> and deduce that every X amount of years the Cascadia Subduction Zone ruptures with some regularity.

I guess this depends on your definition of "regular." If you look a the intervals between events reconstructed from the turbidite record (Table 12 on page 115 of Goldfinger et al., 2012), you'll see that these aren't exactly evenly spaced. E.g., the spacing in years between events is 232, 316, 446, 311, 982, 492, 415, 665, 661, 1189, 508, 715, 443, 548, 733, 195, 117, 577. From this you can calculate an average and it tells you that generally you'd expect an event every few hundred years, but after a given event, there's not necessarily anything to indicate whether the next one is going to be in ~100 years or ~1000 years. I would not describe that as having a particularly "regular" pattern of strain release.

In short, no. Given that people have a weird fascination with 'can we use a bomb to avoid a natural disaster', the USGS has a variety of FAQs centered around these hypotheticals, here's theirs for the bomb to set off a volcano variant.

A "local" scale is specifically calibrated so that some measurable quantity (like the amplitude of seismic waves as measured on a seismometer) gives a somewhat repeatable estimate of earthquake size, but only for a specific area. This is because local scales, like the Richter scale, are effectively a measure of ground shaking. For a given magnitude of earthquake (in the moment magnitude sense, which is a measure of an intrinsic property of the earthquake, i.e., the seismic moment), the details of ground shaking will depend on distance/depth but also details of the rock that the seismic waves passed through between the source and the seismometer. So for the Richter scale and other local magnitude scales, if you try to transport it somewhere else, the magnitude won't be equivalent. I.e., a true Richter magnitude of X in one place won't actually be the same size earthquake of a Richter magnitude of X earthquake somewhere else. That's not a a very useful property for a scale to have.

To clarify, the media isn't using the Richter scale, the media is reporting what ever magnitude a given service (e.g., the USGS or GFZ Potsdam GEOFON, etc) reports and then calling it a "Richter" magnitude. That magnitude is typically a moment magnitude, but depending on the location and details, it might be one of several seismic magnitude scales, e.g., occasionally you'll see a body wave magnitude (mb) or a surface wave magnitude (Ms) reported for a particular earthquake. As to why calling everything a "Richter" magnitude has persisted, it's unclear. The Richter scale was the first, but it was always a local scale (i.e., it was only really calibrated to be used in one part of the world) and it hasn't effectively been used for >50 years.

It's important to remember that the scales we use for earthquakes (which in the US, is typically the moment magnitude scale, i.e. Mw) are logarithmic. Thus, let's say we define a big earthquake as an Mw 8.0 and a little earthquake as an Mw 2.0, the Mw 8.0 is 1,000,000 times larger than the Mw 2.0 (or alternatively if we say a Mw 3.0 is small, the Mw 8.0 is 100,000 larger, and so on).

Now, this is just thinking about the magnitude as represented on a seismogram, if we want to say how many earthquakes of a given small magnitude equal a given single large magnitude earthquake, we need to consider this through the lens of radiated energy. For this purpose we can use the equation on the linked wiki page that relates Mw and radiated energy Es, specifically,

Mw = 2/3 log(Es) - 3.2

So, we can use this to calculate the amount of energy released by a single Mw 2.0 or Mw 3.0 and a Mw 8.0 earthquake and thus just how many Mw 2.0 or 3.0 events we'd need to equal the energy of a single Mw 8.0. If you go through the math, you'll find that to equal the released energy of a single Mw 8, you would need ~31 million Mw 3.0 or ~1 billion Mw 2.0 events. Let's be more generous and consider something of a more moderate event, like a Mw 5.0, but even then you'd need around 32,000 Mw 5.0 events to release the same energy as a single Mw 8.0.

With this, you could play other games, like lets say the fault system in question has stored enough energy to generate a Mw 8.0, but you have 25 Mw 5.0 earthquakes over a given period, how much energy is left? Again, doing the math, enough to generate a Mw 7.9997 earthquake.

