Exploring the interior of our planet, as we explore the solar system, is a frontier that can be reached. The intense pressure and temperature of the Earth make it almost impossible to even imagine how we could explore much of our planet with our own eyes. That doesn’t mean we don’t know a lot about Earth’s internals, but some circumstantial evidence is needed to do so.
If we were to travel from the surface of the Earth to its very center, we would travel nearly 4,000 miles. Although this is the same distance as Boston to Helsinki, it is infinitely more difficult to traverse. In fact, if “straight down” is the direction you want to go, people have only traveled ~7.5 miles – or a paltry 0.2% of the trek. The hole drilled to this depth was the famous one Kolski borehole in northwestern Russia (at the time when it was the Soviet Union). Even then, that hole was no wider than a pie plate.
The very idea of exploring the Earth’s interior directly feels as feasible as faster-than-light travel. So instead, we have to use other clues and data to get a sense of what’s sitting beneath our feet, then fit that into the models we’ve developed of a planet’s interior.
Perhaps the most important data we can gather about Earth’s interior is how fast seismic waves generated by earthquakes travel through the planet. Depending on the layers they cross, the the waves will travel at different speeds. This is because the composition and state of the layer will change these velocities. In general, cold and solid materials mean faster waves, warm and partially molten materials mean slower waves. This is a gross oversimplification, but it works in a broad sense.
Is the core slowing down?
Two recent studies in Natural geosciences uses data from seismic waves to reveal fascinating new information about the planet’s interior. The headlines that have appeared about these studies make it seem that way our planet could go off the railsbut really they just help us understand the dynamics nature of the Earth’s mantle and core.
The internal structure of the Earth. Credit: Wikimedia Commons.
The first study to gain media attention was what was sold as “The Earth’s Core Has Stopped Spinning!” Now, that sounds really dramatic… but it really isn’t. This study, by Yang and Songused it’s repeating the paths of seismic waves to infer the motion of the Earth’s inner core.
Earth’s metallic core is separated in two layers: the liq outer core and solid inner core. The inner core grows slowly like the outer core it crystallizes and rotates … but it rotates independently of the Earth’s rotation. This is because of this liquid outer core. However, there appear to be connections between all layers of the Earth thanks to electromagnetism, gravity and momentum.
What they found by looking at this data is that the rotation of the inner core has likely slowed to a halt and even reversed direction over the past 30 years. This appears to be part of a ~70 year rotation cycle of the inner core – in a sense the core oscillates rather than rotates. This change manifests itself in slight (we’re talking fractions of fractions of fractions of seconds) changes in the length of the day (the Earth’s rotation).
Now, it may sound dramatic that the inner core may have stopped spinning or even gone into reverse, but the authors believe that this is perfectly normal behavior for the inner core. It probably has very little effect on processes at the surface – or even deep in the Earth. Yet this seismic data allows us to understand something new about how the core works.
Is the mantle molten?
The second study by Hua et al uses seismic data to study the state of the Earth’s mantle. Beneath the Earth’s crust (which is only ~45 miles thick) is the mantle, a layer that extends over 1,700 miles down. It’s the mantle that matters, no layer of molten rock. This is the topmost layer called the lithosphere, which is hard, brittle rock (our tectonic plates are made up of crust and lithosphere). The next layer is asthenospherewhich is also hard but malleable — 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 within the Earth to the surface. Some of this hot rock melts beneath mid-ocean ridges and other tectonic boundaries. In places where there is molten rock (magma), seismic waves tend to slow down as they travel through the “liquid”.
Hua and others found that there is a zone about 90-100 miles below our feet that appears to be a globally present lower-velocity layer. They interpret this as a region of partial melting of the mantle that is embedded in the asthenosphere.
Headlines now use phrases like “molten rock layer” or “lurking hidden molten layer“. This is not quite the case. The layer proposed in this study is partially I’m melting That means it’s likely mostly solid, but with a significant portion of magma. They don’t say how much, but it could be as much as 20% of the rock layer is liquid rock. This is far from a huge layer of magma.
What is most interesting about the study is that the layer was observed globally. We know that there are regions of partial melting in the mantle in places where rocks are actively melting, such as mid-ocean ridges, subduction zones (like the Andes), and hotspots (like Hawaii). A global zone means that conditions for least mantle melting exist widely at this depth.
So rest assured, none of this new research suggests Earth is doing anything new and dangerous. Instead, we’re using seismic wave data to reveal more about the final frontier that is Earth’s interior. We may never get there, but at least we can get a glimpse of what might happen.