Conditions for the arrest of a vertical propagating dyke |
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| Authors: | Benoît Taisne Stephen Tait Claude Jaupart |
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| Institution: | 1.équipe de Sismologie, Institut de Physique du Globe de Paris, Sorbonne Paris Cité,Univ Paris Diderot,Paris,France;2.équipe de Dynamique des Fluides Géologiques, Institut de Physique du Globe de Paris, Sorbonne Paris Cité,Univ Paris Diderot,Paris,France |
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| Abstract: | Magma ascent towards the Earth’s surface occurs through dyke propagation in the vast majority of cases. We investigate two
purely mechanical effects unrelated to cooling or solidification that lead to the arrest of propagation, so that no eruption
occurs. The first is that the input of magma from the source is not maintained continuously, such that a fixed volume of magma
is released. Laboratory experiments show that, in this case, the dyke stops at a finite distance from the source. This behaviour
is specific to the fracturing process in 3-D. We derive a relationship for the minimum magma volume required for an eruption
as a function of magma buoyancy and source depth. When large magma volumes are available, eruption may also be prevented by
a thick low density layer in the upper crust. Numerical studies of dyke propagation show that the dyke continues to rise even
though it is negatively buoyant. Magma accumulates in a swollen nose region at the interface between the low density layer
and the dense basement. Magma overpressure is largest at this interface and increases with increasing penetration into the
upper layer. It may become large enough to induce horizontal fractures in the dyke walls and lateral intrusion of a sill,
which prevents eruption. This requires that the thickness of the low density layer exceeds a threshold value that depends
on the density contrast between magma and host rock. If the magma volume is smaller than a threshold value, neither sill intrusion
nor eruption are possible and magma gets stored in a horizontal blade-shaped dyke straddling the interface. Scaling laws for
variations of ascent rate and for the minimum magma volume allow diagnosis of a failed eruption. |
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