The earth is so big, and as I understand, constantly moving and roiling away inside – why isn’t there a constant low level rumble?
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Because the huge size also dampens the vibrations enough for a human to feel it.
This would only be possible with very sensitive equipment.
Because like you said, the earth is BIG. The ‘magma layer’ (simplified wording, of course) starts around 200km beneath our feet. That’s over 20 times the height of mount Everest.
Beneath us and magma is 200km of dense rock/sand/water depending on your location, all of which act as sound insulation.
You know how there’s a big difference between the amount of rocking you feel in a small boat vs something like a cruise ship?
Imagine if your boat was multiple kilometers thick and was kind of spongey and able to act as a shock absorber.
There is a detectable rumbling, and instruments built specifically to measure it can record it. But since everyday tectonic movement doesn’t really make a difference to how we live, there’s no reason for our bodies to have adapted to sense that part of our environment. And when seismic events do affect our lives, it’s often at frequencies that we don’t really consider to be sound. The seismic waves in earthquake events are primarily between 10 hertz and 1/100th of a hertz, which are significantly lower than the lowest frequencies we perceive as sound. Below about 20 hertz we stop ‘hearing’ pressure waves and experience them more as bodily thuds.
Because the huge size also dampens the vibrations enough for a human to feel it.
This would only be possible with very sensitive equipment.
Because like you said, the earth is BIG. The ‘magma layer’ (simplified wording, of course) starts around 200km beneath our feet. That’s over 20 times the height of mount Everest.
Beneath us and magma is 200km of dense rock/sand/water depending on your location, all of which act as sound insulation.
You know how there’s a big difference between the amount of rocking you feel in a small boat vs something like a cruise ship?
Imagine if your boat was multiple kilometers thick and was kind of spongey and able to act as a shock absorber.
Media representations of the earth’s interior are horribly misleading.
The earth’s inner core is solid, the outer core is liquid, the mantle – making up most of the rest of the planet is also solid, but flows on geological timescales (think about a finger width per year)
The crust is largely rigid and sits on top. Nothing there would be described well as “roiling”.
Magma is a separate thing, and represents liquid generated by the partial melting of the mantle. It rises up to feed volcanoes. It’s a tiny miniscule proportion of the Earth, and often in very small pockets. That’s not to say we don’t feel it – volcanic earthquakes and deformation on the scale of mm to cm occurs when magma is intruded near the surface.
The mantle is not magma, and does not behave like magma.
There is a detectable rumbling, and instruments built specifically to measure it can record it. But since everyday tectonic movement doesn’t really make a difference to how we live, there’s no reason for our bodies to have adapted to sense that part of our environment. And when seismic events do affect our lives, it’s often at frequencies that we don’t really consider to be sound. The seismic waves in earthquake events are primarily between 10 hertz and 1/100th of a hertz, which are significantly lower than the lowest frequencies we perceive as sound. Below about 20 hertz we stop ‘hearing’ pressure waves and experience them more as bodily thuds.
[The Kola Superdeep Borehole](https://en.wikipedia.org/wiki/Kola_Superdeep_Borehole) is the deepest hole on Earth at 12,262m deep. Still no magma. The crust is very thick relative to a human. It’s very thin relative to the Earth as a whole.
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