Suffice to say, no, a few small quakes every year are a literal drop in the bucket toward the total strain budget of a system capable of generating a large magnitude earthquake so these do not really do much in terms of preventing an eventual large magnitude event.

EDIT: Writing this answer as I was falling asleep led to me not addressing the "overdue" aspect of the original question. If you would like a deeper dive on why the concept of earthquakes being "overdue" is incredibly problematic, I'll refer to you this FAQ.

Right, I understand what an hourglass looks like, the point is that this type of question probably stemmed from a particular example and so providing that example would help narrow down specifically what OP is asking about.

It's pretty unclear what you're describing, providing an example of a lake with this feature might help.

There are similar things, e.g., Fassett & Thomson, 2014 use estimates of diffusion rates and diffusion modelling to work out estimates of crater ages.

> Do we have any records of meteor impacts on the moon? Is there any way to monitor this?

Yes, and we do monitor this. There may be a deeper record (perhaps others will address that aspect), but NASA runs a lunar impact monitoring program. The basic strategy is to look for "flashes" using specially designed telescopes, where the flashes are a portion of the kinetic energy of the impact converted to visible light. NASA has been running this program since 2006. This page provides some recent candidate impacts and there's a map and links at the bottom that give a more complete accounting of what they've monitored. From a quick glance, you can see that since they started the monitoring program, they've observed ~440 candidate impact events.

> Is there any way we could find out how old the impacts are? Carbon dating?

Yes, but radiocarbon dating is (1) restricted to samples about ~50,000 years old and (2) dates the time that a living thing stopped being in equilibrium with the atmosphere, i.e., it died. Thus, radiocarbon is really not useful for the question (or for the Moon more broadly) since most craters will be much older, there's nothing alive on the Moon, and there's effectively no atmosphere on the Moon. There are however a range of radiometric dating techniques which are more applicable for the moon and impacts more specifically. In terms of dating impacts directly via a radiometric technique, the basic idea is to try to date a sample of "impact glass", i.e., material that was melted and quenched rapidly during the impact process, and thus dating this glass constrains the time of the impact that caused the melting. The common radiometric techniques applied to impact melts are Ar/Ar and a somewhat niche version of Pb-Pb ages (e.g., Zellner, 2019).

In addition to radiometric techniques applied to melts, there have been a variety of other methods proposed to approximately date lunar impacts. For example Ghent et al., 2012 suggested that the breakdown of the ejecta blanket, i.e., the blocks of the lunar surface that are excavated during impacts and strewn around the crater, could provide an estimate of age. As discussed by Ghent, large intact rock chunks within the ejecta from larger impacts are degraded by impacts of mircometeorites (there are a few other processes that also weather material on the moon which might also contribute to some degree) so the degree of preservation of the intact rocks in the ejecta can serve as a proxy for age for younger craters (once all the rocks are degraded, this method no longer works), but where "younger" is used in a geologic context, i.e., it works on craters 10s to 100s of millions of years old, which given the ~4.6 billion year history of the Moon, counts as young.

> Now, this is only accurate a few hundred years either side, due to the nature of carbon dating.

This would be a pretty terrible radiocarbon date, most have uncertainty in the range a few decades at most (e.g., Scott et al., 2007).

It's not something that's been suggested to my knowledge and geochemically it's missing some of the hallmarks. There are suggestions of isolated slab window related magmatism in the Tahoe region (e.g., Cousens et al., 2011), but not Long Valley. Long Valley is generally associated with other magmatic systems in that part of the western Greater Basin. Their exact origins are a bit enigmatic but are largely inconsistent with slab window volcanism seen elsewhere.

There are a couple of different potential outcomes, and there are examples of pretty much all of them in some places. If the ridge is roughly parallel to the subduction zone:

1. Option 1 is that the ridge doesn't actually subduct because subduction stops before the ridge gets there. 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.
2. Option 2 is the ridge subducts and the slab detaches because there's nothing really connecting the other side of the ridge to the slab. The end result of this proceeds largely the same as above.

