Bubnoff unit
The Bubnoff unit (abbreviated B) is a unit of speed equal to 1 m / 106 a. In other words, 1 B is equal to 1 meter in 1,000,000 years, 1 millimeter in 1,000 years, or one micron per year. The Bubnoff unit is employed in geology to measure rates of lowering of earth surfaces due to erosion. Erosion speed of 1 B also means that 1 m3 of earth is being removed from an area of 1 km2 in 1 year (Kearey 2001).
Compared to everyday phenomena, erosion is under most circumstances (excluding rapid events like landslides) an extremely slow process, calling for such a specialized unit. For instance, the current average rate of erosion over Earth's landmasses has been estimated at 30 B (30 m in a million years). There are, however, great regional differences in erosion speed. As an extreme example the watershed area of the Semani River in Albania is eroding at a rate of almost 3000 B (3 millimeters per year), the river having been estimated to transport about 4600 tons of earth per year from the average square kilometer in its watershed (Duff 1993).
References
Duff, D. (1993) Holmes' Principles of Physical Geology. Chapman & Hall, London, 792 pp.
Kearey, P. (2001) The New Penguin Dictionary of Geology. Penguin, London, 336 pp.