What Is The Difference Between A Dike And A Sill

What's the Difference Between a Dike and a Sill? A Comprehensive Guide

Understanding the differences between dikes and sills requires delving into the fascinating world of igneous geology. Both are intrusive igneous formations, meaning they are formed from magma that cools and solidifies beneath the Earth's surface, unlike extrusive formations like lava flows. However, their contrasting orientations and resulting geological structures make them distinct features. This comprehensive guide will explore the defining characteristics of dikes and sills, highlighting their formation, composition, geological significance, and the key differences that set them apart.

Defining Dikes: Discordant Intrusions

A dike is a tabular (sheet-like) intrusion of igneous rock that cuts across the bedding planes of the surrounding rock strata. This is known as a discordant intrusion. Imagine a knife slicing through a layered cake – the knife represents the dike, cutting through the pre-existing cake layers. Dikes can vary drastically in size, from centimeters to kilometers in width, and from meters to hundreds of kilometers in length.

Formation of Dikes:

Dike formation is often associated with tectonic activity and volcanic processes. Magma, under immense pressure, seeks pathways of least resistance to ascend towards the surface. These pathways might be fractures, joints, or faults within the existing rock formations. The magma forcefully injects itself into these pre-existing weaknesses, subsequently cooling and solidifying to form the dike. The solidifying magma is often more resistant to weathering and erosion than the surrounding rock, so it sometimes forms a prominent ridge or wall at the surface.

Composition and Characteristics of Dikes:

The composition of a dike reflects the source magma. They can range from felsic (rich in silica, like granite) to mafic (rich in magnesium and iron, like basalt) depending on the specific geological context. The texture of the rock can vary depending on the cooling rate. Rapid cooling at the margins of the dike often produces fine-grained textures, while slower cooling in the interior might result in coarser-grained textures. Many dikes exhibit columnar jointing – a pattern of interconnected, roughly hexagonal columns – a consequence of contraction during cooling.

Geological Significance of Dikes:

Dikes serve as crucial indicators of past tectonic events and magma pathways. Their orientation and composition provide valuable information about the direction and magnitude of stress fields during their formation. Studying dikes allows geologists to reconstruct past tectonic settings, understand magma plumbing systems, and estimate the timing and nature of magmatic activity. Dikes can also act as conduits for hydrothermal fluids, leading to mineral deposits within or adjacent to the dike.

Defining Sills: Concordant Intrusions

A sill is another tabular intrusion of igneous rock, but unlike a dike, it intrudes along the bedding planes of the surrounding rock strata. This makes a sill a concordant intrusion. Think of a layer of icing being spread between layers of a cake – the icing is analogous to the sill, parallel to the existing cake layers. Sills, like dikes, can exhibit significant variations in size, from thin sheets to massive formations extending for many kilometers.

Formation of Sills:

Sills typically form when magma exploits zones of weakness parallel to the bedding planes. These zones of weakness might be permeable layers within sedimentary sequences or existing bedding planes where less resistance exists for magma to intrude. The magma then spreads laterally, injected into the existing rock strata, cooling and solidifying to create the sill. The pressure of the overlying rocks often plays a crucial role in preventing the magma from pushing upward.

Composition and Characteristics of Sills:

Similar to dikes, the composition of sills varies widely, reflecting the source magma. They can range from felsic to mafic compositions. The texture is also influenced by cooling rates, with fine-grained textures at the margins and coarser-grained textures in the interior. Sills can be massive and extensive, sometimes forming significant landscape features. They often exhibit layering and may contain xenoliths – fragments of the surrounding country rock incorporated into the sill during its emplacement.

Geological Significance of Sills:

Sills provide essential insights into the emplacement processes of magma and the tectonic history of a region. Their concordant nature indicates the presence of horizontally-oriented stress fields or pathways of least resistance. Studying sills helps in understanding the distribution of magma bodies and how they influence the surrounding geology. Large sills can significantly alter the stratigraphy and structural framework of the area, often leading to faulting and fracturing. Similar to dikes, sills can also be associated with hydrothermal alteration and mineral deposits.

Key Differences Between Dikes and Sills: Orientation and Mode of Intrusion

The most fundamental difference between dikes and sills lies in their orientation relative to the pre-existing rock strata. Dikes cut across bedding planes (discordant), while sills intrude along bedding planes (concordant). This difference stems from the distinct pathways taken by the magma during its emplacement. Dikes exploit pre-existing vertical fractures or zones of weakness, while sills utilize horizontally-oriented weaknesses to spread laterally.

This difference in orientation leads to differences in their shapes and sizes. Dikes tend to be more linear and elongated, while sills often exhibit a greater lateral extent. Although both can be extensive, sills are typically more sheet-like, while dikes can be more irregular in form.

The mode of intrusion also differs. Dikes often involve forceful injection of magma into pre-existing fractures, leading to potentially high pressures. Sills often form through less forceful intrusion, spreading along existing bedding planes due to buoyancy and lateral pressure. This often leads to more gradual intrusion and potentially flatter sill geometries compared to the often more abrupt geometry of a dike.

Other Distinguishing Factors: Scale and Associated Features

While orientation is the primary differentiator, other factors can help distinguish dikes from sills, particularly in field settings where structural details might be less clear:

• Scale: While both can range in size, very large, laterally extensive sheet-like intrusions are more likely to be sills, whereas smaller, more sharply defined intrusions often prove to be dikes.
• Associated structures: Dikes are often associated with faulting and fracturing, reflecting the forceful nature of their intrusion. Sills might display less fracturing, though they can cause bending or warping of surrounding strata as they intrude.
• Cooling structures: While both can exhibit columnar jointing, the orientation of these joints might differ reflecting the different geometries of the intrusions.
• Xenoliths: Both dikes and sills might incorporate xenoliths, however the frequency and distribution of these fragments might provide additional information for identification.

Advanced Considerations: Complex Intrusive Bodies and Hybrid Features

It's important to note that in reality, intrusive igneous bodies are often complex. Some intrusions might show characteristics of both dikes and sills, exhibiting both discordant and concordant aspects. This complexity arises from multiple phases of magma intrusion or interaction between different magma bodies and existing structures. Detailed geological mapping and analysis are needed to accurately classify these complex intrusive systems. Furthermore, some intrusive formations can defy simple classification as a dike or sill, falling into categories like laccoliths, phacoliths, or lopoliths, which represent more complex magma emplacement scenarios.

Conclusion: Understanding the Significance of Dikes and Sills

Dikes and sills are fundamental components of the Earth's crust, providing crucial insights into tectonic processes, magma evolution, and geological history. Understanding their defining characteristics, particularly their contrasting orientations and modes of intrusion, is vital for geological interpretation. While the primary distinction lies in their relationship to bedding planes, various other factors, like scale, associated structures, and cooling patterns, contribute to accurate classification in complex geological settings. By recognizing the unique properties and formation processes of dikes and sills, geologists can piece together a clearer picture of our planet's dynamic past. Furthermore, recognizing these formations contributes to a broader comprehension of geological hazards and resource exploration, potentially leading to advancements in these domains.