Most magma never reaches the surface. It stalls underground and freezes into coarse-grained igneous rock in bodies called intrusions, or plutons, which we see only after erosion strips away the rock above. In Cross-Cutting Relationships you learned that whatever cuts across rock is younger than it; intrusions are the classic example. Their names describe their shape and how they relate to the layers around them.
- Dike: a sheet of magma that cut across the layers (discordant), filling a crack.
- Sill: a sheet that squeezed in between layers, parallel to them (concordant).
- Laccolith: a sill-like body that bulged the layers above it into a dome.
- Batholith: a huge pluton exposed over more than 100 km², often granite, the roots of old mountain ranges such as the Sierra Nevada. Smaller ones are stocks.
- Volcanic neck: magma frozen in a volcano's throat, left standing when the softer cone eroded.
Why it works
Magma rises because it is lighter than the rock around it. It pushes into cracks, so where the crust is being stretched it forms upright dikes; where the layers above are weaker to lift than to split, it spreads sideways between them as sills. Hot magma also bakes the rock it touches, a narrow zone of contact metamorphism. That baking is a clue to the age order.
Key idea
Intrusions are named by shape: dikes cut across layers, sills run between them, and batholiths are huge bodies exposed over more than 100 km². An intrusion is younger than every rock it cuts or bakes.
Worked examples
Worked example: Sill or buried lava flow?
A basalt layer lies between two sandstones. The sandstone above and below it is baked, and the basalt has no gas holes. Sill or lava flow?
- A lava flow cools at the surface: it bakes only the ground under it, and gas bubbles collect at its top.
- A sill is injected between layers that were already there, so it bakes both.
- Both sides are baked and there are no bubbles: it is a sill, younger than the sandstone above it.
Answer: A sill.
Worked example: Batholith or stock?
A granite body is exposed over an area about 9 km by 14 km. Batholith or stock?
- Area: $9 \times 14 = 126\,\text{km}^2$.
- More than 100 km², so it is called a batholith.
Answer: A batholith.
Common mistake
A sill is easy to mistake for a buried lava flow: both are flat sheets of igneous rock between layers. But the order of events is different. A lava flow is older than the layer on top of it; a sill is younger than both its neighbors. Check the contacts: baked on both sides means sill; baked below only, with a bubbly, weathered top, means lava flow.