As others have indicated, yes, fossils of deep sea organisms tend to be more rare, or at least are not as well represented as fossils in shallow marine or continental slope areas. The primary reasons for this is subduction of oceanic lithosphere and the general depositional history that characterizes these environments (e.g., Holland, 2016). As highlighted in this paper (and generally in many paleontology textbooks), the fossil record is relatively biased toward organisms that were deposited in environments that have a higher preservation potential (of which the deep sea, along with extremely erosive environments etc., is not one). An additional influence, depending on the geologic period and the type of fossil in question, that can influence preservation of deep sea fossils is the carbonate compensation depth, i.e., basically the depth below which carbonate begins to preferentially dissolve (there is a similar depth for aragonite). Given that the preserved part of many marine invertebrates are their calcite or aragonite hard parts, areas of the sea floor below the CCD are not conducive to preservation of their remains.

For coal, the source material is primarily land based plants, preferentially those that were growing in swamps or other similar environments. There is an oft repeated claim that coal only formed during the Carboniferous because the lack of decomposers able to digest lignin, but this is demonstrably false. As described in this FAQ, the abundance of coal formation during the Carboniferous relates to specific paleoclimatic and paleogeographic conditions (i.e., lignin decomposers were present at this time) that supported large areas of swamps for significant periods of time. For petroleum and natural gas, the source material is primarily marine photosynthetic microorganisms, like algae, phytoplankton, etc.

For both types, in order for the respective fossil fuel to form, the raw organic (in the chemical sense) material needs to be buried quickly and experience the right temperature and pressure conditions for the various reactions that produce either coal or petroleum. The wiki articles on coal and petroleum formation are decent and provide a decent overview of the source material and the relevant chemical processes along with the specific conditions required for them to occur.

I'm not aware of anything like this and I've worked adjacent to the oil & gas industry for all of my career. The vast majority of petroleum is derived from photosynthetic marine organisms (algae, phytoplankton, etc). Maybe you're thinking of the theory that some amount of petroleum is produced through abiotic mechanisms, but these ideas have been thoroughly discredited and have never yielded a successful find (e.g., Glasby, 2008). Even if we ignore that, the concept of depletion and peak oil is arguably independent of the formation mechanism of oil, i.e., it's more controlled by production (in the sense of our extraction) of oil than the way in which oil is formed (e.g., Höök et al., 2010).

One way to approach this is through consideration of the idea of "peak oil", i.e., the idea that at some point there will be a peak in oil production that will never again be reached. The reasoning behind this is a mixture of the finite nature of oil reserves and the economics of oil extraction. One important component of the peak oil concept, which also reflects a misconception within your question, is that we will never ever get the last drop. There will always be petroleum left behind in a reservoir because at some point the cost of extraction greatly exceeds the price to extract that oil. If you look through the linked wikipedia page, you'll see how this plays out when a new way to extract oil becomes available. Specifically, the original peak oil prediction suggested that oil production would peak sometime in the early 1970s and this basically held until about the mid 1990s (in this plot the red is the projected production based on the peak oil concept and the green is actual global production). So what happened in the 1990s that allows production to go back up? A few things, but primarily development of technologies that allowed us to start efficiently extracting petroleum from reservoirs that were previously non-viable (primarily directional/horizontal drilling coupled with hydraulic fracturing that allowed us to extract oil from "tight" reservoirs, but others as well). This highlights that it's hard to project out when we will actually effectively run out of oil because we fundamentally don't know the future in terms of development of new ways of extraction (and to a lesser extent the discovery of additional reservoirs, though both the rate of new finds and the general exploration process is a shadow of what it was in the mid 20th century).

The other important side is demand. Circling back to the beginning, oil companies are not non-profits, they only extract oil if it's economically viable (and profitable) to do so. If factors reduce demand (e.g., taxes on petroleum to reflect the huge environmental cost of continuing to burn oil/gas, decreasing prices of alternative energy sources, etc), demand for, and thus the price of, oil will drop below a point where it's economically viable to extract whatever is left in reservoirs. Analyses attempting to work in some of these ideas of changing patterns in demand and viable energy alternatives still consider the peak oil concept, with some projections now pointing to a peak in production in the mid 2030s (e.g., Delannoy et al., 2021). But again, a lot is riding on projecting out a variety of pretty hard to predict things (basically anytime one group of variables is "collective human behavior", projections are going to be a bit tricky).

