Inner Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 214

Cauldron subsidence,
caldera collapse,
climactic eruption

Surface

Surface upwarping

Development of plume head, bulk crystallization,
cessation of motion

Diffuse collection of melt at
high crustal levels in a magma
chamber and upward motion
of sinusoidal melt front as a
sill-like laccolith

Intrusion of melt as dykes
to form a composite pluton

Bulk upward buoyant motion of entire melt body
as a starting plume

Collection zones form above melt-filled fractures
and bulk upward motion begins

Fig. 5.18 Various possible types of magmatic diapirs or “rock mushrooms.”

orientated in fractures parallel with the largely vertical axis
of maximum principal stress. It is only high in the crust
that the minimum stress direction deviates sufficiently
from the horizontal to allow horizontal, sill-like sheets of
melt to accumulate. Hence it has also been proposed that
the upper crustal magma bodies grow mostly incremen-
tally and in situ as sill-like blisters due to inflation by frac-
ture-induced squirts of melt from below (Fig. 5.18).
However, many ancient plutons show steeply dipping out-
ward contacts that gravity studies reveal to persist down at
least to mid-crustal depths; it seems that the predicted
magma blisters were often more tumor-like in form.
Regardless of the exact mechanism involved, the
accretion of magmatic material into the crust below
volcanic arcs is the chief method by which continental
lithospheric accretion has taken place over geological time.
The essential role of water noted previously in generating
this melt activity brings us back full circle to the planetary
importance of water in ensuring the long-lasting
dynamism of our Cybertectonic planet. More important
for present purposes, magmatic accretion is also a mute
witness to the volumetrically insignificant nature of
volcanism compared to plutonism.

3

2

1

Fig. 5.19 Experimental diapirs produced as thermal plumes by
heating a layer of more viscous oil uniformly from below: Sequence
1-3 in time.