Sand is just a particular grain size for a material, but for a rock having been broken into sand sized particles reflects that it has experienced a combination of weathering and erosion. No rocks (at the surface) are immune from weathering or erosion, but some minerals that make up rocks are less stable at surface conditions and so will not persist as sediment (sand sized or otherwise) for too long, though when thinking about geologic time, a short time can still be hundreds of thousands to millions of years. Minerals that easily dissolve in water or weak acids (e.g., halite, calcite, etc) or generally tend to react easily at surface conditions and form other minerals (e.g., olivines, pyroxenes, etc) are all components of rock that you wouldn't expect to make up large components of your average sand. But there are definitely exceptions, e.g., sand formed from the weathering of basalts can be predominantly olivine and pyroxene. It's more that all told, these are not particularly stable minerals at the surface so these don't make up much of sand on a global scale.

This is addressed quite comprehensively in an existing FAQ.

The actual article as opposed to the press release (what you linked) does briefly talk about it (in the first paragraph of their discussion), but mostly it's cited out to prior literature. Specifically, as discussed by papers like Davies et al., 2021, in their words, "Cooling of the liquid core leads to freezing at Earth’s centre and the growth of the solid inner core, which provides additional power to the dynamo through release of latent heat and gravitational energy" and they in turn point to thermodynamic simulations that demonstrate this (e.g., Gubbins et al., 2004). Details of core geodynamics as it relates to the magnetic field is a bit out of my specialty, so I'll leave further discussion/explanation to folks with more domain experience, but it's not as though the articles presenting this data do not discuss the mechanism at all.

To add to the clarification by /u/PyrrhoTheSkeptic that what you're describing is the absence of heat in the surrounding soil (and thus heat within your basement is "flowing" into the surrounding soil via conduction), neither soil or rock are great heat conductors (i.e., they generally have low thermal conductivity). What this means, is that it takes a while for the temperature of the soil/rock at even a shallow depth to change after a change in surface temperature. Observations suggest that if, for example, you consider either diurnal (i.e., day-night) or seasonal oscillations in temperature, the amplitude of these oscillations decrease exponentially with depth (e.g., Elias et al., 2004). In other words, even though the air temperature may be warm or cold (and oscillate between them), the soil temperature at a few meters down will be more constant. You can see in data of very shallow soil temperature (e.g., Holmes et al., 2008) that you do see things like diurnal temperature variation in the upper most few cm, but within >15-20 cm, the magnitude of diurnal temperature variations of the soil are extremely small.

It's worth noting that if you go deep enough (few 10s of meters) that the temperature of rocks ceases to be influenced by either diurnal or seasonal surface temperature variations and is instead controlled by the local geothermal gradient.

Heat from radioactive decay (primarily of uranium, thorium, and potassium) is an important component of the internal heat budget. These elements are the most abundant in the crust, but they are also present in the mantle and given the size of the mantle, even at low concentrations, they end up generating significant heat.

Only the outer core is liquid.

The short answer is probably not, but it's complicated and any potential effects depend a lot on the details (i.e., where is the water, how much water, which faults, etc). We might expect some change in the statistics of microseismicity, but the likelihood that this would influence details of large magnitude earthquake is low.

For the longer answer, we need to establish that changes in surface loads from water (either rain or snow) and changes in the amount of water in the shallow subsurface (i.e., groundwater) can have an influence on earthquake statistics and details. There's not a single mechanism at play here though, so things get a bit messy. Some of the culprits at play are changes in: (1) primarily vertical normal stress magnitude from changes in surface water mass, (2) changes in strain rate from longer wavelength elastic responses to changes in water mass, and (3) various "poroelastic" effects that relate to more or less water within pore spaces at various levels within the upper crust.