In terms of these geometries, the basic assumption was effectively option 2, but in detail, it's actually hard to get a ridge to subduct and option 1 is more favorable (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).

If instead the ridge is very oblique or orthogonal to the subduction zone, 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.

Thanks for clearly stating what I was trying to express somewhat sloppily. This is largely why in discussions of rheology (for rocks at least), talking about them as either "solid" or "fluid" is uncommon and instead you tend to see them described just as "materials", i.e., when texts introduce useful analogues for thinking about the stress-strain or stress-strain rate relationship (i.e., the various combinations of a sliding frictional block, spring, or dashpot that would produce some sort of equivalent stress-strain or stress-strain rate response) they tend to do so in terms of just materials, e.g., "Maxwell materials" or "Voigt materials" etc. Not all geology texts are good about this though.

This is actually kind of a misleading "clarification" though. Pitch is a useful example, i.e., a viscoelastic solid that will deform on long time-scales under its own weight. At room temperature and timescales sufficiently short (i.e., less than a few years), pitch would meet the simple definitions of a "solid", but observed on long enough time scales, it can be observed to flow.

> How viscous is the magma in the mantle?

The mantle is solid. That being said, even though it is demonstrably solid, the mantle flows like a fluid on geologic time scales and generally tends to behave like a Non-Newtonian fluid (i.e., stress and/or viscosity are a non-linear function of the strain rate). If you want a deep dive on mantle rheology (i.e., how it deforms), this slide-deck has a thorough treatment. Given that it's ideally a Non-Newtonian fluid, it's hard to ascribe a single viscosity to mantle materials, and it will vary as a function of background conditions (i.e., temperature and lithostatic pressure) and the rate of strain the material is experiencing.

> Are there points of greater viscosity and lesser?

As described above, in detail the viscosity will vary as a function of strain rate, i.e., as strain rate increases, viscosity will decrease. Those complications aside, we do have a variety of estimates for what we could sort of think of as the "background" viscosity (or the viscosity that, when assuming the mantle instead behaves like a Newtonian fluid with a single viscosity, best explains observations) for the mantle and these do vary as a function of location. For these, we typically consider the upper mantle (nominally the mantle above 660 km) and lower mantle (mantle between 660 km and the outer core) separately.

For the upper mantle, this has a pretty wide range of viscosities anywhere from 10^(18) Pa s to 10^(21) Pa s (e.g., Dixon et al., 2004) with significant lateral variability. As discussed in that paper (and other sources), this high degree of variability is largely ascribed to differences in water content or temperature that reflect both modern and past tectonic histories and where higher water content and/or higher temperatures lead to lower viscosities (i.e., materials that flow easier / are effectively weaker).

For the lower mantle, depending on the source, similar order of magnitude ranges have been suggested but tend to be uniformly higher and the variations are more in terms of depth as opposed to laterally. For example, both Lau et al., 2016 and Mitrovica & Forte, 2004 suggest viscosities ranging from 10^(21) to 10^(23) Pa s with lower values at the top of the lower mantle and a peak in viscosity near the bottom of the lower mantle (though all estimates suggest that viscosity decreases significantly to <10^(20) Pa s approaching the core-mantle boundary). In contrast, other estimates like Cízková et al., 2012 and van der Meer et al., 2018 suggest less variable lower mantle viscosity mostly between 10^(22) and 10^(23) Pa s but still with a peak near the bottom of the lower mantle.

It's worth briefly discussing where these numbers come from. For the upper mantle, it's primarily from using the response to glacial isostatic adjustment, i.e., the lithosphere flexed down under the weight of large ice sheets and is still flexing back up after they melted and the rate and spatial patterns of that flexing back up (which we can measure) is in part controlled by the viscosity of the mantle (we also use similar response to responses to large loads like lakes, etc to work out viscosity estimates for areas far away from those that were glaciated). Similar data is used for some of the lower mantle estimates (e.g., Lau et al., 2016 and Mitrovica & Forte, 2004), but alternatively, papers like Cízková et al., 2012 and van der Meer et al., 2018 both use the sinking rate of detached subducted slabs imaged by seismic tomography to estimate viscosity.