The final point to consider is that if we consider some hypothetical where we really go for getting as much remaining petroleum as we possibly can and burn it all, even without considering the "what do we do now for energy" challenge, this would basically be a "let's shoot for a RCP 8.5 scenario", which would make for some pretty hellish conditions in the future, to put it mildly.

It's wrong because much of the data is not consistent with your idea if you do the math. The data presented in Leathers et al 1998 is useful here as they consider how Tmax varies with DTR. Even for those with a linear Tmax to DTR relationship, these don't have consistent percentage drops (and so your explanation doesn't work). Additionally, as described in the original answer, many sites have very complicated relationships between Tmax and DTR and with variable timing of seasonal maxes of DTR. For example, one site where at days with a Tmax of nearly 30 C and those with a Tmax of -5 C have the same DTR of ~12 C (with a minimum in DTR with a Tmax of 5 C). I.e., looking at actual data highlights that your explanation is not useful as a broad explanation.

This is not supported by any of the cited literature.

Not sure if this is what you mean, but this ends up being pretty common for empirically derived dimensional constants within equations. As a random example from my field, if you look at Table 2 from this paper you'll see that they're reporting the values and units of a variety of constants for a set of equations to estimate how the rate of rock erosion within a river varies as a function of water discharge (among other things). In there, there are various constants with somewhat nonsensical units, e.g., k_e has units m^(2.5) s^(2) kg ^(-1.5), k_w has units m^(-0.65) s^(0.55), and k_t has units of m^(-7/3) s^(-4/3) kg.

So what does these physically mean? Not much really. These are empirical constants and the units are in effect almost like dummy values to ensure that actual physical values have meaningful units. Take the k_w value for example. This is a constant in the equation w_b = k_w * Q_b^(omega_b), which is relating the bankfull width of a river (w_b) to the banfull discharge (Q_b) as a powerlaw. You'd get the value of k_w and omega_b by fitting observed values of width (in meters) and discharge (in m^(3)/s) and thus the units on k_w will depend on the best fit power law exponent (omega_b) and be appropriate to make sure that when you raise discharge in m^(3)/s to that power and multiply by k_w, you get width in meters.

Some terminology first, what you're describing is usually referred to the 'diurnal temperature range' or DTR (so if you want to poke around more, searching for this along with 'seasonality' will get you much of the relevant literature).

The pattern you're noticing, i.e., a maximum in DTR during the summer and a minimum during the winter, is characteristic of some (but importantly not all) locations. For example, Ruschy et al., 1991 found similar patterns in portions of Minnesota and attributed these to tradeoffs between cloud cover, solar radiation intensity, and albedo (from snow) throughout the year. Schwartz, 1996 found a similar pattern over much of the northeastern US, but added a variety of other possible controls (e.g., seasonal humidity changes, changes in vegetation behavior, etc).

However, the above is not a global pattern, but rather regional. For example, broader consideration of just the contiguous US highlights three general behaviors (1) the northern and western US have a maximum DTR in the summer and a minimum in the winter, (2) the central and southeastern US have a maximum DTR in the winter and a minimum in the summer, and (3) the mid-latitudes of the US have two maxima in DTR in the spring and fall and two minima in the summer and winter (e.g., Leathers et al., 1998, Robinson et al., 1995). As with the geographically more limited view from above, a variety of factors (that vary be region) are proposed as causing these different patters in DTR (and subsequent work has proposed even more, e.g., Durre & Wallace, 2001a, Dai et al., 1999, Portmann et al., 2009). If you browse many of these, you'll find that common suggested controls are cloudiness, various aspects of the hydrologic cycle (e.g., precipitation, soil moisture, etc), and vegetation changes (which are linked to the hydrologic cycle).

Also of note is a wide body of work that highlights that patterns and magnitudes of seasonal DTR are changing as a result of climate change (e.g., Durre & Wallace, 2001b, Balling & Cerveny, 2003, etc.) so longer term historical ranges for your (and other) location(s) may differ from more recent seasonal patterns in DTR.

> If they could say how inaccurate the current maps are they could have made maps that were accurate to begin with.

It's very possible to know something within a given uncertainty, still be able to quantify that uncertainty, but not be able to completely eliminate that uncertainty. If we take for example the paleomagnetic measurements underlying many paleogeographic reconstructions, it is definitely possible to estimate the uncertainty, and propagate that uncertainty into uncertainty in paleogeographic locations (e.g., Heslop & Roberts, 2020). The extent to which that uncertainty can be reduced will be fundamentally limited by both epistemic (which we can reduce by collecting more data) and aleatory uncertainty within the paleomagnetic measurements. Additionally, there will be a fundamental limit in terms of how much we can reduce the epistemic uncertainty because of the limited availability of preserved rocks. Not all steps in paleogeographic reconstructions lend themselves to as direct uncertainty estimation as the paleomagnetic components, but it's not fair to say that there is no way to characterize uncertainty in these products.

