Page 203 - Fundamentals of Physical Volcanology

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180 CHAPTER 12

abnormal climate. For instance, some studies have several years of cooler weather. The amount of
used the Volcanic Explosivity Index (VEI) (see cooling was, however, only of the same scale as the
section 10.6.1) as a way of defining the magnitude nonvolcanically related temperature variations,
of eruptions and then looked to see if the largest and so it is not possible to prove that the cooling
eruptions coincide with periods of unusual climate. was volcanically induced. When the temperature
Other studies have used Lamb’s “Dust Veil Index” deviations after a number of major eruptions were
(DVI). This is an index derived specifically to look examined in detail, though, a consistent pattern
at the effects of volcanic eruptions on the transmis- emerged which suggested that the largest erup-
sion of sunlight through the atmosphere. It defines tions are related to a small, but consistent, decline
a DVI value for known eruptions, with larger values in temperature for 2–3 years after an eruption
indicating a greater effect on light transmission. (Fig. 12.2).
The DVI value for an eruption is derived from a Table 12.1 shows another examination of vol-
range of information including historical accounts, canic and climate data. In this case data from ice-
optical phenomena, radiation measurements, and core records and DVI studies are compared with
eruption volume. The “dry fog” which impeded the years in which trees in the western USA developed
transmission of sunlight after the Laki and Tambora “frost rings”. The study of tree rings (dendrochron-
eruptions is now known to be caused by the injec- ology) is one of many techniques used by climatol-
tion of sulfurous gases into the atmosphere. ogists to build-up records of past climate. Trees pro-
Chemical reactions between the gas and water duce annual growth rings. If the tree experiences
vapor in the atmosphere cause the conversion of a period of unusually cold weather during its grow-
the gas into droplets of sulfuric acid (H SO ) ing season (i.e., not during winter) it may develop
2 4
which are eventually removed from the atmo- an unusually narrow ring known as a frost ring. By
sphere as a natural form of “acid rain” (or, more cor- examining tree rings in living and dead trees, scien-
rectly, “acid precipitation” as it includes snow as tists have been able to develop dendrochronolo-
well as rain). Recently scientists have started to use gies dating back ∼10,000 years. Years in which frost
data from ice cores drilled near the poles to develop rings occurred can be identified and dated precisely
records of which eruptions caused the greatest and can then be compared with records of volcanic
input of sulfurous gases into the atmosphere. activity to see if the two are linked. Table 12.1 shows
When examined in detail, ice cores can be seen to a comparison between these two sets of data for the
be formed of a series of thin layers, each one repre- years 1600 to 1965. There are a number of points

senting snowfall during an individual year. Thus it of note about this study.
is possible to count and date each layer in the core.
The acidity of the ice in each layer can be measured. • Considering just the ice-core and frost-ring data
The deposition of acidic snow following volcanic there are seven eruptions which are recorded in the
eruptions leads to layers which are unusually acid- ice core which correspond with frost rings. There
ic. Examination of ice cores from Greenland and are a further four frost rings which correspond with
Antarctica reveals the presence of acidity spikes known major eruptions which do not have a signa-
which often can be correlated with known vol- ture in the ice-core record. So overall 11 of the 21
canic eruptions (Fig. 12.1). frost rings (52%) correspond to known large-scale
Having categorized known volcanic eruptions eruptions.
using one of these methods, it is possible to then • Not all of the volcanic eruptions which are
compare the most “significant” eruptions with cli- recorded in the ice core correspond to frost rings.
mate records to see if there is correspondence This could be for any number of reasons such
between volcanic eruptions and periods of unusual as: (i) the eruption did not affect climate; (ii) the
climate. One such study examined the effects of the change in climate was one of warming not cooling;
largest eruptions to have occurred in the 19th and (iii) the effect on climate was too localized to affect
20th centuries. It was found that a few eruptions, trees in the western USA; (iv) the eruption is
such as the 1883 Krakatau eruption and the 1963 recorded in the ice core because it occurred near
eruption of Agung, were followed by a period of Greenland and was not actually a large enough