> If it weren't for the heat, could you swim in it? What's an everyday substance that might have comparable viscosity?

For reference, the viscosity of water is around 0.001 Pa s, higher end viscosities of honey will be around 10 Pa s, average peanut butter is around 100 Pa s, and the viscosity of pitch (e.g., pitch drop experiments) is ~10^(8) Pa s, so still at minimum 10 orders of magnitude less viscous than the least viscous part of the mantle.

In short, the viscosity of the mantle is high enough that on human timescales, a material with a similar viscosity at room temperature would be for all intents and purposes a solid, so no, you could not swim in it. This is why we talk about the mantle as a solid. It is in the way we experience solids, and the fact that it has a viscosity and flows is only relevant if you're considering timescales of several thousands or tens of thousands of years at the minimum.

There is a pretty extensive literature (which is not exactly hard to find) of climatic variability, drought, and influences of these on various societies in the Middle East / SW Asia at both long (e.g., Kaniewski et al., 2012, Xoplaki et al., 2016, Flohr et al., 2017, Jones et al., 2019, Fleitman et al., 2022) and short (e.g., Donat et al., 2013, Barlow et al., 2016) time scales. The general point is summed up nicely by the title of the Kaniewski et al., 2012 paper, i.e., Drought is a recurring challenge in the Middle East.

Nuclear winter is discussed in a FAQ that is linked in the answer to which you're responding.

> Maybe it was the same type of crystal with different impurities?

Most likely. You can get things like gradations between amethyst (a purple type of quartz) and citrine (an orange type of quartz) in a single crystal because it's all quartz with different things substituting into the lattice. Some minerals can have really complicated intergrowths and gradations of versions themselves, e.g., tourmaline does all sorts of weird stuff, but importantly all have effectively the same lattice structure.

I am of course incredibly biased as a professional geologist who teaches geology for a living, but I would highly recommend an intro geology class for anyone. Developing a basic understanding of the history and workings of the planet on which we all live has intrinsic value and you'll be surprised how relevant much of the insight gained from an intro class will be for random things in your life (e.g., thinking about where to buy a home, etc.).

Yeah, fixed.

Yep, corrected.

Fun idea, but minerals don't work like that. First, some basic mineralogy stuff. Amethyst is just dirty quartz and sapphire is just dirty corundum, i.e., amethyst is a quartz crystal that has impurities (usually iron, but sometimes other metals) and sapphire is a corundum crystal that has impurities (for a blue sapphire, typically iron and titanium). For reference, there are other color sapphires (with different elements subbing into the crystal lattice, producing different colors) and we give other names to corundum with different impurities (e.g., if corundum has chromium in it, it will tend to have a red color, which we call a ruby).

So lets say you take some amethyst (quartz - SiO2 - with some Fe) and sapphire (corundum - Al2O3 - with some Fe and Ti) and put them into a crucible, how hot would you need to get them to melt? Well, quartz (for a rock forming mineral) melts at relatively low temperature of around ~570 C (assuming we're basically doing this at atmospheric pressures) EDIT depending on the type of quartz and the duration of heating, will melt at ~1750 C (e.g., Folstad et al., 2023), but we need to get our mixture up to ~2000 C to melt corundum. Let's say you have the right equipment to do that and you get both your amethyst and corundum into a melt, you've basically made a "melt" consisting of Si, Al, O, Fe, and Ti (assuming that the amethyst was an amethyst because of Fe and not some other metal).

