This magma intrusion has various forms. In thick crustal areas the magma may only
penetrate part of the way through the crust forming a pluton which hardens, at the
top end, into a granite plug or inselberg (if it is rounded in cross-section) or
a batholith or dike(if it forms as an extended structure).
When a subsequent crustal strain regime thins the overlaying crust, often these granite
structures are hydraulically pushed upwards like a piston in a cylinder. Many inselbergs
and volcanic plugs show lateral marks where the granite has been scraped as it has
risen through sedimentary strata.
Most geology books suggest that these granitic structures have been exposed by erosion
of the overlaying rocks. However, the numerous small inselbergs in East Africa were
formed relatively recently and there has not been enough time for the vast areas
of overlaying strata to erode. Charles Lyell, some 150 years ago, was probably the
first to suggest that the granite blocks were being pushed upwards through the crust.
In the light of a growing Earth model this ‘piston effect’ is just another manifestation
of the fact that tremendous hydrostatic pressures are constantly building up within
the upper mantle.