Igneous intrusions and volcanic forms: concordant intrusions (sills and laccoliths) versus discordant intrusions (dykes, batholiths and stocks); chilled margins, and baked margins and contact aureoles around intrusions, as way-up and relative-age evidence; cross-cutting relationships; and volcanic forms (shield volcanoes, stratovolcanoes or composite cones, cinder cones, calderas and lava plateaux).

What this dot point is asking

Eduqas wants you to classify intrusive bodies as concordant (sills, laccoliths) or discordant (dykes, batholiths, stocks), to recognise chilled margins, baked margins and contact aureoles as evidence of intrusion and as way-up and relative-age clues, to use cross-cutting relationships to order events, and to recognise the main volcanic forms (shield, stratovolcano, cinder cone, caldera, lava plateau). It draws together igneous classification and metamorphism and feeds the structures and plate-margins topics.

The answer

Intrusive forms: concordant and discordant

Magma that solidifies underground forms intrusive (plutonic) bodies, classified by shape and by their relationship to the bedding of the surrounding country rock:

• Sill (concordant). A sheet-like body intruded parallel to the bedding, often near-horizontal where the beds are flat.
• Laccolith (concordant). A body with a flat floor and a domed roof, formed where viscous magma is injected along bedding and arches the overlying strata upwards.
• Dyke (discordant). A sheet-like body that cuts across the bedding, often near-vertical.
• Batholith (discordant). A very large mass of (usually felsic) intrusive rock, commonly granite, formed deep at the roots of mountain belts; coarse-grained because it cooled slowly.
• Stock (discordant). A smaller discordant pluton (exposed over less than about $100\ \mathrm{km^2}$), often an offshoot of a batholith.

Margins and aureoles as evidence of intrusion

You recognise an intrusion (rather than, say, a buried lava flow) from the way it has affected itself and its surroundings:

• Chilled margin. Inside the igneous body: the edges are finer-grained than the centre, because the magma cooled rapidly against the cold country rock while the interior cooled slowly. A chilled margin shows the body was intruded hot.
• Baked margin. Inside the country rock: the rock immediately adjacent is hardened, reddened or recrystallised by heat (contact metamorphism). A baked margin shows the country rock was already there when the magma arrived.
• Contact metamorphic aureole. A zone of altered country rock around a large intrusion, the grade highest at the contact and decreasing outwards.

The logic is symmetrical and very examinable: a chilled margin is in the intrusion, a baked margin is in the country rock, and the presence of a baked margin proves the intrusion is younger than the rock it baked.

Way-up and relative age from intrusions

A sill bakes the rocks both above and below it and has chilled margins on both sides. An extrusive lava flow bakes only the rocks below it (its top was open to the air) and often has a vesicular top. This is how you tell a buried lava flow (which gives the way up, the baked side being the original base) from a sill.

Cross-cutting relationships

Because an igneous body must intrude rock that already exists, any intrusion that cuts another rock is younger than it (the principle of cross-cutting relationships). A dyke cutting a sill is younger than the sill. Combined with the chilled and baked-margin logic, this lets you order a whole cross-section.

Volcanic forms

Magma that reaches the surface builds volcanic landforms, whose shape depends on the magma's composition and the eruption style:

• Shield volcano. A broad, gently sloping cone built of runny, low-viscosity basaltic lava that flows far before solidifying.
• Stratovolcano (composite cone). A steep cone built of alternating layers of viscous andesitic lava and pyroclastic material, from explosive eruptions.
• Cinder cone. A small, steep cone built of loose pyroclastic fragments (cinders, scoria) thrown out around a vent.
• Caldera. A large basin-shaped depression formed when a magma chamber empties in a violent eruption and the roof collapses.
• Lava plateau. A vast flat upland built by repeated, very fluid basaltic flows (flood basalts) spreading over a large area.

Examples in context

Example 1. A granite batholith and its aureole. A large discordant granite batholith is surrounded by a contact aureole, with high-grade hornfels near the contact grading to unaltered country rock outwards: the baked zone proves the granite is younger than the rocks it intruded.

Example 2. Shield versus stratovolcano. Runny basaltic lava flows far to build a broad, gentle shield, while viscous andesitic magma traps gas and erupts explosively, layering lava and ash into a steep stratovolcano: the magma composition controls the landform.

Try this

Q1. State whether a sill is concordant or discordant, and explain why. [2 marks]

• Cue. Concordant; it is intruded parallel to (along) the bedding of the country rock.

Q2. Explain how a baked margin can be used to show that an intrusion is younger than the country rock. [2 marks]

• Cue. The baked margin is altered country rock heated by the magma, so the country rock must already have existed when the intrusion arrived, making the intrusion younger.

Q3. Name the volcanic form built by runny basaltic lava and the form built by viscous, explosive andesitic eruptions. [2 marks]

• Cue. A shield volcano (runny basaltic lava) and a stratovolcano or composite cone (viscous andesitic lava and pyroclastics).