Sill

A sill is a classic example of a concordant igneous intrusion. Unlike dikes, which cut vertically across rock layers, sills squeeze in between horizontal layers of sedimentary rock or volcanic beds. They are named after the timber sills used in construction to support windows or doors, reflecting their horizontal orientation.

Formation Mechanics

Sills form in shallow crustal environments where magma pressure exceeds the vertical weight of the overlying rock (overburden).

1. Magma Injection: Magma rises through a vertical feeder dike until it hits a barrier—often a tough rock layer or a change in rock density.
2. Lateral Spread: Instead of breaking through, the magma takes the path of least resistance, spreading sideways along the bedding plane.
3. Lifting the Roof: The hydraulic pressure of the magma actually lifts the rock layers above it to make space. This effectively thickens the crust.

Identifying Sills in the Field

Distinguishing a sill from a solidified lava flow can be tricky, as both are horizontal sheets of igneous rock. Geologists look for specific clues:

• Baked Contacts: A sill heats the rock both above and below it. This creates a zone of contact metamorphism (baking) on both sides. A surface lava flow only burns the ground beneath it.
• Chill Margins: The edges of a sill cool faster against the cold country rock, forming finer-grained “chill zones” at both the top and bottom contacts.
• Inclusions: Sills may contain fragments (xenoliths) of the rock layer above them, ripped off during intrusion.

Complex Geometries

While idealized sills are flat sheets, reality is more complex:

• Transgressive Sills: These do not stay in one layer but “step up” or “step down” to adjacent layers, creating a staircase pattern.
• Laccoliths: If the magma is viscous and accumulates rapidly, it pushes the overlying rock up into a dome or mushroom shape, forming a related structure called a laccolith.

Geological and Economic Significance

Sills play a surprising role in resource geology.

• Magmatic Differentiation: In very thick sills, cooling is slow enough for crystals to settle by gravity. Heavy minerals like olivine sink to the bottom, while lighter minerals float. This process can concentrate valuable metals.
• Hydrocarbon Traps: Sills intruding into sedimentary basins can act as impermeable seals (“cap rocks”) that trap oil and natural gas beneath them. However, if the magma is too hot, it may “cook” the oil into graphite or gas.

Famous Examples

• The Palisades Sill (USA): A massive, 200-million-year-old formation visible as steep cliffs along the Hudson River in New York and New Jersey. It is a classic natural laboratory for studying crystal settling.
• The Whin Sill (UK): A durable dolerite intrusion in Northern England. Its hardness made it an ideal strategic foundation for parts of Hadrian’s Wall and Bamburgh Castle.
• Ferrar Dolerites (Antarctica): Part of a massive magmatic event related to the breakup of the supercontinent Gondwana.