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Igneous activity and landforms
Published in Richard J. Chorley, Stanley A. Schumm, David E. Sugden, Geomorphology, 2019
Richard J. Chorley, Stanley A. Schumm, David E. Sugden
Another variation of the sill is the laccolith (Greek: laccos – ‘a cistern’), which is thicker and commonly less extensive than the former, generally of an inverted-bowl or mushroom shape, above which the overlying rocks have been updomed. Laccoliths, which have a huge range of sizes, commonly result from the injection of more viscous acid magmas which tend to solidify rapidly and thicken up close to the magma source. They may be considered to be the intrusive equivalent of the acid lava dome. The type example of laccoliths are the Henry Mountains, Utah (Figure 6.10), formed by the intrusion of diorite porphyry into the shales of a flat-lying shale sandstone sequence of the Colorado Plateau of which the largest (Mt Ellen) rises some 1500 m above the plateau surface and has a basal diameter of 24 km. In fact, each peak is an intrusive complex consisting of up to a dozen smaller laccoliths radiating from a central stock, a small diapirlike discordant igneous intrusion punched up through the country rocks (Figure 6.11). Mt Hillers, the central stock in the group, has a partly exposed stock some 6 km in diameter. On the north-west side of Mt Ellen, Table Mountain is an example of a marginally faulted variant of the laccolith, the bysmalith (Plate 4).
Igneous Rocks
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
A laccolith is a small intrusion having a flat floor and domed roof (Fig. 5.13a), the roof having been arched by the pressure of incoming magma. Laccoliths in the Henry Mountains, Utah, were intruded into mainly horizontal strata and are now exposed after denudation. Others are found in Iceland, in Skye (gabbro laccoliths), and elsewhere.
Potential field modelling and U–Pb geochronology reveal the pluton emplacement dynamics of the Lower Devonian Tarnagulla Granodiorite, southeast Australia
Published in Australian Journal of Earth Sciences, 2022
M. P. Sambrooks, M. A. McLean, R. A. Cayley, R. Maas, R. J. Duncan, C. P. Cairns, J. Ramezani
Interpretation of three quantitative model profiles constructed here suggest emplacement mechanisms and 3D geometries for the Tarnagulla Granodiorite. Profile 1 models the northern portion of the Tarnagulla Granodiorite as tabular or pancake-shaped, with contacts sloping at low angles under the host rocks. This suggests that the intrusion occurred in a sill-like manner characteristic of laccolith or sheet intrusions (Cruden, 1998), consistent with earlier, more qualitative modelling of the pluton (Krokowski De Vickerod et al., 2001). Sill-like intrusion morphologies are a characteristic of intrusions at shallow crustal depths where magma buoyancy exceeds the lithostatic pressure of the overlying host rock, and a component of space for the growing laccolith is created by roof uplift (Brown, 2013), facilitated by shear failure in brittle host rocks (Schmiedel et al., 2017).