Yes, though broadly changes in eustatic sea level are one of the things that we attempt to account for when making paleogeographic maps and as mentioned in the original answer, depositional environments can provide some context to the location of coastlines, etc. That being said, the point is valid in that paleogeographic maps tend to be amalgamations of time periods, i.e., at the finest scale we might make a paleogeographic map that represents the "average" of a few tens of thousands to few million of years of time and certainly within that you would expect a decent amount of sea level variation. All and all, a paleogeographic map would give you a rough approximation of things and would be better than nothing, but yeah, you would need to expect it to be pretty wrong at times.

This is addressed in one of our FAQs.

Paleolatitudes can be determined from paleomagnetism. Paleolongitude will more be an outcome of tectonic reconstruction.

Broadly speaking, the accuracy will decrease the further back in time you consider. This is primarily because the completeness of the geologic record as preserved in rocks decreases as a function of age (though not necessarily linearly), but also because uncertainties and/or errors related to various assumptions that are made to construct paleogeographic maps also increase as a function of age. Thus, the level of accuracy is pretty different for a paleogeographic map of the last glacial maximum (~24,000 years ago) compared to the early Cambrian (~530,000,000 years ago) and so on, but not necessarily easily quantifiable.

Within a given map, there will also likely be portions that are much more certain than others. It's worth considering the primary datasets we use to construct paleogeographic maps and what each dataset provides in terms of information. The base for virtually all paleogeographic maps are plate tectonic reconstructions, i.e., at a given time, where individual tectonic plates were, what shape they approximately were, where were their continental and oceanic components were, and what rate and direction they were moving. A variety of data informs these reconstructions, including paleomagnetism (which allows us to reconstruct paleolatitude for locations at a given time), a variety of geophysical data (it's increasingly common to use seismic tomography to reconstruct subducted slabs and "undo" subduction to aid in reconstructing ocean basins), and other geodetic data for more recent reconstructions (for recent reconstructions, the current long-term average plate rates can be extrapolated backwards reliably for 10s of million years). In addition to tectonic reconstructions, the type rocks of a given age in a location can also provide context, e.g., rocks that are diagnostic of near shore depositional environments provide context that at the time those rocks were deposited, the coastline was near that location, etc. What this means is that for most paleogeographic maps (at least within the Phanerozoic, and into portions of the Proterozoic) the broad locations of continents and their first order shapes will be broadly correct and probably in roughly the right location (in the sense of latitude and longitude). In certain places on these continents, there might be higher levels of certainty of the location of the coasts and things like major rivers based on the rocks that are preserved. Similarly, large mountain chains will be approximately in the right place, but many of the details of them (e.g., their exact widths, heights, etc) will be only relatively constrained. Lots of fine scale details would not be preserved, some of which might certainly impact the utility of maps for navigation, so things like smaller rivers (and even some large rivers) and small to moderate sized islands (so things the size of Hawaii, etc) would generally not be preserved so a paleogeographic map would not include it, and again, more so the further back you go.

Finally, when considering paleogeographic maps, it's also important to realize that there is a fair amount of artistic license taken in many cases. A good paleogeographic map will use as much of the available data to constrain as much of the paleogeography as possible, but there will still be a lot of unknowns so choices have to be made. Different people making paleogeographic maps take different strategies for how to make some of these choices. You can compare some commonly used sets of global reconstructions, e.g., those from Christopher Scotese and those from Ron Blakey, to get a sense of the variability. Without a doubt, Blakey's maps are much more aesthetically pleasing and he does a huge amount of work to make sure they're as accurate as they can be, but he also takes a lot more license to make things always look complete and like "real" landscapes (even when that completeness is effectively made up). In comparison, if you look at especially old time slices in Scotese's maps, they look more like blobs on the globe, which is not as pretty, but more accurate in terms of the level of certainty we have with respect to the actual paleogeography.

