What is a Geologic Dike?
A geologic dike is a tabular or sheet-like body of rock that cuts across the pre-existing layers or structures of a host rock. They are formed when molten rock (magma) intrudes into a fracture and solidifies, or when sediment fills a pre-existing crack in another rock. Dikes typically have a discordant relationship with the surrounding rock, meaning they cut across bedding planes or foliation.
Formation Processes of Dikes
Dikes primarily form through two main processes: magmatic intrusion and clastic (sedimentary) infilling. Magmatic dikes result from magma forcing its way into cracks in the Earth's crust, often associated with volcanic activity or spreading centers. Clastic dikes, on the other hand, form when sediment, such as sand or mud, is injected into fissures in unconsolidated or weakly consolidated rock layers, typically due to pressure differences or seismic activity.
A Practical Example
A classic example of magmatic dikes can be seen in many volcanic regions, where vertical or steeply inclined sheets of basalt or dolerite cut through older lava flows or sedimentary strata. For instance, in parts of Scotland, intricate networks of ancient basaltic dikes reveal past periods of intense magmatic activity. Similarly, clastic dikes filled with sandstone can be found cutting through shale layers in areas with historical seismic events.
Importance and Applications in Geology
Geologic dikes are crucial for understanding the structural and magmatic history of an area. They serve as conduits for magma, providing insights into subsurface plumbing systems, and can indicate directions of crustal extension or tectonic stress. Dikes also help in relative dating of rock layers (cross-cutting relationships) and can sometimes host valuable mineral deposits, making them important targets for geological exploration.