(c) 2005 Andrew Alden, licensed to About.com, Inc. (fair use policy)
U.S. Geological Survey photo by Jack Lockwood (fair use policy)
A dike (spelled dyke in British English) is a body of rock that cuts across the layers of its surroundings. Most dikes are made of magma, but sediments can form dikes too. The sandstone dike was emplaced as hydrocarbons and gases moved into the thick sand bed soon after it was buried, and the sand dike rose into the overlying mud while all of the material was not yet hardened into stone. We know this because near the top of these sandstone dikes are preserved fossils of cold seep communities that lived on such hydrocarbons and gases.
The igneous dike is exposed in a Hawaiian volcano crater. There molten basaltic magma rose through and cut across existing layers of basalt. Igneous dikes are common in many bodies of plutonic or highly metamorphosed rock, where molten or fluid materials have invaded preexisting rock formations.
What defines a dike is that it cuts across the bedding planes of the rock it intrudes. When an intrusion cuts along the bedding planes, it is called a sill. In a simple set of flat-lying rock beds, dikes are vertical and sills are horizontal. In tilted and folded rocks, dikes and sills may be tilted too.
The true three-dimensional shape of a dike is not exposed at sites like these, but we can show that dikes must be thin, flat tongues or lobes. Clearly they intrude along the plane of least resistance, where rocks are relatively in tension, so dike orientations give us clues to the local dynamic environment at the time they formed. Commonly dikes are oriented in line with local patterns of jointing.