In short, the level of accuracy will broadly scale with how far back in time the period that is being reconstructed, with paleogeographic maps reconstructing periods further back in time being less accurate than more recent times. Within a time slice, some features will be much more certain than others where as some features will be effectively made up depending on the available data for that time period (and the preceding and following time periods). All of this means that we can't really quantify the accuracy of these maps (and we can't ground truth them either), but depending on the level of "navigation" you had in mind and the time period in question, they could be used to navigate to some degree. Broadly, if you trying to get from one major landmass to another by boat, most all of them would probably be good enough (at least within the last few hundred million years). If you were trying to navigate over land, there would probably be a lot of fine scale details that you would have no idea existed based on the map. Further back in time, even navigation by sea would become a bit more tricky as the probability of a semi-large island chain being in your way (that's not on the map, because it was not preserved in the geologic record) goes up.

>Do you think that the Gorda/ Juan de Fuca plates being locked solid

This is not an accurate depiction of this margin though. Cascadia appears to experience both large slip events, but also so-called episodic tremor and slip (ETS) events (e.g., Dragert et al., 2001, Brudzinki & Allen, 2007), so it's not as though subduction is completely paused along this margin between megathrust events.

> might be creating a delayed onset of volcanic activity? Like we know the last mega thrust quake was 200-ish years ago, could it be that the crust pushed down in that event just hasn’t had time to really melt much yet?

More importantly though, and similar to the original consideration of OP, this is a ridiculously short time frame to consider when we're thinking about processes that when discussing the relevant rates we typically average over 10^(4) to 10^(6) years.

Again, I would start with questioning the validity of the underlying premise, i.e., considering extremely short duration records of activity is not a valid way to compare volcanic arcs. A more valid comparison would be things like total magmatic productivity (including both extrusive and intrusive) per unit arc length, total eruptive products per unit arc length, or similar. The question then becomes whether, when viewed in a valid way, whether the Andes are more magmatically productive than the Cascades. I don't know the answer to this (and it's not readily apparent from a quick glance at the literature), but that's really the first step, but as we'll see, an inherently challenging step.

With respect to the proposed hypothesis, i.e., that subduction rate directly ties to volcanic production rate, while logical this ends up being problematic, or at least not simple to demonstrate. It does have some support in the literature, i.e., there are studies that find relationships between estimates of volcanic activity and subduction rate (e.g., Huang & Lundstrom, 2007, Syracuse & Abers, 2006 - note for this last one, the Cascades are not even included in the compilation), but this correlation breaks down in other studies (e.g., Acocello & Funiciello, 2010). There are a variety of other things that have been argued to influence volcanic productivity, e.g., instead of the rate of subduction, the degree of obliquity of subduction (e.g., Sheldrake et al., 2020, Gazel et al., 2021), regional stress state of the overriding plate (e.g., Takada, 1994) - which is not wholly independent of the degree of obliquity or subduction rate, or the extent of hydration of the subducting slab (e.g., Till et al., 2019, Cooper et al., 2020) - which itself might correlate to things like slab age which in turn could correlate to subduction rate, amongst other controls. As highlighted in several of these papers (most prominently Acocello & Funiciello and Till et al) though, working out any of these controls is problematic because of a variety of challenges that mirror the issues mentioned in the first paragraph. For example Acocello & Funiciello suggest that the degree to which magmatic activity will or will not correlate with subduction rate depends on the time frames, i.e., plate rates are often estimated over long time frames than magmatic activity and whether you see a correlation or not depends on whether you take a long or short term view of magmatic activity. They also touch on the inherent issue with defining magmatic productivity. This is really picked up on in Till et al. where they discuss that (1) estimates of surface eruptive volumes are often highly uncertain and (2) estimates of intrusive volumes are even more uncertain. Importantly, apparent temporal or spatial changes in magmatic productivity based mostly on surface volumes may largely reflect changes in the intrusive vs extrusive ratio (which might depend on things like stress state in terms of whether magma is able to reach the surface or not, etc). So, essentially, we are still stuck with inherent challenges in truly assessing whether a particular arc is more productive than another. Suffice to say and as originally stated, one thing is for sure, specifically that an instantaneous snapshot of volcanic activity is definitely not the appropriate way to compare volcanic activity.

North America geographically is not the same as North America tectonically. Hawaii is on the Pacific plate, but as part of the US, geographically it is often considered as part of North America (though definitions vary). Similarly, the North American Plate contains portions of Russia (which geographically tends to not be considered part of North America), but does not contain the Caribbean (which does tend to be considered part of North America geographically). In my answer, I assumed OP was using the term North America in a geographic context.

The short version is mostly a mixture of random chance + infinitesimally small sampling time (with respect to geologic time) + vastly different relevant areal extent of the regions being compared.