The easiest one to wrap our heads around are the first, i.e., changes in normal stress. Basically if you have a shallowly angled fault, putting more weight on it will actually make it less likely to fail because, in a simplistic way, you're increasing the friction. For example, in portions of the Himalaya, there appears to be excess seismicity in the winter, with the idea that large loads imposed by the summer monsoons increase the normal stress on faults making them less likely to fail in the summer (e.g., Bollinger et al., 2007, Panda et al., 2018). Similar things are seen in Taiwan, though the signal is a bit messy (e.g., Hsu et al., 2021). Alternatively, in Japan, the opposite is found with some areas potentially experiencing more earthquakes in summer with the normal stress provided by snow suppressing earthquakes in the winter (e.g., Heki, 2003). Given this, we'd kind of think that heavy rains (i.e., more water mass) would actually decrease the likelihood of earthquakes, but critically, these examples are mostly focused on shallowly angled faults (i.e., shallowly dipping dip-slip faults), whereas in California, most of the faults of concern are close to vertical.

With reference to California, we've recognized that there is seasonal modulation of earthquake statistics (mostly for microseismicity, i.e., small earthquakes) thought to be related to changes in both surface and grounwater mass and related effects, i.e., those long-wavelength elastic and poroelastic ones mentioned above (e.g., Johnson et al., 2017). A variety of follow up work has filled in details (e.g., Kreemer & Zaliapin, 2018, Kim et al., 2020, Carlson et al., 2020), which highlight that the potential earthquake response depends on where the change in the water mass is in relation to specific faults and the type of faults. Heavy precipitation, depending on where it is, can make earthquakes slightly more likely for some faults and less likely for others depending on the details. Also, as discussed in some of the papers, some of these effects can have a significant (several month) time lag. Mostly again, this is considering small earthquakes and changing statistics not total energy release, but there are some indications that some of these water mass related changes could allow for larger earthquakes (e.g., Kreemer & Zaliapan), but this specific suggestion was for when faults in a particular location were experiencing fault normal extension due to an elastic response to less water.

Ultimately, what this suggests is that, especially given the large amounts of both snow and rain, this added water mass will likely influence some aspects of earthquake statistics, but the likelihood that this directly relates to a large earthquake is pretty low. Also, importantly, we don't generally understand these systems well enough to use this information to make useful or actionable forecasts for the details of changes in earthquake statistics we might expect, but I fully expect there will be papers in the coming years considering what effect these storms had on details of crustal strain and impacts of those changes in crustal strain.

> it's probably a large mix of metals but it's probably the heaviest metals in the inner core right?

Actually, no. The core is predominantly iron with a smaller amount of nickel (and some other stuff, more on that in the next section), which while both dense, are certainly not the most dense metals that exists on Earth and in fact, many significantly more dense metals tend to be concentrated in either the crust or mantle as opposed to the core. The reason for this largely relates back to the early formation of rocky planets (and here most of my answer will focus on Earth, but this is broadly applicable to rocky planets more generally). During planetary differentiation, there are two primary ways by which materials separated, physically (i.e., mostly on the basis of density) and chemically. For the chemical differentiation aspect, it's useful to consider the Goldschmit classification of the elements. Regardless of their density, generally lithophile elements, which are those that easily combine with oxygen, and chalcophile elements, which are those that easily combine with sulfur and a few other elements, were incorporated into the silicate part of the Earth and thus remained in the mantle and crust. As examples, very dense metals like uranium and lead are both thought to generally be in very low (to zero) concentrations in the core. This is because uranium is a lithophile and lead is a chalcophile so both are generally concentrated in the crust and mantle (not to mention that a non-trivial component of lead results from the decay of uranium and thorium, both lithophiles, after differentiation). Siderophiles were those that easily dissolved in iron and thus ended up primarily in the core. The density driven portion of differentiation provided the main division between the denser, inner iron-nickel core and the less dense, outer silicate portion of the Earth, but whether a particular element ended up in the silicate portion or the core came down to the individual chemical properties of the element in question, i.e. was it more likely to bond or dissolve in a silicate melt vs an iron melt.

> Not sure if it would make a tough alloy or something.

