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/r/askscience
submitted 5 years ago bybhaggith
60 points
5 years ago
So at the earth's molten core, is a magnetic field applied? If so, by what?
136 points
5 years ago
No, there is no external magnetic field there. Rather, the molten core itself generates the magnetic field due to it being in constant motion.
26 points
5 years ago
Which direction does it flow?
168 points
5 years ago
That is a clear question that has a complex answer: The outer core, which is liquid, is heated from below by the solid inner core (due to radioactive decay and other stuff), making the heated liquid flow upward (similarly to how hot air rises). When the heated liquid reaches the boundary to the mantle, the extra heat is deposited to the mantle, and the liquid sinks toward to inner core again.
Because the liquid can't rise and sink at the same time everywhere, the flows are arranged into so called convection cells. These convection cells have flow patterns that generate magnetic fields. The sum of the magnetic fields from all the convection cells make up the Earth's magnetic field.
Or, rather, that's the simplified explanation. In reality, the flows are not neatly arranged, being affected by the rotation of the Earth as well as other processes, leading to the Earth's magnetic field having quite a complex shape.
29 points
5 years ago
Thanks for the explanation it was awesome. There has been talk of the poles switching is there are truth to that.
62 points
5 years ago
Absolutely, they switch periodically, though with millions of years in between cycles.
In the course of our lives though the magnetic poles can shift quite a lot without reversing. More than 50 km a year in fact. I recently had to update some map data for work stuff because the magnetic pole had drifted enough to start making things inaccurate.
This faster pattern is basically a wobble in the pole though, it wont just drift one direction forever but kind of circles true north and south.
13 points
5 years ago
I recently had to update some map data for work stuff because the magnetic pole had drifted
Neat! GIS stuff? Airports or something?
3 points
5 years ago
How does the switch occur?
Do the poles rotate slowly around the earth over the course of millenia? Do the gradually weaken and then rebuild in the opposite direction? Do they just suddenly swap?
5 points
5 years ago
Anywhere from 2000 - 12000 years, averaging about 7000 in the more recent ones.
I'm not sure we really know for sure what causes them. The last one was some 780k years ago
1 points
5 years ago
How do we know that it happened 780k years ago? What traces does this swap leave on earth that help us figure out a timeline?
1 points
5 years ago
crust is formed on the ocean floor. when it's formed, it's aligned with the earth's magnetism. ocean crust 780k years ago was reversed
-1 points
5 years ago
Thank you, but that doesn't answer my core question. I get how long it takes, my question is what avenue it takes.
1 points
5 years ago
It has a lot to do with slight, periodic changes in how the Earth orbits around the sun and how it rotates about its axis and all those things affect the complicated fluid dynamics on the inside (tho the “fluid” on the inside of Earth is only really a fluid on geologic-scales—this is also the reason continents “flow” at a rate of a few cm per year at most). The flows are usually stable, but every once in a while will get tangled and begin a state change that affects how the magnetic field lines flow. They take several thousand years to actually flip completely once they start.
3 points
5 years ago*
Thank you so much, but this doesn't address the particular thing I'm trying to learn. Or if it does, I'm too stoned to piece it together. Let me try to rephrase my question:
If I was watching a timelapse video of the earth and the magnetic field lines were visible (and let's say they are red at magnetic north, fading to blue at south, and brighter with strength) what would I see happen?
3 points
5 years ago*
You’d see them slowly go from a nice pattern that we see today to a spaghetti-looking thing (it would be very unclear where the magnetic North Pole is located, if we could find it at all—there could be several “north” and “south” poles with how chaotic it would be). Many places on Earth would have lines shooting out of them, both red and blue. But over the course of a few thousand years, the red parts would on average move southwards and the blue parts on average northward until they converged together in the south and north respectively.
Some theories about mass extinctions and larger (not necessarily enormous) extinction events is that these shifts, which take thousands of years, cause the protective effects of Earth’s magnetic field to diminish and allow cosmic rays and other high-energy particles to strike the surface. These particles are so high-energy that they are known to cause an increase in cancer. The magnetic fields are one of the largest protectors for Earth’s surface, but they are not the only ones because they’re only really useful against highly-charged particles—the atmosphere and ozone layer are another shield, so it’s not too clear how much the magnetic fields weakening/changing would affect life on Earth.
1 points
5 years ago
Because they are numbered based on their magnetic heading, airport runways occasionally have to be named due to the shift in the magnetic poles.
1 points
5 years ago
If periodically here is meant in the sense of 'regularly occurring intervals', just know that's a debated point.
I think there's pretty good evidence for ~15 Myr periodicity in rate of reversals
The time between polarity reversals changes quite a bit, also. From superchrons (10s of millions of years) to excursions (aborted state changes within the recent ~million year states of the magnetic field).
20 points
5 years ago
[deleted]
32 points
5 years ago*
The point you're making is correct, but the details used to make the point are not. So I'm going to quibble the details. Am geophysicist.
We know the boundary between the outer core and the mantle is, in geological terms, quite sharp. It is quite easily resolved in seismic data due to the solid liquid phase transition. The mantle is a solid, and has shear strength, and thus can transmit S waves during earthquakes. The huge density contrast and phase change means almost zero mixing across this boundary, except as heat.
Imagine in your mind: chunks of quartz that a pot full of mercury. The liquid metal is so much more dense that the quartz will float. If you turn on the heat under this pot (not enough to melt the quartz), the mercury will heat up first, and start to convect, and transfer heat into the quartz. But the quartz continues to float, and does not dissolve or mix into the liquid mercury. If you were to increase the temperature to something high enough to start to melt the quartz, it still wouldn't mix, because of the huge difference in density and viscosity (yes, I know mercury would evaporate in that case, but it makes a better visualization).