For the longer answer, let's first take a look at the referenced map here (note this is a live map, so it may not reflect OPs question if viewed later in time). At the time being asked, it does indeed show that there are several volcanoes in Central and South America that are in some stage of eruption and/or unrest, but it's also worth noting that the original premise as stated is wrong from the start, i.e., there is at least one volcano in similar states of unrest in North America unless we exclude Mexico from the traditional definition of North America (not to mention various eruptions throughout the Aleutians and Hawaii, which are both part of North America as well, but would not be part considered here because of the arbitrary limits placed by only looking at "mainland" North America). It's also worth noting on this map, that in North, Central, and South America, there are consistently many more volcanoes not in a state of unrest, so even in Central and South America, the currently active volcanoes represent a small proportion of the total separate volcanic systems.

Next, let's consider the dominant driver of much of the volcanism along the North-Central-South American west coast, i.e., subduction related arc volcanism. If we consider the portions of these coasts that are still active subduction zones, e.g., this map, we can see that (especially if we exclude the Aleutians), a much smaller proportion of the North American west coast contain subduction zones compared to nearly all of Central and South America containing active subduction zones. While there is (geologically) recent volcanism in the southwestern US and northern Mexico, much of this related to other processes or the past history of subduction in these regions, but in general, we broadly expect the activity of many of these systems to be substantially less than active arc volcanism. Thus, in a simple sense, if we assumed that (1) there is broadly more active volcanism in arc settings above subduction zones compared to other volcanic settings within the broad area of interest and (2) over a given distance of arc we'd expect some number of volcanoes to be erupting at any given time - these two together, along with the relatively small amount of arc "real estate" in mainland North America as defined, would predict that generally the probability of seeing activity at any given time in North America is much less than in Central and South America simply because they're bigger.

Finally, time is a critical factor here, and specifically the effectively instantaneous nature of the observation with which we've started. If we consider the history of eruptions in the Cascades, e.g., this graphic, we can see that in the still very recent past the Cascades have been quite (and mostly consistently) active. Now, it is true that in the last ~100 years, the Cascades have largely been quiet (with the exception of the Mt. Saint Helen's eruption). The question of why the Cascades seem to be less active compared to many other arcs is directly discussed in this write up from volcanologist Erik Klemetti, in which he muses on potential geologic/tectonic reasons for the tendency (e.g., differences in the angle of the slab and/or age of the oceanic crust being subducted), but ultimately largely argues for the same thing I'm suggesting here, i.e., random chance and extremely short time sampling of very slow processes.

The two geochemical tracks in Hawaii (and as observed in many other plume related hotspot tracks) are thought to be related to some sort of heterogeneity in the mantle plume itself. As described in the recent review on mantle plumes by Koppers et al., 2021, there are three basic models to explain this:

1. An unzoned, but heterogeneous plume where the double tracks reflect different components with different melting temperatures.

2. A concentrically zoned plume with the hottest and densest portion of the plume material in the center.

3. A bilaterally zoned plume where one half of the plume is more a direct sampling of the source LLSVP and, since most plumes originate from the edge of LLSVPs at the core-mantle boundary, the other half incorporates more "ambient" mantle.

At present, the bilateral model is more favored as a general explanation, but it doesn't explain all of the observations of double tracks at all hotspots, so there may not be a single mechanism. Specific to Hawaii though, the bilateral plume model is the favored one (e.g., Williamson et al., 2019).

From a quick glance, those look like classic spheroidally weathered granite, so the original discussion of PBRs is relevant for these as well.

Thanks for joining us! It seems like one of the underlying goals is to understand freshwater fluxes. I'm curious how you'll convert river level heights to volumes / cross-sectional areas to asses changes through time, i.e., how will you get at the channel cross sectional geometry / wetted perimeter to pair with the surface heights to be able to calculate volumes? I'm also wondering if you're planning on validating these data with discharge measurements from relevant gages?

Depending on the examples in question, there's going to be a few different options, I'll discuss two common/possible ones here.

(1) For balanced rocks, or as their often referred to in a paleoseismic context precariously balanced rocks or just PBRs, these often form in-situ. As described in a variety of publications (e.g., Bell et al., 1998, Balco et al., 2011 with most referencing the Twidale, 1982 book - Granite Landforms with respect to formation mechanisms for PBRs), one common way they can form is basically having a series of vaguely spheroidal shaped boulders in the subsurface defined by sets of intersecting fractures (which form via exhumation jointing, spherodial weathering, etc) surrounded by looser regolith (i.e., much more broken up rocks). As weathering and erosion continues, the looser bits get eroded away, but the boulders (i.e., the corestones) get left behind, and if they're stable, they'll end up staying in their stacked position as the surrounding regolith is evacuated. The Balco et al PDF has a schematic diagram to help visualize this process in their figure 2.