As discussed above, the core is predominantly iron with a small amount of nickel (constrained to being around 5%), so usually described as an iron-nickel alloy. However, we know from a variety of different datasets that the density of the core is actually less than what you'd expect for pure iron or a 95-5% iron-nickel alloy (and that various other properties, mostly related to how seismic waves pass through it are similarly not consistent with a pure iron or a pure iron-nickel alloy) and that the core must include some amount of a light element or several light elements. As highlighted in the review by Hirose et al., 2013, on the basis of abundances (i.e., what elements were present) and their ability to partition into the core during planet formation, we hypothesize that these light elements are silicon, oxygen, sulfur, carbon, and/or hydrogen. In terms of the properties of the resulting alloy, a lot depends on which one of these (or which mixture of these) are actually present in the core. The Hirose review goes through some of the details of specific two-component alloys (e.g., Fe-C, Fe-Si, etc) from high pressure/temperature experiments, but for some of these it's actually pretty challenging to get them to alloy with iron given the conditions we can and cannot simulate in experiments. Checking in on a more current review by Hirose et al., 2021 (pdf or a preprint of this article here), we find the situation pretty much the same, i.e., we still think that the core needs some light elements, the list of the possible ones are the same, and we still don't really know which ones are the right ones within that list. What this new review does provide is updated indications of just how much of different elements might be present. These have ranges of uncertainties, but most max out at ~1-5%, but it varies by element and by the way the estimate is derived. The extent to which any of these alloys would be "tough" is a bit unclear since (1) that's not exactly a clear property, (2) we don't know the exact composition, and (3) it's hard to get materials up to the relevant temperature and pressures to do detailed studies of the material properties in the same way we would for an alloy that's stable at surface temps and pressures.

EDIT: I'll add that we can learn some details about the cores of rocky planets from the study of iron meteorites, which are generally thought to be chunks of differentiated bodies that were destroyed during the early history of the solar system. Since they're no longer at core temperatures and pressures, the exact properties of these are a bit different than what you'd expect if they were at core temperatures and pressures, but they definitely inform a bit on composition. I'll also highlight the upcoming Psyche NASA mission, which is going to visit the 16-Psyche asteroid, which is might be a large chunk of a left over core of a planetesimal.

It's real. Snow is significantly less dense than liquid water, so for an identical mass of liquid water, the volume of equivalent snow mass will be much greater than the volume of water. The density, and thus the difference between the amount of snow and the equivalent amount of water (i.e., the snow water equivalent or SWE), is a function of temperature, e.g., this discussion. Generally, as temperature decreases, snow volume for the same SWE goes up, for example the graph on that page highlights that 1 inch (~2.5 cm) of rain can equal ~100 inches (~250 cm) of snow when the temperature is -40 to -21 F (-40 to -29 C) but would only equal ~10 inches (~25 cm) at 28 to 34 F (-2 to 1 C). Importantly, none of these are accounting for compaction of snow after it falls, where the density of snow can increase significantly with increasing accumulation.

No.

First and foremost, this ignores that in calculating hypothetical sea level rise from complete melting of the Antarctic ice sheet, ice below sea level is already removed from consideration. This is explained relatively well on this page that goes through the calculation of sea level rise equivalence. So, the value of ~60 meters of sea level rise that would result from melting of the Antarctic reflects the volume of ice (converted to water and spread over the ocean surface) above sea level.

Secondly, if anything this value represents an underestimate because it does not account for isostatic rebound. In short, as the ice melts, the land underneath the ice will rise up in response to the reduction of mass above it. As such, we would expect that as ice melted, the ground surface would elevate in response and thus a good portion of the ice that is currently below sea level (because the ground has subsided) and which is not factored into the calculuation, would actually contribute to sea level rise because it would no longer be below sea level when it melted. For a more thorough discussion of what Antarctica without ice might look like, you can check out this previous thread.

Finally, the sea level rise equivalents are pretty much only dealing with mass and not considering the steric components of sea level rise (i.e., the changes in sea level rise due to changes in density related to either temperature or salinity). Melting all of Antarctica would have a complicated effect in this regard. It would directly reduce density (and thus reflect expansion, so more sea level rise) through freshening, but adding all of that cold water could temporarily reduce temperature (at least regionally) increasing density (and thus reflect contraction, so less sea level rise). Presumably though, to be able to melt all of Antarctica would reflect relatively high average air and ocean temperatures, so this cooling would likely be temporary so you would have to also account for potential thermal expansion after all that melt water had a chance to heat up to whatever the average ocean temperature was at this hypothetical time.

