5 ms·

Mantle plumes don't significantly change the shape of the core or mantle, though. They're convection within the mantle. They rise from the core mantle boundary,

by jofer 4y ago

Mantle plumes don't significantly change the shape of the core or mantle, though. They're convection within the mantle. They rise from the core mantle boundary, but they're basically temperature features, not structural features. In other words, mantle plumes are parts of the mantle that are hotter than the other identically composed mantle around them. In contrast, the core and the crust are different compositions than the mantle. Just to clear up another common confusion, the mantle is very much solid, except for a tiny fraction of melt in a narrow and shallow zone called the athenosphere. The mantle flows over time despite being solid, though (think of a glacier). Just like a marble slab will bend over time (see benches in old graveyards that sag in the middle), the mantle slowly flows, but a hammer/etc would bounce right off of it. That's also true of large portions of the crust. As far as how smooth or not smooth the actually core boundaries are, we don't really know in detail. To a first order, they're smooth (i.e. we measure a broadly consistent radius from multiple directions), but that doesn't mean they're necessarily a "billard ball". There's likely fairly complex topography at the boundary that we can't easily measure.

hcrisp 4y ago
Does the athenosphere produce surface lava?
- jofer 4y ago
  That's a surprisingly complex question. The best answer is probably "sometimes". The chemistry of volcanic rocks gives a lot of clues as to the origin of the melt. The athenosphere is actually a bit poorly defined in this sense (it's a mechanical classification, not a chemical classification). Regardless, it's easy to distinguish magmas with a pure mantle source from others. MORB (mid ocean ridge basalt) is a common acronym for volcanic rocks with a chemistry that indicates they're essentially pure mantle melt. In the most common type of volcanoes you see on land (arc volcanoes), magma forms due to the introduction of water and other volatiles that lower the melting point of the upper mantle. That's what happens at arc volcanoes like the Cascades or Mt Fuji in Japan. It's not exactly coming from the athenosphere in that case, and it's often the lithospheric mantle and lower crust that are being melted. It's not heat that causes it, but instead the introduction of water. Volcanoes can also form due to the introduction of extra heat, as happens above hotspot volcanoes (e.g. the Galapagos or Hawaii). In many of those cases, you're basically seeing the athenosphere supplying extra heat to the mantle lithosphere and crust and melting it. The magma usually isn't coming directly from the athenosphere in those cases (though some of it can and does). However, flood basalts are an extreme case of hotspots, and their chemical signatures suggest that it's more or less melt directly from the athenosphere. Finally, one of the most common types of volcano on the planet does come directly from the athenosphere: mid ocean ridge systems. (Where the term MORB comes from) Those are almost entirely deep in the oceans, so non-geologists don't think about them as much, but they make up the bulk of volcanic activity on Earth. In those cases, you're essentially bringing the athenosphere up and melting more and more of it as it rises. Those are the cases where magma is most directly sourced from the athenosphere.
  - bcbrown 4y ago
    I'm curious how water would lower the melting point of solid rock. I'm aware that adding ethanol to water lowers the boiling point of the resulting solution, but that's a combination of two liquids. I can't think of a physical mechanism where adding a liquid to hot solid rock results in hot liquid rock. Where does the water come from? Is it liquid, or gaseous?
    - jofer 4y ago
      It's more or less the same way putting salt on ice causes it to melt. Diffusion still occurs with solids, just at a lower rate than with liquids. As for whether it's liquid or gaseous, I probably shouldn't have called it "water", and should have said "hydrogen and oxygen in silicate minerals". Silicates always have oxygen, but some hydrogen too (hydroxyl groups - i.e. OH). Basically, you have minerals that transform to other minerals at depth due to the pressures and temperatures involve. That transformation leads to the release of hydrogen and oxygen as they're in one crystal structure and not in the other. (Basically, minerals with hydroxyl groups transform into minerals that do not have hydrogen in their composition, releasing hydrogen and oxygen in the process.) That hydrogen and oxygen diffuses into adjacent mineral crystals and can cause them to melt. With that said, any time you have magma (i.e. melt), it's going to have H2O, CO2, halogens, etc dissolved in it, just like water has oxygen and carbon dioxide dissolved in it. We talk about these in simple terms of "H2O" and "CO2" etc, but often the details of how things are bonded are a bit different, just like CO2 dissolved in water isn't exactly CO2, but is carbonic acid (H2CO3) instead.
      - bcbrown 4y ago
        Fascinating stuff, thanks for the explanation.
    - pfdietz 4y ago
      Think of how adding water to hot sugar reduces the melting point of sugar. Sufficiently large amounts of water reduces the melting point to room temperature (also known as dissolving the sugar.) https://foodcrumbles.com/sugar-cooking-temperature-stages/ https://foodcrumbles.com/sugar-cooking-temperature-stages/
hguant 4y ago
>They rise from the core mantle boundary, but they're basically temperature features, not structural features. my understanding was that this was not actually a settled topic, and there was ongoing debate as to whether or not they were thermochemical structures, with the main evidence against them being "basically temperature features" the fact that they're not a classic plume shape
- jofer 4y ago
  Yeah, it's far from settled in detail, and for a long time there was a camp that maintained that mantle plumes did not really exist in the normal sense. However, further imaging work seems to show that 1) they actually do seem to be a classic plume shape in many cases (or, more precisely, have complex shapes compatible with convection), and 2) most do have temperature anomalies associated with them. Some things we thought were plume related may not be, but folks are much more in agreement that they look a lot more like convection-related features. E.g. compare these two papers (which are both basically review papers): https://www.sciencedirect.com/science/article/abs/pii/S0012821X02010932 https://www.sciencedirect.com/science/article/abs/pii/S00128... https://www.nature.com/articles/s43017-021-00168-6 https://www.nature.com/articles/s43017-021-00168-6
valarauko 4y ago
> In other words, mantle plumes are parts of the mantle that are hotter than the other identically composed mantle around them. The parent's link suggests that alternate views exist, that the mantle plumes are indeed chemically distinct from the surrounding rock. From the linked article: This suggests that the edges of the blobs mark a transition between materials, not just temperature. In this view, the blobs are so-called thermochemical piles, clumps of dense rock with a distinct chemical composition. Because of their prolonged contact with the core, they are hotter than the rest of the mantle, causing plumes to sprout.
- jofer 4y ago
  Yes, there's a chemical component as well. I was simplifying. Regardless, though, they're not core material rising up. They're still basically olivine+pyroxenes+etc, same as what's around them. They're not the exact same, and indeed they bring deep mantle material up, but they're not nickle-iron rising up from the outer core, which is what I was trying to get across.
ASalazarMX 4y ago
> Just like a marble slab will bend over time (see benches in old graveyards that sag in the middle), the mantle slowly flows I had to see more of this, and it took me a while because most results are about construction and marble products. It looks like thin slabs of marble bow because of microfractures and internal stress, not by flowing. https://link.springer.com/article/10.1007/s00254-008-1307-z https://link.springer.com/article/10.1007/s00254-008-1307-z
- thehappypm 4y ago
  Microfractures and internal stress are a form of flowing in this case.
tenken 4y ago
So what you're telling me is that 1 of my favorite movies, The Core (2003), is waaaay off on Science :D :(
- jofer 4y ago
  Gotta watch out for those amethyst geodes the mantle!
- jjgreen 4y ago
  No, The Core is actually a documentary
- bcraven 4y ago
  I know the science advisor for The Core. He apologises for it regularly.
  - pfdietz 4y ago
    I hope everyone thanks him for his core competencies.
nine_k 4y ago
If you take a real well-played snooker ball, it has plenty of tiny but visible scratches, say, 0.25 mm deep.the ball's diameter is 52.5 mm, so the scratches depth is about 1/200 of its diameter. If we take Earth's diameter as 12750 km, scratches like that would be features about 63 km tall, nearly an order of magnitude larger than tallest mountains on Earth. If course the mantle is smaller diameter, but unless we cannot detect roughly 50 km-tall ridges (I don't know much about seismic wave registration), it's about as smooth as a billiard ball %)

