A Moment of Science

The Earth’s Crystalline Inner Core

How big is a really big crystal? Learn more on this Moment of Science.

Cobalt blue amythest crystals

Photo: cobalt123 (flickr)

Many geologists believe that crystals like these make up Earth's inner core

How big is a really big crystal? Would a diamond the size of a mini van qualify? How about a mountain sized snow flake?

Actually, there might be a globe-shaped crystal that’s much bigger; more than a thousand miles across.

Is this huge crystal orbiting some obscure corner of the solar system? Not at all. It’s right under our feet. It’s the Earth’s crystalline inner core.

The inner core is impossible to study directly since we can’t exactly dig down and drop in for a visit. Temperatures there are nearly as hot as the surface of the sun, and the pressure is three million times what it is on Earth’s surface. Still, geologists have devised clever ways of studying these depths indirectly.

One way is to study deep seismic waves, caused by earthquakes, that happen to pass through this innermost region. Geologists have found that seismic waves traveling along the north-south axis of our planet pass through the inner core slightly faster than seismic waves going through any other way.

What does this difference in speed tell us?

When vibrations go through ordinary matter, they tend to move the same speed no matter which direction they go. However, in a crystal, the atoms are lined up, so vibrations go through faster some ways than others.

Geologists have long suspected that the inner core is solid iron. If the iron atoms were lined up in a hexagonal crystal pattern, like some tremendous, iron crystal ball, it would neatly explain the way seismic waves pass through. Odd as it might seem, a giant crystal is our best guess so far for Earth’s inner core.

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