|The purported Chinese stealth jet, J-20. Source of photo unknown.|
|U.S. Navy photo of an Air Force F-22 Rapter,|
June 22, 2009 in the Gulf of Alaska.
How does this tie into geology? Some fluids, such as boiling water, bubbly magma, and dusty gases have very low sound speeds (here's a pdf of a paper that I published on this phenomenon). The sound speed of boiling water can be as low as a few meters per second--this means that a fast track athlete could potentially run at speeds greater than Mach 1 if immersed in boiling water! Shock waves would be streaming off that runner like those in the photo above of the F-22!
Another way to envision the aircraft is from the pilot's view: he's not moving, but air is moving past him at Mach 1,2,3 or 4. The same applies to geologic situations. Imagine that a fluid that has a low sound speed is flowing past an obstacle--a rock or a ridge, for example. Shock waves will be generated around this obstacle that make the flow field very different from that created by low speed, subsonic fluids. This fact has been under appreciated in general in the geologic community, and only over the past few decades have studies begun that include the possibility of supersonic flows. I discussed one observation of shock waves earlier on this blog here. Shock waves were observed by a number of people during the eruption at Eyjafjallajokul earlier this summer, for example, this You-Tube video.