by jofer 4y ago

hcrisp 4y ago

Does the athenosphere produce surface lava?

jofer 4y ago
That's a surprisingly complex question. The best answer is probably "sometimes". The chemistry of volcanic rocks gives a lot of clues as to the origin of the melt. The athenosphere is actually a bit poorly defined in this sense (it's a mechanical classification, not a chemical classification). Regardless, it's easy to distinguish magmas with a pure mantle source from others. MORB (mid ocean ridge basalt) is a common acronym for volcanic rocks with a chemistry that indicates they're essentially pure mantle melt. In the most common type of volcanoes you see on land (arc volcanoes), magma forms due to the introduction of water and other volatiles that lower the melting point of the upper mantle. That's what happens at arc volcanoes like the Cascades or Mt Fuji in Japan. It's not exactly coming from the athenosphere in that case, and it's often the lithospheric mantle and lower crust that are being melted. It's not heat that causes it, but instead the introduction of water. Volcanoes can also form due to the introduction of extra heat, as happens above hotspot volcanoes (e.g. the Galapagos or Hawaii). In many of those cases, you're basically seeing the athenosphere supplying extra heat to the mantle lithosphere and crust and melting it. The magma usually isn't coming directly from the athenosphere in those cases (though some of it can and does). However, flood basalts are an extreme case of hotspots, and their chemical signatures suggest that it's more or less melt directly from the athenosphere. Finally, one of the most common types of volcano on the planet does come directly from the athenosphere: mid ocean ridge systems. (Where the term MORB comes from) Those are almost entirely deep in the oceans, so non-geologists don't think about them as much, but they make up the bulk of volcanic activity on Earth. In those cases, you're essentially bringing the athenosphere up and melting more and more of it as it rises. Those are the cases where magma is most directly sourced from the athenosphere.
- bcbrown 4y ago
  I'm curious how water would lower the melting point of solid rock. I'm aware that adding ethanol to water lowers the boiling point of the resulting solution, but that's a combination of two liquids. I can't think of a physical mechanism where adding a liquid to hot solid rock results in hot liquid rock. Where does the water come from? Is it liquid, or gaseous?
  - jofer 4y ago
    It's more or less the same way putting salt on ice causes it to melt. Diffusion still occurs with solids, just at a lower rate than with liquids. As for whether it's liquid or gaseous, I probably shouldn't have called it "water", and should have said "hydrogen and oxygen in silicate minerals". Silicates always have oxygen, but some hydrogen too (hydroxyl groups - i.e. OH). Basically, you have minerals that transform to other minerals at depth due to the pressures and temperatures involve. That transformation leads to the release of hydrogen and oxygen as they're in one crystal structure and not in the other. (Basically, minerals with hydroxyl groups transform into minerals that do not have hydrogen in their composition, releasing hydrogen and oxygen in the process.) That hydrogen and oxygen diffuses into adjacent mineral crystals and can cause them to melt. With that said, any time you have magma (i.e. melt), it's going to have H2O, CO2, halogens, etc dissolved in it, just like water has oxygen and carbon dioxide dissolved in it. We talk about these in simple terms of "H2O" and "CO2" etc, but often the details of how things are bonded are a bit different, just like CO2 dissolved in water isn't exactly CO2, but is carbonic acid (H2CO3) instead.
    - bcbrown 4y ago
      Fascinating stuff, thanks for the explanation.
  - pfdietz 4y ago
    Think of how adding water to hot sugar reduces the melting point of sugar. Sufficiently large amounts of water reduces the melting point to room temperature (also known as dissolving the sugar.) https://foodcrumbles.com/sugar-cooking-temperature-stages/ https://foodcrumbles.com/sugar-cooking-temperature-stages/

