Intrusive Igneous Bodies: Plutons, Sills & Dikes

A pluton is any body of intrusive igneous rock, regardless of shape or size. The term covers everything from thin sheets to massive chambers. In practice, geologists use more specific names based on size:

• Stock — a relatively small, irregular pluton exposed over < 100 km². It may be the top of a larger body still hidden underground.
• Batholith — a very large pluton exposed over > 100 km². Batholiths are composite: they form from many separate magma injections over millions of years, so they contain distinct internal zones. The Sierra Nevada batholith in California is a famous example, built above an ancient subduction zone.

Batholiths are almost always felsic to intermediate (granite to diorite) because the thick continental crust needed to stop a huge magma body from reaching the surface also provides the silica-rich source material.

A dike is a tabular (wall-like) intrusive body that cuts across pre-existing rock layers. Dikes form when magma injects into a fracture and solidifies before it can reach the surface. They range from millimetres to tens of metres thick and can extend for kilometres.

Because dikes are discordant, they are easy to spot in outcrop: a band of igneous rock crossing sedimentary beds at an angle. Sets of parallel dikes called dike swarms record regional tensional stress that opened many fractures at once. The diabase dikes of the Canadian Shield form enormous swarms hundreds of kilometres long.

A sill is a tabular intrusive body that forms parallel to pre-existing rock layers. Sills form when magma exploits weaknesses between bedding planes, spreading horizontally like a fluid injected between pages of a book. They are typically mafic (gabbro or diabase) because basaltic magma is less viscous and can penetrate thin gaps.

Sills can be thin (metres) or thick (hundreds of metres). The Palisades Sill along the Hudson River in New Jersey is a famous example: a 300 m thick sheet of diabase that cooled into a distinctive columnar jointing pattern.

A laccolith begins like a sill — magma injected between layers — but the magma is too viscous to spread far horizontally. Instead, it pushes the overlying rock upward into a blister or dome. Laccoliths are typically small (a few kilometres across) and often form in sedimentary basins where horizontal layering is strong.

The Henry Mountains of Utah are classic laccolithic domes. The magma that formed them never reached the surface; it simply pushed the overlying strata upward and cooled underground.

When hot magma intrudes cooler country rock, it bakes the surrounding rock in a narrow zone called the contact aureole. Within this zone, temperature and chemically active fluids alter the minerals of the country rock, producing contact metamorphism. The width of the aureole depends on the intrusion's size and temperature: a batholith may bake rock for kilometres, while a thin dike leaves only a millimetre-scale skin.

Sometimes fragments of the country rock break off and fall into the magma. If they do not fully melt, they survive as xenoliths (Greek for 'foreign rock') embedded in the igneous body. Xenoliths are valuable clues: they tell geologists what kind of rock the magma intruded, and sometimes how deep the intrusion formed.

Different tectonic settings favour different intrusive shapes:

• Subduction zones — flux melting generates felsic to intermediate magma that stalls in the thick continental crust, building large batholiths and stocks (e.g. Sierra Nevada, Andes).
• Rift valleys and mid-ocean ridges — extension opens fractures that fill with mafic magma, producing dikes and sills (e.g. oceanic Layer 2B is a sheeted dike complex, sitting above the pillow basalts of Layer 2A and below the gabbro of Layer 3).
• Hotspots — mantle plumes inject magma at various depths, producing a mix of sills, dikes, and layered intrusions.