A dike (spelled dyke in British English) is a body of rock, either sedimentary or igneous, that cuts across the layers of its surroundings. They form in pre-existing fractures, meaning that dikes are always younger than the body of rock that they have intruded into.
Dikes are normally very easy to find when looking at an outcrop. For starters, they intrude the rock at a relatively vertical angle. They also have a completely different composition than the surrounding rock, giving them unique textures and colors.
The true three-dimensional shape of a dike is sometimes hard to see at an outcrop, but we know that they are thin, flat sheets (sometimes referred to as tongues or lobes). Clearly, 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. Commonly, dikes are oriented in line with local patterns of jointing.
What defines a dike is that it cuts vertically across the bedding planes of the rock it intrudes. When an intrusion cuts horizontally 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, however, dikes and sills may be tilted too. Their classification reflects the way that they were originally formed, not how they appear after years of folding and faulting.
Often referred to as clastic or sandstone dikes, sedimentary dikes occur whenever sediment and minerals build up and lithify in a rock fracture. They are usually found within another sedimentary unit, but can also form within an igneous or metamorphic mass.
Clastic dikes can form in several ways:
- Through fracturing and liquefaction associated with earthquakes. Sedimentary dikes are most often associated with earthquakes and often serve as paleoseismic indicators.
- Through the passive deposition of sediment into pre-existing fissures. Think of a mudslide or glacier moving over an area of fractured rock and injecting clastic material downward.
- Through the injection of sediment into a not-yet-cemented, overlying material. Sandstone dikes can form as hydrocarbons and gases move into a thick sand bed overlain by mud (not yet hardened into stone). The pressure builds in the sand bed, and eventually injects the bed's material into the above layer. We know this from the preserved fossils of cold seep communities that lived on such hydrocarbons and gases near the top of sandstone dikes.
Igneous dikes form as magma is pushed up through vertical rock fractures, where it then cools and crystallizes. They form in sedimentary, metamorphic and igneous rocks and can force open the fractures as they cool. These sheets range in thickness, anywhere from a few millimeters to several meters.
They are, of course, taller and longer than they are thick, often reaching thousands of meters high and many kilometers in length. Dike swarms consist of hundreds of individual dikes that are oriented in a linear, parallel or radiated fashion. The fan-shaped Mackenzie dike swarm of the Canadian Shield is over 1,300 miles long and, at its maximum, 1,100 miles wide.
Ring dikes are intrusive igneous sheets that are circular, oval or arcuate in overall trend. They form most commonly from caldera collapse. When a shallow magma chamber empties its contents and releases pressure, its roof often collapses into the voided reservoir. Where the roof collapses, it forms dip-slip faults that are nearly vertical or steeply sloping. Magma can then rise up through these fractures, cooling as dikes that make up the outer edge of a collapsed caldera.
The Ossipee Mountains of New Hampshire and Pilanesberg Mountains of South Africa are two examples of ring dikes. In both of these instances, the minerals in the dike were harder than the rock that they intruded into. Thus, as the surrounding rock eroded and weathered away, the dikes remained as small mountains and ridges.
Edited by Brooks Mitchell