hguant 4y ago

>They rise from the core mantle boundary, but they're basically temperature features, not structural features. my understanding was that this was not actually a settled topic, and there was ongoing debate as to whether or not they were thermochemical structures, with the main evidence against them being "basically temperature features" the fact that they're not a classic plume shape

jofer 4y ago
Yeah, it's far from settled in detail, and for a long time there was a camp that maintained that mantle plumes did not really exist in the normal sense. However, further imaging work seems to show that 1) they actually do seem to be a classic plume shape in many cases (or, more precisely, have complex shapes compatible with convection), and 2) most do have temperature anomalies associated with them. Some things we thought were plume related may not be, but folks are much more in agreement that they look a lot more like convection-related features. E.g. compare these two papers (which are both basically review papers): https://www.sciencedirect.com/science/article/abs/pii/S0012821X02010932 https://www.sciencedirect.com/science/article/abs/pii/S00128... https://www.nature.com/articles/s43017-021-00168-6 https://www.nature.com/articles/s43017-021-00168-6

valarauko 4y ago

> In other words, mantle plumes are parts of the mantle that are hotter than the other identically composed mantle around them. The parent's link suggests that alternate views exist, that the mantle plumes are indeed chemically distinct from the surrounding rock. From the linked article: This suggests that the edges of the blobs mark a transition between materials, not just temperature. In this view, the blobs are so-called thermochemical piles, clumps of dense rock with a distinct chemical composition. Because of their prolonged contact with the core, they are hotter than the rest of the mantle, causing plumes to sprout.

jofer 4y ago
Yes, there's a chemical component as well. I was simplifying. Regardless, though, they're not core material rising up. They're still basically olivine+pyroxenes+etc, same as what's around them. They're not the exact same, and indeed they bring deep mantle material up, but they're not nickle-iron rising up from the outer core, which is what I was trying to get across.

ASalazarMX 4y ago

> Just like a marble slab will bend over time (see benches in old graveyards that sag in the middle), the mantle slowly flows I had to see more of this, and it took me a while because most results are about construction and marble products. It looks like thin slabs of marble bow because of microfractures and internal stress, not by flowing. https://link.springer.com/article/10.1007/s00254-008-1307-z https://link.springer.com/article/10.1007/s00254-008-1307-z

thehappypm 4y ago
Microfractures and internal stress are a form of flowing in this case.

tenken 4y ago

So what you're telling me is that 1 of my favorite movies, The Core (2003), is waaaay off on Science :D :(

jofer 4y ago
Gotta watch out for those amethyst geodes the mantle!
jjgreen 4y ago
No, The Core is actually a documentary
bcraven 4y ago
I know the science advisor for The Core. He apologises for it regularly.
- pfdietz 4y ago
  I hope everyone thanks him for his core competencies.

nine_k 4y ago

If you take a real well-played snooker ball, it has plenty of tiny but visible scratches, say, 0.25 mm deep.the ball's diameter is 52.5 mm, so the scratches depth is about 1/200 of its diameter. If we take Earth's diameter as 12750 km, scratches like that would be features about 63 km tall, nearly an order of magnitude larger than tallest mountains on Earth. If course the mantle is smaller diameter, but unless we cannot detect roughly 50 km-tall ridges (I don't know much about seismic wave registration), it's about as smooth as a billiard ball %)