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MAGMA STORAGE 53

Gabbrofjeld
Uttental
Plateau
Kraemer
Fig. 4.12 Schematic of the shape of
the Skaergaard layered intrusion in
Greenland, which appears to have
been intruded as a sill along an
unconformity and later inflated into a
more laccolithic shape. (Adapted from
fig. 5 in Norton, D., Taylor, H.P. and
Bird, D.K. (1984) The geometry and
high-temperature brittle deformation
of the Skaergaard intrusion. 3 km
J. Geophys. Res., 89, 10178–10192.)

Fig. 4.13 Cross-section through the
5 km
Cadillac Mountain Intrusive Complex
in Maine. The top of the intrusion has
been eroded away, but the basin-like Somesville Granite
shape of the lower part is similar to the
Cadillac Mountain Granite
shapes of modern magma chambers
implied by geophysical studies. Intrusive Breccia
(Modified from fig. 2 in Wiebe, R.A.
(1994) Silicic magma chambers as Gabbro-Diorite
traps for basaltic magmas: the Cadillac Gabbro
Mountain intrusive complex, Mount
Bar Harbor Formation
Desert Island, Maine. Journal of
Geology, 102, 423–437. Copyright Ellsworth Schist
University of Chicago Press.)

4.3 Formation and growth of magma and dikes) probably represent the emplacement
chambers of one pulse of magma and thus are not magma
chambers in the sense that we use the term here. A
A magma chamber is a zone of storage that receives fundamental issue in the development of a magma
multiple pulses of magma and stores magma within chamber is that it must be resupplied intermittently
the crust for an extended period of time and, in or continuously with fresh magma. Any intrusive
doing so, significantly influences the character of body within the crust is subject to cooling: a nar-
the particular volcanic system. Study of intrusive row dike intruded into the shallow crust is likely to
bodies and active magma chambers shows that cool and solidify within a matter of hours or days
magma storage zones vary widely in shape and size. (Fig. 4.16). A thicker dike or a sill will take longer
The bodies with the simplest geometries (i.e., sills to cool but is still likely to do so within years to