1 Introduction

Some years ago, my PhD advisor described me as an iconoclast. Very early in my career, I found that if I wanted to build on someone else's hypothesis, it was necessary to ensure that I fully understood it, and was aware of and agreed with all of its assumptions. It was a defense mechanism to avoid being led astray by so-called experts straying off the topic of their supposed expertise. At that stage I had just encountered RL Shreve's air-layer lubrication (Shreve 1968) with some disbelief. Air-layer lubrication was a neat idea, but it was based on the assertion that the landslide mass falls on and traps a layer of air. It sounds so simple, so it ‘must be true.’ If rock avalanches behaved as Autumn leaves that's what they would do. There are some obvious differences in the behavior of falling rocks and falling leaves. Rock-avalanche behavior is dominated by momentum and is appropriately studied as a problem of ballistics and not aerodynamics. That is my first recollected experience in myth busting. My current efforts at myth busting also relate to masses of broken rock, one is the myth of fracture-surface energy, another is the myth of a fragmentation limit.

2 The myth of fracture-surface energy

Conventional wisdom has it that when rocks break they make fracture-surface energy. I was introduced to this wisdom by some wise and well meaning gentlemen who tried to tell me that rock breakage was an energy consuming process and could not possibly contribute to the apparent low friction of rock-avalanche long runout. I tend to ignore both wisdom and convention, especially when I cannot make sense of it. But I did attempt to track down the origin of this conventional wisdom because it seemed to be important. And so I was introduced to Griffith fracture theory (aka. Griffith failure criterion).

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