A lava sill is a distinctive type of igneous intrusion, a tabular sheet of solidified magma that has forced its way horizontally between existing layers of rock. While the term "lava" typically refers to molten rock that has erupted onto the Earth's surface, a "lava sill" commonly refers to either:
- A sill that has intruded into pre-existing layers of volcanic lava or tuff. In this scenario, the sill itself is formed from magma (intrusive igneous rock) and has been emplaced within older, solidified volcanic material.
- A sill composed of igneous rock that is compositionally similar to erupted lava, such as basalt or dolerite, often found in regions with significant volcanic activity. These sills are closely associated with volcanic processes and may be colloquially referred to as "lava sills" due to their rock type.
Essentially, a sill is a concordant intrusive sheet, meaning it lies parallel to the bedding or foliation of the surrounding rock.
Understanding Sills and Lava
To fully grasp the concept of a lava sill, it's important to differentiate between sills and lava:
- Sill: In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. Sills are formed from magma that cools and solidifies beneath the Earth's surface, resulting in an intrusive igneous rock such as dolerite or gabbro.
- Lava: This refers to molten rock that has erupted onto the Earth's surface. When lava cools and solidifies, it forms extrusive igneous rocks like basalt or rhyolite.
The term "lava sill" therefore highlights the close association with volcanic materials, either as the surrounding host rock or through the sill's basaltic composition, which mirrors common lava types.
Key Characteristics of a Lava Sill
Lava sills, like all sills, possess specific geological attributes:
- Tabular Shape: They are relatively thin and extensive, resembling a sheet or slab of rock.
- Concordant Intrusion: They run parallel to the layering of the host rocks, rather than cutting across them. This differentiates them from dikes, which are discordant.
- Composition: Often composed of mafic igneous rocks like basalt (when extruded) or its intrusive equivalent, dolerite (also known as diabase) or gabbro. This composition is common in areas of volcanic activity.
- Host Rock: Can intrude into various rock types, but for a "lava sill," the intrusion into existing layers of volcanic lava, tuff (consolidated volcanic ash), or other volcanic sedimentary rocks is a defining feature.
- Chilled Margins: The edges of the sill, where it contacted the cooler host rock, typically show finer grain sizes due to rapid cooling. The interior of the sill cools more slowly, resulting in larger crystals.
Formation Process
The formation of a lava sill involves a series of geological events:
- Magma Generation: Molten rock (magma) forms deep within the Earth's crust or upper mantle.
- Magma Ascent: This magma begins to rise towards the surface due to its buoyancy.
- Lateral Injection: When the upward pressure of the magma is insufficient to break through overlying rock layers, it can exploit weaknesses or existing bedding planes, particularly within horizontal layers of sedimentary rock or, in this case, pre-existing volcanic layers.
- Solidification: The magma spreads laterally, forming a sheet-like body that cools and crystallizes in situ, beneath the surface. The resulting rock is an intrusive igneous rock, forming the sill.
- Erosion: Over geological time, overlying rocks may erode away, exposing the sill at the Earth's surface.
Types and Composition
Many sills associated with volcanic environments are mafic in composition, meaning they are rich in magnesium and iron. This includes:
- Dolerite Sills: These are very common and are the intrusive equivalent of basaltic lavas. They have a fine- to medium-grained texture and are often dark gray to black.
- Basaltic Sills: While less common to call them "basaltic sills" as basalt is extrusive, the term is sometimes used to emphasize their direct compositional link to basaltic lava flows.
- Gabbro Sills: These are coarser-grained intrusive rocks, also mafic, and represent a slower cooling of basaltic magma.
The composition dictates the color, density, and mineralogy of the sill, often appearing dark due to minerals like pyroxene and olivine.
Where are Lava Sills Found?
Lava sills are frequently found in regions with a history of significant magmatic activity. Notable examples include:
- Large Igneous Provinces (LIPs): These vast areas of magmatic rock, often associated with mantle plumes, are prime locations for extensive sill complexes. Examples include the Siberian Traps, the Deccan Traps, and the Karoo Basin in South Africa.
- Continental Flood Basalt Provinces: Sills often form part of the plumbing system that feeds massive flood basalt eruptions.
- Rift Zones: Areas where the Earth's crust is pulling apart, allowing magma to rise, often feature numerous sills.
- Oceanic Crust: Sills are also found within the oceanic crust, often forming part of the lower crust or intruding into overlying pillow lavas.
Sill vs. Dike
Understanding the difference between sills and dikes is crucial for classifying intrusive igneous bodies:
| Feature | Sill | Dike |
|---|---|---|
| Orientation | Parallel (concordant) to existing rock layers | Cuts across (discordant) existing rock layers |
| Formation | Magma intrudes along bedding planes or foliation | Magma intrudes through fractures or fissures |
| Shape | Tabular, sheet-like, typically horizontal or gently inclined | Tabular, sheet-like, typically vertical or steeply inclined |
| Exposure | Often forms cliffs or ledges when exposed by erosion | Can form walls or ridges, sometimes resistant to erosion |
Identifying a Lava Sill
Geologists identify sills through field observations and laboratory analysis:
- Field Mapping: Observing the relationship between the intrusive body and the surrounding host rocks. A concordant relationship is key.
- Contact Metamorphism: Sills often cause slight metamorphic changes in the host rock immediately adjacent to the intrusion due to heat.
- Chilled Margins: The presence of finer-grained rock at the edges indicates faster cooling against the host rock.
- Petrographic Analysis: Examining thin sections of the rock under a microscope to determine mineralogy, texture, and grain size, which helps classify the rock as intrusive (e.g., dolerite) and confirm its origin.
Significance in Geology
Lava sills are important for several reasons:
- Understanding Magma Pathways: They provide insights into how magma moves through the Earth's crust.
- Paleoenvironmental Reconstruction: The study of sills can help reconstruct ancient stress fields and tectonic settings.
- Resource Exploration: Sills can host valuable mineral deposits (e.g., platinum, nickel, chromium) or act as traps for hydrocarbons.
- Geochronology: They can be dated to determine the age of specific magmatic events and help constrain the timing of regional geological processes.
