A new theory of how granite reaches the surface has been researched by Dr Nick Petford and colleagues at Kingston University, by considering the fluid dynamics of magma. The effects predicted for the view currently held by most petrologists, that pear-shaped ‘diapirs’ of magma rise slowly through the Earth’s crust, are not found in the field. The Kingston group have found that granite’s viscosity is orders of magnitude lower than had previously been thought, and believe it could have risen instead as a fluid magma in the form of veins and dykes.

It’s really rather queer that a granite diapir
Doesn’t strain the ground as physics says it ought;
Yet countless textbook pages have pictured them for ages
As pear-shaped blobs, as geologists have taught.

When they’ve looked at granite stumps they’ve seen worn-down dome-topped lumps –
Clear signs of diapirric form below?
And those acres glowing red on our geo-maps, they said,
Were ‘proof’ their big idea was really so.

But there’s another view that’s relatively new:
It’s based on detailed thermal computations
Plus Stokes’s Law for flows, and it’s led some to suppose
That granite’s flow rates beat all expectations.

A centimetre a second it can go, or so it’s reckoned;
It can wind its upward way through veins and dykes.
(So the earlier idea of a blobby diapir
Is one the Kingston group no longer likes.)

And when it finds a fault, the granite doesn’t halt.
Instead, it puddles out to fill the gaps,
Forming horizontal blocks of intruded granite rocks.
And that’s what’s coloured red on all the maps.

[Images: Sydney University School of Geosciences]
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