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MAGMA STORAGE 49

at a distance, d, from the center of uplift (Fig. 4.7c). tem. The simplest such bodies are sills and lac-
If the uplift ∆h is divided by its value ∆h immedi- coliths (Fig. 4.10). Figure 4.11 shows an outcrop
0
ately above the buried sphere, the relationship is of the Whin Sill, which crops out across much of
northern England and which varies in thickness
2
2
(∆h/∆h ) = 1/[1 + (d /h )] 3/2 (4.1) from 2–3 m to greater than 60 m. Larger sills occur:
0 0
the Basement Sill in Antarctica, for example, is typic-
So, when d = h , (∆h/∆h ) = 0.353, and when ally 300–400 m thick with a maximum thickness
0 0
d = 2 h , (∆h/∆h ) = 0.089, and so on. Figure 4.7c of ∼700 m. Occasionally sills can exceed 1 km in
0 0
shows the results of using a Mogi-type model to fit a thickness.
set of uplift patterns observed at Krafla. This indi- Many large intrusive complexes may have started
cates that the deformation recorded during activity as sills but gradually evolved in shape and size as
at Krafla during the 1970s is consistent with the more magma was added to them. For example, the
inflation and deflation of a magma chamber cen- Skaergaard layered intrusion in Greenland appears
tered at a depth of ∼3 km beneath the surface. to have started as a sill which was intruded along
Similar patterns of deformation have been an unconformity but later developed a more lac-
observed at other volcanoes and successfully mod- colithic shape (Fig. 4.12). A laccolith, like a sill,
eled using Mogi-type models. For instance, Fig. 4.8a intrudes between layers in the country rocks, but
shows the uplift at Kilauea volcano in Hawai’I a laccolith has a less tabular shape because the
between January 1966 and October 1967. Figure intrusion causes updoming of the overlying rocks
4.8b shows the uplift of a single benchmark within (Fig. 4.10b). Many large intrusions have a generally
the caldera between August and October 1967 sheet-like shape (e.g., Bushveld in South Africa,
which has been fitted using a Mogi model. The best Dufek in Antarctica, Newark Island in Canada)
fit suggests that the center of deformation is located suggesting that the formation of sills often plays
at a depth of ∼3 km. Figure 4.9 shows similar mod- an important part in the initial formation of magma
eling for Mauna Loa volcano, also in Hawai’I. The chambers. The largest known basaltic intrusions
center of deformation at Mauna Loa is at a depth of on Earth (Dufek and Bushveld) have volumes of
3
5
∼3.1 km. Seismic activity there defines a storage zone ∼10 km , which is far in excess of the ∼10 km 3
at depths of ∼3 to ∼8 km. The center of deformation volumes of the magma reservoirs beneath current
is thus within the seismically defined storage zone basaltic centers such as Krafla, Kilauea and Mauna
but is located towards its top. This is also the case Loa.

at Kilauea where the deformation center is located Not all intrusions have this sheet-like form.
at a depth of ∼3 km while the seismically defined Figure 4.13 shows, for example, a cross-section
chamber extends from ∼2 to ∼6 km (Fig. 4.3). through the Cadillac Mountain Intrusive Complex
Table 4.2 summarizes geophysical observations in Maine which has a more basin-like shape. The
of magma chamber sizes and locations made on a top of this intrusion has been lost to erosion so it is
number of currently active volcanoes. not known what the geometry of the upper bound-
ary was like, but the basin-like shape appears to
be more like the relatively equant shape usually
4.2.4 Geological evidence for magma storage
inferred for modern magma chambers from geo-
While geophysical and petrological techniques allow physical studies. The various Tertiary intrusive
us to infer the presence of magma chambers complexes of the west coast of Scotland also
beneath active volcanoes, geological studies allow exhibit this more equant shape (Fig. 4.14). These
us to examine the intrusive bodies left behind when centers were some of the earliest intrusive com-
a volcanic system cools and is eroded. Numerous plexes to be studied in detail and are wonderful
intrusive bodies have been identified and studied in examples of what a currently active volcanic center
detail by geologists. Of interest to us here are those may look like after final solidification and erosion.
which are large enough to have played a significant Figure 4.14 shows the association of the central
role in controlling the dynamics of a magmatic sys- complex of Mull with contemporaneous lavas