Landslides - Extreme Events: A Physical Reconstruction and Risk Assessment - page 185

50

40

Rhizocarpon
Alpicola

30

Section

20

Lecidea atrobrunnea

10

0

1980

2020

1700 1740 1780 1820 1860 1900 1940

Calendric age AD

Figure 7.2. Growth rates of two lichen species growing on fresh rock faces in rock
fall deposits, New Zealand (from Bull and Brandon, 1998 ).

growing on these freshly exposed rock surfaces. If the rate of growth of a partic-
ular species of lichen can be determined, then the measurement of the size of
lichens growing on a particular surface can provide an indication of the time
of exposure of that surface, and hence the age of the deposit and the landslide
event. Many studies have examined the rates of growth of various species of
lichens growing on tombstones whose age is usually inscribed as the year of
death of the individual. By examining tombstones of different ages, the rate of
growth, which need not always be constant for a given species, can be ascer-
tained. With the same micro- to macro-climatic conditions the size of the same
species of lichen growing elsewhere can be measured and plotted against a curve
of known growth rate to determine an approximate age for a surface (Fig. 7. 2 ).
Rock falls, rock avalanches and debris flows generate new rock exposures
upon which lichens can grow. Dating of the event or events that led to this
exposure is possible because lichen sizes record both the initial time of expo-
sure of the rock surface upon which they grow and subsequent disturbances
that provide further fresh substrate where new lichens can colonise. The use of
lichenometry provides a direct date for the geomorphic event rather than an
indirect date like that provided by other methods such as radiocarbon dating.
In this sense, it is more precise and accurate than radiocarbon dating espe-
cially over the past few hundred years. Events only 2--4 years apart can be dated.
Bull and Brandon (1998) concluded that they could obtain event ages with 95%
uncertainty margins of ± 10 years in their lichenometry study of rock falls gen-
erated by earthquakes in New Zealand. But the range of possible ages using this
technique is largely determined by the frequency of reworking of the rock fall
deposit. As blocks are broken and covered by newly deposited blocks, or when