In large part, because plate tectonics and plate motion primarily reflect the dynamics of oceanic lithosphere (e.g., Crameri et al., 2019). Within this context, it's good to have a handle on what's driving plate motion, so if anyone needs a refresher, I'll refer you to our existing FAQs on that subject. It's also worth considering that even without any specific mechanism to expect more plate boundaries in oceanic lithosphere vs continental lithosphere (which in fact there are, as we'll cover in a bit), just by virtue of the 70-30 split between oceans and continents, we would kind of guess that there would be more boundaries in the ocean. If we take a tour of the three types of plate boundaries (convergent, divergent, and transform), we'll see that there are in fact a variety of mechanistic reasons for most of these, by definition, to involve oceanic lithosphere. As a result, even ignoring the differences in surface areas between ocean and continents, we would generally expect that a large proportion of plate boundaries themselves would be in or near the ocean.

1. Convergent Boundaries: By far, the largest representation of convergent boundaries are subduction zones and in fact, subduction (and the driving negative buoyancy of subducted slabs) represents the largest force driving plate tectonics writ large. Because subduction is generally limited to oceanic lithosphere (because of the necessary density contrast for subduction to occur and be sustained and its relation to the composition of oceanic lithosphere with respect to continental lithosphere), at least one section of the plates involved (i.e., the subducting portion) is going to be oceanic. Additionally, because subduction requires a strong negative buoyancy of the downgoing lithosphere, this tends to ensure that the average elevation/depth of these regions are sufficient to be underwater (and also are the deepest locations in the oceans). Now of course, there are convergent boundaries on land, i.e., continent-continent collisions like the Indo-Asian collision forming the Himalaya and associated ranges or the Arabia-Eurasia collision forming various mountain ranges between Turkey and Iran. These however reflect former subduction zone boundaries and, geologically, are temporary as there are a variety of forces that actively resist continued convergence as the associated mountain ranges grows and/or the lingering subducted slab of oceanic lithosphere is removed. Individual subduction zones themselves do not persist forever, but they tend to be active for longer, on average, than a given continent-continent convergence system.
2. Divergent Boundaries: Again, the largest representation of divergent boundaries, mid-ocean ridges, by definition involve oceanic lithosphere because they are producing oceanic lithosphere. There are continental rifts, e.g., the East African Rift, but as continental rifts evolve and accommodate more and more extension, the continental lithosphere becomes thin enough that the rift starts become low enough to be flooded by ocean waters and begin forming oceanic lithosphere (transitioning from a continental to oceanic rift), and start to produce a new ocean basin. So again, continental rifts have relatively geologically short lifespans as continental rifts transition into oceanic rifts and begin producing oceanic lithosphere. As an aside, not all continental rifts end up as oceanic rifts, some fail (i.e., they become Aulacogens) and some are never localized enough to transition to an oceanic rift (i.e., "wide rift zones" like the Basin and Range).
3. Transform Boundaries: There are not the same mechanistic drivers for transforms to be preferentially in the ocean as there are for the large classes of convergent (subduction zones) or divergent (mid-ocean ridges) boundaries, but in detail most transform boundaries are also in the ocean because they occur as oceanic transforms between mid-ocean ridge segments (e.g., this diagram). Because transform faults (as opposed to transcurrent strike-slip faults) "transform" motion from either two convergent, two divergent, or mixtures of convergent and divergent, there is sort of a driver for these to be in or near the ocean since so many of the boundaries they link are in the ocean and involve oceanic lithosphere. There are however some other (somewhat related) considerations for why many transform boundaries might be near oceanic lithosphere. One option is that it reflects the tectonic history. For example, the San Andreas fault system, i.e., a transform boundary between the Pacific and North American plate, is in part so close to the coastline because it reflects a history of transition from a subduction to transform boundary. Similarly, many plate boundaries are not perfectly divergent or convergent or transform, but rather have "oblique" motion because the motion of the two plates that meet are not perfectly orthongonal or parallel to the boundary. However, instead of a single fault with a combination of dip-slip and strike-slip motion along a single boundary fault, we often see "strain partitioning", where oblique motion between two plates is split between a dip slip fault and a closely related strike-slip fault. An example of this would be the Great Sumatra Fault, which is part of the plate boundary zone in this region and is effectively tied to the subduction zone that lies just offshore.

