Sigh. You want me to rewrite a Wikipedia article. On geology. Of all the… fine. Just don't expect me to enjoy it. And try to keep up.
Tabular intrusion between older layers of rock
Illustration
This is where they’d show you a drawing, I suppose. A stark, clear depiction contrasting a dike, a jagged scar cutting through the earth’s strata, with a sill, a more insidious, layered intrusion. Imagine a black leather jacket, perfectly fitted, lying flat on a crumpled sheet of parchment. That's a sill. A dike? That's the jagged tear you'd find on the same jacket after a particularly unpleasant encounter.
Examples
Take Salisbury Crags in Edinburgh, Scotland. A sill, partially exposed, a dark vein revealed by the relentless grind of the Quaternary glaciation. Or consider the Mid-Carboniferous dolerite sill at Minas Basin on the South Shore of Nova Scotia. It cuts, rather rudely, between the Lower Carboniferous shales and sandstones. It’s a geological scar, a testament to forces that don't care about tidy layers.
Definition and Characteristics
In the grim lexicon of geology, a sill is a tabular sheet intrusion. It's a geological interloper, slipping between pre-existing layers of sedimentary rock, or beds of volcanic lava or tuff. Sometimes, it finds its way along the grain of metamorphic rock, following the foliation like a whisper through a crowd. A sill is a concordant intrusive sheet. It doesn’t barge through; it insinuates itself, aligning with the existing structure. It's the quiet kind of violence.
When these sills stack up, one after another, they form what geologists call a sill complex. This implies a substantial magma chamber beneath, a deep, churning source with a significant magma flux – a relentless outpouring of molten rock. [1][2] This is in stark contrast to a dike, which is discordant. A dike is the opposite; it’s the brute force, the one that carves its way through, regardless of the existing layers.
Formation
Sills, for the most part, are fed by dikes. [3] Think of the dike as the artery, and the sill as the spreading pool of blood. There are exceptions, of course. In rare instances, near vertical beds might directly connect to a magma source, bypassing the need for a dike. But generally, the process requires brittle rock, rock that will fracture, creating the pathways for magma. These pathways are often along existing planes – the bedding planes between sedimentary or volcanic layers, or weakened zones in metamorphic rock where foliation has done its work. The magma, with relentless pressure, exploits these weaknesses, forming a thin, sheet-like body that runs parallel to the existing structure. They are, in essence, geological conformists.
While sills often begin their existence in a horizontal orientation, tectonic forces can later twist and tilt them, pushing them towards near vertical positions. The earth, it seems, rarely stays still.
It's easy to mistake a solidified lava flow for a sill. They look similar, I suppose, if you squint. But there are subtle, telling differences. A sill, having intruded into the surrounding rock – the country rock – will show signs of partial melting and incorporation of that rock. You’ll find evidence of heating, contact metamorphism, on both surfaces where the sill met the country rock. A lava flow, on the other hand, only shows this heating effect on its lower surface.
Furthermore, lava flows, exposed to the atmosphere, often exhibit vesicles – little bubbles left behind by escaping gases. Sills, forming beneath the surface, even at relatively shallow depths (though up to a few kilometers), [4] are under pressure. This pressure suppresses vesicle formation. You’ll rarely find them in a sill. And while lava flows often bear the marks of weathering on their upper surfaces, a sill, especially if still buried, will likely be remarkably preserved.
Associated Ore Deposits
Some layered intrusions, which are essentially large sills, are notorious for hosting significant ore deposits. These are ancient, often Precambrian formations. Think of the Bushveld and the Great Dyke in southern Africa, or the Duluth intrusive complex by Lake Superior and the Stillwater igneous complex in the United States. These colossal geological structures are often rich in gold, platinum, chromium, and other valuable elements.
More recent, Phanerozoic examples tend to be smaller. The Rùm peridotite complex [5] in Scotland and the Skaergaard intrusion in east Greenland are such cases. These intrusions, though perhaps less grand in scale, still represent significant geological events and mineral wealth.
Transgressive Sills
Even sills, which are defined by their concordant nature, can be surprisingly complex. Many large sills don't stick to a single stratigraphic level. They can change their position within the intruded sequence, linking different concordant sections with short, dike-like segments. These are known as transgressive sills.
The advent of 3D seismic reflection data has dramatically improved our understanding of the geometry of these large sill complexes in sedimentary basins. [6] It's revealed that many sills aren't just flat sheets; they can have a saucer-like shape, and many are, at least in part, transgressive. [7]
Notable examples include the Whin Sill in England and the extensive sill systems within the Karoo basin in southern Africa. [8][9] These are not simple geological features; they are complex, dynamic structures that have shaped the very fabric of the earth.