Molten rock, or magma, continuously moves beneath the surface, feeding and shaping volcanic structures. These hidden pathways are known as igneous intrusions, and understanding their geometry is fundamental to comprehending a volcano’s plumbing system. The volcanic dike is one of the most common subsurface features, providing a direct link between the deep magma reservoir and the surface.
Defining Volcanic Dikes
A volcanic dike is a sheet-like body of igneous rock that forms when magma solidifies within a fracture that cuts across the existing layers of rock. This cross-cutting relationship, known as discordance, is the defining characteristic of a dike. Unlike intrusions that spread out along bedding planes, a dike forces its way through the structure of the surrounding host rock.
Dikes are typically oriented in a near-vertical or steeply inclined position, which gives them their distinctive wall-like or tabular shape. Their thickness can range from a few centimeters to many meters, and their lateral extent can stretch for kilometers, sometimes forming vast networks known as dike swarms. The rock material depends on the original magma’s chemistry, but many dikes form from basaltic magma, resulting in dark, fine-grained rock like basalt or diabase.
Formation and Injection Mechanics
The formation of a dike begins with the immense pressure exerted by magma within a subsurface chamber. Magma naturally seeks to migrate upward toward the Earth’s surface, overcoming the strength of the surrounding crustal rock by exploiting or creating fractures. The magma is forcibly injected into existing zones of weakness, such as faults or joints, or it can generate new cracks by applying tensional stress that pulls the host rock apart. This tensional stress is often oriented perpendicular to the direction of the dike’s propagation. Once the injection stops and the magma stalls underground, it cools and crystallizes, solidifying into the hard igneous rock that forms the dike.
Distinguishing Dikes from Sills and Other Intrusions
The difference between a dike and a sill is based on their geometric relationship to the surrounding rock layers. A dike is a discordant intrusion, cutting across the host rock layers at a steep angle, while a sill is a concordant intrusion, intruding horizontally parallel to the bedding planes. This distinction is crucial for interpreting the stress fields that existed during the magma’s emplacement. Dikes reflect a crustal state allowing for vertical fracturing, whereas sills form where the path of least resistance is lateral. A dike often acts as the feeder conduit that supplies the magma to form a sill.
Geological Significance and Surface Exposure
Dikes represent the primary feeder channels that transport magma from deep sources to the upper crust and sometimes to the surface. A volcano’s internal structure is often a complex network, with hundreds of these tabular intrusions radiating outward from the central magma chamber. Studying the orientation of dikes allows geologists to reconstruct the ancient stress fields and tectonic movements that were operating when the magma was injected. Dikes exposed on the Earth’s surface are a result of differential erosion. The igneous rock of the dike is often much harder and more resistant to weathering than the surrounding host rock, so the softer material erodes away, leaving the solidified dike standing as a prominent, wall-like ridge. These exposed structures provide direct evidence of the subsurface plumbing system.