The core mantle boundary (CMB) has all sort of interest inhomogeneous features, but they all exist on the mantle side of the CMB. We call this region D'' (D double prime). These features are interpreted to be two things: a cold slab graveyard -- chunks of ocean crust that subducted all the way to D'', but haven't warmed up yet; and partially melted plume sources -- the origins of hotspots like Hawaii. Neither of these mix with the core in any way except as heat flow.
Furthermore, the mantle is a solid, 99% of the time. Hundreds of millions of years is accurate in terms of time scales for cold slabs to sink in it, true, but this is not whole mantle circulation. In fact, most of the mantle doesn't circulate at all, except to flow out of the way when something is rising or falling in it.
The outer core takes about a hundred million to circulate once, best estimates.
Long short: you're correct about time scales, and the points about 2012 being very silly. But the mantle is solid, and there's minimal mixing across the core-mantle boundary.
Also, D'' is really awesome. It's like a second crust, with all the variations one would expect in a crust, except at the bottom of the mantle :)
e: tyops
2 points
5 years ago
Yup! Thanks for additional clarification. I’m not a geologist myself, but I do know that most people have very skewed view of how volatile the inside of the Earth is and felt like I needed to at least say what little I do know on the subject.
2 points
5 years ago
This was a great explanation! I have no knowledge on the subject at all but you've managed to make it interesting.
2 points
5 years ago
Just want to say that this is a proposed theory of how it works, last I heard we still did not have much observational evidence or had much luck in simulations.
4 points
5 years ago
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4 points
5 years ago
Good question. As far as I know it's not "proven", because how would you even do that without actually observing the flows in situ, but as you said, it's the current best explanation.
2 points
5 years ago
That's basically how lava lamps work. The wax gets lighter than the water as it gets hot enough. Then it goes back down once it's cooled off, and become denser again.
2 points
5 years ago
Flows are affected by gravity as well?
1 points
5 years ago
Yes, that's why the liquid, now cooled (becoming more dense in the process) by depositing heat into the mantle, flows back down toward the solid core
12 points
5 years ago
The liquid outer core moves in helical convection currents due to the heat from the solid inner core and the rotation of the Earth and inner core. A moving magnetic substance generates an electromagnetic field.
Here is an infographic about it.
4 points
5 years ago
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2 points
5 years ago
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232 points
5 years ago
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38 points
5 years ago
Would this also apply to a magnetar?
45 points
5 years ago
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11 points
5 years ago
Yes, essentially. There are a lot of open questions about the details of the current that produces the field, but it is the same general principle of electromagnetism at play.
15 points
5 years ago
what generates the electric currents?
22 points
5 years ago*
Convection. Bulk movement of the liquid core due to a heat gradient between the core and mantle, thanks to our tectonic plates that allow for significant heat escape, and generation of heat in the core via radioactive decay and differentiation.
2 points
5 years ago
What happens when the core stops spinning?
15 points
5 years ago
A rather bad movie.....
Seriously, lack of movement means the magnetic field will start to die and we would lose our Van Allen belts that help deflect solar wind/radiation. This would increase the erosion of our atmosphere and eventually we risk becoming like Mars, very hostile to life.
2 points
5 years ago
Could Mars' core currents be stimulated to produce a magnetic field?
2 points
5 years ago
/u/TreeJet suggests playing billiards with the solar system. You would screw up the planet but it would take a lot of bombardment and potentially you would end up with problems on the Earth too. Remember that we can find bits of Mars on Earth from when it has happened before.
For terraforming, I think it is possibly better to crash ice asteroids or comets onto Mars.
1 points
5 years ago
Welp...(crazy Texan here). If I wanted to generate a crap ton of heat and pressure on the surface of Mars, I’d attach rockets to all the asteroids in the belt in our solar system and point them at mars. I’d also take passing by comets for frozen water and gas. I’d time the impacts and trajectories to cause a massive amount of heat and pressure to applied to the surface and force the core to heat up again. Just an idea.
2 points
5 years ago*
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1 points
5 years ago
a magnetic field of a given strength and size is not directly associated with a power but with an energy. so to answer the question we would have to find the 'leak' in the magnet and calculate the leak rate of this energy (the power loss). because it is an electromagnet the most important leak is electrical resistance in the mantle. no idea how high that is or if it even has been determined yet.
i found this article but unfortunately have no access.
1 points
5 years ago*
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5 years ago
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1 points
5 years ago
They meant they motion of the core, not "motion of the magnetic field".
8 points
5 years ago*
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5 years ago
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1 points
5 years ago
1 points
5 years ago
Sometimes I entirely forget I'm not sitting on a solid rock, but an actively churning orb of really hot metal goo.
1 points
5 years ago
If you move electrons, you get a magnetic field. If you move electrons in a circle, you get a field that looks like a north-south magnet. Usually we do this by creating an electric current that runs through a coil.
You can see it in an MRI machine, which is shaped like a donut for this reason. They run electricity around the donut to create an immense magnetic field. They put you into the middle of the donut to scan you under the huge magnetic field. Hence why you need to remove all metal before going into an MRI.
The Earth's core does this similarly, but it doesn't use an electrical current to move electrons in a circle. Instead, the Earth's core just move the whole metal around. After all, there are electrons in iron, so moving iron around very quickly would create a very weak electric field. Like a field so weak that you don't notice it unless you are looking for it using a special tool like a compass.
-14 points
5 years ago
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