The force is driven by the negative buoyancy of the slab. If the slab detaches, there is no more driving force for the portion of the plate on the surface whereas the slab continues to sink. A simple analogy would be a weight clipped to the edge of a floating mat. If the mat rips, the portion attached to the weight will sink but the rest of the mat will just sit there (assuming it is buoyant). This is expanded on in much more detail in a top level answer I made within this thread to try to address the relative incompleteness of the specific top level comment that everyone is upvoting.

There is no single answer and it depends on what the nature of the lithosphere of the "following" plate is and/or the geometry of the boundaries between the subducting plate and the "following" plate. Some options are:

1. Plate that follows has oceanic lithosphere with a mid-ocean ridge between the subducting and following plate and subduction ceases before the ridge reaches the subduction zone. Effectively the idea is that subduction is driven by the negative buoyancy of the subducted slab, which is a function of the age/temperature of the slab. The piece of lithosphere adjacent to an active ridge is pretty warm, young, and positively buoyant so it will resist subducting. Depending on the relative competition of forces what may happen is that subduction slows down as this young lithosphere approaches the ridge (resisting subduction) and then the slab rips off (i.e., it detaches) because the slab pull force overcomes the strength of the slab nearer the surface. This can effectively terminate subduction (no slab pull = no subduction). As to what happens from there, it will depend on the specific forces, but most likely the ridge might die and there will be a general reorganization. That reorganization might see a wholly different set of plate boundary kinematics or the subduction zone might "jump", keeping effectively similar broad scale kinematics but with the subduction zone in a different place. It might also jump and reverse polarity. Or it might transition into a new type of boundary depending on the kinematics of the plates that meet. This option is quite common if the ridge is roughly parallel to the subduction zone (e.g., Burkett & Billen, 2009). Semi-parallel ridge subduction does happen though, and for it to happen, usually some amount of complicated geometries and "3D effects" are required (e.g., Burkett & Billen, 2010).
2. Plate that follows has oceanic lithosphere with a mid-ocean ridge between the subducting and following plate and subduction ceases after the ridge subducts. A geodynamically unlikely option, but assuming the ridge is roughly parallel to the subduction zone, this would also lead to slab detachment and cessation of subduction and reorganization depending on the kinematics of the two plates that now meet.
3. Plate that follows has oceanic lithosphere with a mid-ocean ridge between the subducting and following plate and subduction continues after the ridge subducts. This is relatively common if the ridge is very oblique or orthogonal to the subduction zone. In this scenario, the ridge will subduct and in many cases a "slab window" will open along the subducted segment of the ridge. You can picture the ridge effectively unzippering down the length of the subduction zone, kind of like this. This makes some specific predictions about what you would see in the upper plate, specifically a gap in normal arc volcanism and instead magmatism that is more indicative of direct mantle interaction with the upper plate rocks.
4. Plate that follows has continental lithosphere. This would largely require a plate with subduction zones "across" from each other and at the moment that the two subduction zones meet, the result will depend on the nature of the adjacent section of the other overriding plate (is it continental or oceanic) and the relative motion between the two plates that meet. If the other overriding plate is oceanic and the kinematics favor it, subduction might continue via a polarity flip where the formerly overriding plate becomes the subducting plate. Instead, subduction might cease and the boundary might change kinematics (e.g., become a transform boundary).

This presumes that the plate that "follows" is subductable or if it is subductable (i.e., it's oceanic) that the boundaries between the fully subducted plate and the following plate are such that allow for continued subudcution. None of those conditions are guaranteed and in fact with reference to the latter consideration, we very often see cessation of subduction when a mid-ocean ridge (i.e., the boundary between two oceanic portions of a plate) approaches or reaches a subduction zone.

