Extreme events over time - Extreme Events: A Physical Reconstruction and Risk Assessment - page 270

chain of these isotopes can be determined accurately. Thermally ionised mass spectrometry
(TIMS) allows direct measurement of the concentration of these isotopes. There are very small
analytical errors associated with this method and it can be used to date events ranging from
less than 1000 years old to more than 500 000 years. This method can be used to estimate an
age range of mass movements by dating carbonates or sinter crusts from above or below the
body of a landslide, or by dating carbonates infilling cracks related to such events. Carbonates,
from raised coral reefs, during earthquakes are also amenable to the U-series technique.

Argon--Argon (Ar--Ar) dating

This dating method is based on the radioactive decay of 40 Kto 40 Ar. The 40 Ar
content of tephra ash is determined by the time that has elapsed since the mineral cooled
below the temperature at which the system became closed and 40 Ar is retained. The method
utilises laser fusion combined with noble gas spectrometry and is useful for Quaternary
deposits as the 40 Ar content in these deposits is usually low and difficult to detect through
more conventional methods due to the half-life of 1.2 × 10 10 . The method is useful for dating
tephra layers that have been deposited following major volcanic eruptions and which act as
stratigraphic markers in a sedimentary sequence. Hence, a minimum age for landslides and
flood deposits for example can be determined where the tephra overlies the event sediments.

Alpha-recoil-track (ART) dating

The method is deemed to be promising in dating the mineral formation of mica
and thus allows for the dating of tephras. As with Ar--Ar dating it can be useful in dating
stratigraphic chronologies. The method identifies particle tracks (alpha-recoil-tracks) in the
crystal lattice by the recoil of the nucleus during the emission of an -particle. The -decay of
238 U, 235 U and 232 Th is used to conduct ART dating.