Magma that cools and solidifies beneath the Earth’s surface creates igneous intrusions. Dikes and sills are two common forms of these intrusions, both characterized as tabular, sheet-like bodies of igneous rock. Understanding the difference between them relies primarily on how the magma interacted with the surrounding, pre-existing rock layers. Geologists study these formations to decipher the stress fields and magmatic histories of ancient crustal environments.
The Fundamental Distinction: Orientation and Host Rock
The defining characteristic separating a dike from a sill is the geometry of the intrusion relative to the host rock layers, known as the country rock. A dike is a discordant intrusion, meaning it cuts across the bedding planes or foliation of the surrounding rock. This cross-cutting relationship often results in a steep or near-vertical orientation.
In contrast, a sill is a concordant intrusion, meaning the magma flowed parallel to the layers of the country rock. Sills typically exploit pre-existing planes of weakness, such as sedimentary bedding planes, resulting in a near-horizontal orientation. The relationship with the host rock at the time of emplacement determines its classification, even if tectonic forces later tilt the feature.
Dikes: Formation and Geological Significance
Dikes form when magma is injected into fractures that open up in the Earth’s crust, typically under tensional stress. This tensional environment creates vertical cracks which the buoyant magma exploits to ascend toward the surface. Dikes are generally high-angle or vertical, ranging in thickness from a few centimeters to many meters, and their lateral extent can stretch for many kilometers.
The primary significance of dikes is their role as conduits, transporting magma from a deep source to the near-surface or feeding overlying volcanic eruptions. They are often found in linear or radial clusters known as dike swarms, which are associated with continental rifting events. Most dikes are composed of basaltic or mafic rock, reflecting magma derived from the mantle.
Sills: Formation and Geological Significance
Sills form when rising magma, often supplied by a feeder dike, encounters a plane of weakness within the crust. The magma spreads laterally along this weakness, such as the contact between two different sedimentary layers, creating a sheet-like body parallel to the surrounding strata. The weight of the overlying rock layers (lithostatic pressure) often assists in confining the magma to this horizontal plane.
Sills can be extensive, sometimes covering hundreds of square kilometers, and are generally more laterally expansive than dikes. A key distinction between a sill and a surface lava flow is the presence of contact metamorphism on both the top and bottom surfaces. This two-sided alteration confirms the feature was intruded beneath the surface, rather than extruded upon it.
Identifying Intrusions in the Landscape
When dikes and sills are exposed by erosion, they create distinct topographic features. Dikes commonly form prominent linear ridges or wall-like structures because the igneous rock, such as basalt, is typically more resistant to weathering than the surrounding host rock. These narrow features cut abruptly across the regional geological grain.
Sills, due to their near-horizontal attitude and greater lateral extent, tend to create flat-topped benches or cap rock layers on hillsides, protecting the softer rock beneath them. Geologists use the Law of Cross-cutting Relationships to determine the relative age of these features. This law states that any rock unit that cuts across another feature must be younger than the feature it cuts, helping reconstruct the sequence of intrusive events.