Abstract
Field studies indicate that nearly all eruptions in volcanic edifices and rift zones are supplied with magma through fractures (dykes) that are opened by magmatic overpressure. While (inferred) dyke injections are frequent during unrest periods, volcanic eruptions are, in comparison, infrequent, suggesting that most dykes become arrested at certain depths in the crust, in agreement with field studies. The frequency of dyke arrest can be partly explained by the numerical models presented here which indicate that volcanic edifices and rift zones consisting of rocks of contrasting mechanical properties, such as soft pyroclastic layers and stiff lava flows, commonly develop local stress fields that encourage dyke arrest. During unrest, surface deformation studies are routinely used to infer the geometries of arrested dykes, and some models (using homogeneous, isotropic half-spaces) infer large grabens to be induced by such dykes. Our results, however, show that the dyke-tip tensile stresses are normally much greater than the induced surface stresses, making it difficult to explain how a dyke can induce surface stresses in excess of the tensile (or shear) strength while the same strength is not exceeded at the (arrested) dyke tip. Also, arrested dyke tips in eroded or active rift zones are normally not associated with dyke-induced grabens or normal faults, and some dykes arrested within a few metres of the surface do not generate faults or grabens. The numerical models show that abrupt changes in Young's moduli(stiffnesses), layers with relatively high dyke-normal compressive stresses (stress barriers), and weak horizontal contacts may make the dyke-induced surface tensile stresses too small for significant fault or graben formation to occur in rift zones or volcanic edifices. Also, these small surface stresses may have no simple relation to the dyke geometry or the depth to its tip. Thus, for a layered crust with weak contacts, straightforward inversion of surface geodetic data may lead to unreliable geometries of arrested dykes in active rift zones and volcanic edifices.
Highlights
What are the mechanical conditions that make it possible for magma to flow from a source chamber to the surface of a volcano, resulting inField observations indicate that almost all eruptions in volcanic edifices and rift zones are supplied with magma through fractures that are generated by magmatic overpressure
In host rocks that can be modelled as homogeneous and isotropic, the stress fields associated with ideal spherical and cylindrical magma chambers may commonly satisfy the conditions for dyke injection and propagation in the vicinity of the chambers while at certain limited distances from the chambers satisfy the conditions for dyke arrest
The results of field observations and analytical and numerical models presented in this paper suggest that the conditions for sheet and dyke arrest or, alternatively, their propagation to the surface, are primarily controlled by the local stress fields in the rift zone or volcanic edifice
Summary
Field observations indicate that almost all eruptions in volcanic edifices and rift zones are supplied with magma through fractures that are generated by magmatic overpressure. Observational data show that dykes commonly become arrested or, more generally, offset on meeting with contacts between layers of contrasting mechanical properties in volcanic edi-. These data indicate that mechanical, rather than thermal, conditions are of primary importance in controlling the arrest of dykes or, alternatively, their propagation to the surface as feeder dykes These mechanical conditions are reflected in the local stress fields associated with individual layers and contacts in the volcanic edifices. The second part of the paper presents analytical and numerical models as to how local stress fields in layered rocks affect dyke propagation, dyke arrest and surface deformation in rift zones and volcanic edifices
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