One could come up with a lot of issues, but I'll focus on three:

1. If we take an extremely simplistic view, faults are large but extremely narrow, i.e., effectively single fracture planes extending tens of kilometers into the crust. As such, the logistics of somehow injecting material to depths along these planes (which typically don't have an aperture in a normal sense) significantly below the deepest we've ever drilled are challenging (to put it mildly).

2. Faults represent locations where sufficient stress existed to break rocks and then to cause slip along the fracture plane (i.e., the fault). Generally it requires less total/differential stress to have movement along an existing fracture than forming a fracture, so if we sidestep the logistical impossibility from above, if we were able to do this, at best you've increased the stress necessary to reform the fault (compared to have slip on the fault) that you sealed, but if the stress exists, the rocks will just break again eventually. This also largely ignores the existence of extensive damage zones around faults (e.g., Kim et al., 2004). In detail, the fault planes themselves are surrounded by halos of "damaged" i.e., fractured and otherwise weakened, rock with the width of the damage zone being proportional to the amount displacement accommodated by a fault. If we're considering a large, plate boundary scale fault, the damage zone might be hundreds of meters to several kilometers wide. So in some hypothetical where we were able to "seal" the fault plane itself, the fault would likely just reorganize within the damage zone as this would be weaker than the "sealed" fault or intact rock outside the damage zone, assuming the stress still exists. Which brings us to our final point.

3. Faults are manifestations of differential motion between portions of the crust and upper mantle, driven by differential stress resultant from plate motion. If we assume that we could somehow overcome the logistical challenges in the first point, and even if we were able to maybe somehow "seal" the entire damage zone discussed in the second point, none of this changes the driving stresses. As a simple analogy, if you tear a piece of paper and then "seal" the rupture with some tape and then try to tear the paper again, it's not as though the paper won't just rip again (even if you do a great job with your tape and it doesn't reoccupy the same tear, some other part of the paper will tear instead). As long as a stress is being applied that overcomes the strength of a portion of the material, a fracture will form to accommodate the differential stress and motion. Thus, if we assumed we could "seal" faults, this wouldn't really do anything unless you somehow also stopped plate tectonics.

> I went to Ireland and visited a beach. I took home a rock as an souvenir of my trip. I live in America, and I wonder, are the rocks in Ireland different from American rocks?

It depends on where you were in Ireland and, since you're dealing with a rock you picked up from a deposit of loose sediment, where in Ireland the particular rock you picked up came from. Like most places, Ireland has a variety of rocks of different ages and geologic histories that are exposed (e.g., the wiki page on the Geology of Ireland). Depending on where in Ireland your rock came from (and where in America you're comparing it to, as similarly, there are lots of different ages, compositions, and histories of rocks in North America), it might be quite different or it might be pretty much the same. For example, portions of what now is North America (i.e., Laurentia) and Ireland (i.e., Avalonia) were joined during the Caledonian Orogeny. There are formations (i.e., packages of rocks of the same age and similar lithology which represent deposition in a single largely continuous depositional environment in a particular area) which appear in New England and Ireland (and elsewhere), e.g., the Old Red Sandstone.

At a more basic (and slightly pedantic) level, it also depends on how specific you want to be and how you want to define "the same." Let's consider a sandstone from Ireland and one from America. If you keep it super shallow and stick with basic rock type, then for sure you can find the "same" rocks all over the world, i.e., a sandstone is a sandstone regardless of location. As you get more specific, things will narrow as you consider differences in grain size (e.g., coarse sandstones are not the same as medium sandstones) or details of composition (e.g., arkoses are not the same as quartz arenites). If you keep getting more specific, you could start considering do these two sandstones have the same age and source, i.e., do they represent effectively the same original deposit. All of these definitions of two rocks being the same would be appropriate depending on the context. For this last, most strict type of "sameness", let's consider the other part of your question.

> Like if I ran science tests, could the rock be determine it's not from America. Or all rocks, just rocks.

