length is around 0.01 to 0.001 near the surface, but at depth it ranges from 0.001 to 0.0001. A surface dike 10 meters in thickness will extend about 3 km, while a dike of similar thickness at depth will extend about 30 km. This tendency of intruding magma to form shorter fissures at shallower depths has been put forward as an explanation of en echelon dikes. However, en echelon dikes have also been explained as a consequence of the direction of minimum principal stress changing as the magma ascends from deep to shallow levels in the crust. An en echelon dike set may evolve into single dike with bridges connecting the formerly separate segments and horns showing former segment overlaps. In ancient dikes in deformed rock, the bridges and horns are used by geologists to determine the direction of magma flow. Where there is rapid flow of molten magma through a fissure, the magma tends to erode the walls, either by melting the wall rock or by tearing off fragments of wall rock. This widens the fissure and increases flow. Where flow is less rapid, the magma may solidify next to the wall, narrowing the fissure and decreasing flow. This causes flow to become concentrated at a few points. At Hawaii, eruptions often begin with a curtain of fire where lava erupts along the entire length of a fissure several kilometers long. However, the length of erupting fissure diminishes over time, becoming focused on a short segment of less than half a kilometer. The minimum possible width of a dike is determined by the balance between magma movement and cooling. Multiple and composite dikes There may be more than one injection of magma along a given fissure. When multiple injections are all of similar composition, the dike is described as a multiple dike. However, subsequent injections are sometimes quite different in composition, and then the dike is described as a composite dike. The range of compositions in a composite dike can go all the way from diabase to granite, as is observed in some dikes of Scotland and northern Ireland. After the initial formation of a dike, subsequent injections of magma are most likely to take place along the center of the dike. If the previous dike rock has cooled significantly, the subsequent injection be characterized by fracturing of the old dike rock and the formation of chilled margins on