Page 62 - Fundamentals of Physical Volcanology

9780632054435_4_003.qxd 12/10/2007 12:18PM Page 39

MAGMA MIGRATION 39

Flood basalt

Ocean
One member of a Continent
giant dike swarm
Partial
melt
zone

Partial Partial
melt melt
zone zone
Fig. 3.5 Three likely scenarios for the fate of magma rising in a dike from a partial melt zone. In the first two, magma is
produced in the mantle beneath continental lithosphere. A dike may penetrate the crust–mantle boundary but fail to reach the
surface, at least immediately, in which case the dike propagates laterally, possibly for many hundreds of kilometers, to form
one of the dikes of a giant dike swarm. In the second continental case, magma may stall at the crust–mantle boundary, lose
some heat and undergo some chemical changes; subsequently a dike grows from this melt body and reaches the surface to
form an extensive flood basalt province. The third case involves melting beneath oceanic lithosphere, in which case dike
intrusions and lava eruptions on the ocean floor build new oceanic crust at a mid-ocean spreading centre.

of 19 km. For a melt source at 45 km, the magma reservoirs form at neutral buoyancy levels within
should be able to rise to a depth of 1.3 km, but the the crust. We discuss shallow magma reservoirs in
dike tip would cease to fracture the crustal rocks at the crust in detail in Chapter 4, but for the moment
a depth of 5 km. Finally, for a melt source at a depth Fig. 3.5 shows some general examples of possible
of 50 km, the situation appears to be particularly scenarios for dike growth and trapping.
strange because the pattern described so far is By far the most impressive type of geological sys-
reversed: the stress conditions will just allow the tem that appears to be a consequence of the vert-

dike tip to reach the surface, even though Table 3.1 ical trapping and lateral spreading of a large volume
implies that the magma inside the dike should not of magma at a deep neutral buoyancy level is called
be able to rise above a depth of ∼800 m. In practice, a giant dike swarm. Many of these structures are
when account is taken of the release of magmatic found in parts of the continental crust on Earth
volatiles in the low-pressure region near the tip of greater than 2 Gyr old, and even the youngest
the dike, it is found to be likely that some sort of formed more than 10 Myr ago. Analogous features
short-lived eruption would take place. also exist on Venus and Mars (Chapter 13). The key
characteristic of a giant dike swarm is that a series
of dikes is seen to radiate outward from a relatively
3.6 Consequences of dike trapping small region for distances of at least several hun-
dred and up to 2000 km. The dikes do not neces-
If the conditions controlling a growing dike can no sarily propagate to all points of the compass – they
longer allow it to grow upward, it may still be able may just occupy a sector as shown in Fig. 3.6.
to expand sideways to accommodate more magma. Exposures of these dikes in regions that have under-
It is easy to show by considering the stresses acting gone great uplift and erosion suggest that the dikes
along all the edges of the dike that it is most likely commonly extend vertically for at least 10 km, and
to grow sideways at the depth where it is neutrally in some cases 20 km. Dike thicknesses range from
buoyant, and this is the basic reason why magma several tens of meters to as much as 200 m, and the