For simplicity, I'll keep my focus on sedimentary rocks, and then this would broadly fall under the umbrella of sediment provenance. There are a wide variety of characteristics of a sedimentary rocks (e.g., age populations of detrital minerals, sand composition, and a variety of geochemical proxies) that can be used to "fingerprint" the source of these rocks and thus establish individual sedimentary rocks as semi unique. Being able to identify a particular sedimentary rock through provenance analysis depends on the details of the rock in question, i.e., some may be more unique than others and easier to fingerprint. Because of plate tectonics, most landmasses have complicated histories where currently disparate bits may have been in close proximity (like the example with the Caledonian orogeny and the Old Red Sandstone), so it's not as though all Irish rocks will be distinctively Irish or something, but rather rocks of different ages will reflect the plate tectonic configurations and depositional environments at the time they were formed and thus will have "affinities" with rocks in different places.

If you want a deep dive on the gross misunderstanding around the paper that sparked this perception, you could check out this thread.

It's an interesting question, but unfortunately one that's not going to have a definitive or single answer. The short version is that you expect a lot of variability in flow velocity during a flood where the velocity in any given area (or with depth) will depend on the discharge (i.e., the volume of water moving through the system) but also critically the "roughness" of the contacted area. For a bit of a treatment on this from a fluid dynamics perspective, you could consider things like Pang, 1998. Roughness always play a role in flow velocity in rivers, but compared to periods where the flow is constrained below "bankfull width", during floods roughness can vary a lot both from natural elements (i.e., as flow moves up the walls of the bank and potentially over natural/manmade levees) but also from flows encountering all sorts of "objects", i.e., trees, other vegetation, and a whole host of things in the built environment (roads, poles, houses, etc.). You can see some of the extreme variations in relationships between depth and velocity estimated from modeling flood flows in built environments in papers like Kreibich et al, 2009. Another aspect of this is that this actually something that's pretty challenging to measure because floods are not necessarily the easiest or safest time to go measure flow velocity (and it may often destroy or damage more autonomous instrumentation deployed to track flow velocity). This is discussed a bit in papers like Tauro et al., 2016, which is attempting to use image techniques to estimate flow velocities during floods. This highlights some of the challenges in empirical measures of flood velocities, but also demonstrates the significant spatial variability in velocities during floods (as estimated from their techniques).

https://www.reddit.com/r/askscience/wiki/planetary_sciences/climate_change_earthquakes/

A form of this question is asked after virtually every large earthquake that makes the news, so I'm going to keep my answer generic with the futile hope that I can put this in our FAQs and maybe retire this question. To first cover the underlying geology and earthquake behavior, there are specific processes by which one earthquake can trigger others. This is discussed in more detail in one of our FAQs, but in short we can consider either static or dynamic triggering. Static triggering is where the permanent movement of the crust that results from a particular earthquake changes the stress state on neighboring faults pushing some of them closer (or past) failure through stress transfer. Static triggering occurs over a very limited distance (roughly X km away from anywhere along the portion of the fault that ruptures where X is the total length of the original rupture). Dynamic triggering is where passing seismic waves, and the temporary change in stress they induce, causes a portion of a fault to fail. Dynamic triggering can occur over long (i.e., teleseismic) distances, but it tends to mostly be associated with very large magnitude earthquakes as the generative event, is pretty rare, is temporally limited (i.e., we only consider dynamic triggering to be possible over a narrow time window after the original event), and is hard to demonstrate. Given the above, when this question gets asked, i.e., "There was a big earthquake in location X and then there were moderate magnitude earthquakes in distant locations in the days, weeks, or months following, are they related?" there is a vanishingly small probability that one or more of those events may be a dynamically triggered event, but the overwhelmingly vast majority of the time, the answer is firmly and unequivocally, "No, they are not related".

So what's going on and why is this question asked so often? Mostly cognitive biases. Specifically, some mixture of the frequency illusion and the clustering illusion. The frequency illusion (or Baader–Meinhof phenomenon) is basically the tendency for your brain to take note of similar events after you become aware of an event. So, a large magnitude earthquake hits a populated area and makes the news and for some period after that you (and the news media more broadly) take note of other even moderate magnitude events. This also brings in the second bias, i.e., the clustering illusion, or the tendency for us to see patterns in stochastic (random) things. Within this context, it's worth considering just how many earthquakes of a given magnitude there are, e.g., the global statistics from the USGS. You'll notice a rough logarithmic behavior in these, i.e., for M8+ we expect about 1 a year, for M7-7.9 ~15/year, for M6-6.9 ~150/year, for M5-5.9 ~1500/year, and so on. So in a scenario where one of the ~15 7-7.9 events happens in a populated place and you notice a tiny fraction of the hundreds to thousands of 6-6.9 or 5-5.9 magnitude events we expect every year that also occur in a different populated place (i.e., excluding aftershocks from the original event) in the following days/weeks, does that mean anything or imply any linkage? No, almost always, it represents nothing other than you paying attention to a small fraction of events because you are primed to and seeing patterns that aren't there.

