Despite the continual round of annual conferences, special sessions and symposia that provide ample opportunity for researchers to get together and talk about igneous processes, the origin of laccoliths and sills continue to inspire and confound geologists. In one sense this is surprising. After all, don't we know all we need to know about these rocks by now? As testified by the diverse range of topics covered in this Specical Publication and elsewhere (Breitkreuz & Petford 2004), the answer is clearly no.

This Special Publication contains 13 papers that cover a diversity of perspectives relating to the geology and emplacement of sills, dykes and laccoliths that together help advance our understanding of their formation. Ablay et al. describe a new fracture-mediated intrusion model that attempts to resolve the sequence of magma and rock displacements comprising felsic magma systems coupled with a thermal model for the lower crust, arguing that the system is driven fundamentally by partial melting at source. Thomson & Schofield report on the relationship between sills, dykes, laccoliths and pre-existing basin structure in the NW European Atlantic margin. Using three-dimensional (3D) seismic data, they interpret the sills as predominantly concave-upwards in shape with flat inner saucers connected to an outer rim by a steeply inclined sheet structure. Magma flow patterns, as revealed by opacity rendering, suggest that sills propagate upwards and outwards away from the magma feeder. Magma emplacement below the level of neutral buoyancy would allow sill inflation and country rock deformation. Fracturing of country rock allowed magma to move upwards and feed shallower-level intrusions.

In a study of sills in the Ferrar large igneous province, Leat surmises from field observations and geochemical relationships (consistent with fractional crystallization during magma flow) that magma may have been transported laterally over large distances, possibly in excess of 3000 km. No feeder dyke swarm has been identified and the implication is that the Ferrar sills are some of the longest subterranean lateral magma flows on Earth. Németh et al. describe magma–water interactions from Pálháza, NE Hungary, and show that the rock succession is made up of a complex association of Miocene rhyolitic shallow intrusions, cryptodomes and endogenous lava domes emplaced into and onto soft, wet pelitic sediment in a shallow submarine environment. Németh & Cronin have examined pit craters and high-level magma feeding systems of a mafic island-arc volcano in the South Pacific. One of the Marum craters, Niri Taten, exposes portions of solidified lava lakes and magma pods that fed spatter cones, small shallow-level intrusions and larger sills that connect through a complex network of dykes to the surface. These features show that shallow-level infiltration of degassed and low-viscosity melts into pyroclastic deposits can play an important role in the growth of scoria and spatter cones. Cone collapse results in lateral escape of magma to form lava flows. Vinciguerra et al. present the results of experiments to determine melt concentration and strain distributions around basalt dykes determined from image analysis and chemical profiles. Melt migration is enhanced by porosity of the microstructure and by the loading conditions. Highest melt concentrations (and presumably highest stress concentrations) occur at the dyke tip. Matrix deformation appears to be controlled by granular flow, but dilatancy occurs near the tip of the dyke, implying coupled magma transport and granular flow. Bunger et al. present the results of analogue experiments and quantitative analysis designed to gain better insight into the mechanics of formation of saucer-shaped sills. In their experiments, fractures that govern final sill geometry are seen to undergo three separate transitions. Each transition is governed by a characteristic timescale relating to viscous (magma) flow and energy dissipation, the time lag between fracture and magma propagation fronts, and the time for a sill to grow as long as it is deep from the surface. Mazzarini & Musumeci have studied sheet-like intrusions emplaced close to the surface on Elba Island, Italy. Field data show that sill and dyke emplacement was controlled by a combination of host-rock fracturing and magma overpressure resulting in hydraulic fracturing. Analysis of the spatial distribution and geometry of the sills and dykes gives clues on fluid pressure conditions and the stress state during magma emplacement, as well as on the depth of emplacement. Stress ratios were used to estimate a magma overpressure of 6–54 MPa at an emplacement depth of approximately 2 km.

Continuing with the Elba theme, Dini et al. have examined a magmatic centre exposed in the western part of Elba Island forming part of a late Miocene complex of nested Christmas-tree laccoliths, plutons and dykes. Igneous activity, comprising of hybridized mantle-derived magmas, occurred in an extensional regime following eastward-migrating compression of the Apennine front. It seems that the magmatic centres of northern Tyrrhenian–Tuscan are distributed along lineaments and developed as a wave moving northeastward across the region. Magmatism was focused by transfer-zone development as back-arc extension migrated in that direction and reactivated former faults. Morris et al. use AMS (anisotropy of magnetic susceptibility) data to look again at the Etive Dyke Swarm, the largest Caledonian dyke swarm in Britain and Ireland. New data are presented that suggest dyking resulted from passive upwelling of magmas into fractures created by regional tectonic stresses around 415 Ma and linked to episodes of sinistral transpression on the Great Glen and other regional faults that lie subparallel to the dyke swarm. The AMS data suggest that the dykes fed a volcanic field much larger than that now preserved as the Glencoe Volcano complex.

Stevenson et al. present the results of a structural study of the Slieve Gullion Igneous Centre, County Armagh, Northern Ireland, carried out to reassess the traditional mechanisms proposed for the emplacement of ring dykes. Using a combination of petrofabric analysis, field measurements and AMS they show that the complex was emplaced as a series of subhorizontal sheets, contrary to the standard ring-dyke emplacement model. The authors caution against the use of applying standard (one-size fits all) theories to explain ring-dyke emplacement. Bermúdez & Delpino provide evidence of the economic importance of high-level intrusions. They have studied the formation of concentric and radial joint systems in basaltic sills from the Neuquén Basin, Argentina, a system of shallow intrusions that contain gas and oil. Their capacity to act as reservoir rocks is due to an increase in effective porosity via simultaneous development of alteration and fracture processes during cooling.

Finally, Winter et al. present new textural and petrofabric analysis of Late Palaeozoic pyroclastic rhyolitic dykes from Saxony, Germany. The orientation of both fiamme and cooling columns and other field relations suggest that the rhyolites represent welded fall-back tuffs formed in vents positioned above an active magmatic dyke system.

In addition to the papers presented here, abstracts of the meeting are published in Visual Geosciences (http://www.springerlink.com/content/110377/).