(2) Another possible option are glacial erratics. These are large rocks that are dropped by large ice sheets and glaciers. These were rocks that were picked up (or fell into) a large ice sheet/glacier and transported, usually significant distances, until they were dropped by the ice sheet/glacier, usually as a result of melting. Most erratics don't appear as balanced rocks (more just kind of large, random boulders, often in areas that otherwise do not have many exposed rocks), but theoretically it's possible for a glacial erratic to end up as a balanced rock.

In terms of distinguishing between the two, classic PBRs usually will all be the same rock, i.e., the individual boulders balanced on each other will be largely derived from the same original bedrock and have the same composition, etc. Also, in a given area, there will probably be several PBRs as their formation reflect the right sets of conditions and processes that allow for their formation (and thus you would likely expect it to have happened in more than one spot in that area, though not always). In contrast glacial erratics, by definition, are very different rock than the surrounding bedrock. Similarly, a stacked erratic is probably going to be just one rock high, i.e., a single glacial erratic sitting on top of the local bedrock, as the ability to form actual columns of corestones (like with in-situ formed PBRs) is not really present. Also, while you might expect to see groups of erratics in a particular area, you wouldn't necessarily expect groups of balanced erratics as an erratic ending up as a balanced rock is going to be a bit more "by chance" as opposed to by process like with traditional PBRs.

> From what I gather, it's believed that there is significant but not extremely substantial heat generated from ongoing rasioactive decay.

This depends on where you are. I.e., this is broadly correct for the core specifically in that we don't consider there to be many radioactive elements in the core. However, in terms of the total internal heat budget, radioactive heat production accounts for roughly half of the heat budget, but this is primarily from elements in the mantle and crust.

Gravitational acceleration decreases with depth as you move from the outer core inward (e.g., this estimate using PREM), but overburden (i.e., the mass above a given point) is still increasing, especially given the large density increase from the mantle to the core, so pressure is definitely still increasing, especially considering the inner core boundary.

EDIT: If you want to directly see estimates of pressure with depth, you can look at table 2 starting on page 312 of the original PREM paper by Dziewonski & Anderson, 1981. Looking at this, you can see that pressure broadly is predicted to increase with depth despite the decline of gravitational acceleration as you approach the center of the planet.

The state of a material very generally is a function of both temperature and pressure, e.g., this phase diagram for iron. Temperature and pressure both increase with depth and the transition from the outer to the inner core reflects where you cross from a liquid to a solid in the phase diagram for core material based on the pressure and temperature conditions. Referencing the above phase diagram, the estimated temperature and pressure near the inner core boundary is ~5700K and 330 GPa, which would put us in the solid part of that diagram (though in reality, we'd need to consider an Fe-Ni alloy phase diagram that also accounts for all the potential minor components that would influence the phase diagram, but this broadly gets the point across).

As for the nature of the inner core boundary, the review by Deuss, 2014 provides some detail. In short, based on the observations we have from seismic waves, it appears to be a relatively sharp, nearly perfectly spherical boundary with perhaps a small amount of "topography". As mentioned by Deuss however, there is a layer at the base of the outer core called the "F layer" that has been described as a "slurry", i.e., it's a mixture of liquid and solid components, though as discussed by Wong et al., 2021, this layer itself is likely stratified into more liquid rich vs more crystal/solid rich sections. This F layer is considered part of the outer core, so we would still describe the inner core boundary as being sharp (and here, we're using the very specific behaviors of some components of seismic waves to define and describe the inner core boundary) despite the existence of this "slurry" above it, i.e., we don't describe the inner core boundary as gradational.

But this (and similar) doesn't actually answer the question, i.e., this would really only explain why you would generally expect oceanic lithosphere to underthrust continental lithosphere, not why you would expect oceanic lithosphere to be able to subduct, i.e., sink into the mantle. Similarly, this explanation would not really help to explain how ocean-ocean convergent boundaries work. The key is that the subducting portion is denser than the underlying mantle, and so in the simplest sense, the density of the overriding plate (i.e., the non-subducting one) doesn't matter for subduction to be able to occur (though it matters in the sense of which plate is able to subduct, at least in the ocean-continent boundary example).