe, its distinct properties may point to an unknown, dramatic event in the Earth’s history. “We found evidence that may indicate a change in the structure of iron,” at a depth of 5,800 kilometers.

Earth’s inner core is solid despite the high temperatures surpassing 5,000 degrees Celsius because the high pressure doesn`t allow the nickel-iron-alloy to melt. At the center of the Earth, the pressure could be high enough to transform the amorphous alloy into a crystalline form, explaining the differences in seismic wave paths through Earth.

But the change in structure could also be explained by two separate cooling events in Earth’s history, Stephenson said. The first-generation of iron minerals that crystallized out of the magma and which compose the innermost core have different structural alignments as the later crystallized and deposited.”It’s very exciting—and might mean we have to re-write the textbooks!”

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