Technically, it is any geologic body which cuts across: a) flat wall rock structures, such as bedding. b) massive rock formations, usually igneous in origin. Dykes can therefore be either pushed in between (intrusive) or laid down (sedimentary) in origin.
The most usual thing that happens is that later volcanic activity pushes lava through strata which were laid down earlier in a sedimentary fashion, or through earlier igneous rocks. On the Isle of Arran, for example, there are hundreds of igneous dykes giving rise to the term dyke swarm.
Alternatively, sedimentary rocks can be laid down in vertical gaps between strata. Or, after underwater earthquakes, gaps caused by the earthquake can be filled in with breccia, that is, broken rocks.
Dykes are a common, almost universal, feature of the older Palaeozoic rocks. Another type of intrusion is the sill, where later rock is formed between older layers, not through them. So dykes are vertical intrusions, and sill are horizontal intrusions. Many magmatic events produce both vertical and horizontal intrusions.
Dyke swarms can be seen on other planets.
- Ernst R.E; Grosfils E.B.; Mège D. 2001. Giant dike swarms: Earth, Venus, and Mars. Annual Review of Earth and Planetary Sciences'. 29: 489–534. Bibcode:2001AREPS..29..489E. CiteSeerX 10.1.1.473.1821. doi:10.1146/annurev.earth.29.1.489.