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 toward 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.

The orientation of a dike is not random. Dikes open in the direction of least compressive stress in the crust—the path of easiest fracturing. This means that studying the orientation of a dike swarm provides direct information about the stress field of the crust at the time of intrusion, which is valuable data for reconstructing ancient tectonic environments.

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—sometimes filling with water to form a small linear pond.

The width of a dike reflects the amount of magma that intruded: a wide dike indicates a sustained supply of magma, while a thin dike may represent a brief pulse. The texture of the dike rock also varies with depth: fine-grained margins (chill zones) form where the magma cooled quickly against cold country rock, while the center may be coarser-grained if the supply of heat was sustained long enough for crystals to grow.

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, emplaced ~1.27 billion years ago when a large mantle plume impacted the lithosphere.
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. Radial dike patterns are commonly observed in eroded volcanic systems, beautifully exposed at Shiprock in New Mexico and at the island of St. Helena in the South Atlantic.
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. Ring dike complexes of Paleogene age in the British Isles (such as those on Mull and Ardnamurchan in Scotland) represent deeply eroded calderas.

Dike Intrusion as a Precursor to Eruption

In the weeks or days before a volcanic eruption, magma propagates toward the surface as a dike intrusion. This movement creates characteristic seismic swarms—rapid sequences of small earthquakes—and can cause measurable ground deformation detectable by GPS and satellite radar (InSAR).

The 2018 eruption of Kīlauea in Hawaii was directly preceded by a dike intrusion that traveled approximately 40 km along the East Rift Zone in a matter of days. Seismometers tracked the swarm of small earthquakes as the dike tip propagated eastward, giving scientists and civil protection authorities a window to issue warnings before the eruption opened along new ground cracks in the lower East Rift Zone. Similarly, the 2014 Holuhraun eruption in Iceland was preceded by a dike that traveled 40–45 km from the Bárðarbunga volcano in just a few days, tracked in real time by Iceland’s seismic network.

Dikes and Mineralization

Dikes play an important economic role in ore formation. As hot magma forces its way through fractures, it heats groundwater and other fluids in the surrounding rock, driving hydrothermal circulation. These mineral-rich fluids can deposit valuable metals along the margins of the dike and in surrounding fractures. Many gold and silver vein deposits are spatially associated with dike systems, as are some copper and molybdenum ore bodies.

In some geological settings, dikes intruding into coal-bearing sequences have thermally altered (“coked”) the coal into natural coke, a process that has been both economically exploited and studied as a natural analogue to industrial coking.

Importance in Volcanology

Studying ancient dikes allows geologists to reconstruct the stress fields of past tectonic environments. Furthermore, dikes are crucial for understanding hazard assessments: they can transport magma horizontally for vast distances, potentially creating new eruptive fissures far from the main summit of a volcano—often in areas with no volcanic history and therefore no prepared emergency response plans.

Dikes in the Deep Record: Ophiolites and Sheeted Dike Complexes

One of the clearest places to study dikes in their natural three-dimensional context is in ophiolites—ancient fragments of oceanic crust that have been obducted (pushed up) onto continental margins by tectonic forces, exposing what was once the seafloor in cross-section.

A classic ophiolite sequence, like those in Oman or on the island of Cyprus (the Troodos Ophiolite), exposes a vertical slice through oceanic crust from the mantle up to the seafloor. Moving upward through the sequence, geologists encounter layered gabbro (the deep crustal cumulates), then a zone of nearly 100% dikes—the sheeted dike complex—where dike after dike has been intruded so densely that each new dike had only the previous dike to intrude into. This sheeted dike zone is the “frozen plumbing” of the former mid-ocean ridge. Every one of those dikes represents a pulse of seafloor spreading, a moment when the plates pulled apart and magma filled the gap. Studying the Troodos Ophiolite in Cyprus directly shaped early understanding of seafloor spreading and mid-ocean ridge architecture long before submersibles could observe the process directly on the modern ocean floor.

Related Terms

Sill is the concordant equivalent of a dike—a sheet intrusion that runs parallel to existing rock layers rather than cutting across them. Ring dike refers to a circular dike associated with caldera collapse. Dike swarm describes a regional arrangement of many parallel or fan-shaped dikes. Volcanic tremor is the seismic signal associated with fluid (including magma) moving through conduits, including propagating dikes. Ophiolite refers to a fragment of ancient oceanic crust and mantle thrust onto a continental margin, exposing the sheeted dike complexes of ancient mid-ocean ridge systems.