Dyke

A dyke is a sheet of rock that formed in a crack in a pre-existing rock body. However, when the crack is between the layers in a layered rock, it is called a sill, not a dike. It is a type of tabular or sheet intrusion, that either cuts across layers in a planar wall rock structures, or into a layer or unlayered mass of rock. they intrude along the plane of least resistance, where rocks are in relative tension; therefore, dike orientations give us clues to the local dynamic environment at the time they formed. 

An intrusion is a body of igneous (created under intense heat) rock that has crystallized from molten magma. Gravity influences the placement of igneous rocks because it acts on the density differences between the magma and the surrounding wall rocks (country or local rocks). In general, silica-rich magmas are less dense than wall rocks, while silica-poor magmas are similar in density to wall rocks. Because of this , low density intrusions take different shapes to higher density intrusions . 

Low-density magmas (such as granitic magmas) are more buoyant in their invasions and cause subsidence of the surrounding wallrocks. High-density magmas (such as basaltic magmas) are closer in hydrostatic equilibrium with the surrounding wallrocks.

Dyke swarms reflect stress fields or basement fabric.

Intersecting Dikes in Black Canyon of the Gunnison

Vertical basalt dikes cutting horizontal lava flows, Lord Howe Island, Australia

Magmatic dikes radiating from West Spanish Peak, Colorado, Usa

Clastic dike (left of notebook) in the Chinle Formation in Canyonlands National Park, Utah.

Eroded channel of a dike

Mafic sill and dike intruding foliated marble

These two mafic intrusions nicely demonstrate the difference between dikes and sills. The lower intrusion is the dike, as it cuts across layering (which in this case is foliation in marble); the upper intrusion is the sill, as it runs parallel to the layering. 

Basalt dyke

The dark-coloured basalt dyke was intruded as molten magma that rose almost vertically through the surrounding horizontal layers of Carboniferous limestone. This contains fossils showing that the lime sediments were originally laid down in shallow tropical seas around 350 million years ago.

Fractures caused by stretching of the crust, allowed basalt magma to feed up to volcanoes above.
Some forms of basalt, like this one, weather quite quickly and are easily removed by wave erosion to form trenches.

Examples of the ideal dike shape for trachybasalt magmas. (A) and (C) are a photograph and accompanying sketch showing a teardrop-shaped trachybasalt dike that provides evidence of a magma flow process that separated the magma into distinctive dike head, body, and tail sections. The bulging head shape formed as a result of the concentration of stress caused by excess magma pressure, with the inferred flow direction from right to left. Note the pale orange-colored (the grey in greyscale) pyroclastic material that fills the tip cavity space adjacent to the dike head. (B) and (D) are a photograph and accompanying sketch showing a bifurcating trachybasalt dike that flowed from right to left.