hypabyssal igneous rocks Hypabyssal igneous rocks are defined as those rocks that lie between the two categories plutonic and volcanic. Plutonic rocks form in large igneous intrusions, deep in the Earth's crust, and are consequently coarse-grained (lentil-sized grains and larger). Volcanic rocks, by contrast, are those that were erupted on or near the surface of the Earth and are fine-grained (grains not visible to the naked eye), having cooled quickly. There is a continuous range of sizes and depths of intrusions between plutonic and volcanic and it is here that the category ‘hypabyssal’ lies. The boundaries of the group of hypabyssal rocks are thus blurred and the term has been dropped by some geologists.
In practice, the term ‘hypabyssal’ is used to encompass dykes and sills (Fig. 1), which are parallel-sided, sheet-like igneous intrusions that form as a result of dilation, that is, a rock mass splits to allow magma to force its way through the country rock (i.e. the rock through which the magma is flowing) without otherwise deforming it. Dykes and sills propagate as the ‘hydrostatic’ pressure of magma flowing through them splits the country rock in the direction of least resistance i.e. at right angles to the least principal stress. During the initial intrusion of the dyke, magma chills on to the sides of the conduit to form a chill margin, but if there is prolonged transport of magma the chill margin and even part of the country rock may be melted. Where oceanic crust is forming at the mid-ocean ridges, the new crust consists entirely of dykes and each successive intrusion is inserted through the centre of its predecessor. This type of intrusive body is called a sheeted dyke complex and is characterized by multiple half-dykes, each of which has a single chill margin.
The word ‘dyke’ (‘dike’ in American usage) is often used to describe a sheet intrusion that cuts across strata (sedimentary beds), in contrast to a sill which was intruded parallel to the strata. Most dykes were intruded vertically, whereas sills were intruded horizontally. Dykes range in thickness from a matter of centimetres to over 100 m but are generally about a metre thick. Smaller dyke-like intrusions (less than 10 cm thick) are known as igneous veins.
Confusion between the terms ‘hypabyssal’ and ‘plutonic’ arises when a large igneous body is formed by the intrusion of a dyke or a sill. For example, the Palisades sill of New Jersey in the USA is over 300 m thick and is unusually large. It shows many features such as chilled margins, cumulates, and pegmatites that, had its lower contact been obscured, would lead to its having been classified as plutonic. Sills can also be of large areal extent. Those of the Karroo, in South Africa, extend over an area of more than 500 000 km
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Dykes, belonging to dyke swarms comprising many parallel and closely spaced dykes commonly propagate along planes of weakness that are fractures formed at right angles to the least pressure and parallel to the regional direction of the greatest horizontal pressure. The orientation of such a regional stress field can be perturbed by the parent igneous intrusion itself and, in extreme cases, radial and concentric dyke swarms can be produced. Dykes may travel large distances under the ground before they break the surface as a fissure eruption. The Cleveland dyke, part of the Skye dyke swarm, extended from the Isle of Skye on the west coast to Scotland, 400 km across mainland Britain to Yorkshire. The surface of the planet Venus also shows linear features that are interpreted as dykes and might express the pattern of stresses on the planet. Sills are less common than dykes, because for magma to occupy a horizontal sheet all the overlying strata have to be lifted up; in contrast, the crust has merely to extend horizontally for a dyke to form. In the north of England, the Whin (hard) sill underlies an area of at least 6000 square kilometres.
Judith M. Bunbury
