New video of Halema'uma'u Crater; and a great roll cloud in Tennessee

There is a 4-minute long UAS (Unoccupied Aircraft Systems) video released by the USGS today showing the enormous collapse features around the rim of Halema'uma'u crater. Filming was done two days ago on July 24. The trailer at the end of the video (which flashes by before you can read it!) says that the two flat surfaces that have subsided are the former caldera floor and the former floor of Halema'uma'u.
   

Photo as credited in text.

      On another topic, Colby Hutton of Adamsville, Tennessee took the adjacent photo of a roll cloud in Tennessee after a thunderstorm. A roll cloud is a subclass of "arcus clouds," low horizontal clouds (the other main type is a shelf cloud).  Roll clouds, according to Wiki, form along the leading edge of thunderstorm outflow where cold outrushing air lifts warmer air up to a level where condensation occurs, giving rise to the cloud. The most famous occurrences of roll clouds are the so-called Morning Glory clouds that form in Queensland, typically in September and October. These clouds form where air temperature reverses from its normal state (warm air at the bottom of the atmosphere, cooling upward), resulting in warm air on top of cool air. Shear across the inversion point (where the gradient changes) sets up the rolling motion, giving rise to the roll cloud.  They can last for several hours, be several hundred kilometers in length, and occur in sets. Conditions for the inversion are most likely to happen in the morning and hence the name "Morning Glory" in Queensland. They are not common--conditions for their formation are common in the spring in the midwest, but I lived there for 10 years and never saw one.  If there is too much moisture around, as in a thunderstorm, any roll cloud may be hidden amongst other clouds. Here's a link to a National Geographic short video of a roll cloud in Texas, and here's another spectacular compilation, not sure where it is from.

What in the world is "geofoam?"

In the past week, I have traveled the 135 mile route between Seattle and Vancouver, B.C. twice, each trip taking more than 6 hours when going north.  A significant part of the problem is waiting at immigration at the border, but an equally significant problem is the crazy highway maze from the border into Vancouver: continuous spots of merging 3,4,5,6 lanes down to 1,2 or 3.  Amidst the frustration, my suppressed desire to become a highway engineer emerged: either to design the interchanges properly using fluid mechanics principals, or to design them to punish the many drivers who skirt to the right-hand side "truck only" or "exit for duty-free" places to cheat and move way up in line.
     With this recent thinking about traffic engineering, I was interested today in an article on the WSDOT blog site regarding "geofoam" which I had never heard of. It is used in place of dirt because the use of real dirt takes longer and has higher costs. Dirt has to settle, and crews have to wait until the ground has settled before building a structure on top of it. The use of geofoam also reduces the weight on underlying soils: blocks of geofoam are placed on a section of ground that is greater than the weight of the geofoam. The blocks are glued together with a quick-setting roofing adhesive and then secured to the ramp walls with reinforcing steel. Ready now for concrete to be poured on top!
     So, what is geofoam? Does it have anything to do with "geo"? Not as far as I can tell, though it's probably more stable than the "ghost poop" foam beads that are used to cushion materials in packaging! It is "expanded polystyrene" manufactured into large lightweight blocks. I couldn't find a list of the elements/compounds in it, but according to one site, it is a "closed-cell, strong, but lightweight premium quality expanded polystyrene (EPS) foam." It is used in a variety of "residential, commercial, and industrial applications." Polystyrene has the general chemical formula (C8H8)n with a density of  1%-2% of that of soil of equal strength. 
     Geofoam is used because, for engineers, it has predictable properties compared to soil fills. It is basically (but not completely, see below) inert so that it won't spread into surrounding soils, e.g., lawns, gardens, farmlands...  It can even be installed by hand and does not require heavy machinery for installation.  It's quick and easy to install and can be installed in any type of weather and night or day, i.e., cheaper installation costs.  It can be dug up and reused.
     On the other hand, there are some serious disadvantages: It is a fire hazard if untreated.  It is soluble in petroleum solvents, eek.....if it comes into contact with a 'petroleum solvent' it turns into a "glue-type substance unable to support any load." Engineers have to figure out buoyancy problems because of it's low density: cars were crushed against a ceiling in one instance after floodwaters below a carpark floated it.  It is susceptible to insect infestations when used in buildings.
    Conclusion: The "geo" in geofoam has nothing to geology!

Note added 2/28/2020: Paul Martin has created a more detailed guide to geofoam that can be found here. He is at SpecialChem.com which has "been publishing articles relevant to the Specialty Chemicals Industry since 2000."