Dike

A dike (or dyke) is a type of sheet intrusion that cuts discordantly across older “country rock.” In geological terms, discordant means the intrusion fractures or cuts through the existing bedding planes or foliation, rather than running parallel to them (which would be a sill). Dikes are fundamental components of a volcano’s plumbing system, serving as the conduits that transport magma from deep reservoirs to the surface.

Formation Mechanics

Dikes form through a process called hydraulic fracturing. Pressurized magma pushes against the surrounding rock until the stress exceeds the rock’s tensile strength, creating a crack. The magma then injects itself into this opening.

This process is self-propagating: the tip of the magma-filled crack concentrates stress, allowing the fracture to extend further as long as there is sufficient magma pressure. Once the magma cools and solidifies, it forms a tabular body of rock that can range from a few centimeters to tens of meters in width, and can extend laterally for kilometers.

Structural Geometry and Erosion

Because dikes are formed from igneous rock (often basalt or diabase), they are typically harder and more resistant to weathering than the sedimentary or metamorphic rocks they intrude. Over millions of years, as the softer country rock erodes away, the dike remains standing as a prominent, wall-like ridge. This differential erosion creates striking natural barriers across landscapes.

Conversely, if the dike rock is more susceptible to chemical weathering than the host rock, it may erode faster, leaving a narrow trench or ditch.

Large-Scale Systems

Dikes rarely occur in isolation. They are often part of massive geological complexes:

1. Dike Swarms: These are major geological features consisting of hundreds or thousands of parallel dikes. They represent episodes of massive crustal extension and magma generation. The Mackenzie Dike Swarm in Canada is the largest known example, creating a fan shape over 500 km wide.
2. Radial Dikes: Around a central volcanic vent, dikes often radiate outward like spokes on a wheel. This occurs because the pressure from the central magma chamber exerts stress equally in all directions, fracturing the surrounding edifice.
3. Ring Dikes: These are curved dikes that form a circle or ellipse. They are associated with caldera collapse, forming when a block of the crust sinks into an emptying magma chamber, and magma squeezes up into the circular fracture created by the collapse.

Importance in Volcanology

Studying ancient dikes allows geologists to reconstruct the stress fields of past tectonic environments. The orientation of a dike indicates the direction of crustal extension at the time of its formation—the dike opens in the direction of least compressive stress. Furthermore, dikes are crucial for understanding hazard assessments, as they can transport magma horizontally for vast distances, potentially creating new eruptive fissures far from the main summit of a volcano.