If you start cooling this melt, what's going to happen? Well, you'll start to crystallize things, and effectively you'll crystallize things in the reverse order. I.e., whatever melted first EDIT: last - will start to crystallize first. So in a super simple scenario, as the temperature of our mixture drops below ~2000 C, you might start to get bits of sapphire to crystallize. This is effectively a reflection of one of the basic things we teach in an intro geology class, i.e., Bowen's reaction series, which basically is a progression of minerals you'd expect to crystallize out of a melt containing a mixture of common mineral forming elements (or in reverse, what order you'd expect minerals within a rock to melt as you ramp up the temperature). This progression effectively relates back to the melting/crystallization temperature of different minerals, but also the evolution of a melt, i.e., when a particular mineral crystallizes from a cooling melt because it is thermodynamically favorable to do so, depending on what constituents it "takes up", the composition of the melt will change.

With that in mind, and returning to our specific example, importantly, you've got a melt that has some extra components compared to your original sapphire, namely Si, so chances are you might not even get sapphire (or corundum) back, for example, you might start to instead crystallize an aluminosilicate, i.e., Al2SiO5, specifically probably andalusite since we're doing this experiment at atmospheric pressures) or some other minerals depending on the exact mixtures and conditions as you reduced the temperature. As you continue to cool the melt, finally, you'd probably get quartz, basically using up what ever Si and O were left. Whether this quartz looked anything like amethyst would depend on whether the minerals that crytallized before it had left any iron around. Effectively, what you've done is made an artificial rock, i.e., a mixture of one or more minerals but where the individual minerals are distinct crystals. Also of note, it tends to take relatively specific conditions to grow large crystals that we could consider "gem quality", and chances are, our experiment would not result in this, but instead a relatively fine grained rock with lots of little crystals, so probably not a very pretty rock.

You also might be asking, instead of cooling our melt slowly and letting crystals form, what if we cooled it really quickly, i.e., if we "quenched" our melt? Well, then you've basically formed glass. Chances are it's going to look basically like obsidian, which is a natural form of glass from rapid cooling of melts rich in silicon, oxygen, and aluminum (along with some other bits) kind of like our melt.

Finally, it's worth noting that the material properties for minerals and metals tend to be very different. Those differences in material properties allow metals to be "worked", i.e., you can deform them in a "ductile" manner even at low temperature and pressure to form things like rings. At atmospheric temperatures and pressures, most naturally occurring minerals instead deform "brittlely", i.e., they fracture. So, you would not really be able to form a mineral into something like a band, unless you had a single crystal large enough to just cut a ring shaped object out of this crystal. You can get minerals to deform in a ductile manner, but it takes relatively intense temperature and pressure conditions to do so and not exactly something you can do in your kitchen, unless for some reason you have a diamond anvil cell in your kitchen.

EDIT: For all the people asking me various forms of, "what if you did this other kind of manufacturing technique on minerals/resin/other stuff to get the desired effect?" this is a fundamentally different question than "can you melt two minerals together." The former question is relevant for what OP wants, but is not really for a geologist to answer (i.e., most of us are not professional jewelers, oddly enough). I.e., stop asking me how to make jewellery, I don't know how to make jewellery.

> For accommodation space to be created in the continental realm, the continental part of a tectonic plate needs to break apart, known as a rift basin.

While it's true that continental rifts are definitely locations where accommodation space is made, it's demonstrably false that these are the only continental environments where accommodation space is made either in the geologic past or in the modern. To start, there are several other tectonic environments where tectonic components of subsidence generate (many times very significant) amounts of accommodation space. The largest by far would be in convergent environments where loads associated with the growth of large mountains and/or negative buoyancy from underthrust lithosphere generates significant subsidence and thus accommodation space. Any region with mountain ranges still experiencing active convergence (e.g., Himalaya, Andes, Greater Caucasus, Taiwan, etc) will largely also be actively generating tectonic subsidence in portions of their respective foreland basins and thus actively generating accommodation space.

Other tectonic environments can also generate subsidence and accommodation space, though they tend to be more localized. For example releasing or transtensional step-overs in strike slip systems tend to also produce large amounts of (very localized) subsidence and thus accommodation space. Ridge Basin in southern California is a classic example of this type of environment (though no longer actively forming accommodation space), but there are many releasing step overs and transtensional bends in large, modern continental strike slip systems (e.g., along the San Andreas, North and East Anatolian, Altyn Tagh, etc.)

