Intrusions & unconformities

Two kinds of interruption in the rock record: magma pushing in, and time going missing.

By the end of this module, you will be able to
  • Recognise dikes, sills, laccoliths and batholiths
  • Tell the three kinds of unconformity apart, and see how one forms
  • Read the sequence of events in a real-style cross-section

Intrusions: magma that never reached the surface

Not all magma erupts. Much of it forces its way into existing rock and cools underground, forming intrusive igneous bodies (large, deep ones are called plutonic). Erosion may expose them millions of years later. Their shape tells you how the magma moved.

1 2 3 4 5
  1. Volcano, fed by the dike when it was still molten.
  2. Dike: a sheet that cuts straight across the layers.
  3. Sill: a sheet that slides between two layers.
  4. Laccolith: pushes the layers above into a dome, which can raise a hill.
  5. Batholith: a huge body of granite that cooled slowly at depth.
All the intrusions are shown as they are today, cooled to solid rock. Dikes and sills are often basalt, which cools quickly in thin sheets. Batholiths are usually granite, which cools slowly over hundreds of thousands of years and grows large crystals. By the cross-cutting rule, every intrusion is younger than the rock it cuts into. Not to scale: batholiths can be hundreds of kilometres across.

Concordant vs discordant

Concordant intrusions (sills, laccoliths) run parallel to the existing layers. Discordant intrusions (dikes, batholiths) cut across them.

Contact metamorphism

Hot magma "bakes" the rock next to it, leaving a rim of metamorphic rock along the contact. This is contact metamorphism. If the rock above a sheet of basalt is baked too, it's a sill. If it isn't, it may be an old lava flow that was buried later.

Sill Buried lava flow 1 2 2 3 baked above and below baked below only
  1. Baked roof. The magma squeezed in under rock that was already there, and heated it.
  2. Baked floor. Both sheets heat the rock they lie on.
  3. Unbaked cover. The shale arrived after the lava had cooled. Its rubbly, weathered top and the basalt pebbles in the shale (inclusions) tell the same story.
BasaltBaked rim
A sill and a buried lava flow can look the same in a cliff. The baked contacts tell them apart, and so tell you whether the basalt is younger or older than the rock above it.

Unconformities: missing time

The rock record has gaps. Sometimes sediment stops being deposited, or the land is lifted up and eroded, and then deposition starts again. The surface where the gap sits is an unconformity, a buried erosion surface. It might represent a thousand years or a billion. James Hutton first set out his idea of an immensely old Earth in 1785. The unconformity he found at Siccar Point in Scotland in 1788 became its most famous evidence.

Angular unconformity

Tilted or folded layers below, flat layers above. The old rocks were deposited, tilted, eroded flat, and then buried again.

Disconformity

Parallel layers above and below, separated by an uneven erosion surface. These are the hardest to spot. Often a sudden jump in fossil types is the only clue: here, trilobites below and ammonites above, with the species in between missing.

Nonconformity

Sedimentary layers resting on eroded igneous or metamorphic rock. The granite formed deep underground, so kilometres of overlying rock must have eroded away before the sediment arrived.

Try it · build an angular unconformity

An angular unconformity takes four steps, and a lot of time. Drag the slider, or step through, to watch one form.

sea unconformity

Not to scale, and each step is drawn as if it were quick. In reality each can take millions of years, and the erosion step can remove kilometres of rock. Tilting usually happens during mountain building, which is also what lifts the rocks above sea level.

An unconformity tells you that rock was once there and is now gone. The thing you're looking at is the missing time.

Field exercise: read the cross-section

Use the stratigraphic rules to put the events that built this landscape in order, from oldest to youngest. A, B and C are tilted sedimentary layers (A is the lowest of the three). D, E and G are flat-lying sediments. F is a basalt dike. Once you solve it, you can replay the history.

A B C D E F G
Orange line: an erosion surface. F stops at the base of G.
Select events in order: oldest first
Your sequence

Select an event in your sequence to send it back. Use ▲ ▼ to move it.