ISSN 1028334X, Doklady Earth Sciences, 2010, Vol. 430, Part 2, pp. 176–180. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © N.S. Zhatnuev, 2010, published in Doklady Akademii Nauk, 2010, Vol. 430, No. 6, pp. 787–791.
176
There are different views on the depth of magma
origin. Until recently, it was generally accepted that
the mantle magmas were probably generated by the
melting asthenosphere. However, most researchers
have eventually concluded that the mantle sources of
magmas were almost certainly formed at much greater
depths.
Various possible mechanisms of magma ascent,
although widely discussed and analyzed, are still a sub
ject of controversy. Some authors speculate that the
dominant driving force for the ascent of magma is
given by hydrostatic stress conditions [6]. According to
other models, magma may move upwards through the
crust by zone melting [3] or, as proposed in the over
view study by N. Rast [4], magma ascent is driven by
either the buoyancy force, pressure build–up in the
bubbles due to boiling, or tectonic stress and magma
overpressure due to the melting of rocks within the
mantle and crust. The buoyancydriven (hydrostatic)
models imply that the medium is a high viscosity fluid
which allows the lower viscosity magma to float on top
of it.
This study attempts to substantiate a model of
magma ascent driven by hydrostatic forces, but in a
solid elastic medium through processes of hydraulic
fracturing (magmadriven fracturing) of the rock
mass. Our previous model considered the propagation
of fluidfilled fractures (hydraulic fractures) in a solid
elastic medium by overpressure resulting from the
density difference between the wall rock and the fluid
[2]. By analogy with this model and based on the
mechanism of overpressure development presented in
[8], here we evaluate the possibility of ascent of mag
mas with different densities and from different mantle
depths. Furthermore, we consider conditions of the
formation of intermittent (or peripheral) magma res
ervoirs at strength barriers.
Figure 1 shows the mechanism of overpressure
development in a rising magma column. Figure 1a
presents data on the crustal and mantle densities after
[5], which were used to calculate the lithostatic pres
sure. This figure also shows pressures exerted by the
column of magmatic fluid at constant densities of 2.5
and 3.0 g/cm
3
. In this case, we used the generic values
of magma density, because natural magma may have
different densities varying with the column height.
Figure 1b shows pressures produced by the magma
column at the respective density and assuming a
magma chamber depth of 400 km, which are the cal
culated projections of pressure curves in Fig. 1a. Fig
ure 1c depicts conditional models of magma cham
bers. The filled points on the curves in Fig. 1b depict
the pressure at the head of the magma column at the
respective level. For example, assuming the depth of
the magma chamber to be 400 km and the column
height to be 300 km (or 100 km from the surface), the
magmatic column may produce an overpressure (in
excess of the lithostatic pressure) of 1153 MPa, if the
magma density is 3.0 g/cm
3
, or 2624 MPa at a density
of 2.5 g/cm
3
. These values are the difference between
the magma pressure and the lithostatic pressure at this
level. The lithostatic pressure at this level is 3113 MPa.
The Dynamics of Deep Magmas
N. S. Zhatnuev
Presented by Academician F.A. Letnikov December 22, 2008
Received February 25, 2009
Abstract
—This study attempts to substantiate a model of magma ascent from deepseated sources driven by
the density difference between magmas and wall rocks in a plastic solid medium via hydraulic fracturing
(magmadriven fracturing). The difference between magma and wall rock densities causes overpressure devel
opment in the head of a magma column. With increasing height of the column or decreasing magma density,
the overpressure can build significantly, which is valid for the case of continuous magma conduit from a deep
seated source. The depth of mantle chambers, their vertical extent, and magma densities seem to be the key
factors in the formation of volcanoes and intracrustal intrusions. Intermittent (or peripheral) magma cham
bers under volcanoes and crustal–mantle intrusive bodies may be formed at strength barriers, in the zone of
elastoplastic transition and at the mantle–crust boundary.
DOI:
10.1134/S1028334X10020066
Geological Institute, Siberian Branch, Russian
Academy of Sciences, Ulan–Ude, Russia
GEOLOGY