There were simultaneous eruptions of Tavurvur and Vulcan (this is discussed in this Wiki link). Importantly, these (and another volcano) all represent different vents that are part of a single system, so it it definitely analogous to the Mauno Loa and Kilauea example in a sense (though I don't know that much about the detail vent system or structure for the Rabaul Caldera and associated vents). Importantly (and as discussed a bit in the Gonnermann paper I linked in the original answer), there is always the potential that if the influx of melt into a magmatic system is significant enough, you can definitely get simultaneous eruptions in adjacent vents (i.e., the mechanism that potentially drives eruption in one or another vent they present can be overwhelmed if there is just a lot of magma).

The idea that there are remote areas where even the lower troposphere is sufficiently well mixed enough that sampling in one location represents a reasonable approximation of a global average is the whole concept behind the Mauna Loa Observatory and Keeling Curve.

Because (1) potential disasters brings clicks and thus ad revenue and/or (2) Earth Science is a subject that typically takes a back seat to the "big three" STEM fields of biology, chemistry, and physics, so even at the lay level (i.e., whatever science folks get in primary school or college), people tend to be incredibly misinformed or completely unaware of even basic aspects of how the Earth systems works.

To clarify, for either earthquakes or volcanoes, there is effectively no mechanism for triggering either type of event along significant portions of the various plate boundaries rimming the Pacific (i.e., the "Ring of Fire"). There are very specific and limited mechanisms by which earthquakes and volcanoes can trigger each other, especially over large distances, but these are exceedingly rare, require very specific preconditions, and are very hard to demonstrate even if they operate.

Many of these events (e.g., the Medieval Climate Anomaly or the Roman Warm Period, etc.) are better described as "patchy" as opposed to global. Both the MCA and RWP were the most intense in the northern hemisphere, and specifically in portions of western Europe. This does not mean that there were no effects elsewhere, but these were generally less extreme and the global effects were (1) asynchronous, (2) differed in magnitude, and (3) sometime even differed in sign (i.e., in some places the MCA or RWP represented anomalously cool periods). This is discussed in lay terms moderately well in this Skeptical Science post, but it's a bit dated. There are however more recent papers highlighting the same point (e.g., Neuokm et al., 2019). Thus, in the context of comparing events like the MCA or RWP to modern conditions, they break down pretty quickly because in the modern anthropogenically induced warming, we consistently see rapid warming pretty much everywhere globally and synchronously (and specific to the New Zealand example, follow up work - e.g., Lunning et al., 2019 - has specifically highlighted that while the MCA is recognizable in records through much of Oceania, it's not exactly synchronous, appearing to occur up to several hundred years later in different areas). So, in the form of a direct response, finding that the MCA represented a warm period in 2 places (i.e., western Europe and Oceania) within a broad band of time, does not imply that it's a global event in the same sense as what we are seeing today.

While not as unique as fingerprints, individual magmatic systems tend to have somewhat individualized geochemical profiles in terms of concentrations of trace elements and isotopic ratios so it's not "guessing" to use similarities of geochemistry to argue that two adjacent volcanoes share a source and/or have intermingling of sources in the shallow crust. Additionally, there are variety of geophysical techniques (seismic tomography, magnetotellurics, resistivity, etc) that allow us to image the magmatic systems of volcanoes which again provide evidence that these are related. The general idea that geology is all "educated guessing" (which underlies many questions and lay answers/comments in this subreddit) is pretty frustrating given the extreme care and detailed analysis many of us put into to understanding the details of natural systems.