Planet Earth

Let's Set a Few Things Straight About Our Planet's Interior

We have to explore the Earth's interior by reading the seismic waves produced by earthquakes. What they have shown us a dynamic planet, but don't worry, it is all normal.

Rocky Planet iconRocky PlanetBy Erik KlemettiFeb 20, 2023 8:10 PM
Seismic Waves passing through Earth
A schematic diagram showing how seismic waves generated by an earthquake pass through Earth's interior, revealing its structure. Credit: Wikimedia Commons.


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Exploring inside of our planet like we explore the solar system is a frontier that may never be reached. The intense pressure and temperature in the Earth makes it nearly impossible to even conceive of how we could explore much of our planet with our own eyes. That doesn't mean we don't know a lot about the inner workings of Earth, but it takes some circumstantial evidence to pull it off.

If we were to travel from the surface of the Earth to the very middle, we'd travel nearly 4,000 miles. Although that is the same distance as Boston to Helsinki, it is infinitely harder to traverse. In fact, if "straight down" is the direction you want to go, humans have only made it ~7.5 miles -- or a paltry 0.2% of the trek. The hole drilled to that depth was the famous Kola Borehole in northwestern Russia (back when it was the Soviet Union). Even then, that hole was no wider than a pie plate.

The very idea of exploring the interior of the Earth directly feels about a feasible as traveling faster than light. So, instead we have to use other clues and data to get an idea of what sits under our feet, then fit those to models that we've developed for the interior of a planet.

Probably the most important data we can collect about Earth's interior is how quickly seismic waves generated by earthquakes pass through the planet. Depending on the layers they intersect, the waves will move at different speeds. That's because the composition and the state of the layer will alter those velocities. In general, cold and solid materials mean faster waves, warm and partially melted materials mean slower waves. This is a gross simplification, but it works in a broad sense.

The Core is Slowing Down?

Two recent studies in Nature Geosciences used seismic wave data to reveal some new, fascinating information about the interior of the planet. The headlines that popped up about these studies made it seem like our planet might be coming off the rails, but really they are just helping us understand the dynamic nature of the mantle and the core of Earth.

The internal structure of the Earth. Credit: Wikimedia Commons.

The first study to catch the media's attention was one that was sold as "Earth's core stopped spinning!" Now, that does sound dramatic ... except that it really isn't. That study, by Yang and Song, used repeated seismic wave travel paths to deduce the motion of the Earth's inner core.

The Earth's metallic core is divided into two layers: the liquid outer core and solid inner core. The inner core is slowly growing as the outer core crystallizes and it is spinning ... but it is spinning independent of the rotation of the Earth. That's because of that liquid outer core. However, there does appear to be connections between all of Earth's layers thanks to electromagnetism, gravity and momentum.

What they found looking at these data is that the inner core rotation likely slowed to a stop and even change direction over the past 30 years. This appears to be part of a ~70 year cycle for the rotation of the inner core -- in a sense, the core is oscillating rather than spinning. This change is manifested in slight (we're talking fractions of fractions of fractions of seconds) changes in the length of the day (Earth's rotation).

Now, it might sound dramatic that the inner core may have stopped spinning or even went in reverse, but the authors think this is perfectly normal behaviour for the inner core. It likely have very little impact on surface -- or even deep Earth -- processes. Yet, these seismic data let us know something new about how the core works.

The Mantle is Molten?

The second study by Hua and others used seismic data to examine the state of the Earth's mantle. Underneath the Earth's crust (which is only up to ~45 miles thick) is the mantle, a layer that extends over 1,700 miles down. The mantle is, importantly, not a layer of molten rock. It is a top layer called the lithosphere that is solid, brittle rock (our tectonic plates are made of crust and lithosphere). The next layer is the asthenosphere, which is also solid but ductile -- it can bend and flow.

Schematic model for a partially molten layer in the asthenosphere. Credit: Hua et al. (2023), Nature Geosciences.

The mantle will convect as hot rock rises from deep in the Earth to the surface. Some of that hot rock melts under mid-ocean ridges and other tectonic boundaries. In places where molten rock (magma) is present, seismic waves tend to slow down as they pass through the "liquid".

Hua and others found that there is a zone about 90-100 miles beneath our feet that seems to be a globally-present lower velocity layer. They interpret this as an area of partial melt of the mantle that is embedded in the asthenosphere.

Now, the headlines used phrases like "molten rock layer" or "hidden molten layer lurking". That isn't entirely the case. The layer suggested in this study is partial melt. That means that it is likely mostly solid, but with some significant portion of magma interspersed. They don't say how much, but it could be 20% of the rock layer is liquid rock. That's far from a vast layer of magma.

What was most interesting in the study is that the layer was observed globally. We know that there are areas of partial melt in the mantle in places where rock is actively melting, like mid-ocean ridges, subduction zones (like the Andes) and hotspots (like Hawai'i). A global zone means that conditions to, at the very least, melt the mantle exist widely at that depth.

So, rest assured, none of these new studies imply that the Earth is doing something new and hazardous. Instead, we are using seismic wave data to reveal more about the final frontier that is the interior of the Earth. We may never get there, but at least we can catch a glimpse of what might be happening.

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