Outside of tectonic sources of subsidence, sediment deposition tends to beget more sediment deposition because the deposited mass (1) induces flexure of the lithosphere - generating accommodation space and (2) induces compaction / dewatering of underlying sediments - generating accommodation space. Additionally, in environments with large amounts of organic material incorporated into deposits (e.g., marshes, wetlands, etc.), organic decomposition can lead to large amounts of subsidence. Thus, many coastal environments, especially those adjacent to large deltas (which represent massive amounts of sediment being deposited) experience significant subsidence and accommodation space generation. As an example of this, consider the gulf coast of the US and its rapid subsidence. This is in no small part due to the combined effects of (1) continued subsidence from the mass of the Mississippi delta, (2) subsidence of compaction and organic decomposition, and (3) large scale levee systems keeping the Mississippi along its current course and thereby preventing sediment deposition in the adjacent areas. I.e., in part the elevation of the gulf coast is actively lowering because accommodation space is being generated but virtually no sediment is allowed to fill it through artificial means.

> Are there any types of sedimentary rock that form in terrestrial environments, and what are the processes?

Yes. For this, it's better to discuss this in terms of depositional environment instead of the exact rock type as the latter are non-unique. Common terrestrial depositional environments are related to rivers or lakes. The types of rocks deposited by those systems will depend a lot on the system in question. For rivers, it can be quite varied. In rivers that are close to a high relief sediment source (e.g., a mountain range), conglomerates (representing river that were carrying gravels) are common. Moving down the system, grain sizes generally fine, so sandstones to silstones would be common. Most rivers will also have large amount of mudstones/shales associated with them as these represent flood plain (overbank) deposits. Lakes are a little less varied, primarily being represented by shales/mudstones, but you can even get carbonates in lakes as well. As you can see, rock types are non-unique, e.g., you can get mudstones associated with fluvial (river), pluvial (lake), or marine settings. Similarly, deep water clastic systems produce rock types and deposits that look pretty similar to terrestrial fluvial systems, but are deposited offshore. There are some types of deposits that are pretty unique to terrestrial environments, e.g., paleosols, loess, ergs, etc., but in terms of raw rock type, these would still be kind of generic mudstone, siltstones, or sandstones broadly. They would largely be distinguished on the basis of primary features. E.g., Erg deposits that become sandstones will tend to have massive crossbeds, like the Navajo Sandstone.

> Generally it seems that lithification happens in marine environments, the rock is then uplifted into the terrestrial environment, where it then erodes back to the sea. Does lithification generally not happen on land because the accumulation of new sediment is not enough to replace or overtake sediment loss to erosion?

So the focus on lithification vs deposition is kind of misplaced. Regardless of whether we're talking terrestrial vs marine, lithification is not happening at the surface, it's only after burial. Depending on location and the progression of environments, marine deposits could be lithified after being buried by terrestrial deposits (during regressions, i.e., sea level falls) or terrestrial deposits could be lithified after being buried by marine deposits (during transgressions, i.e., sea level rise). Thus, it would be better to move your focus away from lithification in this context.

The main difference between marine and terrestrial environments is accommodation space, i.e., the difference in elevation between the current surface of the Earth in that location and the maximum height to which sediments could be deposited (usually sea level, but not always). Effectively, accommodation space is a hole. For marine environments, there's pretty much always accommodation space. For terrestrial environments, accommodation space is more rare and will be typically localized where there is some process driving subsidence, i.e., a force making a hole. If there's no hole for sediment to fill, it will "bypass", i.e., it will keep moving until it reaches an area where it can fill a hole. This ends up meaning that there are lots of areas in terrestrial environments that are not conducive to sediments depositing. But there definitely are areas where sediments can (and do) deposit in terrestrial environments.

Yes, references to Superman and/or the potential necessity for us to make some Unobtanium and send Aaron Eckhart, Hillary Swank, and Stanley Tucci et al., into the bowels of the Earth with a bunch of nukes abound today.